Chemical Changes Flashcards
1
Q
Why is the reactivity series used?
A
The series can be used to place a group of metals in order of reactivity based on the observations of their reactions with water, acids and salts.
2
Q
What is potassiums reaction with water?
A
Reacts violently
3
Q
How does sodium react with water?
A
Reacts quickly
4
Q
How does calcium react with water?
A
Reacts less strongly
5
Q
What is potassium’s reaction with acid?
A
Reacts violently
6
Q
What is sodium’s reaction with acid?
A
Reacts violently
7
Q
What is calcium’s reaction with acid?
A
Reacts vigorously
8
Q
What is magnesium’s reaction with acid?
A
Reacts vigorously
9
Q
What is zinc’s reaction with acid?
A
Reacts less strongly
10
Q
What is iron’s reaction with acid?
A
Reacts less strongly
11
Q
What happens when a metal reacts with water?
A
• It produces a metal hydroxide and hydrogen.
• The reactions of potassium and sodium are covered in more detail in another section, but the reaction with calcium and water is given here for reference:
Ca (s) + 2H20 (l) - Ca(OH)2 (aq) + H2 (g)
calcium + water - calcium hydroxide + hydrogen
• The reactions with magnesium, iron and zinc and cold water are very slow.
12
Q
What happens when a metal reacts with a dilute acids?
A
• Only metals above hydrogen in the reactivity series will react with dilute acids.
• The more reactive the metal then the more vigorous the reaction will be.
• Metals that are placed high on the reactivity series such as potassium and sodium are very dangerous and react explosively with acids.
13
Q
What happens when a metal reacts with salt?
A
• The reactivity between two metals can be compared using displacement reactions in salt solutions of one of the metals.
• This is easily seen as the more reactive metal slowly disappears from the solution, displacing the less reactive metal.
• For example, magnesium is a reactive metal and can displace copper from a copper sulfate solution:
Mg + CuSO4-> MgSO4 + Cu
14
Q
What happens when magnesium displaces copper from a copper sulfate solution?
A
• The blue color of the Cuso4 solution fades as colourless magnesium sulfate solution is formed.
• Copper coats the surface of the magnesium and also forms solid metal which falls to the bottom of the beaker.
15
Q
What are the products of magnesium and iron sulfate?
A
Magnesium Sulfate and Iron
16
Q
What are the products of Zinc and Sodium Chloride?
A
No reaction as Sodium is above Zinc
17
Q
What are the products of Lead and Silver Nitrate?
A
Lead Nitrate and Silver
18
Q
What are the products of Copper and Calcium Chloride?
A
No Reaction as Calcium is more reactive than Copper
19
Q
What are the products of Iron and Copper Sulfate?
A
Iron Sulfate and Copper
20
Q
What happens to the more reactive metals in reactions?
A
In all these reactions the more reactive metals lose electrons to become cations.
21
Q
What is the loss of oxidation called?
A
The loss of electrons is oxidation.
22
Q
What does it mean if the metal is high up on the reactivity series?
A
• The higher up the metal is in the reactivity series the more easily it will undergo oxidation.
23
Q
Are unreactive metals more resistant or less resistant for oxidation?
A
Unreactive metals are therefore more resistant to oxidation.
24
Q
What does OIL stand for?
A
Oxidation = is the loss of electrons
25
What does RIG stand for?
Reduction = is the gain of electrons
26
What is oxidation?
The loss of electrons
27
What is reduction?
The gain of electrons
28
By identifying the displacement reaction, what does this mean?
• By identifying what has happened to the electrons in displacement reaction we can deduce the redox changes taking place.
29
What would happen if there was a reaction between zinc and copper sulfate?
Zinc displaces copper from a solution of copper(Il) sulfate.
30
What are the steps to deducing redox change in displacement reactions?
1. Write out the full equation.
2. Write out the ionic equation.
3. Species oxidised.
4. Species reduced.
31
What is a tip to make sure you are correctly writing half equations?
After writing half equations, you can see if they are correct by checking that the number of electrons on either side is the same, which should combine to give 0 charge.
32
What does the earths crust contain?
The Earth's crust contains metals and metal compounds such as gold, copper, iron oxide and aluminium oxide.
33
What happens to useful metals?
Useful metals are often chemically combined with other substances forming ores.
34
What is a metal ore?
A metal ore is a rock that contains enough of the metal to make it worthwhile extracting.
35
Why do metals have to be extracted from their ores?
They have to be extracted from their ores through processes such as electrolysis, using a blast furnace or by reacting with more reactive material.
36
Is the ore being extracted an oxidation or reduction reaction?
In many cases the ore is an oxide of the metal, therefore the extraction of these metals is a reduction process since oxygen is being removed.
37
What are examples of oxide ores?
Common examples of oxide ores are iron and aluminium ores which are called hematite and bauxite respectively.
38
Do unreactive metals have to be extracted chemically?
• Unreactive metals do not have to be extracted chemically as they are often found as the uncombined element.
• They are known as native metals.
• This occurs as they do not easily react with other substances due to their chemical stability.
39
What are examples of native metals?
Examples include gold and platinum which can both be mined directly from the Earth's crust.
40
Where are the most reactive metals on the reactivity series?
The most reactive metals are at the top of the series.
41
How does being a reactive metal link to the tendency to become oxidised?
The tendency to become oxidised is thus linked to how reactive a metal is and therefore its position on the reactivity series.
42
What happens to metals that are higher on the reactivity scale?
Metals higher up are therefore less resistant to oxidation than the metals placed lower down which are more resistant to oxidation.
43
What does the position of the metal determine on the reactivity series?
Determines the method of extraction.
44
How are higher placed metals extracted on the reactivity series?
Higher placed metals (above carbon) have to be extracted using electrolysis as they are too reactive and cannot be reduced by carbon.
45
How are metals lower on the reactivity series extracted?
Lower placed metals can be extracted by heating with carbon which reduces them.
E.g. The oxides of metals which are below carbon can be reduced by heating them with carbon
The carbon removes the oxygen from the metal oxide
46
What is the order of the reactivity series?
Potassium - please
Sodium - send
Lithium - little
Calcium - Charlie's
Magnesium - monkeys
Aluminium - and
Carbon - cute
Zinc - zebras
Iron - in
Hydrogen - hard
Lead - lead
Copper - cages
Silver - safely
Gold - guided
47
Which metals are extracted by electrolysis of the molten chloride?
- potassium
- sodium
- lithium
- calcium
- magnesium
- aluminium
48
Which metals are extracted by heating with a reducing agent such as carbon?
- zinc
- iron
- hydrogen
- copper
49
Which metals are found as pure elements?
- silver
- gold
50
When can a metal reduce another metal?
A metal can reduce another metal (remove oxygen) only if it is more reactive than the metal that is bonded to the oxygen.
51
What do displacement reactions do?
Displacement reactions can be analysed in terms of redox reactions by studying the transfer of electrons.
52
What is an example of a displacement reaction?
- For the example of magnesium and copper sulfate, a balanced equation can be written in terms of the ions involved:
Mg (s) + Cu2+ (aq) + SO42- (aq) → Mg2+ (aq) + SO42- (aq) + Cu (s)
- The sulfate ions, SO42-, appear on both sides of the equation unchanged hence they are spectator ions and do not participate in the chemistry of the reaction so can be omitted:
Mg (s) + Cu2+ (aq) → Mg2+ (aq) + Cu (s)
- This equation is an example of a balanced ionic equation which can be further split into two half equations illustrating oxidation and reduction individually:
Mg → Mg2+ + 2e–
Cu2+ + 2e–→ Cu
- The magnesium atoms are thus oxidised as they lose electrons
- The copper ions are thus reduced as they gain electrons
53
What should you remember about oxidation and reduction?
OIL RIG
Oxidation Is Loss, Reduction Is Gain of electrons
54
What happens when zinc displaces copper from a solution of copper(II)sulfate?
1. Write the full equation
Zn (s) + CuSO4 (aq) → ZnSO4 (aq) + Cu (s)
2. Write the ionic equation
Zn (s) + Cu2+ (aq) + SO42- (aq) —> + SO42- (aq) + Cu (s)
3. Use the ionic equation to rule out / ignore spectator ions that are present as reactants and products
- SO42- is present as a reactant and a product so it can be ignored.
4. Use the ionic equation to identify the species that is oxidised (OIL)
- Zn (s) → Zn2+ (aq) + 2e–
5. Use the ionic equation to identify the species that is reduced (RIG)
- Cu2+ (aq) + 2e– → Cu (s)
55
Which metals on the reactivity series will react with dilute acids?
- Only metals above hydrogen in the reactivity series will react with dilute acids.
56
If the metal is high in the reactivity series, how does this relate to the strength of its reaction?
The more reactive the metal then the more vigorous the reaction will be.
57
What does it mean if a metal is at the top of the reactivity series?
Metals that are placed high on the reactivity series such as potassium and sodium are very dangerous and react explosively with acids.
58
What happens when acids reacts with metals? 
When acids react with metals they form a salt and hydrogen gas:
- The general equation is:
metal + acid ⟶ salt + hydrogen
59
When sulfuric acid is reacted with metals, what does it produce?
Sulfuric acid reacts with metals and produces sulfate salts while hydrochloric acid produces chloride salts.
60
What are metal-acid reactions called?
They are called redox reactions.
61
What does redox mean?
Redox means reduction and oxidation at the same time.
62
What is the ionic equation for the reaction between Zinc and Hydrochloric acid?
Zn + 2H+⟶ Zn2+ + H2
63
What can the equation Zn + 2H+⟶ Zn2+ + H2 be further split into?
Zn → Zn2+ + 2e–
2H+ + 2e–→ H2
- The zinc atoms are thus oxidised as they lose electrons.
- The hydrogen ions are thus reduced as they gain electrons.
- Both reactions are occurring at the same time and in the same reaction chamber hence it is a redox reaction
64
What happens to metal atoms when they undergo oxidation?
Remember metal atoms tend to lose electrons and in these reactions are usually the species that undergoes oxidation.
65
What happens when an acid reacts with a base?
A neutralisation reaction occurs.
66
What ph do bases have?
Bases have pH values above 7.
67
What are alkalis?
Many bases do not dissolve in water but the ones that do dissolve in water are called alkalis.
68
What do bases that do dissolve in water form?
They form an alkaline solution.
69
What are examples of alkalis?
Examples of alkalis are soluble metal hydroxides such as NaOH and Ca(OH)2.
70
What is produced in all acid-base neutralisation reactions?
In all acid-base neutralisation reactions, salt and water are produced:
acid + base ⟶ salt + water
71
What is produced if a metal carbonate is a base?
If the base is a metal carbonate, carbon dioxide is also produced:
acid + base ⟶ salt + water + carbon dioxide
72
What does the identity of the salt produced depend on?
The identity of the salt produced depends on the acid used and the positive ions in the base.
73
What does Hydrochloric acid produce?
Chlorides
74
What does sulfuric acid produce?
Sulfate salts
75
What does nitric acid produce?
Nitrates
76
What do metal oxides and metal hydroxides act as?
They can act as bases.
77
What happens when metal oxides and metal hydroxides react with an acid?
When they react with acid, a neutralisation reaction occurs.
78
What is produced in all acid-base neutralisation reactions?
Salt and water
79
What do acids react with to produce salt, carbon dioxide and water?
Acids will react with metal carbonates.
80
Are reactions of acids with metal carbonates easily distinguishable?
- These reactions are easily distinguishable from acid – metal oxide/hydroxide reactions due to the presence of effervescence caused by the carbon dioxide gas.
81
If there is an acid-base reaction what is produced?
If in an acid-base reaction there is effervescence produced then the base must be a metal carbonate which produces carbon dioxide gas.
82
What is a salt?
A salt is a compound that is formed when the hydrogen ion in an acid is replaced by a metal ion.
83
Why are salts important?
- Salts are an important branch of chemistry due to the varied and important uses of this class of compounds.
84
What important items are salts included in?
- These uses include fertilisers, batteries, cleaning products, healthcare products and fungicides.
85
How many parts of the name of the salt are there?
- The name of a salt has two parts.
86
Where does the first part of the name from the metal come from?
The first part comes from the metal, metal oxide or metal carbonate used in the reaction.
87
Where does the second part of the name come from?
The second part comes from the acid.
88
How can the name of the salts be determined?
The name of the salt can be determined by looking at the reactants.
89
What are examples of naming salts?
- Sodium hydroxide reacts with hydrochloric acid to produce sodium chloride.
- Zinc oxide reacts with sulfuric acid to produce zinc sulfate.
90
What is the formula for group 1 metals?
M+
91
What is the ion for group 1 metals?
Iron(II)
92
What is the formula for Group 7 halide ions?
X-
93
What is the formula for calcium?
Ca2+
94
What is the formula for magnesium?
Mg2+
95
What is the charge of salts?
Salts have no overall charge since the sum of the charges on the ions is equal to zero.
96
What is the formula of magnesium phosphate?
Step 1: Write out the formulae of each ion, including their charges
Mg2+ and PO43-
Step 2: Balance the charges by multiplying them so that the charges are equal but opposite and they cancel out:
(Mg2+) x 3 = 6+ and (PO43-) x 2 = 6-; so (6+) + (6-) = 0
Step 3: The multiplying number for each ion tells you how many ions there must be present in the formula so use these to construct the formula:
The formula is Mg3(PO4)2
Note: Use brackets around the ion if there is more than one ion and the ion contains more than one element
97
How can a soluble salt be made?
A soluble salt can be made from the reaction of an acid with an insoluble base.
98
What happens during the precipitation of the soluble salts?
During the preparation of soluble salts, the insoluble reactant is added in excess to ensure that all of the acid has reacted.
99
What would happen if the insoluble reactant was not added in excess?
If this step is not completed, any unreacted acid would become dangerously concentrated during evaporation and crystallisation
100
What happens to the excess reactant?
The excess reactant is then removed by filtration to ensure that only the salt and water remain.
101
What is left after filtration has taken place?
Since all of the acid has reacted and the excess solid base has been removed then the solution left can only be salt and water.
102
What would have happened to the carbon dioxide produced, if a carbonate was used?
If a carbonate was used as the solid base instead of an oxide or hydroxide, then any carbon dioxide gas produced would have been released into the atmosphere.
103
What is the objective of the preparation of a soluble salt practical?
To prepare a pure, dry sample of a soluble salt from an insoluble oxide or carbonate using a Bunsen burner and dilute acid.
104
What is the hypothesis of the preparation of a soluble salt required practical?
A salt can be prepared and separated by an acid-base neutralisation reaction.
105
What are the materials that should be used in the Preparation of a Soluble Salt Required Practical?
Materials:
- 1.0 mol/dm3 dilute sulfuric acid
- Copper(II) oxide
- Spatula & glass rod
- Measuring cylinder & 100 cm3 beaker
- Bunsen burner
- Tripod, gauze & heatproof mat
- Filter funnel & paper, conical flask
- Evaporating basin and dish.
106
What is the method for Preparation of a Soluble Salt, Required Practical 1?
1. Add 50 cm3 dilute acid into a beaker and warm gently using a Bunsen burner.
2. Add the insoluble oxide slowly to the hot dilute acid and stir until the base is in excess (i.e. until the base stops dissolving and a suspension of the base forms in the acid).
3. Filter the mixture into an evaporating basin to remove the excess base.
4. Gently heat the solution in a water bath or with an electric heater to evaporate the water and to make the solution saturated.
5. Check the solution is saturated by dipping a cold glass rod into the solution and seeing if crystals form on the end.
6. Leave the filtrate in a warm place to dry and crystallise.
7. Decant excess solution and allow the crystals to dry.
107
What should the results of Preparation of a Soluble Salt practical be?
Hydrated copper(II) sulfate crystals should be bright blue and regularly shaped.
108
What should the evaluation of Preparation of a Soluble Salt practical be?
Describe how your crystals compare to the description in the results section. If different, suggest an explanation.
109
What is the conclusion of the Preparation of a Soluble Salt?
Acid-base reactions produce salt and water with the regular shape of the salt reflecting the ionic lattice structure in its bonding.
110
What do acids form when added to water?
When acids are added to water, they form positively charged hydrogen ions (H+)
111
What does the presence of H+ ions make?
It makes a solution acidic.
112
What happens when alkali’s are added to water?
When alkalis are added to water, they form negative hydroxide ions (OH–)
113
What does the presence of OH- ions make?
It makes the aqueous solution an alkali
114
What is the ph scale?
The pH scale is a numerical scale which is used to show how acidic or alkaline a solution is, as it is a measure of the amount of the hydrogen ions present in solution
115
What is the range of numerical values on the ph scale?
The pH scale goes from 1 – 14 (extremely acidic substances can have values of below 1).
116
What is it called for the numbers that are under 7 on the ph scale?
All acids have ph values below 7.
117
What is it called for the numbers that are above 7 on the ph scale?
All alkalis have ph values of above 7
118
What does the lower the pH mean?
The lower the pH then the more acidic the solution is.
119
What does the higher the pH mean?
The higher the pH then the more alkaline the solution is.
120
Where on the pH scale is described as neutral?
A solution of pH 7 is described as being neutral.
121
How can the pH of something be measured?
pH can be measured using an indicator or a digital pH meter.
122
What do pH meters contain?
- pH meters contain a special electrode with a thin glass membrane that allows hydrogen ions to pass through; the ions alter the voltage detected by the electrode.
123
What is a universal indicator?
An indicator is a substance which changes colour depending on the pH of the solution to which it is added.
124
What is the use of a natural indicator?
Generally, natural indicators are wide range indicators contain a mixture of different plant extracts and so can operate over a broad range of pH values.
125
What are the uses of synthetic indicators?
- Synthetic indicators mostly have very narrow pH ranges at which they operate.
- They have sharp colour changes meaning they change colour quickly and abruptly as soon as a pH specific to that indicator is reached.
126
How sensitive are indicators?
Indicators are intensely coloured and very sensitive so only a few drops are needed.
127
When does a neutralisation reaction occur?
- A neutralisation reaction occurs when an acid reacts with an alkali.
128
What happens when an acid reacts with an alkali?
When these substances react together in a neutralisation reaction, the H+ ions react with the OH– ions to produce water.
129
How does the reaction of an acid or alkali lead to a pH of 7?
This is the net ionic equation of all acid-base neutralisations and is what leads to a neutral solution, since water has a pH of 7:
H+ + OH- ⟶ H2O
130
How would you know if a reaction is a neutralisation reaction?
- Not all reactions of acids are neutralisations.
- For example when a metal reacts with an acid, although a salt is produced there is no water formed so it does not fit the definition of neutralisation.
131
Why are neutralisation reactions important in plants?
- Neutralisation is very important in the treatment of soils to raise the pH as some crops cannot tolerate pH levels below 7.
- This is achieved by adding bases to the soil such as limestone and quicklime.
132
What is a universal indicator?
Universal indicator is a wide range indicator and can give only an approximate value for pH.
133
How is a universal indicator made?
It is made of a mixture of different plant indicators which operate across a broad pH range and is useful for estimating the pH of an unknown solution.
134
How does a universal indicator work?
A few drops are added to the solution and the colour is matched with a colour chart which indicates the pH which matches with specific colours.
135
Why do universal indicator colours vary slightly?
Universal indicator colours vary slightly between manufacturer so colour charts are usually provided for a specific indicator formulation.
136
What can not be achieved with a universal indicator?
A common error is to suggest using universal indicator as a suitable indicator for an acid-base titration. This is incorrect as a sharp colour change is required to identify the end-point, which cannot be achieved with Universal Indicator.
137
How can you differentiate if an acid is weak or strong?
Acids can be either strong or weak, depending on how many ions they produce when they dissolve in water.
138
When acids are added to water what happens?
When added to water, acids ionise or dissociate to produce H+ ions:
Hydrochloric acid: HCl ⟶ H+ + Cl–
Nitric acid: HNO3 ⟶ H+ + NO3–
139
What happens to strong acids in water?
Strong acids such as HCl and H2SO4 dissociate completely in water, producing solutions with a high concentration of H+ ions and thus a very low pH.
140
What happens to weak acids in water?
Weak acids such as ethanoic acid, CH3COOH, and hydrofluoric acid, HF, only partially ionise in water, producing solutions of pH values between 4 – 6.
141
What does a high concentration of H+ ions mean for the pH?
Low pH
142
What is set-up for weak acids?
For weak acids, there is an equilibrium set-up between the molecules and their ions once they have been added to water
143
What symbol represents a reversible reaction?
The ⇌ symbol indicates that the process is reversible, as the products can react together forming the original reactants.
144
Where does the equilibrium lie for weak acids?
The equilibrium lies to the left, meaning there is a high concentration of intact acid molecules and therefore a low concentration of ions in solution, hence the pH is that of a weak acid and closer to 7 than a strong acid.
145
What does the term strong and weak acid refer to?
The terms strong and weak refer to the ability to dissociate whereas the term concentration refers to the amount of acid present in solution. A dilute solution of a strong acid can have a lower pH than a concentrated solution of a weak acid, due to the stronger acid undergoing complete dissociation.
146
What does a concentrated solution contain?
A concentrated solution of either an acid or a base is one that contains a high number of acid or base molecules per dm3 of solution so would produce pH values below 4 and above 10.
147
What does a dilute solution contain?
A dilute acid or base solution is therefore one that has much fewer acid or base molecules per dm3 of solution, hence the pH value would lie between 5 and 9.
148
What is an example of a acid or base being strong as it may be made from a weak acid or base?
For example, a dilute solution of HCl will be more acidic than a concentrated solution of ethanoic acid, since most of the HCl molecules dissociate but very few of the CH3COOH molecules do.
149
What does concentration describe?
Remember concentration describes the total number of acid molecules added to the solution but does not consider those that dissociated. This is measured using the pH scale.
150
What does the pH measure?
The pH is a measure of the concentration of H+ ions in a solution.
151
How is the pH scale logarithmic?
The pH scale is logarithmic, meaning that each change of 1 on the scale represents a change in concentration by a factor of 10.
- Therefore an acid with a pH of 3 has ten times the concentration of H+ ions than an acid of pH 4.
152
Why would the strong acid have a lower pH if the two acids were of equal concentration?
- From this we can summarise that for two acids of equal concentration, where one is strong and the other is weak, then the strong acid will have a lower pH due to its capacity to dissociate more and hence put more H+ ions into solution than the weak acid.
153
What does acid strength indicate?
Acid strength indicates the proportion of acid molecules that dissociate while concentration is a measure of how much acid there is per unit volume of water.
154
What happens when an electric current is passed through a molten ionic compound?
- When an electric current is passed through a molten ionic compound the compound decomposes or breaks down.
- The process also occurs for aqueous solutions of ionic compounds.
155
What are electrolytes?
Liquids and solutions that are able to conduct electricity.
156
Can covalent compounds conduct electricity or not?
Covalent compounds cannot conduct electricity hence they do not undergo electrolysis.
157
What is an electrolytic cell?
An electrolytic cell is the name given to the set-up used in electrolysis and which consists of the following:
- electrode
- electrolyte
- anode
- anion
- cathode
- cation
158
What is an electrode?
a rod of metal or graphite through which an electric current flows into or out of an electrolyte.
159
What is an electrolyte?
ionic compound in molten or dissolved solution that conducts the electricity
160
What is anode?
the positive electrode of an electrolysis cell
161
What is an anion?
negatively charged ion which is attracted to the anode
162
What is a cathode?
the negative electrode of an electrolysis cell
163
What is a cation?
positively charged ion which is attracted to the cathode
164
Cathode
Catherine = attracts negative ions
165
What does PANIC stand for?
Positive (is) Anode Negative Is Cathode
166
Can ionic compounds in a solid state conduct electricity?
- Ionic compounds in the solid state cannot conduct electricity since they have no free ions that can move and carry the charge.
167
What are the conditions needed for ions to carry charge?
The ions must be able to move and can only do so in the molten state or when dissolved in a solution, usually aqueous.
168
What happens when the cell is turned on and current is passed through an electrolyte?
When the cell is turned on and an electric current is passed through an electrolyte the ions in the solution start to move towards the electrodes
169
Why are cations attracted to the cathode and anions are attracted to the anode?
Cations are attracted to the cathode and anions are attracted to the anode. Electron flow in electrochemistry occurs in alphabetical order as electrons flow from the anode to the cathode.
170
During electrolysis, where does the power supply move towards?
- During electrolysis the electrons move from the power supply towards the cathode.
171
How do electrons flow in electrochemistry?
- Electron flow in electrochemistry thus occurs in alphabetical order as electrons flow from the anode to the cathode.
172
Where do positive ions with the electrolyte migrate?
Positive ions within the electrolyte migrate towards the negatively charged electrode which is the cathode.
173
Where do the negative ions go in the electrolyte migrate towards?
Negative ions within the electrolyte migrate towards the positively charged electrode which is the anode.
174
What is moving through the metal in electrolysis?
When a metal conducts it is the electrons that are moving through the metal. When a salt solution conducts it is the ions in the solution that move towards the electrodes while carrying the electrons.
175
What is lead(II) bromide?
- Lead(II) bromide is a binary ionic compound meaning that it is a compound consisting of just two elements joined together by ionic bonding.
176
What happens when lead bromide is heated?
- When these compounds are heated beyond their melting point, they become molten and can conduct electricity as their ions can move freely and carry the charge.
- These compounds undergo electrolysis and always produce their corresponding element.
177
How do you predict the products of any binary molten compound?
To predict the products of any binary molten compound first identify the ions present.
178
Where do both the positive and negative ions migrate towards?
- The positive ion will migrate towards the cathode and the negative ion will migrate towards the anode.
- Therefore the cathode product will always be the metal and the product formed at the anode will always be the non-metal.
179
How does electrolysis of lead bromine take place?
1st. Add lead(II) bromide into a crucible and heat so it will turn molten, allowing ions to be free to move and conduct an electric charge.
2nd. Add two graphite rods as the electrodes and connect this to a power pack or battery.
3rd. Turn on the power pack or battery and allow electrolysis to take place.
4th. Negative bromide ions move to the positive electrode (anode) and lose two electrons to form bromine molecules. There is bubbling at the anode as brown bromine gas is given off.
5th. Positive lead ions move to the negative electrode (cathode) and gain electrons to form grey lead metal which deposits on the bottom of the electrode.
180
What are the anode products of electrolysis of lead(II) bromide?
Bromine gas
181
What are the anode products of electrolysis of lead(II) bromide?
Lead metal
182
What does the position of the metal on the reactivity series determine?
The position of the metal on the reactivity series determines the method of extraction.
183
How are higher placed metals extracted?
Higher placed metals (above carbon) have to be extracted using electrolysis as they are too reactive and cannot be reduced by carbon.
184
How are lower placed metals extracted?
Lower placed metals can be extracted by heating with carbon which reduces.
185
What are the negatives of electrolysis?
Electrolysis is very expensive as large amounts of energy are required to melt the ores and produce the electrical current.
186
What is an example of a substances extracted by electrolysis on a large scale?
Aluminium
187
What are the raw materials used in electrolysis?
- aluminium ore (bauxite)
- cryolite (sodium aluminium fluoride)
188
Why is aluminium ore (bauxite) used in electrolysis?
- The bauxite is first purified to produce aluminium oxide, Al2O3.
- Aluminium oxide has a very high melting point so it is first dissolved in molten cryolite producing an electrolyte with a lower melting point, as well as a better conductor of electricity than molten aluminium oxide.
- This reduces the costs considerably making the process more efficient.
- The electrolyte is a solution of aluminium oxide in molten cryolite at a temperature of about 1000 °C.
- The molten aluminium is siphoned off from time to time and fresh aluminium oxide is added to the cell.
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What voltage does the cell in electrolysis work at?
The cell operates at 5-6 volts and with a current of 100,000 amps.
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What happens to the heat generated by the current in electrolysis?
- The heat generated by the huge current keeps the electrolyte molten.
- A lot of electricity is required for this process of extraction which is a major expense.
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What is the overall equation for the extraction of aluminium by electrolysis?
The overall equation is:
2Al2O3 (l) ⟶ 4Al (l) + 3O2 (g)
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What equation represents the oxygen produced at the positive electrode subsequently reacting with graphite electrode to produce carbon dioxide gas?
- Some of the oxygen produced at the positive electrode then reacts with the graphite (carbon) electrode to produce carbon dioxide gas:
C (s) + O2 (g) ⟶ CO2 (g)
- This causes the carbon anodes to burn away, so they must be replaced regularly.
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What is cryolite?
This is the compound sodium aluminium fluoride which is a naturally occurring mineral found mainly in Greenland.
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What will aqueous solutions always contain?
Water molecules
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What do water molecules produce when they dissociate?
They produce H+ and OH- ions
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What can the concentration of solution effect in electrolysis?
The concentration of the solution can affect the products of electrolysis, however, this is beyond the scope of this course and you are not expected to know the specific details of this.
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What is attracted to the positive electrode?
Negatively charged OH– ions and non-metal ions are attracted to the positive electrode.
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What happens if halide ions are present at the anode?
If halide ions (Cl-, Br-, I-) and OH- are present then the halide ion is discharged at the anode, loses electrons and forms a halogen (chlorine, bromine or iodine)
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What happens if there are no halide ions present at the anode?
If no halide ions are present, then OH- is discharged at the anode, loses electrons and forms oxygen.
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What is attracted to the cathode?
Positively charged H+ and metal ions are attracted to the negative electrode but only one will gain electrons.
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What will be produced at the cathode?
- Either hydrogen gas or the metal will be produced.
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What happens if the metal is above hydrogen on the reactivity series in electrolysis?
- If the metal is above hydrogen in the reactivity series, then hydrogen will be produced and bubbling will be seen at the cathode.
- This is because the more reactive ions will remain in solution, causing the least reactive ion to be discharged.
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What gas will be produced at the cathode?
Therefore at the cathode, hydrogen gas will be produced unless the positive ions from the ionic compound are less reactive than hydrogen, in which case the metal is produced
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Work out what ions are attracted to the cathode. In the electrolysis of magnesium iodide solution.
Hydrogen and Magnesium
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Work out which element will be discharged. In the electrolysis of magnesium iodide solution.
Hydrogen is discharged at the cathode as it is less reactive than magnesium.
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Work out what ions are attracted to the anode in the electrolysis of magnesium iodide solution.
Hydroxide (OH-) and iodide (I-)
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What element will be discharged in the electrolysis of magnesium iodide solution?
Iodine is discharged at the anode as iodide ions are present
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What ions will be attracted to the cathode in the electrolysis of copper sulphate solution?
Hydrogen (H+) and copper (Cu2+)
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Which element will be discharged in the electrolysis of copper sulphate solution?
Copper is discharged at the cathode as it is less reactive than hydrogen.
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Work out what ions are attracted to the anode in the electrolysis of copper sulphate solution?
Hydroxide (OH-) and sulfate (SO42-)
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Which element will be discharged in the electrolysis of copper sulphate solution?
Oxygen is discharged at the anode as no halide ions are present.
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If a gas produced at the cathode burns with a ‘pop’ what gas is this?
Hydrogen
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If the gas at the anode relights a glowing splint what gas is present?
Oxygen
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If the gas produced at the anode turns damp blue litmus paper red what gas is present?
Chlorine
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What colour does bromine gas produce?
Red-brown
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What colour does chlorine produce?
Yellow-green
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What are the materials required for Electrolysis of Aqueous Solutions?
Materials:
- Test tubes
- Electrolyte solutions
- 100 cm3 beaker
- Stand and clamp
- Two carbon rod electrodes
- Two crocodile / 4 mm plug leads
- Low voltage power supply
- Blue litmus paper
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What are the tips when completing the Electrolysis of Aqueous Solutions practical?
1. Make sure the test tubes do not cover the electrodes completely and fall to the bottom of the cell or the conductivity will fall considerably and the rate of electrolysis will be very slow.
2. Replacing the test tubes with graduated test tubes or measuring cylinders means that the volume of gas produced over time can be measured.
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What is the method for Electrolysis of Aqueous Solutions?
1. Set up the apparatus as shown in the diagram.
2. Add the aqueous solution to the beaker.
3. Add two graphite rods as the electrodes and connect this to a power pack or battery.
4. Turn on the power pack or battery and allow electrolysis to take place.
5. Record the results in a suitable table (see below) and repeat for another solution, checking the electrodes in between runs to see if any metal has been deposited.
6. The following aqueous solutions are suitable for this investigation: copper chloride, copper sulfate, sodium chloride, sodium bromide, sodium nitrate.
7. The gases produced can be collected in the test tubes to be tested later.
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What is the evaluation for the Electrolysis of Aqueous Solutions practical?
The gases and corresponding tests are:
Hydrogen – lighted splint goes out with a squeaky pop.
Oxygen – a glowing splint relights.
Chlorine – damp blue litmus paper turns red and is then bleached white.
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What is the observation of the dilute NaCl at the anode?
Effervescence, no colour, splint relights, gas is O2
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What is the observation at the cathode of dilute NaCl?
Effervescence, no colour, squeaky pop, gas is H2.
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What are we concerned with in electrochemistry?
In electrochemistry we are mostly concerned with the transfer of electrons, hence the definitions of oxidation and reduction are applied in terms of electron loss or gain rather than the addition or removal of oxygen
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What is oxidation?
Oxidation is when a substance loses electrons and reduction is when a substance gains electrons.
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What happens when the ions come into contact with the electrode?
Electrons are either lost or gained and they form neutral substances.
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What happens when electrons are either lost or gained and they form neutral substances?
These are then discharged as products at the electrodes.
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What happens to negatively charged ions at the anode?
Ions lose electrons and are thus oxidised.
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What happens to positively charged ions at the cathode?
Ions gain electrons and are thus reduced
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How can reduction and oxidising be shown?
- This can be illustrated using half equations which describe the movement of electrons at each electrode.
- It is important to make sure that the charges as well as the number of atoms/ions on each side of the equation are balanced.
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What is the half equation of molten lead(II) bromide?
Pb2+ + 2e– ⟶ Pb
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What is produced at the positive electrode of molten lead(II)?
At the positive electrode (anode) bromine gas is produced by the discharge of bromide ions:
2Br- – 2e– ⟶ Br2
OR
2Br- ⟶ Br2 + 2e–
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What is the half equation of molten aluminium oxide?
- Aluminium ions are discharged at the negative electrode (cathode) and the aluminium is collected at the bottom of the cell:
Al3+ + 3e– ⟶ Al
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What is produced at the positive electrode of molten aluminium oxide?
At the positive electrode (anode) oxygen gas is produced:
2O2- – 4e– ⟶ O2
OR
2O2- ⟶ O2 + 4e–
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What is the half equation at the negative electrode(cathode), when the metal is more reactive than hydrogen leading to it being discharged?
2H+ + 2e– ⟶ H2
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What is the equation for when the metal is less reactive than hydrogen, meaning it is discharged?
Cu2+ + 2e– ⟶ Cu
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What is the equation for when a halide ion is present at a positive electrode?
2Cl– – 2e– ⟶ Cl2
OR
2Cl– ⟶ Cl2 + 2e–
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What is the equation for when a halide ion is not present?
4OH– ⟶ O2 + 2H2O + 4e–
OR
4OH– - 4e– ⟶ O2 + 2H2O
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What do half equations show?
Half equations illustrate the transfer of electrons during a chemical process as they provide a more detailed picture of the redox processes taking place.
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What do half equations combine to give?
Half equations combine to give the ionic equation for an electrolytic cell.
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What are the steps to electrode half equations? 
1. Write down the anode half equation.
2. Write down the cathode half equation.
3. Combine both equations, keeping the same species on either side of the arrow.
4. Cancel out the electrons on either side.
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What is the formula for salt and a hydrogen gas?
• When acids react with metals they form a salt and hydrogen gas:
The general equation is:
metal + acid - salt + hydrogen
