Topic 4 - Chemical Changes Flashcards
1
Q
What do acids produce in aqueous solutions?
A
Acids produce hydrogen ions (H+) in aqueous solutions.
2
Q
What do alkalis produce in aqueous solutions?
A
Aqueous solutions of alkalis contain hydroxide ions (OH–).
3
Q
What is the pH scale and how can it be measured?
A
The pH scale, from 0 to 14, is a measure of the acidity or alkalinity of a solution, and can be measured using universal indicator or a pH probe.
4
Q
What do the different numbers of pH scale mean?
A
- A solution with pH 7 is neutral.
- Aqueous solutions of acids have pH values of less than 7.
- Aqueous solutions of alkalis have pH values greater than 7.
5
Q
How does an indicator work in terms of pH?
A
An indicator is a dye that changes colour depending on whether it’s above or below a certain pH.
6
Q
What is a wide range indicator and why are they useful?
A
Some indicators contain a mixture of dyes that means they gradually change colour over a broad range of pHs. These are called wide range indicators and they’re useful for estimating the pH of a solution, for example Universal indicator.
7
Q
How do universal indicators work?
A
Acid = red = 0
neutral = green = 7
Alkaline = pink/purple = 14
These are the extreme colours on the scale.
8
Q
How do pH probes work?
A
- A pH probe attached to a pH meter can measure pH electronically.
- The probe is placed in the solution you are measuring, and the pH is given on a digital display as a numerical value, meaning its more accurate than an indicator.
9
Q
What is a neutralisation reaction and what is the general equation?
A
An acid will react with an alkali to form a salt and water.
acid + alkali => salt + water
10
Q
What is a neutralisation reaction in terms of H+ and OH- ions and what is the equation?
A
In neutralisation reactions between an acid and an alkali, hydrogen ions react with hydroxide ions to produce water.
H+(aq) + OH-(aq) => H(2)O(l)
11
Q
What is a titration?
A
A titration is an experiment that lets you see what volume of a reactant is needed to react completely with a certain volume of another reactant. For example, you can use a titration to find out exactly how much acid is needed to neutralise a certain quantity of alkali or vice versa. You can also use the results of a titration to find the concentration of one of the reactants.
12
Q
What is the 12 step method for the titration practical?
A
- Use a pipette with a pipette filler to measure out a volume of alkali.
- Put the alkali in a flask with a few drops of indicator.
- Using a funnel, fill a burette with some acid of a known concentration. Run a small amount through the tap, then turn the tap off. Place the flask containing the alkali under the burette.
- Take a reading of the volume of acid in the burette by reading off the value where the acid touches the scale.
- The first titration you do should be a rough titration. This helps to make your final results more accurate by giving you an approximate idea of how much acid is needed to neutralise the alkali. Add the acid to the alkali a bit at a time, giving the flask a regular swirl.
- The indicator changes colour when all the alkali has been neutralised. This is the end-point of the reaction.
- Record the volume of acid left in the burette.
- Calculate the amount of acid that was needed to neutralise the alkali by subtracting the initial reading of acid in the burette from the final reading of acid in the burette. This is your rough titre.
- Now do an accurate titration. Take an initial reading of how much acid is in the burette. Then run the acid in to within 2 cm of the end point.
Continue to add the acid drop by drop - you need to spot exactly when the colour of the indicator changes for your result to be accurate. - Record the amount of acid left in the burette at the end-point of the titration and use this, along with the initial reading, to calculate the volume of acid used to neutralise the alkali.
- Repeat the process of making an accurate titration a few times, until you have at least three results that are within 0.10 cm(3) of each other.
- Calculate the mean volume of acid that was needed to neutralise the alkali, ignoring the rough titre and any anomalous results.
13
Q
What are single indicators?
A
Indicators that only contain one colour changing compound, so has one distinct colour change at a particular pH.
14
Q
What is the colour change for methyl orange in an alkali to neutral titration?
A
- Methyl orange is yellow in alkalis and red in acids.
- If there’s methyl orange and an alkali in the flask at the beginning of the titration, the solution will be yellow.
- When all of the alkali has reacted with the acid that’s added, the indicator will turn red - end point.
15
Q
What are the main hazards in titrations?
A
The acids and the alkalis.
16
Q
What happens when acids are added to an aqueous solution/water?
A
They ionise to produce H+ ions.
17
Q
What is dissociation?
A
The breaking up of a molecule into ions when dissolved in water.
18
Q
What does the strength of an acid tell you? (3)
A
- How strong or weak an acid is tells you what proportion of the acid molecules ionise in water.
- It also tells you the proportion of acid particles that will dissociate to produce H+ ions.
- Strong acids have a higher proportion than weak acids.
19
Q
What happens to strong acids in aqueous solutions?
A
A strong acid is completely ionised in aqueous solution - all the particles dissociate to release H+ ions.
20
Q
What are some examples of strong acids?
A
Examples of strong acids are hydrochloric, nitric and sulfuric acids.
21
Q
What happens to weak acids in aqueous solutions?
A
A weak acid is only partially ionised in aqueous solution.
22
Q
What are some examples of weak acids?
A
Examples of weak acids are ethanoic, citric and carbonic acids.
23
Q
What kind of reaction is ionisation of a weak acid?
A
A reversible reaction, which sets up an equilibrium between the undissociated and dissociated acid.
24
Q
What is the effect of acid strength on reactivity?
A
- If the concentration of H+ ions is higher, the rate of reaction will be faster.
- Since strong acids dissociate more than weak acids, a strong acid will be more reactive than a weak acid of the same concentration.
25
What is the effect of acid strength on pH? (2)
* For a given concentration of aqueous solutions, the stronger an acid, the lower the pH.
* As the pH decreases by one unit, the hydrogen ion concentration of the solution increases by a factor of 10.
26
How can you calculate the change in H+ concentration?
Factor H+ ion concentration changes by = 10 to the power of (- difference in pH)
27
What does the concentration of acid tell you?
Concentration measures the total number of dissolved acid molecules there are in a certain volume of water, not the number of molecules that are ionised to produce hydrogen ions at any given moment - basically tells you how watered down the acid is.
28
What does the reaction of a metal and acid produce?
acid + metal => salt + hydrogen
29
How can you see how reactive different metals are? What is the speed of the reaction indicated by? (3)
* By monitoring the rate of hydrogen production when they react with an acid.
* The more reactive the metal, the faster the reaction - more rapid production of hydrogen bubbles.
* The speed of the reaction is indicated by the rate at which hydrogen bubbles are given off.
30
What are some examples of very reactive metals and how do they react with acids?
Potassium, sodium, lithium and calcium react explosively.
31
What are some examples of less reactive metals and how do they react with acids?
Magnesium, zinc and iron react less violently. Copper won't react with cold, dilute acids.
32
How can the production of hydrogen be tested?
Putting a lit splint at the mouth of the tube containing the acid and metal. If its positive you hear a 'squeaky pop' - louder = more reactive.
33
How can you determine which salt is produced from an acid and metal reaction?
* The metal is the first part of the salt name.
* The second part comes from the acid.
34
How are acid and metal reactions redox reactions?
* The metal atoms lose electrons which go to the hydrogen - oxidation.
* The hydrogen ions gain electrons from the metal - reduction.
35
What is a base?
Bases are substances that can react with acids in neutralisation reactions. All metal oxides and hydroxides are bases.
36
What are soluble metal hydroxides - acid/alkali?
Alkalis because they dissolve in water/are soluble to form OH- ions.
37
What is the reaction of acids with metal oxides/hydroxides (alkalis/bases)?
acid + metal oxide or metal hydroxide (alkalis or bases) => salt + water
38
What is the reaction of acids with metal carbonates?
metal carbonate + acid => salt + water + carbon dioxide
39
What does the particular salt produced in any reaction between an acid and a base or alkali depend on?
* the acid used (hydrochloric acid produces chlorides, nitric acid produces nitrates, sulfuric acid produces sulfates)
* the positive ions in the base, alkali or carbonate - usually metal ions
40
How can soluble salts be made from acids?
* Soluble salts can be made from acids by reacting them with solid insoluble substances, such as metals, metal oxides, hydroxides or carbonates.
* The solid is added to the acid until no more reacts and the excess solid is filtered off to produce a solution of the salt.
* Salt solutions can be crystallised to produce solid salts.
41
What is the 5 step method for producing soluble salts from acids and insoluble bases - required practical?
1. Put the acid in a beaker. Gently warm the dilute acid using a Bunsen burner, then turn off the Bunsen burner.
2. Add the insoluble reactant (the metal oxide or metal carbonate) and stir — it will form a soluble product in the acid as it reacts.
3. Keep adding the insoluble reactant until it is in excess. You'll know when this is because there will be some left over that won't react — this shows that all the acid has been neutralised and the reaction has finished.
4. Then you need to filter out the excess insoluble reactant to get the salt solution. This is done using filter paper and a filter funnel.
5. You'll be left with a salt solution at the end of the filtration. You can convert this into pure, solid crystals of salt using crystallisation. To do this you first need to heat the salt solution using a water bath or an electric heater, in order to evaporate some of the water and make the solution more concentrated. Then, stop heating it and leave the solution to cool. Crystals of the salt should form, which can be filtered out of the solution and then dried.
42
How are metal oxides produced and what kind of reaction is this?
Metals react with oxygen to produce metal oxides. The reactions are oxidation reactions because the metals gain oxygen.
43
What is oxidation and reduction in terms of oxygen?
* Oxidation can be defined as the gain of oxygen by an element or compound.
* Reduction can be defined as the loss of oxygen by an element or compound.
44
What happens to metals when they react, and what is the reactivity of a metal related to?
When metals react with other substances, the metal atoms lose electrons to form positive ions. The reactivity of a metal is related to its tendency to form positive ions.
45
What is the reactivity series?
A list of metals that are arranged in order of how reactive they are. The most reactive metals are at the top and the least reactive are at the bottom.
46
What non-metals are often included in the reactivity series?
The non-metals hydrogen and carbon are often included in the reactivity series.
47
How can you place the metals in the reactivity series in order?
If you compare the relative reactivity of different metals with either an acid or water, and put them in order from most reactive to the least reactive - reactivity series.
The higher a metal is in the reactivity series, the more easily it reacted with water or acid.
48
How do potassium, sodium and lithium react with water at room temperature?
Very rapidly.
49
How does calcium react with water at room temperature?
Quite rapidly.
50
How do magnesium, zinc, iron and copper react with water at room temperature?
No reaction.
51
How do potassium, sodium and lithium react with acids?
Dangerously fast reaction.
52
How do calcium, magnesium, zinc, iron and copper react with dilute acids - all are different?
* Calcium - extremely vigorous reaction.
* Magnesium - rapid reaction.
* Zinc - quite rapid reaction.
* Iron - quite slow reaction.
* Copper - no reaction
53
How is the reactivity of metals with water or dilute acids related to the tendency of the metal to form its positive ion?
The more reactive a metal is or the higher up the reactivity series it is, the more likely/able it is to form its positive ion.
54
How are metals found?
Unreactive metals such as gold are found in the Earth as the metal itself but most metals are found as compounds that require chemical reactions to extract the metal.
55
What is a displacement reaction?
A displacement reaction occurs when a more reactive element displaces, or pushes out, a less reactive element from a compound that contains the less reactive element - e.g, displacing iron from iron oxide with potassium.
56
How can metals less reactive than carbon be extracted from their oxides?
Metals less reactive than carbon can be extracted from their oxides by reduction with carbon.
57
What is an ore?
Ores are naturally occurring rocks that contain metals or metal compounds in sufficient amounts to make it worthwhile extracting them.
58
What is oxidation and reduction in terms of electrons?
* Oxidation is the loss of electrons.
* Reduction is the gain of electrons.
59
What are electrolytes?
When an ionic compound is melted or dissolved in water, the ions are free to move about within the liquid or solution. These liquids and solutions are able to conduct electricity and are called electrolytes.
60
What is electrolysis? (4)
* Passing an electric current through electrolytes causes the ions to move to the electrodes.
* Positively charged ions (cations) move to the negative electrode (the cathode).
* Negatively charged ions (anions) move to the positive electrode (the anode).
Ions are discharged at the electrodes producing elements.
* This process is called electrolysis.
61
What is produced at the electrodes when a simple ionic compound in the molten state is electrolysed? What would be produced for lead bromide as an example?
When a simple ionic compound is electrolysed in the molten state using inert electrodes, the metal is produced at the cathode and the non-metal is produced at the anode.
Lead is produced at the cathode and bromine is produced at the anode.
62
When is electrolysis used to extract metals from molten compounds?
Electrolysis is used if the metal is too reactive to be extracted by reduction with carbon, or if the metal reacts with carbon.
63
Which redox reactions occur at the electrodes in electrolysis?
* Reduction is occurring at the negative electrode as the positive ions are gaining electrons.
* Oxidation is occurring at the positive electrode as the negative ions are losing electrons.
64
How do you write a half equation for the reactions that take place at the negative electrodes? (4 steps)
1. Write the symbol for the positive ion in the electrolyte on the left hand side of the equation.
2. Write the symbol for the neutral atoms or molecules produced, on the right-hand side of the equation.
3. Balance the number of atoms in this equation.
4. Balance the charges by adding or subtracting electrons (e-).
65
How do you write a half equation for the reactions that take place at the positive electrodes? (4 steps)
1. Write the symbol for the negative ion in the electrolyte on the left hand side of the equation.
2. Write the symbol for the neutral atoms or molecules produced, on the right-hand side of the equation.
3. Balance the number of atoms in this equation.
4. Balance the charges by adding or subtracting electrons (e-).
66
How is aluminium extracted, and what from?
Aluminium is extracted from aluminium oxide by electrolysis.
67
What is the ore of aluminium oxide?
Bauxite which has many impurities.
68
What is the purified bauxite ore known as?
Aluminia - Al(2)O(3)
69
What is aluminium oxide mixed with as the first step of electrolysis? Why?
* Aluminium oxide is mixed with cryolite, a sodium based compound.
* Lowers melting point from 2000 degrees C to 1000 degrees C.
* This reduces the amount of energy needed and saves money.
70
In the electrolysis of aluminium oxide, what state is it in and what are the anodes made of?
* Molten state.
* Graphit - carbon.
71
What happens at the cathode (-) in the electrolysis of aluminium oxide - including equation?
The cathode attracts the aluminium cations, forcing them to gain electrons and become neutral aluminium atoms. The overall equation is;
Al(3+) + 3e- => Al
72
What happens at the anode (+) in the electrolysis of aluminium oxide - including equation?
The anode attracts oxide anions, forcing them to lose electrons and become neutral. The overall equation is;
2O(2-) => O(2) + 4e-
73
What happens to the carbon electrodes in the electrolysis of aluminium oxide at 1000 degrees C?
The carbon electrodes slowly burn away, reacting with the oxygen.
This adds to the cost, as the carbon anode needs to be replaced.
74
What is the overall equation for the electrolysis of aluminium oxide?
2Al(2)O(3) => 4Al + 3O(2)
75
What do the ions discharged, when an aqueous solution is electrolysed using inert electrodes, depend on?
The ions discharged, when an aqueous solution is electrolysed using inert electrodes, depend on the relative reactivity of the elements involved.
76
When an aqueous solution is electrolysed, what is produced at the cathode?
At the negative electrode (cathode), hydrogen is produced if the metal is more reactive than hydrogen.
77
When an aqueous solution is electrolysed, what is produced at the anode?
At the positive electrode (anode), oxygen is produced unless the solution contains halide ions, then the halogen is produced.
78
What is produced at the electrodes in the electrolysis of aqueous solutions?
In an aqueous solution water molecules break down producing hydrogen ions and hydroxide ions that are discharged.
79
How do you set up the electrolysis of an aqueous solution for required practical 3? (5 steps)
1. Get two inert electrodes (e.g. platinum or carbon). Clean the surfaces of the electrodes using a piece of emery paper (or sandpaper).
2. From this point on, be careful not to touch the surfaces of the metals with your hands - you could transfer grease back onto the electrodes.
3. Place both electrodes into a beaker filled with your electrolyte and position them so they are inside inverted test tubes containing the aqueous solution.
4. Connect the electrodes to a power supply using crocodile clips and wires.
5. As the reaction progresses, gases may form and collect inside the test tubes.
80
After the experiment for required practical 3 is finished, how can you test gaseous products to work out what was produced? (3)
* Chlorine bleaches damp litmus paper turning it white.
* Hydrogen makes a 'squeaky pop' with a lighted splint.
* Oxygen will relight a glowing splint.
