4.4- Chemical Changes

Question 1

What is oxidation and reduction in terms of electrons?

Answer 1

When an atom loses electrons, it has been oxidised (oxygen has been added). When an atom gains electrons, it has been reduced (oxygen has been removed).

Question 2

What is the reactivity series?

Answer 2

The reactivity series is a list of metals in order of their reactivity, with the most reactive at the top and the least reactive at the bottom.

Question 3

How can metals be placed in order of reactivity?

Answer 3

Metals can be placed in order of reactivity based on their reactions with either water or a dilute acid. They react to form hydrogen. (The test for hydrogen is the squeaky pop).

Question 4

Explain the method for how you would place metals in order of reactivity?

Answer 4

To do this, you measure a set volume of a dilute acid and measure its initial temperature. You add one of the metals to (dilute) hydrochloric acid, stir and measure the highest temperature reached of the solution. You then calculate the temperature difference. This is then repeated for the other metals. For a non reactive metal like copper, there will be no temperature change, and for reactive metals like magnesium and iron there will be a temperature, however magnesium will increase in temperature more than iron, since it is more reactive. The most reactive metal will have the largest temperature change.

Control variables for this experiment:
*   same concentration of hydrochloric acid
*   same volume of hydrochloric acid
*   same mass / moles of metal
*   same particle size of metal
*   same temperature of acid (if comparing rate of bubbling)

Question 5

What is the order of metals in the reactivity series

Answer 5

Potassium
Sodium
Calcium
Magnesium
Aluminium
Carbon
Zinc
Iron
Tin
Lead
Hydrogen
Copper
Silver
Gold

Question 6

What is the rule for displacement reactions?

Answer 6

A more reactive metal displaces (pushes out) the less reactive metal from its compound. The equation for this reaction is:
More reactive metal + less reactive metal compound → more reactive metal compound + less reactive metal

These reactions are called displacement reactions.

Question 7

What happens when magnesium reacts with copper sulfate?

Answer 7

A more reactive metal displaces (pushes out) the less reactive metal from its compound. The equation for this reaction is:
More reactive metal + less reactive metal compound → more reactive metal compound + less reactive metal

These reactions are called displacement reactions.

Question 8

What is a metal ore?

Answer 8

Metal ores contain enough of the metal to make it economic to extract the metal. Ores are mined and might need to be concentrated before the metal is extracted and purified.

Question 9

How do you determine whether it is economical to extract an ore?

Answer 9

How easy it is to extract from its ore
How much metal the ore contains
The changing demands for a particular metal

However, these factors can change over time (eg. you might find a new, cheaper method to extract metals later on)

Question 10

How are gold and silver found?

Answer 10

A few metals, for example gold and silver are so unreactive, that they are found as the metals (elements) themselves, existing in their natural state.

Question 11

Which metals are extracted by carbon?

Answer 11

The way that a metal is extracted depends on its place in the reactivity series, as a more reactive metal will displace a less reactive metal from its metal compound. This applies to carbon (a non-metal) which will also displace less reactive metals from their oxides. Copper is used to extract some metals from their ores in industry.

Question 12

How does extraction with carbon work?

Answer 12

In order to do so, the metal oxide (eg. zinc, copper, iron) must be heated with carbon. The carbon removes the oxygen from the metal oxide to form carbon dioxide. Therefore metal extraction via carbon is a reduction reaction.

Question 13

How is tungsten extracted?

Answer 13

Another metal extracted by reduction of its oxide is tungsten (W). However, here, the non-metal used as the reducing agent is hydrogen, not carbon. This is because carbon forms a compound tungsten carbide with the metal formed by reduction, where as hydrogen forms a very pure metal.

The equation for this is:
Tungsten oxide + hydrogen →(heat above arrow) tungsten + water (as steam)
WO3 (s) + 3H2 (g) → W (s) + 3H2O (g)

Question 14

How are metals that are more reactive than carbon extracted?

Answer 14

The metals that are more reactive than carbon are not extracted by reduction, but instead with electrolysis.

Question 15

What is an ionic equation?

Answer 15

An ionic equation shows only the atoms and ions that change in a reaction. In this reaction, the sulfate ions SO4 2- ions remain the same, so they do not appear in the ionic equation.

The correct ionic equation would be:
Mg(s) + Cu2+ (aq) → Mg2+ (aq) + Cu (s

Question 16

What is the ionic equation for the displacement of copper by iron?

Answer 16

Fe (s) + Cu 2+ (aq) → Fe 2+ (aq) + Cu (s)

Question 17

What do half equations show?

Answer 17

Half equations can be used to show what happens to each reactant:

Question 18

Explain what these two half equations show:
Fe (s) → Fe 2+ (aq) + 2e-
Cu 2+ (aq) + 2e- → Cu (s)

Answer 18

Fe (s) → Fe 2+ (aq) + 2e-
Here, the iron atoms lose two electrons to form iron (II) ions. This is oxidation (loss of electrons)

Cu 2+ (aq) + 2e- → Cu (s)
Here, the two electrons from the iron are gained by the copper ions to form copper atoms. This is reduction (gaining of electrons)

Question 19

What type of reaction is a displacement reaction?

Answer 19

This is why displacement reactions are known as redox reactions (reduction and oxidation).

Question 20

When can reactions between metals and acids occur?

Answer 20

Reactions between metals and acids can only occur when the metal is more reactive than the hydrogen than the acid.

Question 21

What do all acids contain?

Answer 21

All acids contain hydrogen, which is released as hydrogen ions when the acid is dissolved in water.

Question 22

What is a salt?

Answer 22

. A salt is the name for a compound formed when the hydrogen in an acid is wholly or partially replaced by metal (or ammonium) ions.

Question 23

What is the reaction between a metal and an acid?

Answer 23

metal + acid → salt + hydrogen
eg. iron + hydrochloric acid → iron chloride + hydrogen
Fe + 2HCl → FeCl2 + H2

Question 24

What happens if the metal is very reactive?

Answer 24

If the metal is very reactive, the reaction with acid is too violent to be carried out safely. That’s why alkali metals are never added to acids.

Question 25

What type of salt does hydrochloric acid make?

Answer 25

Hydrochloric acid (HCl) makes chlorides (containing chloride ions)

Question 26

What type of salt does sulfuric acid make?

Answer 26

Sulfuric acid (H2SO4)makes sulfates (containing sulfate ions)

Question 27

What type of salt does nitric acid make?

Answer 27

Nitric acid (HNO3) makes nitrates (containing nitrate ions)

Question 28

What is a base?

Answer 28

Bases are compounds that can neutralise acids

Question 29

What is an alkali?

Answer 29

A base that is soluble in water, eg hydroxides

Question 30

What happens when an acid reacts with a base?

Answer 30

Acid + base →salt + water Hydrochloric acid + iron oxide → iron chloride + water

Question 31

What is the charge of a salt?

Answer 31

Salts are made up of positive metal ions, and a negative ion from an acid and like all ionic compounds, a salt has no overall charge, as the sum of the charges on their ions equals 0.

Question 32

How do metal oxide, metal hydroxides and metal carbonates react with acids?

Answer 32

Metal oxide (insoluble) + acid → salt + water Most metal oxides are not soluble, and require initial heating to make them react Metal hydroxide (soluble) + acid → salt + water Metal carbonate (soluble) + acid → salt + water + carbon dioxide

Question 33

Required Practical: Making soluble salts from an insoluble base

Answer 33

1. Using a measuring cylinder, measure 20 cm3 of sulfuric acid into the beaker. 2. Stand the beaker on a tripod and gauze and warm gently. The glass should not be too hot to handle. 3. Carefully place the beaker of acid on the bench and add half a spatula of copper(II) oxide power into the acid and stir using the glass rod. 4. Continue adding the copper(II) oxide until no more dissolves (this should be most of the solid you have been provided with). The copper oxide is now in excess, which is shown by the excess solid 5. Allow to cool and then filter the mixture and discard excess copper oxide 6. Pour the filtrate into an evaporating basin. Heat the solution until the volume is halved, using a water bath. 7. Remove from the heat and leave to crystallise. 8. Pat the crystals dry with filter paper

Question 34

What is a neutralisation reaction?

Answer 34

A neutralisation reaction is where an acid reacts with a base (alkali) to form a salt dissolved in water.

Question 35

What do all alkalis release in water?

Answer 35

All alkalis form aqueous hydroxide ions, OH-, when added to water. It is these excess OH- ions that make a solution alkaline.

Question 36

What is the neutralisation ionic equation?

Answer 36

H⁺(aq) + OH⁻(aq) → H₂O(l)

Question 37

What is an indicator?

Answer 37

Indicators are substances that change colour when you add them to acids and alkalis. Litmus is a well-known indicator (red in acid, blue in alkali), but there are many more.

Question 38

What is the pH scale?

Answer 38

You use the pH scale to show how acidic or alkaline a solution is. The scale runs from 0 (most acidic) to 14 (least acidic). pH stands for ‘power of hydrogen’ as the acidity of an aqueous solution depends on the number of H+ ions in solution.

Question 39

What type of scale is the pH scale?

Answer 39

The pH scale is a logarithmic scale with base 10. Each value is 10 times the value below it. Eg. pH 5 is 10 times more acidic than pH 6.

Question 40

A 1.0 x 10-3 mol/dm3 solution of HCl has a pH of 3.0. What is the pH of a 1.0 x 10-5 mol/dm3 solution of HCl?

Answer 40

1.0 x 10-3 x 102 = 1.0 x 10-5 Therefore, the pH 3 will increase by 2, to make a pH of 5.0.

Question 41

What is neutral on the pH scale?

Answer 41

Anything in the middle of the pH scale (pH 7) is neutral

Question 42

What can you use to determine the pH of a solution?

Answer 42

Universal indicator- turns a range of colours as the pH changes pH meter- glass probe to dip in the solution being tested Electronic pH sensors- give a digital display of the pH directly

Question 43

What is a titration?

Answer 43

Titration = A technique to measure the exact volumes of acid and alkali needed to react with each other. This can be used to find concentrations of unknown solutions.

Question 44

What is a titre?

Answer 44

Titre: volume of solution needed to reach the end point in a titration (difference between the start and end volume on the burette)

Question 45

What is the end point?

Answer 45

End point: Point at which neutralisation occurs (sudden colour change)

Question 46

What are concordant titres?

Answer 46

Concordant titre: Titres within 0.10 cm³ of each other.

Question 47

Give an example of an indicator you can use in titrations?

Answer 47

Phenolphthalein (goes from pink in alkali to colourless in acid)

Question 48

Why can universal indicator not be used in titrations?

Answer 48

Universal indicator cannot be used for titrations as it only provides a gradual change in colour as the pH of the solution changes. The aim of the titration is to observe the end-point of the reaction which can only be indicated as a single, sharp colour change.

Question 49

Required Practical: Titrations

Answer 49

1. Collect some hydrochloric acid in a beaker and label it. 2. Rinse your burette with distilled water and then with some of the hydrochloric acid. 3. Fill a burette with the hydrochloric acid beyond the zero mark and then let the solution run out until the bottom of the meniscus is exactly on the zero mark. 4. All bubbles should be removed from the jet. 5. Collect some sodium hydroxide in another beaker and label it. 6. Rinse the 25.0 cm3 pipette with distilled water and then with the sodium hydroxide. 7. Use the pipette and the pipette filler to transfer 25.0 cm3 of the sodium hydroxide into a clean dry conical flask. 8. Add three to four drops of phenolphthalein indicator into the flask and swirl. 9. Place the conical flask on the white tile directly below the burette. 10. Record the initial burette reading in the table below (this should be 0.00 cm3). 11. Carry out a rough titration by adding the acid to the alkali in small amounts at a time. Swirl the flask after every addition and continue until the indicator changes from pink to colourless. Note the final burette reading and record it in the table. 12. Repeat the titration accurately by adding the acid drop-wise near the end point. Make sure you record the initial and final burette readings in the appropriate column in the table. Repeat the accurate titrations until you have two concordant results (within 0.10 cm3 of each other). 13. Record all your readings in the results table.

Question 50

What is a dilute solution?

Answer 50

A dilute solution contains a relatively small amount of dissolved solute. A concentrated solution contains a relatively large amount of dissolved solute.

Question 51

What must acids do before they show their acidic properties?

Answer 51

Acids must dissolve in water before they show their acidic properties, since in water, acids ionise (split up). The molecules split up into H+ ions (which all acids have) and negative ions.

Question 52

What is a strong acid?

Answer 52

Strong acids (like hydrochloric, nitric and sulfuric acid) fully dissociate (ionise) in water.

Question 53

What is a weak acid?

Answer 53

Weak acids (like ethanoic acid, citric acid, carbonic acid) partially dissociate (partially ionise) in water. This reaction is reversible, so the molecules of weak acid split to form ions and then recombine to form the original molecule.

Question 54

What happens to the hydrogen concentration as the pH value decreases by one?

Answer 54

As the pH value decreases by one unit, the hydrogen ion concentration of the solution increases by a factor of 10.

Question 55

What does the pH of an acid depend on?

Answer 55

he strength of the acid and the concentration of the acid.

Question 56

How does acid strength impact pH?

Answer 56

The strength of an acid refers to its ability to donate hydrogen ions to a solution. Strong acids, such as hydrochloric acid (HCl), ionize completely in water and release a large amount of H+ ions, resulting in a low pH. Weak acids, on the other hand, only partially ionize and release fewer H+ ions, resulting in a higher pH.

Question 57

How does concentration of an acid impact pH?

Answer 57

The concentration of an acid refers to the amount of acid molecules present in a solution. Higher concentrations of acid mean there are more acid molecules available to release H+ ions, leading to a lower pH. Lower concentrations of acid result in fewer H+ ions and a higher pH.

Question 58

What is electrolysis?

Answer 58

Electrolysis is the process of breaking down ionic substances into simpler substances using electricity.

Question 59

What is an electrolyte?

Answer 59

The substance broken down by electricity is referred to as an electrolyte. It is a liquid/solution that contains molten/dissolved ionic compound (so that it can conduct electricity).

Question 60

How do you set up for electrolysis?

Answer 60

To set up an electrical circuit for electrolysis, you have two electrodes that dip into the electrolyte, with a gap in between them. These electrodes are electricity conducting rods, and they are connected to a battery. The one connected to the positive terminal of the battery is positively charged, and is known as the anode. The other negative electrode connected to the negative terminal is the cathode.

Question 61

What are electrodes normally made of?

Answer 61

Electrodes are normally made of an inert (unreactive) substance, like graphite, so that the electrodes do not react with the electrolyte or the products made in electrolysis.

Question 62

What happens during electrolysis?

Answer 62

During electrolysis, positively charged ions move to the cathode (negative electrode) and at the same time, negatively charged ions move to the anode (positive electrode). This is because opposites attract. When the ions reach the electrodes, they lose their charge, and become elements. Gases may be given off, or metals deposited, depending on the compound and whether it is molten or dissolved in water.

Question 63

What state do ionic compounds need to be in before being electrolysed?

Answer 63

Ionic compounds can only be electrolysed when they are molten or dissolved in water. This is because their ions are now free to move around and carry their charge to the electrodes. Covalent compounds cannot usually be electrolysed unless they react (or ionise) in water to form ions.

Question 64

What happens at the cathode during electrolysis?

Answer 64

At the negative electrode, positively charged ions gain electrons to become metal atoms. This is classed as reduction. Reduction is gaining electrons: Na + + e- → Na (s) (sodium ions become sodium atoms) Cu2+ (l ) + 2e- → Cu (s) Al3+ (l )+ 3e- → Al (s)

Question 65

What happens at the anode during electrolysis?

Answer 65

At the positive electrode, negatively charged ions lose electrons. This is classed as oxidation. Oxidation is losing electrons: 2Cl- (l) → Cl2 (g)+ 2e- (chloride ions becomes chlorine) 2Br- (l) → Br2 (l)+ 2e- (bromide ions becomes bromine) 2I- → I2 + 2e- (Iodide ions becomes Iodine) 2O2- (l) → O2 (g)+ 4e- (oxide ions becomes oxygen)

Question 66

When is electrolysis used to extract metals?

Answer 66

Metals can be extracted from molten compounds using electrolysis. Electrolysis is used if the metal is too reactive to be extracted by reduction with carbon or if the metal reacts with carbon.

Question 67

Why is electrolysis expensive?

Answer 67

Large amounts of energy are used in the extraction process to melt the compounds and to produce the electrical current. This makes electrolysis very expensive.

Question 68

What is electrolysed in the electrolysis of aluminium?

Answer 68

Aluminium oxide, from the ore bauxite, is electrolysed in the extraction of aluminium metal. This aluminium oxide is mixed with molten cryolite.

Question 69

Why is aluminium oxide mixed with cryolite?

Answer 69

Cryolite is used in this process to lower the melting point of the aluminium oxide which therefore reduces the energy.

Question 70

What forms at the cathode during the electrolysis of aluminium?

Answer 70

Aluminium forms at the negative electrode (cathode) Al3+ (l )+ 3e- → Al (s) (reduction)

Question 71

What forms at the anode during the electrolysis of alumnium?

Answer 71

Oxygen forms at the positive electrode (anode). 2O2- (l) → O2(g)+ 4e-

Question 72

Why must the anodes be continually replaced during the electrolysis of aluminium?

Answer 72

During this process, the positive electrode (anode) is made of carbon, which reacts with the oxygen to produce carbon dioxide and so must be continually replaced, because otherwise they burn away.

Question 73

What is the rule for the cathode?

Answer 73

At the negative electrode, the least reactive element discharges. In aqueous solutions of reactive metal salts where the metal is above hydrogen in the reactivity series, the metal will not therefore be evolved at the cathode. (e.g. sodium chloride, calcium fluoride) Hydrogen gas will be evolved at the cathode instead. 2H+ (aq) + 2e- → H2 (g) In aqueous solutions of unreactive metal salts e.g. copper chloride or silver fluoride the metal will be evolved at the cathode. Cu2+ (aq )+ 2e- → Cu (s)

Question 74

What is the rule for the anode?

Answer 74

At the positive electrode (anode), oxygen is produced unless the solution contains halide ions when the halogen is produced. 4OH- (aq)→ O2 (g)+ 2H2O (l) + 4e- or 4OH- – 4e- → O2 + 2H2O If a halide ion is present then the halogen is produced E.g. 2Cl- (aq) → Cl2 (g)+ 2e

Question 75

What happens to the water in electrolysis of aqueous solutions?

Answer 75

water molecules break down producing hydrogen ions and hydroxide ions that are discharged.

Question 76

What happens when you electrolyse sodium chloride solution?

Answer 76

When you electrolyse sodium chloride solution (brine) you get three products, chlorine gas and hydrogen given off at the electrodes, plus sodium hydroxide solution (an alkali) left at the bottom.

Question 77

Required Practical: Electrolysis

Answer 77

1. Pour copper(II) chloride solution into the beaker 2. Add the lid and insert carbon rods through the holes. The rods must not touch each other. 3. Attach crocodile leads to the rods. Connect the rods to the dc (red and black) terminals of a low voltage power supply. 4. Select 4 V on the power supply and switch on. 5. Look at both electrodes. Is there bubbling at neither, one or both electrodes? Write your observations in the results table. 6. Use tweezers to hold a piece of blue litmus paper in the solution next to the positive electrode (the one connected to the red terminal). You will need to lift the lid temporarily to do this. Chlorine bleaches blue litmus paper white. Write your observations in the results table. 7. Write what is being formed at the positive electrode (anode) in the results table. 8. After no more than five minutes, switch off the power supply and examine the negative electrode (the one connected to the black terminal). Is there evidence of a metal coating on it? Write your observations in the results table. 9. Write what is being formed at the negative electrode (cathode) in the results table. 10. Clean the equipment carefully. 11. Repeat steps 1‒6 for sodium chloride