Topic 4: Chemical Change

Question 1

What is oxidation/reduction?

Answer 1

• Oxidation - When a substance gains oxygen
• Reducation - When a substance loses oxygen

Question 2

What is the reactivity series of metals? What are the trends in reactivities of metals in reactions with acids/water?

Answer 2

• The series shows the metals in order of their reactivity.
• Metals above H2 in reactivity series react with acid to produce H2. The more reactive the metal is, the quicker and more violent reaction with acid occurs.
• Metals below H2 don’t react with acids.
• Not all metals above H2 react with water - mostly Group I and II metals. Aluminium is the borderline case.

Question 3

What is a displacement reaction?

Answer 3

A reaction where a more reactive metal displaces a less reactive metal from a compound

Question 4

How are unreactive metals found in Earth?

Answer 4

In their natural state (well, they are unreactive…)

Question 5

How can metals less reactive than carbon be extracted?

Answer 5

• Reduction with carbon.
• Carbon displaces the metal in a metal oxide - gets oxidised to carbon oxides.
• Metal from the metal oxide gets reduced to the pure metal.

Question 6

How are metals more reactive than carbon extracted?

Answer 6

By electrolysis

Question 7

How are oxidation and reduction defined in terms of electron transfer ?

Answer 7

• Oxidation – loss of electrons
• Reduction – gain of electrons

Question 8

What is the general equation for a reaction between metals and acids? What type of reaction is this?

Answer 8

Metal + acid → salt + hydrogen

Redox reaction, also a displacement reaction

Question 9

Which metals in the reactivity series will react with acid?

Answer 9

Those above hydrogen

Question 10

What is the general equation for a neutralisation reaction?

Answer 10

Base + acid → salt + water

Question 11

What is the general equation for the reaction between metal carbonate and acid?

Answer 11

Metal carbonate + acid → salt + water + carbon dioxide

Question 12

What is the general equation for the reaction between metal oxides and acids?

Answer 12

Metal oxide + acid → a salt + water

Question 13

What is a redox reaction?

Answer 13

A reaction where both oxidation and reduction occurs

Question 14

Explain in terms of gain or loss of electrons which species has been oxidised and which species has been reduced when magnesium reacts with hydrochloric acid.

Answer 14

• Magnesium has lost electrons and thus has been oxidised (Mg to Mg2+)
• The hydrogen in HCl has gained electrons and thus has been reduced (H+ to H2)

Question 15

How is a soluble salt formed?

Answer 15

a) React the excess acid with some insoluble chemical (e.g. metal oxide)
b) Filter off the leftovers
c) Crystallise the product

Question 16

What do acids and alkalis produce in aqueous solutions?

Answer 16

Acids produce hydrogen ions, alkalis produce hydroxide ions

Question 17

What are bases, acids and alkalis?

Answer 17

Bases are compounds that neutralise acids, acids produce hydrogen ions in aqueous solutions, alkalis are soluble bases - produce hydroxide ions in aqueous solutions

Question 18

What is the pH scale and what does a pH of 7 show?

Answer 18

The measure of acidity/alkalinity of a solution; neutral solution

Question 19

State the general equation for a neutralisation reaction in a short, ionic form.

Answer 19

H+ + OH− → H2O

Question 20

What is a strong acid? What is a weak acid?

Answer 20

Strong acid is completely ionised in aqueous solution; weak acid is only partially ionised in aqueous solution

Question 21

What happens to pH as concentration of H+ increases?

Answer 21

The pH decreases

Question 22

What is a concentrated acid and what is a dilute acid? Is this the same as a strong and weak acid?

Answer 22

• Concentrated acid has more moles of acid per unit volume than dilute (dilute refers to solutions of low concentrations)
• It is not the same - concentration is not the same thing as strength of an acid.
• Strength refers to whether the acid is completely ionised in water (strong) or
  only partially (weak).

Question 23

As the pH is decreased by one unit, what change is seen in the hydrogen ion concentration?

Answer 23

Increases by a factor of 10

Question 24

Name the following salts: LiNO3, K2CO3, MgBr2, BaSO4

Answer 24

Lithium nitrate
Potassium carbonate
Magnesium bromide
Barium sulfate

Question 25

What is electrolysis?

Answer 25

The passing of an electric current through ionic substances that are molten or in solution to break them down into elements; ions are discharged (they lose/gain electrons) at electrodes to produce these

Question 26

What is an electrolyte?

Answer 26

The liquid/solution which conducts electricity

Question 27

What is a cathode and what is an anode?

Answer 27

Cathode is the negative electrode, anode is the positive electrode

Question 28

What occurs at the cathode and what occurs at the anode during electrolysis?

Answer 28

• Reduction occurs at the cathode
• Oxidation occurs at the anode

Question 29

In aqueous electrolysis, which element is discharged at the cathode? Oxygen is produced at the anode unless what?

Answer 29

The less reactive element discharges at the cathode. Hydrogen is produced unless there is a less reactive metal, in which case the said metal is produced. Oxygen is produced at the anode unless the solution contains halide ions, in which case halogen molecules are produced.

Question 30

How is aluminium manufactured? Why is it expensive?

Answer 30

Aluminium is made through the electrolysis of aluminium oxide and cryolite.

Lots of energy is needed to produce the current in electrolysis which makes this process expensive.

Question 31

What are the half equations in the extraction of aluminium?

Answer 31

Al3+ + 3 e− → Al (cathode)

2 O2− → O2 + 4 e− (anode)

Oxygen reacts with C of the anode producing CO2.

Question 32

Why is cryolite used in this process?

Answer 32

Used as a catalyst as It lowers the melting point of aluminium oxide, reducing energy costs

Question 33

What are the half equations in electrolysis of the aqueous Na2SO4?

Answer 33

2 H+ + 2 e− → H2 (cathode)

4 OH− → 2 H2O + O2 + 4 e− (anode)

Question 34

What are the half equations in electrolysis of the molten and
aqueous KCl?

Answer 34

K+ + e− → K (cathode)

2 Cl− → Cl2 + 2 e− (anode)

2 H+ + 2 e− → H2 (cathode)

2 Cl− → Cl2 + 2 e− (anode), respectively

Question 35

What are the half equations in electrolysis of the aqueous CuBr2?

Answer 35

Cu2+ + 2 e− → Cu (cathode)

2 Br− → Br2 + 2 e− (anode)