Topic 4 – Chemical Changes

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 H, in reactivity series react with acid to produce H. The more reactive the metal is, the quicker and more violent reaction with acid occurs.
Metals below H, don’t react with acids.
Not all metals above H, 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 are metals more reactive than carbon extracted?

Answer 5

By electrolysis

Question 6

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

Answer 6

Metal + acid - › salt + hydrogen
Redox reaction, also a displacement reaction

Question 7

Which metals in the reactivity series will react with acid?

Answer 7

Those above hydrogen

Question 8

What is the general equation for a neutralisation reaction?

Answer 8

Base + acid - › salt + water

Question 9

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

Answer 9

Metal carbonate + acid -› salt + water + carbon dioxide

Question 10

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

Answer 10

Metal oxide + acid -› a salt + water

Question 11

What is a redox reaction?

Answer 11

A reaction where both oxidation and reduction occurs

Question 12

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 12

Magnesium has lost electrons and thus has been oxidised (Mg to Mg?+)
The hydrogen in HCI has gained electrons and thus has been reduced (H* to H2)

Question 13

How is a soluble salt formed?

Answer 13

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

Question 14

What do acids and alkalis produce in aqueous solutions?

Answer 14

Acids produce hydrogen ions, alkalis produce hydroxide ions

Question 15

What are bases, acids and alkalis?

Answer 15

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

Question 16

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

Answer 16

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

Question 17

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

Answer 17

H*+ OH -+ HO

Question 18

What is a strong acid and weak acid?

Answer 18

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

Question 19

What happens to pH as concentration of H+ increases?

Answer 19

The pH decreases

Question 20

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

Answer 20

• 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 21

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

Answer 21

Increases by a factor of 10

Question 22

Name the following salts: LiNO2, K,CO2, MgBr BasO,

Answer 22

Lithium nitrate
Potassium carbonate
Magnesium bromide
Barium sulfate

Question 23

What is electrolysis?

Answer 23

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 24

What is an electrolyte?

Answer 24

The liquid/solution which conducts electricity

Question 25

What is a cathode and what is an anode?

Answer 25

Cathode is the negative electrode, anode is the positive electrode

Question 26

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

Answer 26

Reduction occurs at the cathode
Oxidation occurs at the anode

Question 27

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

Answer 27

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 28

How is aluminium manufactured? Why is it expensive?

Answer 28

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 29

What are the half equations in the extraction of aluminium?

Answer 29

Al3* + 3 e -› Al (cathode)
202- - O2 + 4 e (anode)
Oxygen reacts with C of the anode producing COz

Question 30

Why is cryolite used in manufacturing of aluminium?

Answer 30

It lowers the melting point of aluminium oxide, reducing energy costs

Question 31

What are the half equations in electrolysis of the aqueous Na,SO?

Answer 31

2 H* + 2 e- -> H, (cathode)
4 OH- - 2 HO + 02 + 4 e (anode)

Question 32

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

Answer 32

K+ + e- -› K (cathode)
2 CI- -> Cl2 + 2 e- (anode)
2 H+ + 2 e- -› H2 (cathode)
2 CI- -> Cl + 2 e+ (anode), respectively

Question 33

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

Answer 33

Cu2+ + 2 e -› Cu (cathode)
2 Br- -> Br2 + 2 e- (anode)