Chemical changes

Question 1

What is oxidation/reduction?

Answer 1

Oxidation - when a substance gains oxygen.

Reduction - when a substance loses oxygen.

Question 2

What is the reactivity series of metals? What are the trends in reactivities of metals in reaction 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 reacts with water. It is mostly group 1 and 2 metals that react. 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.

Question 5

How can metals less reactive than carbon be extracted?

Answer 5

Reduction with carbon. Carbon displaces the metal in a metal oxide - carbon 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

It is a redox reaction and 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 → 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 electron which species has been oxidised and which species has been reduced when magnesium reacts with hydrochloric acid

Answer 14

Magnesium has lost electrons and this 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

1) React the acid with an excess of an insoluble base (such as metal oxide).
2) Filter off the leftovers.
3) 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 - produces hydroxide ions in aqueous solutions.

Question 18

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

Answer 18

The pH scale is a measure of acidity/alkalinity of a solution.
a pH of 7 is a neutral solution.

Question 19

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

Answer 19

H⁺ + OH⁻ → H₂O

Question 19

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

Answer 19

Strong acids are completely ionised in aqueous solutions.

Weak acids are only partially ionised in aqueous solutions.

Question 20

What happens to pH as concentration of H⁺ increases?

Answer 20

pH goes to more acidic scale.

Question 21

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

Answer 21

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

Question 22

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

Answer 22

Increases by a factor of 10.

Question 23

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

Answer 23

• Lithium nitrate.
• Potassium carbonate.
• Magnesium bromide.
• Barium sulphate.

Question 24

What is electrolysis?

Answer 24

The passing of an electric current through ionic substance that are molten or in solution to break them down into simpler substances. Ions lose or gain electrons at electrodes to produce elements.

Question 25

What is an electrolyte?

Answer 25

Ionic compound in molten or aqueous solution which conducts electricity.

Question 26

What is a cathode and what is an anode?

Answer 26

Cathode is the negative electrode, anode is the positive electrode.

Question 27

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

Answer 27

Reduction occurs at the cathode.

Oxidation occurs at the anode.

Question 28

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

Answer 28

H⁺ OH⁻ ⇌ H₂O
In aqueous, there will always be water which disassociate to produce hydrogen and hydroxide ions.

H⁺ and metal ions attract to the cathode. Hydrogen is produced unless there is a less reactive metal, in which case the said metal is produced at cathode.

OH⁻ and non-metal ions attract to the anode. Oxygen is produced at the anode unless the solution contains halide ions, in which case the halogen molecules are produced.

Question 29

How is aluminium manufactured? Why is it expensive?

Answer 29

Aluminium is made through the electrolysis of a mixture of aluminium oxide and cryolite.
Lots of energy is needed to produce the current in electrolysis which makes this process expensive.

Question 30

What are the half equations in the extraction of aluminium?

Answer 30

Al³⁺ + 3e⁻ → Al (cathode)
2O²⁻ → O₂ + 4e⁻ (anode)

Oxygen reacts with the graphite (carbon) of the anode producing CO₂.

Question 31

Why is cryolite used in this process?

Answer 31

It lowers the melting point of aluminium oxide, reducing energy costs. It also provides a better conductor of electricity than molten aluminium oxide.

Question 32

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

Answer 32

2H⁺ + 2e⁻ → H₂ (cathode)

4OH⁻ → 2H₂O + O₂ + 4e⁻ (anode)

Question 33

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

Answer 33

Molten:
K⁺ + e⁻ → K (cathode)
2Cl⁻ → Cl₂ + 2e⁻ (anode)

Aqueous solution:
2H⁺ + 2e⁻ → H₂ (cathode)
2Cl⁻ → Cl₂ + 2e⁻ (anode)

Question 34

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

Answer 34

Cu²⁺ + 2e⁻ → Cu (cathode)

2Br⁻ → Br₂ + 2e⁻ (anode)