C4: Chemical Changes

What is oxidation in terms of oxygen?

Oxidation is the gain of oxygen.

What is reduction in terms of oxygen?

Reduction is the loss of oxygen.

What do metals form when they react with other substances?

Positive ions.

Write the word equation for a metal reacting with oxygen.

Metal + oxygen → metal oxide

List the metals in the reactivity series from most to least reactive (including hydrogen and carbon).

Potassium, sodium, lithium, calcium, magnesium, (carbon), zinc, iron, (hydrogen), copper (and gold below)

How does magnesium react with cold water?

Very slowly; it reacts with water to produce magnesium hydroxide and hydrogen.

How does calcium react with cold water?

Vigorously; it produces calcium hydroxide solution and bubbles of hydrogen gas.

How do potassium and sodium react with cold water?

Very vigorously; they fizz rapidly, producing hydrogen gas and the metal hydroxide solution. Potassium ignites the hydrogen.

How does iron react with cold water or dilute acid?

Iron reacts very slowly with cold water (rusting); it reacts slowly with dilute acids to produce a salt and hydrogen.

How does copper react with dilute acids or cold water?

Copper does not react with dilute acids or cold water - it is below hydrogen in the reactivity series.

What is a displacement reaction?

A more reactive metal displaces (takes the place of) a less reactive metal from its compound.

Give an example of a displacement reaction.

Iron + copper sulfate solution → iron sulfate solution + copper (iron displaces copper because iron is more reactive).

How are unreactive metals such as gold found in the Earth?

As the metal itself (native/free metal), not in compounds.

How are metals less reactive than carbon extracted from their ores?

By reduction of their metal oxides with carbon (e.g. iron oxide reduced by carbon in a blast furnace).

Why are metals more reactive than carbon (e.g. aluminium) extracted by electrolysis rather than reduction with carbon?

They are too reactive to be reduced by carbon; electrolysis of the molten compound is used instead.

Define oxidation in terms of electrons.

Oxidation is the loss of electrons.

Define reduction in terms of electrons.

Reduction is the gain of electrons.

What is a redox reaction?

A reaction in which both oxidation (loss of electrons) and reduction (gain of electrons) occur simultaneously.

In the displacement reaction Fe + CuSO₄ → FeSO₄ + Cu, which species is oxidised?

Iron (Fe) is oxidised: Fe → Fe²⁺ + 2e⁻ (it loses electrons).

In the displacement reaction Fe + CuSO₄ → FeSO₄ + Cu, which species is reduced?

Cu²⁺ is reduced: Cu²⁺ + 2e⁻ → Cu (it gains electrons).

What products are formed when an acid reacts with a metal?

A salt and hydrogen gas.

Which metals react with dilute hydrochloric and sulfuric acids? (from the reactivity series)

Magnesium, zinc and iron (metals above hydrogen in the reactivity series).

What is the test for hydrogen gas?

Hold a burning splint at the mouth of the test tube - hydrogen burns with a squeaky pop.

What products are formed when an acid reacts with a metal oxide or hydroxide (base/alkali)?

A salt and water.

What products are formed when an acid reacts with a metal carbonate?

A salt, water and carbon dioxide.

What salt is produced when hydrochloric acid reacts with a substance?

A chloride salt.

What salt is produced when sulfuric acid reacts with a substance?

A sulfate salt.

What salt is produced when nitric acid reacts with a substance?

A nitrate salt.

Write the word equation for the neutralisation of sodium hydroxide with hydrochloric acid.

Sodium hydroxide + hydrochloric acid → sodium chloride + water

Write the ionic equation for any acid-alkali neutralisation.

H⁺(aq) + OH⁻(aq) → H₂O(l)

What does an acid produce in aqueous solution?

Hydrogen ions, H⁺(aq).

What do alkalis contain in aqueous solution?

Hydroxide ions, OH⁻(aq).

What does pH 7 indicate?

A neutral solution.

What pH do acidic solutions have?

Less than 7.

What pH do alkaline solutions have?

Greater than 7.

How can approximate pH be measured?

Using universal indicator solution or a wide-range indicator paper; or more precisely with a pH probe.

How are soluble salts made from an insoluble solid (oxide, carbonate or hydroxide) and an acid?

Add excess solid to the acid until no more reacts, then filter off the excess solid to produce the salt solution; evaporate/crystallise to produce the solid salt.

What is a strong acid?

A strong acid is completely ionised (dissociated) in aqueous solution. Examples: hydrochloric, nitric and sulfuric acids.

What is a weak acid?

A weak acid is only partially ionised in aqueous solution. Examples: ethanoic, citric and carbonic acids.

If the pH of a solution decreases by 1, what happens to the hydrogen ion concentration?

The hydrogen ion concentration increases by a factor of 10.

What is the difference between a dilute acid and a weak acid?

Dilute/concentrated refers to the amount of substance (how much acid is dissolved); weak/strong refers to the degree of ionisation.

What is a titration?

A technique used to find the volumes of acid and alkali that react exactly with each other, using a burette, pipette and a suitable indicator.

How do you calculate concentration from a titration?

Use moles = concentration × volume for both solutions; from the balanced equation, find the molar ratio; calculate the unknown concentration.

What is an electrolyte?

A liquid or solution that can conduct electricity because it contains ions that are free to move.

What is electrolysis?

The process of using an electrical current to decompose an ionic compound (electrolyte) into its elements.

What is the cathode and what charge does it carry?

The cathode is the negative electrode.

What is the anode and what charge does it carry?

The anode is the positive electrode.

Which ions move to the cathode during electrolysis?

Positively charged ions (cations) move to the negative cathode.

Which ions move to the anode during electrolysis?

Negatively charged ions (anions) move to the positive anode.

What happens at the cathode during electrolysis of molten lead bromide?

Lead ions (Pb²⁺) are discharged and lead metal is produced.

What happens at the anode during electrolysis of molten lead bromide?

Bromide ions (Br⁻) are discharged and bromine gas is produced.

State the rule for predicting what is produced at the cathode when electrolyzing an aqueous solution.

The metal is produced if it is less reactive than hydrogen; if the metal is more reactive than hydrogen, hydrogen gas is produced instead.

State the rule for predicting what is produced at the anode when electrolyzing an aqueous solution.

Oxygen is produced unless the solution contains a high concentration of halide ions, in which case the halogen (e.g. chlorine) is produced.

Why is electrolysis used to extract aluminium rather than reduction with carbon?

Aluminium is more reactive than carbon and reacts with carbon, so it cannot be extracted by reduction; electrolysis of molten aluminium oxide (in cryolite) is used instead.

Why is cryolite mixed with aluminium oxide in the electrolysis of aluminium?

Cryolite lowers the melting point of the mixture, saving energy (pure Al₂O₃ melts at ~2000°C).

Why must the carbon anode in aluminium extraction be continually replaced?

The carbon anode reacts with oxygen produced at the anode to form carbon dioxide, so it gradually burns away.

What does Required Practical 3 investigate?

The electrolysis of aqueous solutions using inert electrodes, including investigating what happens at each electrode for different solutions.

What type of reaction occurs at the cathode during electrolysis?

Reduction - positive ions gain electrons.

What type of reaction occurs at the anode during electrolysis?

Oxidation - negative ions lose electrons.