c4: chemical changes

What is an acid?

A substance that produces hydrogen ions (H⁺) in aqueous solution.

What is an alkali?

A soluble base that produces hydroxide ions (OH⁻) in aqueous solution.

What is a base?

A substance that neutralises an acid to form a salt and water

What are the three main types of acid reactions?

• Acid + metal → salt + hydrogen

• Acid + base → salt + water

• Acid + carbonate → salt + water + carbon dioxide

What salt is formed when hydrochloric acid reacts?

A chloride salt.

What salt is formed when nitric acid reacts?

A nitrate salt.

What is the pH scale used for?

To measure how acidic or alkaline a solution is

What ion is responsible for acidity?

Hydrogen ion, H⁺.

What ion is responsible for alkalinity?

Hydroxide ion, OH⁻.

What happens to H⁺ concentration when pH decreases by 1?

The H⁺ concentration increases by a factor of 10.

What is a strong acid?

An acid that fully ionises in water

What is a weak acid?

An acid that only partially ionises in water

What is the difference between acid strength and concentration?

• Strength: how much it ionises.

• Concentration: how much acid per unit volume

What is the ionic equation for neutralisation?

H⁺ + OH⁻ → H₂O

How do you make a soluble salt from an acid and an insoluble base?

• Add excess base to acid until no more reacts.

• Filter off excess solid.

• Heat solution to crystallise the salt.

What is the reactivity series of metals (top to bottom)?

Potassium, Sodium, Calcium, Magnesium, Aluminium, Carbon, Zinc, Iron, Hydrogen, Copper, Silver, Gold, Platinum.

What gas is produced when metals react with acids?

Hydrogen gas.

Write the general equation for metal + acid.

Metal + acid → salt + hydrogen.

What is a displacement reaction?

When a more reactive metal displaces a less reactive metal from its compound.

How are metals below carbon extracted from their ores?

By reduction with carbon.

How are metals above carbon extracted?

By electrolysis

What does OIL RIG stand for?

Oxidation Is Loss, Reduction Is Gain (of electrons).

In redox reactions, what happens when a substance is oxidised?

It loses electrons or gains oxygen.

In redox reactions, what happens when a substance is reduced?

It gains electrons or loses oxygen.

What is electrolysis?

The decomposition of an ionic compound using electricity.

What is the electrolyte?

A molten or aqueous ionic compound that conducts electricity.

What happens at the cathode (negative electrode)?

Reduction — positive ions gain electrons.

What happens at the anode (positive electrode)?

Oxidation — negative ions lose electrons.

What ions are present in water?

H⁺ and OH⁻.

In electrolysis of aqueous solutions, what is formed at the cathode?

Hydrogen (if metal more reactive than hydrogen) or the metal (if less reactive).

In electrolysis of aqueous solutions, what is formed at the anode?

Oxygen (unless halide ions present, then chlorine/bromine/iodine gas).

Why is cryolite used in aluminium extraction?

To lower the melting point of aluminium oxide and reduce energy costs.

Write the electrode reactions for aluminium extraction.

• Cathode: Al³⁺ + 3e⁻ → Al

• Anode: 2O²⁻ → O₂ + 4e⁻

Why do the carbon anodes in aluminium extraction need to be replaced?

They react with oxygen to form carbon dioxide and wear away.

What are the two required practicals in this topic?

• Making a soluble salt from an acid and base.

• Electrolysis of aqueous solutions (identify products at electrodes).