Chemical Changes

What is an acid?

A substance that forms H⁺ ions in solution

What is a base?

A substance that can neutralise an acid

What is an alkali?

A soluble base that produces OH⁻ ions in solution

What is the pH scale?

A scale from 0 to 14 measuring acidity/alkalinity

What is the pH of a neutral solution?

7

What pH range do acids have?

Less than 7

What pH range do alkalis have?

More than 7

How do you measure pH accurately?

Use a pH probe

What does a universal indicator show?

Approximate pH by colour change

What is neutralisation?

Reaction between acid and base to form salt and water

What is the word equation for neutralisation?

Acid + base → salt + water

Give an example of an acid.

Hydrochloric acid (HCl)

Give an example of an alkali.

Sodium hydroxide (NaOH)

Give an example of a base.

Copper(II) oxide (CuO)

What ions do acids produce in water?

H⁺ ions

What ions do alkalis produce in water?

OH⁻ ions

What is the ionic equation for neutralisation?

H⁺ + OH⁻ → H₂O

What is a salt?

Compound formed when H⁺ from acid is replaced by metal or ammonium ion

How is sodium chloride made?

HCl + NaOH → NaCl + H₂O

Name a practical for making pure, dry salt.

React acid with insoluble base, filter, evaporate water

What is a strong acid?

An acid that ionises completely in water

What is a weak acid?

An acid that only partially ionises in water

Give an example of a strong acid.

HCl, sulfuric acid, nitric acid

Give an example of a weak acid.

Ethanoic acid, citric acid

What is the difference between concentration and strength?

Strength: degree of ionisation; concentration: amount of acid per unit volume

What is reactivity series?

List of metals in order of their reactivity

Which metal is most reactive?

Potassium

Which metal is least reactive (common ones)?

Gold

How is reactivity tested?

By reaction with acid, water, or oxygen

What is displacement reaction?

More reactive metal replaces less reactive one from compound

What is the word equation for metal plus acid?

Metal + acid → salt + hydrogen

What is observed when magnesium reacts with HCl?

Fizzing (hydrogen produced), metal dissolves

What does "dilute" mean in chemistry?

A solution with a low concentration of solute

What does "concentrated" mean?

A solution with a high concentration of solute

What is an ore?

A rock containing enough metal for extraction

What is reduction?

Loss of oxygen, gain of electrons

What is oxidation?

Gain of oxygen, loss of electrons

What is a redox reaction?

Reaction involving both oxidation and reduction

How are metals extracted from ores?

By reduction with carbon or by electrolysis

What is electrolysis?

Splitting up a compound using electricity

When is electrolysis used?

When metal is too reactive for reduction by carbon

What is the electrolyte?

Compound melted or dissolved for electrolysis to occur

What are the electrodes?

Conductive rods where reactions occur in electrolysis

What happens at the cathode in electrolysis?

Reduction (gain of electrons)

What happens at the anode in electrolysis?

Oxidation (loss of electrons)

In electrolysis of molten NaCl, what is produced at the cathode?

Sodium metal

In electrolysis of molten NaCl, what is produced at the anode?

Chlorine gas

What is half-equation for sodium at cathode?

Na⁺ + e⁻ → Na

What is half-equation for chlorine at anode?

2Cl⁻ → Cl₂ + 2e⁻

Why must cryolite be used in aluminium extraction?

Lowers melting point of aluminium oxide, saves energy

What is the cathode reaction in aluminium extraction?

Al³⁺ + 3e⁻ → Al

What is the anode reaction in aluminium extraction?

2O²⁻ → O₂ + 4e⁻

Why must carbon anodes be replaced in aluminium extraction?

They react with oxygen to form carbon dioxide

What is the test for hydrogen gas?

Lit splint makes "pop" sound

What is the test for oxygen gas?

Glowing splint relights

What is the test for carbon dioxide?

Turns limewater milky

What determines whether hydrogen or metal is produced at cathode in aqueous electrolysis?

Metal reactivity compared to hydrogen

What is formed at the anode in aqueous sodium sulfate electrolysis?

Oxygen gas

What is formed at cathode in aqueous copper sulfate electrolysis?

Copper metal

What is the use of electrolysis?

Extracting metals, electroplating, making chemicals

What practical involves investigating pH of acid and alkali?

Titration with indicator or pH probe

What is universal indicator used for?

Measuring approximate pH

Why is a strong acid a good conductor of electricity?

Many free H⁺ ions in solution

How is a salt named?

First part from metal/alkali, second part from acid

What salt does sulfuric acid make?

Sulfates

What salt does nitric acid make?

Nitrates

What is the ionic equation for reaction of magnesium with sulfuric acid?

Mg + 2H⁺ → Mg²⁺ + H₂

How is pure, dry copper sulfate made?

React copper oxide with sulfuric acid, filter, crystallise

In electrolysis, what does "inert electrode" mean?

Does not react (e.g., graphite)

How is the rate of gas produced measured in metal-acid reactions?

Collection over water, syringe, or measuring cylinder

Why do potassium, sodium, lithium react violently with water?

Very reactive, produce lots of heat and hydrogen

What is the general pattern for reactions of metals with acids?

More reactive = faster, more fizzing, more heat

What is precipitation?

Formation of insoluble solid from two solutions

How are neutralisation reactions useful?

Making salts, treating acidic soils, indigestion remedies

Why does the reactivity series predict displacement reactions?

More reactive metals displace less reactive metals from compounds

What does "anode" mean?

Positive electrode (oxidation)

What does "cathode" mean?

Negative electrode (reduction)

Why does magnesium displace copper from copper sulfate?

Magnesium is more reactive than copper

What is used to make soap in industry?

Alkalis (e.g., sodium hydroxide)

What is the main use of electroplating?

Coating objects for appearance or corrosion resistance

How is bauxite purified to extract aluminium?

Dissolved in sodium hydroxide solution (Bayer process)

Why are metal oxides basic?

React with acids to form salt and water

What effect does increasing acid concentration have on reaction rate?

Increases rate, more particles available

What's a spectator ion?

Ion that doesn't take part in a chemical reaction

Why don't copper, silver, gold react with dilute acids?

They are less reactive than hydrogen

Why do iron nails rust?

Oxygen and water cause oxidation

How can rusting be prevented?

Painting, oiling, galvanising

Why is electrolysis of brine important industrially?

Makes chlorine, hydrogen and sodium hydroxide

What is the overall equation for brine electrolysis?

2NaCl + 2H₂O → Cl₂ + H₂ + 2NaOH

Why do ionic compounds only conduct when molten or in solution?

Ions can move freely

What is the purpose of a water bath in neutralisation practicals?

Controls reaction temperature safely

Why use excess base in making salts?

Ensures all acid reacts and is used up

How are crystals of salt obtained after reaction?

Evaporate water slowly from solution