C4 - Chemical Changes

What is an acid?

A substance that forms hydrogen ions (H⁺) when dissolved in water

Acids have a pH below 7

What is an alkali?

A soluble base that produces hydroxide ions (OH⁻) when dissolved in water

What is a base?

Any substance that reacts with an acid to form a salt and water

What is a neutralisation reaction?

The reaction between acids and bases

When hydrogen (H⁺) ions react with hydroxide ions (OH⁻) to form water

What is the equation for a neutralisation reaction?

Acid + base → salt + water

What does the pH scale measure?

How acidic or alkaline a solution is

How can pH be measured?

With a Universal Indicator or a pH probe

What is a strong acid?

An acid that is completely ionised in water. Particles dissociate to release H⁺ ions

What is a weak acid?

An acid that does not fully ionise in water.

Only a small proportion of particles dissociate to release H⁺ ions

What are some examples of strong acids?

Hydrochloric Acid (HCl), Nitric Acid (HNO₃), and Sulfuric acid (H₂SO₄)

What are some examples of weak acids?

Ethanoic acid, citric acid, and carbonic acid

Explain the difference between the strength and concentration of acids

The strength of an acid tells you the proportion of particles that ionise in water

The concentration of an acid measures how much acid there is in a given volume

What does pH measure?

The pH is a measure of the concentration of H⁺ ions in the solution

For every decrease of 1 on the pH scale, the concentration of H⁺ ions increases by a factor of 10

What do acids react with metal oxides to form?

Acid + metal oxide → salt + water

What do acids react with metal hydroxides to form?

Acid + metal hydroxide → salt + water

What is the equation for an acid and metal?

Acid + metal → salt + hydrogen

What do acids react with metal carbonate to form?

Acid + metal carbonate → salt + water + carbon dioxide

What do metals react with water to make?

Metal + water → metal hydroxide + hydrogen

What does the reactivity of a metal depend on?

How easily it loses its electrons to form positive ions

What is the reactivity series?

A list of metals arranged by their reactivity with water or an acid

What is the order of the reactivity series

How can you tell the reactivity of a metal when they react with an acid?

The rate at which bubbles of hydrogen are given off

The more reactive the metal is, the faster the reaction will happen

What is a displacement reaction?

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

How are most metals found in nature?

As compounds in ores

How can metals be extracted from their oxides?

By reduction using carbon, or by electrolysis

Which metals can be extracted by reduction with carbon?

Metals that are less reactive than carbon

What metals can be extracted by electrolysis?

Metals that are more reactive than carbon

What is reduction in terms of oxygen?

Loss of oxygen

What is oxidation in terms of oxygen?

Gain of oxygen

What happens to a metal oxide when reduced by carbon?

The metal is produced, and carbon dioxide is formed

What is oxidation in terms of electrons?

Loss of electrons

What is reduction in terms of electrons?

Gain of electrons

What is a redox reaction?

A reaction where reduction and oxidation happen at the same time

In a metal displacement reaction, what happens to the more reactive metal?

It is oxidised as it loses electrons

What happens to the less reactive metal in a displacement reaction?

It is reduced as it gains electrons

What is electrolysis?

Using electricity to break down a compound

What type of substance can be electrolysed?

Ionic substances when molten or dissolved in water

What is the positive electrode called?

Anode

What is the negative electrode called?

Cathode

Where do the positive ions (cations) go?

To the negative electrode (cathode)

Where do the negative ions (anions) go?

To the positive electrode (anode)

What happens to the positive ions (cations) at the cathode?

They gain electrons (reduced)

What happens to the negative ions (anions) at the anode?

They lose electrons (oxidised)

What products are formed at each electrode?

Metal at the cathode

Non-metal at the anode

Why are the electrodes made of graphite?

• Conducts electricity (delocalised electrons which can carry the current)
  

• Withstands high temperatures (strong covalent bonds), without breaking down
  

• It’s inert, meaning it does not easily react with other substances

What happens when an ionic compound is molten?

The ions are free to move

What happens during electrolysis of molten lead bromide (PbBr₂)?

Metals form positive ions, so Lead is attracted to the negative electrode and gains electrons. Lead is formed at the cathode

Non-metals form negative ions, so Bromine is attracted to the positive electrode and loses electrons. Bromine gas forms at the anode

What additional ions are present in aqueous solutions

H⁺ ions and OH⁻ ions from the water

How is the product at the cathode decided?

If the metal is more reactive than hydrogen, hydrogen is produced

If the metal is less reactive than hydrogen, a solid layer of the pure metal will form

How is the product at the anode decided?

If OH⁻ and halide ions are present, molecules of chlorine, bromine, or iodine form.

If no halide ions are present, the OH⁻ ions are discharged, oxygen is produced 

What are the half equations for water electrolysis?

At the cathode: 2H⁺ + 2e⁻ → H₂

At the anode: 4OH⁻ → O₂ + 2H₂O + 4e⁻.

Why are inert electrodes used?

To avoid them reacting with the products