1.5 Energy changes

What happens to energy in chemical reactions?

It is conserved

What does conservation of energy mean?

Total energy before = total energy after

What happens if energy is transferred to surroundings?

Products have less energy than reactants

What is an exothermic reaction?

A reaction that releases energy to the surroundings

What happens to temperature in exothermic reactions?

It increases

Why do products have less energy in exothermic reactions?

Energy has been released

Give examples of exothermic reactions.

Combustion, oxidation, neutralisation

Give everyday examples of exothermic reactions.

Hand warmers, self-heating cans

What is an endothermic reaction?

A reaction that takes in energy from the surroundings

What happens to temperature in endothermic reactions?

It decreases

Why do products have more energy in endothermic reactions?

Energy has been absorbed

Give examples of endothermic reactions.

Thermal decomposition, citric acid + sodium hydrogencarbonate

Give everyday examples of endothermic reactions.

Instant cold packs

What must happen for a reaction to occur?

Particles must collide with enough energy

What is activation energy?

The minimum energy needed for a reaction

What do reaction profiles show?

Energy levels of reactants and products, activation energy, overall energy change

How do you identify an exothermic reaction on a graph?

Products have lower energy than reactants

How do you identify an endothermic reaction on a graph?

Products have higher energy than reactants

What happens when bonds are broken?

Energy is absorbed

What happens when bonds are formed?

Energy is released

How do you calculate energy change?

Energy in (breaking bonds) − energy out (forming bonds)

When is a reaction endothermic (bond energy)?

More energy absorbed than released

When is a reaction exothermic (bond energy)?

More energy released than absorbed

What is a cell?

A device that produces electricity from chemical reactions

What affects the voltage of a cell?

Type of electrode and electrolyte

How can a simple cell be made?

Two different metals in an electrolyte

What is a battery?

Two or more cells connected in series

What are non-rechargeable batteries?

Batteries that cannot be reused once reactants are used up

Give an example of a non-rechargeable battery.

Alkaline battery

What are rechargeable batteries?

Batteries where reactions can be reversed using electricity

What is a fuel cell?

A cell supplied with fuel (e.g. hydrogen) and oxygen to generate electricity

What is the overall reaction in a hydrogen fuel cell?

Hydrogen + oxygen → water

What are advantages of hydrogen fuel cells?

Produce only water, can run continuously with fuel supply

What are disadvantages of hydrogen fuel cells?

Hydrogen storage is difficult and requires high pressure

What is an advantage of rechargeable batteries?

Can be reused by reversing reactions

What is a disadvantage of rechargeable batteries?

Eventually stop working and disposal issues

What happens at the anode in a hydrogen fuel cell?

H₂ → 2H⁺ + 2e⁻ (oxidation)

What happens at the cathode?

O₂ + H⁺ + e⁻ → H₂O (reduction)