chemistry - energy changes

topic 5

What are exothermic reactions?

Reactions that transfer energy to the surroundings, usually as heat.

What are examples of exothermic reactions?

Combustion, many oxidation reactions, neutralisation.

What are endothermic reactions?

Reactions that take in energy from the surroundings.

What are examples of endothermic reactions?

Thermal decomposition, photosynthesis, reaction of citric acid and sodium hydrogen carbonate.

What is an everyday use of an exothermic reaction?

Hand warmers and self-heating cans.

What is an everyday use of an endothermic reaction?

Sports injury cold packs.

What does an energy level diagram show?

The relative energies of reactants and products and the energy change during a reaction.

What is activation energy?

The minimum amount of energy needed for a reaction to occur.

In an exothermic reaction profile, how do energy levels compare?

Products are at a lower energy level than reactants.

In an endothermic reaction profile, how do energy levels compare?

Products are at a higher energy level than reactants.

What is bond energy?

The energy needed to break a bond or released when a bond is formed.

What happens to bonds in chemical reactions?

Energy is needed to break bonds (endothermic); energy is released when bonds form (exothermic).

How do you calculate the overall energy change?

Energy needed to break bonds - energy released when new bonds form.

What does a negative energy change mean?

The reaction is exothermic.

What does a positive energy change mean?

The reaction is endothermic.

What is a chemical cell?

A device that produces a voltage from the chemical reactions between two different metals in a salt solution (electrolyte).

What affects the voltage of a chemical cell?

The type of metals used and the difference in their reactivities.

What is a battery?

Two or more cells connected in series to produce a greater voltage.

Why do batteries go flat?

Because the chemical reactions stop when one or more of the reactants is used up.

What is a fuel cell?

A cell that produces a voltage continuously as long as it is supplied with a fuel (e.g. hydrogen) and oxygen.

How does a hydrogen fuel cell work?

Hydrogen is oxidised at the anode and oxygen is reduced at the cathode, producing water and releasing electrical energy.

What are the advantages of hydrogen fuel cells?

Produce only water, lightweight, compact, no need to be recharged.

What are the disadvantages of hydrogen fuel cells?

Hydrogen is flammable, difficult to store, and often produced from non-renewable sources.