C5.5. Energy Changes

What is an exothermic reaction?

A reaction that transfers energy to the surroundings, causing the temperature of the surroundings to increase.

What is an endothermic reaction?

A reaction that takes in energy from the surroundings, causing the temperature of the surroundings to decrease.

What happens to the temperature of the surroundings during an exothermic reaction?

It increases.

What happens to the temperature of the surroundings during an endothermic reaction?

It decreases.

What are examples of exothermic reactions?

Combustion, many oxidation reactions and neutralisation.

What are examples of endothermic reactions?

Thermal decomposition and the reaction between citric acid and sodium hydrogencarbonate.

What are some everyday uses of exothermic reactions?

Self-heating cans and hand warmers.

What are some everyday uses of endothermic reactions?

Some sports injury packs.

How can you identify whether a reaction is exothermic or endothermic experimentally?

Measure the temperature change of the surroundings.

What happens to energy in a chemical reaction?

Energy is conserved; it is transferred between the reacting substances and their surroundings.

What happens to the energy of the products in an exothermic reaction?

The products have less energy than the reactants by the amount of energy transferred to the surroundings.

What happens to the energy of the products in an endothermic reaction?

The products have more energy than the reactants because energy has been taken in from the surroundings.

What is activation energy?

The minimum amount of energy that reacting particles must have for a reaction to occur.

Why is activation energy needed?

Reacting particles need enough energy for successful collisions to result in a reaction.

What is a reaction profile?

A diagram showing the relative energies of reactants and products and the energy changes during a reaction.

What does the vertical axis of a reaction profile show?

The energy of the reacting substances.

What does the horizontal axis of a reaction profile show?

The progress of the reaction.

How is activation energy shown on a reaction profile?

It is the energy difference between the reactants and the peak of the curve.

How is the overall energy change shown on a reaction profile?

It is the difference in energy between the reactants and products.

How can you identify an exothermic reaction from a reaction profile?

The products are at a lower energy level than the reactants.

How can you identify an endothermic reaction from a reaction profile?

The products are at a higher energy level than the reactants.

What happens to the energy of the surroundings in an exothermic reaction?

The surroundings gain energy.

What happens to the energy of the surroundings in an endothermic reaction?

The surroundings lose energy.

What happens when bonds are broken?

Energy is absorbed.

What happens when bonds are formed?

Energy is released.

Why does breaking bonds require energy?

Energy must be supplied to overcome the forces holding the atoms together.

Why does forming bonds release energy?

The atoms move to a lower-energy, more stable arrangement.

In an exothermic reaction, how does the energy released by bond formation compare with the energy required for bond breaking?

More energy is released forming new bonds than is required to break existing bonds.

In an endothermic reaction, how does the energy required for bond breaking compare with the energy released by bond formation?

More energy is required to break existing bonds than is released when new bonds form.

What is bond energy?

The energy required to break a particular covalent bond.

How can bond energies be used to calculate the overall energy change of a reaction?

Energy change = total energy required to break bonds − total energy released when bonds are formed.

What happens if more energy is released forming bonds than is required to break bonds?

The reaction is exothermic.

What happens if more energy is required to break bonds than is released forming bonds?

The reaction is endothermic.

Why can bond-energy calculations give an estimate rather than an exact value?

Bond energies are average values for particular types of bonds.

What is the required practical for Energy Changes investigating?

Investigating the variables that affect temperature changes in reacting solutions, such as acid plus metals, acid plus carbonates, neutralisation and displacement reactions.

What variables could affect the temperature change in a reacting solution?

Variables such as the substances used, their concentrations and their volumes can affect the temperature change.

How can you make a temperature-change investigation more reliable?

Repeat measurements and calculate a mean, while controlling relevant variables.

How can you improve the accuracy of a temperature-change investigation?

Reduce heat loss to the surroundings, for example by using insulation and a lid.

Why is energy transferred to or from the surroundings during chemical reactions?

Breaking and forming chemical bonds involves energy transfers.

What is the key difference between exothermic and endothermic reactions?

Exothermic reactions transfer energy to the surroundings, whereas endothermic reactions take energy from the surroundings.