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Energy changes for SQA Higher Chemistry

Energy changes

This page covers the following topics:

1. Thermal energy
2. Enthalpy of combustion
3. Exothermic and endothermic reactions
4. Reaction profile

Thermal energy released in a chemical reaction can be obtained by heating a known mass of water in an isolated system and measuring the change in temperature. The energy released is then found by multiplying the mass of water, specific heat capacity of water and the change in temperature.

Thermal energy

Enthalpy change of combustion is the amount of energy gained by a reaction mixture when per one mole of a reactant is burnt in oxygen. Negative enthalpy change suggests that a reaction mixture is losing energy, while positive enthalpy change means that a reaction mixture is gaining energy.

Enthalpy of combustion

In chemical reactions energy is conserved meaning that the energy within a reaction mixture and its surroundings before and after the reaction is equal. An exothermic reaction is a reaction during which it takes less energy to break bonds than to make new ones. Thus, exothermic reactions emit heat to the surroundings. An endothermic reaction is a reaction during which it takes more energy to break bonds than to make new ones. Therefore, endothermic reactions take heat from the surroundings.

To determine if a reaction is endothermic or exothermic, a temperature change throughout the course of a reaction can be measured. If the temperature increases, the reaction is exothermic; if it decreases, the reaction is endothermic. Exothermic reactions are widely used for heating, electricity production, cooking food. Endothermic reactions are involved in cold packs used for cooling down injuries.

Exothermic and endothermic reactions

Reaction profile shows the energies of reactants, intermediates and products in a reaction. The difference between the initial energy of the reactants and the highest point in a reaction profile is called activation energy which is the energy required for a reaction to happen. If reactant molecules have less energy than the activation energy, a reaction between them does not happen. Activation energy can be thought of as a hill that particles need to climb.

Reaction profile can be used to determine if a reaction is exothermic or endothermic. Exothermic reactions have the energy of reactants higher than the energy of products. Endothermic reactions have the energy of reactants lower than the energy of products.

Reaction profile

1

Provide an application of an endothermic reaction.

Endothermic reactions can be involved in the reactions within cold packs used for cooling down injuries.

cold packs

Provide an application of an endothermic reaction.

2

Name A and B shown in the reaction profile.

Reaction profile shows the energies of reactants, intermediates and products in a reaction.

A = reactants, B = products

Name A and B shown in the reaction profile.

3

George has drawn a reaction profile for a photosynthesis reaction provided below. What words should he include instead of X and Y in the diagram?

photosynthesis: water + carbon dioxide โ†’ glucose + oxygen

Reaction profile shows the energies of reactants, intermediates and products in a reaction. Reactants are on the left side of the photosynthesis equation provided, products are on the right.

X = water, carbon dioxide; Y = glucose, oxygen

George has drawn a reaction profile for a photosynthesis reaction provided below. What words should he include instead of X and Y in the diagram?

photosynthesis: water + carbon dioxide โ†’ glucose + oxygen

4

Provide the meaning of an enthalpy of combustion being negative.

Negative enthalpy change suggests that a reaction mixture is losing energy.

The reaction mixture is losing energy.

Provide the meaning of an enthalpy of combustion being negative.

5

Define activation energy.

Activation energy is the energy required for a reaction to happen.

energy required for a reaction to happen

Define activation energy.

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