Topic 5.1: Exothermic and endothermic reactions

5.1.1 Define the terms exothermic reaction, endothermic reaction and standard enthalpy change of reaction.

These words are often used to describe the energy changes that take place during a chemical reaction.

Reactions that release heat energy are called exothermic reactions. These causes a rise in temperature because chemical bonds are broken

When heat energy is taken in form the surroundings by the chemicals, causing a temperature drop, this is called an endothermic reaction. Energy is required because the bonds made exceed the energy levels of the bonds broken.

The absolute enthalpy stored is very difficult to measure. Usually, the enthalpy change of reactants compared to the products is measured.

If this is carried out in the lab at atmospheric pressure (101 kPa) and 298 degree kelvins, then this is called the standard enthalpy change of reaction.

A release of energy is better because the products are more stable than the reactants.


5.1.2 State that combustion and neutralization are exothermic processes.

Combustion and Neutralization are both exothermic processes. The energy required to hold the organic substance, acids and alkali is much higher than salts and atmospheric gases.

The products of combustion are carbon dioxide and water, which is much lower than the organic substances.

The products of neutralization are salts and water, which is much lower than acid and alkali substances.

5.1.3 Apply the relationship between temperature change, enthalpy change and the classification of a reaction as endothermic or exothermic.

Temperature change

- Positive change is exotheric - release heat
- Negative change is endothermic - takes in heat

Enthalpy Change

- Positive change is endothermic - takes in energy
- Negative change is exothermic - release energy

The ideas do not contradict because energy is equal to heat energy, unless the energy released is in a different form such as sound.


5.1.4 Deduce, from an enthalpy level diagram, the relative stabilities of reactants and products, and the sign of the enthalpy change for the reaction.

A substance with too much energy isn't as stable as substances with less energy.

This is how a enthalpy level diagram should look like for exothermic and endothermic