Exothermic and Endothermic Reactions
Define the terms exothermic reaction, endothermic reaction and standard enthalpy change of reaction
Exothermic Reaction – A reaction that causes the temperature of the surroundings to increase.
Energy is lost, or released, in the reaction, as the enthalpy of the products is less than the enthalpy of the reactants.
Endothermic Reaction – A reaction that causes the temperature of the surroundings to decrease.
Energy is used up in the reaction, as the enthalpy of the products is greater than the enthalpy of the reactants. Most endothermic reactions are not spontaneous
Standard Enthalpy Change of Reaction – The difference between the products and the enthalpy of the reactants under standard conditions.
This is affected by temperature, pressure, concentration of solutions and physical state. Therefore, we have to make sure that these are specified, otherwise results are not comparable. The standard conditions of heat energy transfer are pressure 101.3kPa and temperature 298K. Only the change in H can be measured for a given for a reaction, not H for the initial or final state of the species.
State that combustion and neutralisation are exothermic reactions
Combustion is an exothermic reaction as heat energy is released into the surroundings
Neutralisation reactions are exothermic
These are the reactions between acids and bases to form salt and water
Apply the relationship between temperature change, enthalpy change and the classification of reactions as endothermic or exothermic
When the enthalpy of the products is greater than the reactants, energy is absorbed from the surroundings: an endothermic reaction. The temperature of the surroundings decreases
o This kind of reaction is used for ice packs in sports injuries
This is represented mathematically as the enthalpy of the products minus that of the reactants to find the change
When the change is negative, energy is lost to the surroundings, making the reaction exothermic
When the change is positive, energy is used from the surroundings, making the reaction endothermic
A thermochemical equation is when the equation is written with an associated change in enthalpy
Deduce, from an enthalpy level diagram, the relative stabilities of reactants and products, and the sign of the enthalpy change for the reaction
Enthalpy level diagrams, or energy profile diagrams, allow us to visualise what happens to the enthalpy of a reaction as it proceeds
The total enthalpy of the reactant species is labelled HR, the products HP
The bonds between the reactants must first be broken before they can be converted into products. They must collide with sufficient energy for this to happen. Some of this kinetic energy is converted into vibrational energy, which overcomes the bonds when it reaches a certain magnitude. Only then can the reaction be initiated
This amount of energy is known as the activation energy, Ea, seen as a hump on the diagram
The distance between HR and the top of the hump is the total amount of activation energy required. This is always a positive value
When bonds are made to form the products, this always releases energy, causing the diagram to drop at HP
We can also deduce the relative stabilities of the reactants and products
The higher the enthalpy, the less stable the substance
In exothermic reactions, the products have a lower enthalpy than the reactants, and so are more stable, and vice versa for endothermic reactions. Large activation energies indicate strong bonds in the reactants, i.e.
Other reactions have small activation energies. The reaction with white phosphorus below is exothermic. White phosphorus is highly reactive and dangerous, so the less reactive red phosphorus is used in safety matches
The instability of the reactants makes it easier for their bonds to be broken, which is why the activation energy is so low.
Here are a few more reactions you need to be aware of:
Ionisation – Endothermic
Change of State: Melting, Evaporation and Sublimation – Endothermic
Change of State: Vaporisation and Condensation – Exothermic
Remember that bond breaking is endothermic and bond making is exothermic
Summary
| Type of Thermochemical Reaction | Exothermic | Endothermic |
| Enthalpy change | Products < Reactants | Products > Reactants |
| ΔH | negative | positive |
| Change in surrounding temperature | increases | decreases |
| Enthalpy level diagram
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