AS-Level Energy and Enthalpy
During chemical reactions, energy is exchanged with the surroundings.
In exothermic reactions, products are lower energy than the reactants and the temperature of the surroundings increases as energy is released overall.
In endothermic reactions, products are higher energy than the reactants and the temperature of the surroundings decreases as energy is absorbed overall.
The actual amount of energy in a compound or atom is not measured in chemistry but the change in energy during a reaction can be measured and is called enthalpy.
Standard enthalpy changes refer to the thermal energy change that occurs during a reaction, where 1 mole of product is formed in standard conditions (100 KPa and 298K).
When chemical reactions happen, the products produced have different energies to the reactants. This means energy has either been gained or lost by the reactants.
Energy cannot be created or destroyed, only transferred from one ‘store’ to another.
If energy is released in a reaction process, then the reaction is described as being exothermic. Exothermic reactions have negative energy changes.
If energy is absorbed in a reaction process, then the reaction is described as being endothermic. Endothermic reactions have positive energy changes.
During a reaction, energy is transferred in the form of thermal energy. In an exothermic reaction, energy is released from the reactants to the surroundings – the temperature of a reaction’s surroundings increases. In an endothermic reaction, energy is taken from the surroundings and absorbed by the reactants – the temperature of a reaction’s surroundings decreases.
The first law of thermodynamics tells us that the total energy in a closed system remains constant. Energy may be transferred within a closed system, but no energy can enter or leave the system.
In chemistry, we do not measure the actual amount of energy in a particular substance, but we measure the energy change that occurs when the substance reactants and forms a new species. This energy change is called the enthalpy change (∆H).
Enthalpy change (∆H) is measured as a change in temperature (heat energy) at a constant pressure.
If we want to measure and compare enthalpy changes for different processes, then we have to make sure they are being measured in the same conditions.
We call these conditions ‘standard’. Standard enthalpy changes are enthalpy changes that have been measured at a particular pressure and stated temperature. The symbol for standard enthalpy change is ∆H .
Standard conditions are 100KPa and 298K (25°C). This is NOT the same as room temperature and pressure, check your exam board and make sure you know the values required.
Standard states refer to the state of a substance under standard conditions.
We also need to know how much of our substance is reacting!
The standard amount of substance used is always 1 mole.
The standard enthalpy of combustion (∆H ) is the change in enthalpy that occurs when 1 mole of a substance undergoes complete combustion under standard conditions.
The standard enthalpy of formation (∆H ) is the change in enthalpy that occurs when 1 mole of a substance is formed from its constituent elements in their standard states.