Energy Changes in Chemical Reactions

Inquiry question: What energy changes occur in chemical reactions?

When heat is absorbed into a system from the surroundings during a physical or chemical change the process is endothermic. The melting of ice and the evaporation of liquid water are endothermic processes. The temperature of the surroundings decreases as a result of heat transfer to the system. In nature green plants absorb solar energy to power photosynthesis.

When heat is liberated from a system into the surroundings during a physical or chemical change the process is exothermic. When acids neutralise bases or when fuels undergo combustion, heat is released. The temperature of the surroundings increases as a result.

The amount of heat liberated or absorbed in a chemical reaction or physical change can be measured using a calorimeter.

When various ionic crystals dissolve in water, the ions in the crystal dissociate to produce independent aquated ions. This process is an example of dissolution where a solute dissolves in a solvent to produce a solution.

The temperature of the water may increase or decrease. The heat liberated or absorbed is called the heat of dissolution (or heat of solution). 

The molar heat of dissolution can be calculated by dividing the quantity of heat (q), which is absorbed or liberated by the number of moles (n) of solute undergoing dissolution.

Energy level profiles (or reaction profiles) show how the amount of energy that the chemicals have changes during a  reaction.  It shows the activation energy, which is the minimum amount of energy required to start a reaction.  Think of it like a speed hump in a car park; you need a certain speed in order to go over the speed hump.  An example is heating a strip of magnesium so it is hot enough to sustain cumbustion.  The overall energy change is calculated by subtracting the energy of the reactants from the energy of the products.  See the diagram below (Source: https://www.freeexamacademy.com/chemical-energetics/)

The term enthalpy (H) is used to specify the heat content of a substance. The enthalpy change (ΔH) is related to the experimentally measured heat changes that occur when the reaction is performed in a calorimeter. The calorimetry equation is modified (for n moles of material) to allow the enthalpy change to be measured in kJ/mol, as shown below:

Enthalpy and Hess’s Law

Inquiry question: How much energy does it take to break bonds, and how much is released when bonds are formed?

Hess's Law, also known as "Hess's Law of Constant Heat Summation," states that the total enthalpy of a chemical reaction is the sum of the enthalpy changes for the steps of the reaction. So, to find the total enthalpy change, we can break up a reaction into its component steps that have known enthalpy values.

Entropy and Gibbs Free Energy

Inquiry question: How can enthalpy and entropy be used to explain reaction spontaneity?

Gibbs free energy is a measure of the potential for reversible or maximum work that may be done by a system at constant temperature and pressure. It is a thermodynamic property that was defined in 1876 by Josiah Willard Gibbs to predict whether a process will occur spontaneously at constant temperature and pressure. Gibbs free energy G is defined as: