Energy changes in chemical reactions
Energy changes are always involved in a chemical reaction. The energy changes can be Exothermic or endothermic. This energy can be seen by their increase or decrease in energy system of some reactions.
During a chemical reaction, bonds in the reactants are broken, while new bonds are formed in the product. The following example may help to explain this.
Hydrogen reacts with oxygen to form water, according to the following equation:
2H2 + O2 → 2H2O
In this reaction, the bond between the two hydrogen atoms in the H2 molecule will break, as will the bond between the oxygen atoms in the O2 molecule. New bonds will form between the two hydrogen atoms and the single oxygen atom in the water molecule that is formed as the product.
For bonds to break, energy must be absorbed. When new bonds are formed, energy is released. The energy required to break a bond is called the bond energy or bond dissociation energy. Bond energies are measured in units of kJ.mol−1.
Bond energy is a measure of bond strength in a chemical bond. It is the amount of energy (in kJ.mol−1) that is needed to break the chemical bond between two atoms.
Exothermic and endothermic reactions
Exothermic reactions: An exothermic reaction is one that releases energy in the form of heat or light. In exothermic reaction, the energy of the product is less than the energy of the reactants, because energy has been released during the reaction. We can represent this using the following general formula:
Reactants→ Product + Energy
Examples of exothermic reaction includes:
Combustion reactions – The burning of fuel is an example of a combustion reaction, and we as humans rely heavily on this process for our energy requirements.
The following equations describe the combustion of a hydrocarbon such as methane (CH4):
Fuel + Oxygen→ Heat + Water + Carbon Dioxide CH4 + 2O2 → Heat + H2O + CO2
Respiration – Respiration is the chemical reaction that happens in our bodies to produce energy for our cells. The equation below describes what happens during this reaction:
C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
Endothermic reaction: An endothermic reaction is one that absorbs energy in the form of heat. In an endothermic reaction, the energy of the product is greater than the energy of the reactants, because energy has been absorbed during the reaction. This can be represented by the following formula:
Reactants + Energy→ Product
Examples of endothermic reaction includes:
Photosynthesis: This is an endothermic reaction because it will not happen without an external source of energy, which in this case is sunlight. The equation for this reaction is:
6CO2 + 12H2O + energy → C6H12O6 + 6O2 + 6H2O
Thermal decomposition: The thermal decomposition of limestone In industry, the breakdown of limestone into quicklime and carbon dioxide is very important. The equation for the reaction is shown below:
CaCO3 → CaO + CO2
The heat of reaction
The diﬀerence in energy (E) between the reactants and the products is known as the heat of the reaction. It is also sometimes referred to as the enthalpy change of the system
The heat of the reaction is represented by the symbol ∆H, where:
∆H = Eprod −Ereact
- In an exothermic reaction, ∆H is less than zero because the energy of the reactants is greater than the energy of the product. For example,
H2 + Cl2 → 2HCl ∆H = -183 kJ
- In an endothermic reaction, ∆H is greater than zero because the energy of the reactants is less than the energy of the product. For example,
C + H2O → CO + H2 ∆H = +131 kJ
Enthalpy is the heat content of a chemical system, and is given the symbol ’H’.
Note: while writing equations using ∆H
There are two ways to write the heat of the reaction in an equation
For the exothermic reaction C(s) + O2(g) → CO2(g), we can write:
C(s) + O2(g) → CO2(g) ∆H = -393 kJ.mol−1
C(s) + O2(g) → CO2(g) + 393 kJ.mol−1
For the endothermic reaction,
C(s) + H2O(g) → H2(g) + CO(g), we can write:
C(s) + H2O(g) → H2(g) + CO(g) ∆H = +131 kJ.mol−1
C(s) + H2O(g) + 131 kJ.mol−1 → CO + H2
The units for ∆H are kJ.mol−1. In other words, the ∆H value gives the amount of energy that is absorbed or released per mole of product that is formed. Units can also be written as kJ, which then gives the total amount of energy that is released or absorbed when the product forms.
A spontaneous reaction is a physical or chemical change that occurs without the addition of energy. Ie a reaction that occurs on its own. An example of a spontaeneous reaction is the addition of a solution of dilute sulfuric acid with a solution of sodium hydroxide.
H2SO4 +2NaOH → Na2SO4 + H2O
Activation energy and the activated complex
From the understanding of spontaneous and non-spontaneous reactions, it is clear that most reactions will not take place until the system has some minimum amount of energy added to it. This energy is called the activation energy. Activation energy is the ’threshold energy’ or the energy that must be overcome in order for a chemical reaction to occur.
It is possible to draw an energy diagram to show the energy changes that take place during a particular reaction. Let’s consider an example:
H2(g) + F2(g) → 2HF(g)
The reaction between H2(g) and F2(g) in the diagram above needs energy in order to proceed, and this is the activation energy. Once the reaction has started, an in-between, temporary state is reached where the two reactants combine to give H2F2. This state is sometimes called a transition state and the energy that is needed to reach this state is equal to the activation energy for the reaction. The compound that is formed in this transition state is called the activated complex. The transition state lasts for only a very short time, after which either the original bonds reform, or the bonds are broken and a new product forms. In this example, the ﬁnal product is HF and it has a lower energy than the reactants. The reaction is exothermic and ∆H is negative.
Activated complex is a transitional structure in a chemical reaction that results from the eﬀective collisions between reactant molecules, and which remains while old bonds break and new bonds form.
In endothermic reactions, the ﬁnal products have a higher energy than the reactants. An energy diagram is shown below for the endothermic reaction XY +Z → X +Y Z.
In this example, the activated complex has the formula XYZ. Notice that the activation energy for the endothermic reaction is much greater than for the exothermic reaction.