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Chemistry: Energy changes

What you need to know

Reflections and Exam tips


Energy changes

Representing energy changes

The energy changes in a chemical reaction can be conveniently  represented using energy level diagrams.

Energy level diagrams make it easier to decide whether a reaction is exothermic (gives out heat and gets hotter) or endothermic (takes in heat and gets cooler).

Measuring enthalpy changes

To determine the amount of heat a substance produces or absorbs we often use   q = cmT. Where C is the specific heat capacity, m is the mass of solution and delta T is change in temperature.

Specific heat capacity = heat capacity of 1 g of a substance. It takes 4.2 J to heat 1 ml/g of solution by 1 °C, so specific heat capacity for water is 4.2 J/ml/°C.

To determine the enthalpy change of the reaction the following reaction is used.

Bond energies

Bond Energy- energy required to break apart a bond between two particular atoms,  measured in kJ/mol and used to work out ΔH in energy calculations.

Chemical reactions are in two stages:

  1. Breaking bonds, an endothermic process.
  2. Making new bonds, an exothermic process.

To calculate energy change we need to know:

  a) the amount of energy needed to break the bonds between the atoms; 

  b) the amount of energy released in the formation of new chemical bonds.

Consider the reaction between hydrogen and chlorine:

H2 + Cl2 ——> 2HCl

In this reaction one H-H bond and one Cl-Cl bond is broken and two H-Cl bonds are formed.

The H-H bond energy is 436kJ/mol

The Cl-Cl bond energy is 242kJ/mol

So, the energy needed to break these bonds is 436 + 242 = 678kJ

The H-Cl bond energy is 431kJ

So the energy given out when these bonds are

formed is 2 x -431 = -862kJ

Overall change = 678 + (– 862) = -184kJ, an exothermic reaction.