Exothermic and Endothermic, Collision Theory, Enthalpy


4.10 Understand that chemical reactions in which heat given out are described as exothermic and those in which heat energy is taken in are endothermic. 

  • In an exothermic reaction heat is given out. Because bonds are made which gives out energy.
  • In a endothermic reaction heat is taken in. Because bonds are Broken which requires energy.


Breaking bonds - Endothermic 

Making bonds - Exothermic 

4.11 Describe simple calorimetry experiements for reactions such as combusion, displacement, dissolving and neutralisation in which heat energy changes can be calucated from measured temperature changes. 

Combustion Apparatus - Measure the Temperatur of Water at the ed and beginning to fidn the difference in temperature (caliometry) 



4.11 Calculate Molar Enthalpy Change from Heat Energy Change 

ΔH is measured in J/mol or kJ/mol.

  • ΔH = molar enthalpy change
  • T = temperature increase or decrease caused by the reaction
  • mass of H2O = mass of water in grams; (we assume 10ml is 10g) 
  • 4.2J/g C = specific heat capacity of water

To work our energy change use the following equation - 

Energy transferred = mass of water heated × specific heat capacity of water × temperature rise

Bond energy calculations

You can calculate the energy change in a reaction using average bond energies. Bond energy is the amount of energy needed to break one mole of a particular bond.

To calculate bond energy

  • Add together the bond energies for all the bonds in the reactants – this is the ‘energy in’.
  • Add together the bond energies for all the bonds in the products – this is the ‘energy out’.
  • Calculate the energy change = energy in – energy out.

Worked example – an exothermic reaction

Hydrogen and chlorine react to form hydrogen chloride gas:

H−H + Cl−Cl → 2 × (H−Cl)

BondBond Energy (kJ/mol) H−H 436 Cl−Cl 243 H−Cl 432

  1. Energy in = 436 + 243 = 679 kJ/mol
  2. Energy out = 2 × 432 = 864 kJ/mol
  3. Energy change = in – out = 679 – 864 = –185 kJ/mol

The energy change is negative, showing that energy is released to the surroundings in an exothermic reaction.

ΔH = ΔT x mass of H2O (g) x 4.2 / number of moles 

ΔH is the symbol that represents the amount of energy lost or gained in a reaction.

+ΔH is endothermic (because it gains heat)


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