Exothermic and Endothermic, Collision Theory, Enthalpy

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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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