Chemistry Unit 2

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  • Created by: Ali
  • Created on: 15-05-12 17:00


Endothermic and Exothermic Reactions


  • If at the end of a reaction energy has been given out, the reaction is exothermic
    • an example of this is neutralisation
  • If at the end of a reaction energy has been taken in, the reaction is endothermic
    • an example of this is the breakdown of limestone (calcium carbonate) into lime (calcium oxide) and carbon dioxide


Enthalpy change (ΔH)

  • heat energy change at constant pressure
  • standard condition (100KPa, 298K)

Physical States

  • the physical states of the reactants and products affects the enthalpy change of a reaction

Measuring enthalpy changes

Standard enthalpies

  • Standard enthalpy of combustion is the enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions, all in their standard states
  • Standard enthalpy change of formation is the enthalpy change when one mole of a compound is formed from it's elements under standard conditions, all in their standard states

Measuring the enthalpy change

q = m.c.ΔT

q = measured enthalpy change

m = mass of substance

c = specific heat capacity

ΔT = temperature change

The Simple Calorimeter

  • can use a simple calorimeter to find the enthalpy change when a fuel burns
  • burn the fuel in a known mass of water and then measure the temperature rise

Hess's Law

  • the enthalpy change for a chemical reaction is the same, independent of the route taken
  • energy can't be created or destroyed

Enthalpy of formation

  • FOPOMIR (formation = products  - reactants)

Enthalpy of combustion

  • CRAMP (combustion = reactants - products)


Collision Theory

  • most collisions between molecules do not lead to reaction because they don't have enough energy or are the wrong orientation 

Factors that affect the rate of chemical reactions

  • Increasing the temperature - the speed and therefore kinetic energy
  • Increasing the concentration of a solution - more particles means more chance of collisions, however then all particles are used up which slows the rate of reaction
  • Increasing the pressure of a gas reaction - more particles so bigger chance of collisions
  • Increasing the surface area of solid reactants - more sites for reaction

Activation Energy

  • for a collision to result in a reaction, the molecules must have a certain minimum energy to start breaking bonds

The Maxwell-Boltzmann distribution

The effect of temperature on reaction rate

  • at higher temperatures the peak of the curve is lower, and it moves to the right
  • the number of particles with very high energy increases
  • the graphs show that at higher temperatures more of the molecules have energy greater than Ea (activation energy) so more reactions will take place
  • a small increase in temperature produces a large increase in the number of particles with energy greater than Ea


  • substances that affect the rate of chemical reactions without being chemically changed themselves
  • used in industry to speed up reactions, which is cheaper than increasing the temperature or pressure

How catalysts work

  • provide a different pathway for the reaction with a lower activation energy
  • don't affect the enthalpy change or…




Thanks :) 



Fantastic :)

Hayley Osgood


wow :) thank you!



I use **** and FLIPPER for the enthaplies



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