Chemistry Unit 2 Module 3, 2.3.1 - 2.3.5

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  • Created by: R
  • Created on: 19-03-13 10:40

Standard enthalpy changes

Standard conditions 

  • Pressure of 100kPa 
  • A stated temperature -298K is usually used 
  • a concetration of 1 mol dm-3 (for reactions with aqueous solutions 

Standard states 

  • Magnesium has the standard state; Mg (s)
  • Hydrogen has the standard state; H2 (g)
  • Water has the standard state; H2O (l) 

Standard enthalpy changes

  • An equation - this gives the amounts, in mol, of reactants and products
  • An enthalpy change, kJ mol-1 - this gives the enthalpy change for the molar qunatites in the stated equation
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Standard enthalpy changes

Standard enthalpy change of combustion 

  • 1 mole of a substance reacts completely with oxygen under standard conditions, 
  • All reactants and products being in their standard states 

Standard enthalpy change for formation 

  • Takes place when one mole of a compound is formed from its constituent elemetns in their standard states under standard conditions
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Determination of enthalpy changes

Specific heat capacity 

Q = m x c x   T  (Joules) 

m = the mass of the surrounds involved in the heat exchange 

c = the specific heat capacity of the surroundings 

__T = the temperature change of the surroundings __ T = Tfinal - Tinitial 

__H = Q / Moles of substance

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Enthalpy H - is the heat content that is stored in a chemical system

Exothermic - refers to a reaction in which the enthalpy of the products is smaller than the enthalpy of the reactant; resulting in heat loss to the surroundings 

Negative enthalpy change!

eg Self heating cans, Oxidation of fuels, Respiration

Endothermic - refers to a reaction in which the enthalpy of the proudcts is greater than the enthalpy of the reactant; resulting in heat being took in from the surroundings, 

Postive enthalpy change!

eg Self cooling cans, Thermal decompostion of limestone, Photosysthensis 

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Enthalpy profile diargrams

Enthalpy profile diagrams - is a diagram for a reaction to compare the enthalpy of the reactants with the enthalpy of the products 

Exothermic reactions;                                          Endothermic reactions;

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Activation energy

Activation enery - is the minimum energy required to start a reaction by the breaking of bonds 

Exothermic;                                                 Endothermic;

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