Equilibria - AQA Chemistry

Summary of equilibria for CHEM2 

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  • Created by: Kim
  • Created on: 04-04-13 16:48

Enthalpy change

  • Definition:- amount of heat energy released/absorbed when a chemical or physical change occurs at constant pressure 
  • Exothermic:- system gives out heat energy to surroundings, enthalpy lost = negative 
  • Endothermic:- system takes in heat energy from surroundings, enthalpy gained = positive
  • Standard enthalpy changes:- occur at standard pressure of 100kPa and temp 298k
  • Standard enthalpy of combustion:- (http://upload.wikimedia.org/math/9/f/c/9fc9d8d0fcb6c7527bf33cde834dd98a.png) = enthalpy change when 1 mol of a substance is burned completely in oxygen, all reactants and products in standard states of 298k and 100kPa, under standard conditions
  • Standard enthalpy of formation:- (http://upload.wikimedia.org/math/e/5/8/e588f36cd32aff3af94ea52caf90cdaf.png) = enthalpy change when 1 mol of a compound is formed from its constituent elements, all reactants and products in standard states of 298k and 100kPa, under standard conditions
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  • Calorimeter:- apparatus used to measure heat change
  • q = m x cΔT
  • q = heat energy kJ
  • m = mass of substance kg
  • c = specific heat capacity kJ K-1 kg-1
  • ΔT = change in temperature K

In an experiment, 1.00g of methanol (CH3OH) was burned in air and the flame was used to heat 100g of water, which rose in temperature by 42.0oC.
CH3OH(l) + 3/2O2(g) --> CO2(g) + 2H2O(g)
Calculate the enthalpy change.

- Heat energy gained by water --> q = mcxΔT = 0.100x4.18x42.0 = 17.6kJ
- Heat energy lost by methanol --> = -17.6kJ
- Moles of methanol burned --> Mass/Mr
                                             = 1.00/32.0 = 0.0313mol
- Enthalpy change per mole --> 
ΔH = heat energy lost by methanol/moles of methanol
                                                   = -17.6/0.0313kJ mol-1 = -563kJ mol-1 

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Hess's Law

  • First law of thermodynamics:- energy can be neither created nor destroyed, but can be converted from one form into another
  • Hess's Law:- the enthalpy change of a reaction depends only on the initial and final states of the reaction and is independent of the route by which the reaction occurs
  • Can be used to determine ΔH values for reactions where direct determination is difficult: 
  • --> E.g. the enthalpy change of formation can be calculated from standard enthalpies of combustion. 
  • Remember ****:- Combustion = reactants and(-) products
  • Similarly the enthalpy change of combustion can be calculated from standard enthalpies of formation values.
  • Formation = products - reactants
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Bond Enthalpies

  • More commonly known as bond dissociation enthalpy (diatomic molecules) --> 
    Definition:- the enthalpy change for the breaking of a covalent bond, with all species in the gaseous state
    E.g. --> A--B(g) ---> A(g) + B(g) 
  • Mean bond enthalpy (polyatomic molecules) --->
    Definition:- the average of several values of the bond dissociation enthalpy for a given type of bond, taken from a range of different compounds
  • Mean bond enthalpies can be used to calculate the enthalpy change for simple reactions:
  • ΔH = Σ(mean bond enthalpy of bonds broken) - Σ(mean bond enthalpy of bonds formed)
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Ninja Steak


Useful notes, thanks! :)

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