Equilibria - AQA Chemistry

• 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:-  = 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:-  = 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

• Calorimeter:- apparatus used to measure heat change
• q = m x c x ΔT
• q = heat energy kJ
• m = mass of substance kg
• c = specific heat capacity kJ K-1 kg-1
• ΔT = change in temperature K

Example:
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 

• 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

• 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)