AQA Chemistry Unit 2: 7 Energetics

• Thermochemistry is the study of heat changes due to chemical reactions
• Energy is put in to break bonds
• Energy is given out when forming bonds
• If overall energy is given out it is exothermic
• If overall energy is taken in it is endothermic
• Exothermic example: neutralising an acid with an alkali
• Endothermic example: the breakdown of limestone

Quantities

• Energy given out or taken in is measured in kilojoules per mole (kJmol-1)

Burning Fuels

• These are very exothermic reactions

• Enthalpy Change: Heat energy change measured at constant pressure
• Activation Energy: Minimum energy required for a reaction to occur
• In exothermic reactions the system loses energy to the surroundings and the energy of the products is lower than the reactants
• In endothermic reactions the system removes energy fro the surroundings, energy in products is higher than the reactants

Bond Enthalpies

• Bond enthalpy: Energy attributed to every bond (measured in constant pressure)
• Different bonds have different enthalpies
• Mean Bond Enthalpy: The average of many bond enthalpy values for the same bond taken from different environments

Standard Enthalpy Change: Enthalpy change measured at standard pressure (100kPa) and standard temperature (298K)

Standard States: Elements and compounds are said to be in standard states if they are in their normal stable state at 298K and 100kPa

Standard Enthalpy Change of Formation: Enthalpy change when 1 mole of compound produced from its elements. Under standard conditions and all reactants and products in standard states

Standard Enthalpy Change of Combustion: Enthalpy change when 1 mole of substance (element/compound) is burnt completely in oxygen. Under standard conditions and all reactants and products in standard states

• Determines heat absorbed, evolved or transferred
• 1st Law of Thermodynamics: Energy cannot be created or destroyed only converted
• Heat energy (q) required to change the temperature of a substance is given by:

q=m X c X /\T

• m: Mass of substance whose temperature is being changed
• /\T is temperature change
• c is the specific heat capacity

Assumptions Made

• All the heat energy produced goes to increasing the temperature
• None is lost to the surroundings
• None is transferred in phenomena
• Heat capacity for water is always used, which is good for when fuel is alight and used to heat know mass of water, okay when 2 solutions are mixed and the temperature of the mixture is monitored, less good using concentrated solutions 

Molar Enthalpy Change =Heat Energy change per Mole

This is q/n ( Heat energy divided by mole)

Definition: Enthalpy change of a reaction depends only on the initial and final states of the reaction and is independent of the route by which it may occur

Mean Bond Enthalpy Calculations

• Draw out the displayed formula of reactants and products
• Calculate the enthalpy change to break bonds
• Calculate the enthalpy change to form bonds

Strategy for Enthalpy Calculations

• Write the equation for the unknown enthalpy change across the top of the thermochemical cycle
• Write the elements or combustion products on the bottom
• Make sure arrows go in direction of enthalpy change
• Look out for multiples and negative negatives (form a positive)