Energetics

• Describe an exothermic reaction

  - surroundings get hotter, so temperature of surroundings increases

  - more energy is released when new bonds form than needed to break bonds in reactants

  - products have less enthalpy than reactants

  - reactants are less stable than the products

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• Describe endothermic reactions


  - surroundings get colder

  - more energy is taken to break the existing bonds than is given out when the new bonds in products form

  - reactants have less enthalpy than the products

  - reactants are more stable than the products

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• Define enthalpy change *

  - the heat energy change measured under conditions of constant pressure
• Define standard enthalpy change *

  - refer to reactions done under standard conditions, and with everything present in their standard states
• What are the standard conditions?

  - temperature of 298K (25°C)

  - pressure of 100kPa

  - when solutions involved, concentration of 1.0 moldm-3

  - remember state symbols when writing equations
• What is the symbol for standard enthalpy change?

  - delta H standard
• Define standard enthalpy change of combustion (3)

  - the enthalpy change when 1 mol of a substance (1)

  - is burned completely in excess oxygen under standard conditions (2)

  - with all reactants and products in their standard states (3)

  - when writing equations, make sure to only make 1 mol of substance
• Define standard enthalpy change of formation (3)

  - the enthalpy change when 1 mol of a substance (1)

  - is formed from its elements under standard conditions (2)

  - with all reactants and products in standard states (3)

  - when writing equations, make sure to only make 1 mol of substance
• Define specific heat capacity

  - the amount of heat needed to raise the temperature of 1g of a substance by 1 Kelvin
• How to calculate molar enthalpy change

  - calculate the enthalpy/energy change in kJ

  - calculate number of moles

  - divide enthalpy change by moles to get molar enthalpy change
• What are the errors of the results when investigating the combustion of alcohols?

  Heat loss - gives a less exothermic answer

  Incomplete combustion - less exothermic anser

  Alcohol loss by evaporation - suggests a larger amount of alcohol is needed to get the temperature rise - leads to a higher heat capacity and so more exothermic answeR
• define enthalpy of neutralisation

  the enthalpy change when solutions of an acid and alkali react together under standard conditions to produce one mole of water

  neutralisation reactions are always exothermic
• define Hess’s law

  states that the enthalpy change in a reaction is independent of the route

  • when doing calculations, look at the data given and this is the one that goes on the bottom
• Define mean bond enthalpy

  Enthalpy change needed in breaking covalent bonds, average for that type of bond over a range of compounds