Enthalpy Changes

• Define Enthalpy:

  The heat energy stored in a system
• Define Enthalpy Change:

  The change in heat energy between the products and the reactants of a system during a reaction
• Define Exothermic reaction:

  A reaction that releases more energy by making bonds than the energy required for breaking bonds. The enthalpy of the products is less than the enthalpy of the reactants. The system loses heat energy to its surroundings. ΔH < 0
• Define Endothermic reaction:

  A reaction that releases more energy by breaking bonds than the energy required for making bonds. The enthalpy of the products is greater than the enthalpy of the reactants. The system gains heat energy from its surroundings. ΔH > 0
• Give examples of exothermic reactions:

  • combustion of fuels
  • respiration
  • neutralisation
• What type of reaction is this profile diagram of?

  Exothermic

  
• What type of reaction is this profile diagram of?

  Endothermic

  
• Define Activation Energy:

  The minimum energy required for a reaction to take place
• State standard conditions:

  • 298K
  • 100KPa
  • 1 mol dm^-3
• Define Standard States:

  The physical state of a substance under standard conditions
• Define Enthalpy Change of Combustion:

  The enthalpy change that takes place when one mole of a substance is completely combusted in excess oxygen
• What is the symbol of enthalpy change?

  ΔH
• Name two endothermic reactions:

  • Photosynthesis
  • Thermal Decomposition
• Define Enthalpy Change of Formation:

  The enthalpy change that takes place when one mole of a compound is formed from its elements in their standard states under standard conditions
• Define Enthalpy Change of Neutralisation:

  The enthalpy change that takes place when one mole of liquid water is formed from a neutralisation reaction of an acid by a base
• Define Enthalpy Change of Reaction:

  The enthalpy change that is associated with a particular chemical equation
• Define Hess' Law:

  The enthalpy change of a reaction is independent of the route taken
• Define Average Bond Enthalpy:

  The energy required to break one mole of gaseous bonds
• The arrows in an exothermic and endothermic profile diagram always go from the reactants to the products
• Limitation of average bond enthalpy:

  • The actual bond enthalpy is calculated in different chemical environment
  • The average bond enthalpy is the mean of different enthalpies
• If the reactants have very strong bonds the activation energy will be very high and the reaction will be slow
• If the reactants had very weak bonds the activation energy will be very low and the reaction will fast
• All reactions have a positive activation energy
• Most enthalpies of formation are negative - most substances form exothermically
• Enthalpies of combustion are negative - substances burn exothermically
• What does Δ stand for and mean?

  Delta = change
• What does the formula Q=mcΔT work out?

  the heat energy transferred
• What does the m stand for in the formula Q=mcΔT?

  the mass of the surroundings/solution
• What does the c stand for in the formula Q=mcΔT?

  the specific heat capacity of water
• What does ΔT stand for in the formula Q=mcΔT?

  the change in temperature
• In the formula Q=mcΔT, the unit of the change in temperature can be in Celsius or Kelvin
• If the change in temperature from the formula Q=mcΔT is negative, the enthalpy change from the formula ΔH=Q/moles will be positive
• If the change in temperature from the formula Q=mcΔT is positive, the enthalpy change from the formula ΔH=Q/moles will be negative
• Define The Specific Heat Capacity of Water:

  The amount of energy required to raise the temperature of 1kg of water by one celsius or kelvin
• State the unit for Q in Q=mcΔT?

  J
• 1 cm^3 = 1 g for aqueous solutions
• 1 cm^3 = 1 g for aqueous solution
• Outline the reasons why it is not always possible to measure an enthalpy change directly:

  • the reaction may have a high activation energy
  • the rate of reaction may be slow
  • more than one reaction may take place, producing lots of unwanted products
• What does Σ mean?

  the sum of
• Formula for calculating bond energy:

  Sum of Reactants - Sum of Products