Energetics

• What is the focus of this revision video?

  Energetics in AQA chemistry
• What does the symbol Delta H represent?

  Enthalpy change at constant pressure
• What are the standard conditions for measuring enthalpy change?

  100 kPa and 298 K
• What characterizes endothermic reactions?

  They absorb energy from surroundings
• How does the energy profile of endothermic reactions appear?

  Reactants lower in energy than products
• What is the enthalpy change for the decomposition of calcium carbonate?

  +178 kJ per mole
• What do exothermic reactions do?

  Release heat energy to surroundings
• How does the energy profile of exothermic reactions appear?

  Reactants higher in energy than products
• What is the enthalpy change for burning ethane?

  -1560 kJ per mole
• What happens during bond breaking?

  Energy is absorbed to break bonds
• What is the enthalpy change when bonds are formed?

  Energy is released when bonds are formed
• How can we determine if a reaction is endothermic or exothermic based on bond energies?

  Compare energy needed to break bonds
• What is mean bond enthalpy?

  Average energy required to break bonds
• Why do bond enthalpies vary in a molecule?

  Different bonds have different energy requirements
• How do you calculate the energy change for a reaction?

  Energy needed to break bonds minus formed
• How do you calculate the total energy needed to break bonds in methane?

  4 times 435 kJ for CH bonds
• What is the total energy released when forming bonds in the combustion of methane?

  3466 kJ per mole
• What is calorimetry used for?

  To determine enthalpy change of combustion
• What is the formula for calculating energy transferred in calorimetry?

  Q = mcΔT
• What is the specific heat capacity of water used in calculations?

  4.18 J/g°C
• How do you calculate the number of moles of ethanol used in a calorimetry experiment?

  Mass of ethanol divided by its molar mass
• What is the difference between energy and enthalpy?

  Enthalpy is kJ per mole, energy is kJ
• What is the purpose of using a polystyrene cup in calorimetry?

  To minimize heat loss to surroundings
• How do you measure temperature change in a calorimetry experiment?

  Measure before and after adding reactants
• What is the formula for calculating entropy change?

  Energy divided by moles minus 14212
• Why is the entropy change negative in this context?

  Because it is an exothermic reaction
• What is the entropy change of the fuel mentioned?

  -364.5 KJ per mole
• What is the purpose of a polystyrene cup in calorimetry experiments?

  • Prevents heat loss to surroundings
  • Contains heat within the beaker
• What is the first step in a calorimetry experiment with a solution?

  Add acid and measure the temperature
• What is the temperature change after adding sodium hydroxide?

  Raised to a maximum of 26° C
• What mass is used in the energy change calculation?

  Mass of both liquids combined
• How do you calculate the number of moles of HCl?

  Concentration times volume
• How do you convert volume from cm³ to dm³?

  Divide by 1000
• How is the entropy change calculated for neutralization?

  Energy divided by number of moles
• What is the entropy change of neutralization for hydrochloric acid?

  -5016 K per mole
• What is Hess's Law?

  • Total energy change is independent of the route taken
  • Useful for calculating energy changes not easily measured
• Who formulated Hess's Law?

  Jérôme Hess
• What is a Hess cycle used for?

  To visualize energy changes in reactions
• What are the two types of Hess cycles?

  1. Formation cycle
  2. Combustion cycle
• What data do you use for a formation cycle?

  Formation data provided in the problem