Enthalpy

• When you measure a heat change at a constant pressure, it is called enthalpy change. (ΔH)
• Standard conditions for measuring enthalpy changes:

  • Pressure of 100kPa
  • Temperature of 298K (room temperature)
• Exothermic

  • Products end up with less heat energy than the starting materials. This means ΔH is negative
• Endothermic

  • Products end up with more heat energy than the starting materials. This means ΔH is positive
• Pressure affects the amount of heat energy given out by reactions that involve gasses
• The physical states of the reactants can affect the enthalpy of a reaction

  e.g. Energy put into a solid to turn it into a liquid or liquid into a gas