Energetics

• What is enthalpy?

  when a chemical reaction occurs, there is a change in energy
• What is ΔH?

  enthalpy change- heat change in a reaction at constant pressure (measured in KJmol-1)
• What are standard conditions?

  100kPa

  298K (25 degreees)
• What is endothermic?


  Absorbs energy from surroundings (feels colder)

  Positive ΔH, products higher in energy than reactants

  e.g. thermal decomposition
• What is exothermic?


  Release energy to the surroundings (feels warmer)

  Negative ΔH, products lower in energy than rectants

  e.g. combustion
• What reaction is more energy needed to break bonds then make them?

  ENDOTHERMIC
• In what reaction is more energy released when bonds are formed then when they are broken?

  EXOTHERMIC
• what is the equation of ΔH?

  ΔH = break - make
• What is calorimetry used for 


  to work out the ΔH of combustion of a fuel
• What is the equation for heat change (Q)? and give the units

  Q = m c ΔT

  
  

  Q = J

  m = g

  ΔT = K
• What is the equation for ΔH involving moles?

  -Q / n = ΔH
• What is Hess's Law?


  the total enthalpy change of a reaction is independent of the route taken
• What is enthalpy of formation?


  1 mol of products formed from its pure elements in standard conditions
• What is the equation for enthalpy of formation?

  ΔHr = ΔHf (products) - ΔHf (reactants)
• What is enthalpy of combustion?


  1 mol of reactants burned completely in sufficient oxygen under standard conditions
• What is the equation for enthalpy of combustion?

  ΔHr = ΔHc (reactants) - ΔHc (products)