Enthalpy changes

• An exothermic chemical reaction where the products have less energy than the reactants. Energy is given out to the surroundings

  (temperature goes up).
• Exothermic Demos

  • Chlorine gas reacting with sodium metal
  • Concentrated sulphuric acid and sugar
• Endothermic Demos

  • Dissolving ammonium nitrate
  • Reaction between ammonium nitrate and barium hydroxide.
• An endothermic chemical reaction is where the products have more energy than the reactants. Energy is taken in from the surroundings

  (temperature goes down).
• The products of a chemical reaction have less energy than the reactants so an exothermic reaction must have taken place.
• Heat taken in (endothermic)

  Energy level of products is higher than reactants so heat taken in.
• Heat given out (exothermic)

  Energy level of products is lower

  than reactants so heat given out.
• The overall energy change is referred to as the enthalpy of reaction
• Standard enthalpy of reaction, ΔHrΦ- is the enthalpy change when 1 mole of a compound if formed from its elements under standard condition
• standard conditions - 1 atmospheric pressure 100 KPa, room temperature 25c or 298k
• H2(g) + ½ O2 (g) --> H2O(l) ΔHr = -286kJmol-1 exothermic
• The standard enthalpy change of reaction ΔHr is the enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states.
• The standard enthalpy change of combustion ΔHc is the enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, all reactants and

  products being in their standard states.

  C2H6(g) + 3½ O2 (g) --> 2CO2(g) + 3H2O(l)
• The standard enthalpy change of formation ΔHf is the enthalpy change that takes place when one mole of a compound is formed from its constituent elements in their standard states under standard conditions.

  H2(g) + ½ O2 (g) --> H2O(l) ΔHf = -286kJmol-1
• Sum (bonds broken) – Sum (bonds made) =Energy change
• activation energy is the energy needed to start the

  reaction by breaking chemical bonds in the

  reactants. Energy is given out when new

  chemical bonds form.

  The overall energy change for a reaction can be

  worked out as the energy required for all bonds

  broken minus the energy gained by making bonds.
• How does calorimetry work?

  A known volume of cold water is measured

  into the beaker/can

  The starting temperature of the water is

  recorded

  The water is heated using the flame from the

  burning fuel

  The final temperature of the water is recorded

  The spirit burner containing the fuel is

  weighed before and after the experiment.
• ΔE = mcΔT

  Energy transferred (joules, J) = mass of water

  heated × 4.2 × temperature rise
• reasons for errors in calorimetry

  • Energy lost to the surroundings
  • Incomplete combustion
  • Conditions which are not standard (ie not 298K and 1atm pressure)
• Hess’ law states that - if a reaction can take place by

  more than one route and the initial and final

  conditions are the same, the total enthalpy change

  is the same.
• combustion

  ΔHr = Σ ΔHc(reactants) – Σ ΔHc(products)
• Hess' law

  ΔH(Route A) = ΔH(Route B) – ΔH(Route C)
• formation

  ΔHr = - Σ ΔHf(reactants) + Σ ΔHf(products)