Enthalpy changes

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Slide 1: Slide
ChemistrySecondary EducationAge 12,13

This lesson contains 18 slides, with interactive quiz and text slides.

Items in this lesson

Text

Slide 1 - Slide

Entalpia
  • total chemical energy inside a substance/heat content

Slide 2 - Slide

Enthalpy change
  • happens when chemical reactions take place, changes in chemical energy take place 
  • ΔH (Δ= change; H = enthalpy)
  • An enthalpy change can be positive or negative

Slide 3 - Slide

Exo vs Endothermic Reactions

Slide 4 - Slide

Exo vs Endothermic Reactions

Slide 5 - Slide

Reaction pathway diagrams
  • A diagram that shows the energies of the reactants, the transition state(s) and the products of the reaction with time
  • Transition state = stage during the reaction when chemical bonds are partially broken and formed.

Slide 6 - Slide

Reaction pathway diagrams
  • The transition state is very unstable – it cannot be isolated and is higher in energy than the reactants and products.
  • The activation energy (Ea) is needed to reach the transition state.

Slide 7 - Slide

Reaction pathway diagrams
  • Activation energy = ‘the minimum amount of energy needed for reactant molecules to have a successful collision and start the reaction.

Slide 8 - Slide

Exothermic Reactions

Slide 9 - Slide

Endothermic Reactions

Slide 10 - Slide

Sample
  • CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l)

Slide 11 - Slide

Sample

Slide 12 - Slide

Standard Enthalpy Change
  • To fairly compare the changes in enthalpy between reactions, all reactions should be carried out under standard conditions:

Slide 13 - Slide

Standard Enthalpy Change

Slide 14 - Slide

Sample
  • Since two moles of water molecules are formed in the question above, the energy released is:

  • ΔHr= 2 mol x (-286 kJ mol -1)
  • =   - 572 kJ mol -1

Slide 15 - Slide

Sample
  • Since two moles of Fe2O3 (s) are formed the total change in enthalpy for the reaction above is:

  • ΔHf = 2 x (-824.2 kJ mol-1)
  • = - 1648 kJ mol -1

Slide 16 - Slide

Sample
  • Answer 1: ΔHr 
  • Answer 2: ΔHf  as one mole of CO2 is formed from its elements in standard state and ΔHc as one mole of carbon is burnt in oxygen
  • Answer 3: ΔHneut as one mole of water is formed from the reaction of an acid and alkali

Slide 17 - Slide


Type two things you learned today.

Slide 18 - Open question