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Reaction feasibility

Reaction feasibility. AH Chemistry, Unit 2(d). Thermodynamics. Helps understand and predict the behaviour of substances and their reactions based on energy changes. Does not rely on knowing about atoms, molecules, ions etc. Questions chemists are interested in….

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Reaction feasibility

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  1. Reaction feasibility AH Chemistry, Unit 2(d)

  2. Thermodynamics • Helps understand and predict the behaviour of substances and their reactions based on energy changes. • Does not rely on knowing about atoms, molecules, ions etc.

  3. Questions chemists are interested in… 1. Will a reaction go “spontaneously” in the direction written? 2. Will the composition of the reaction mixture contain enough product at equilibrium? 3. Will the reaction occur at an reasonable speed?

  4. Thermodynamics predicts if a reaction can happen, given enough time (sometimes a very long time e.g. billions of years). • Kinetics predicts if a reaction happens at a reasonable rate.

  5. Thermodynamically favoured • But not kinetically favoured

  6. Why do chemical reactions happen? To make the Universe more disordered.

  7. Important terms: system and surroundings

  8. Important terms: spontaneous process

  9. Hydrogen and oxygen • Is the reaction of hydrogen and oxygen spontaneous? • YES. It is a processes that will happen without any outside intervention.

  10. True or false? • A spontaneous process is one which moves to minimise it’s energy? • All exothermic reactions are spontaneous? • No endothermic reaction is spontaneous?

  11. Ice • Ice forms liquid water spontaneously at temperatures above 0C (an endothermic process) but the reverse is spontaneous – water to ice (an exothermic process) – at temperatures below 0C. • There must be another factor (other than energy) which plays a part in determining whether changes or reactions happen.

  12. Barium hydroxide and ammonium chloride

  13. This factor is ENTROPY, S. A measure of disorder. The larger the entropy, the greater the disorder. ∆S = S(products) – S(reactants) Entropy

  14. To define the entropy of a substance in absolute terms, a reference point is needed. • A perfectly ordered system would have zero entropy. • The Third Law of thermodynamics states that: “ the entropy of a perfect crystal at zero kelvin is zero”.

  15. Standard Entropy • The standard entropy of a substance, Sº, is the molar entropy at 298K and 1 atmosphere pressure. • The units are J K-1 mol-1

  16. Standard entropy change of a reaction • ∆rSº = ∑Sº(products) – ∑Sº(reactants) Practise Calculate the standard entropy change of reaction at 298K for: • The reaction of hydrogen and oxygen

  17. Why do chemical reactions happen? • To make the Universe more disordered.

  18. Important terms: system and surroundings

  19. Second Law of Thermodynamics • Spontaneous processes are those that increase the total entropy of the Universe. • ∆S(total) = ∆ S(system) + ∆ S(surroundings)

  20. Gibbs energy change • Gibbs energychange, ∆G, (free energy change) combines changes in entropy and enthalpy into a single equation to describe the spontaneity of a process at constant temperature and pressure, using only information from the reaction system. • ∆G = ∆H - T∆S

  21. A negative ∆G means that a reaction CAN happen but does NOT mean that it WILL happen – the reaction is thermodynamically feasible but could be kinetically unfavourable i.e. it is very slow due to a high activation energy.

  22. Practise

  23. Gibbs energy and equilibrium • No chemical reaction proceeds in only one direction. • The reaction of hydrogen and oxygen at 298K nearly does (∆Gº = -237.1 kJ mol-1).

  24. N2O4(g) 2NO2(g) Gibbs energy and equilibrium • The reaction of dinitrogen tetroxide to form nitrogen dioxide (∆Gº = -5.8 kJ mol-1) is different. The reaction is spontaneous but does not go to completion.

  25. Gibbs energy and equilibrium • When the Gibbs energy of the reactants has become the same as the Gibbs energy of the products, the mixture is at equilibrium. • Therefore, at a point in a reaction when ∆G = 0, the reaction is at equilibrium.

  26. Coupling reactions • A positive ∆G does not mean that a reaction can never happen, just that it will not occur spontaneously to any significant extent. • If it is coupled to a reaction with a larger, negative ∆G, the reaction can be made to occur. • This is common in biochemical processes.

  27. The blast furnace

  28. Ellingham diagrams • The opposite of iron rusting. • A +ve G doesn’t mean that it can’t happen, just that work must be done to make it happen. • It can be coupled to a reaction which has a more negative G in order to make it happen. 2Fe2O3(s) → 4Fe(s) + 3O2(g) G = +1487 kJ mol-1

  29. x3 2CO(g) + O2(g) 2CO2(g) G = -514.4 kJ mol-1 2Fe2O3(s) 4Fe(s) + 3O2(g) G = +1487 kJ mol-1 2Fe2O3(s) + 6CO(g) 4Fe(s) + 6CO2(g) G = -56 kJ mol-1 Overall, the reduction of iron(III) oxide has become thermodynamically feasible.

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