Tang 01 reversible reactions 2

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  • Tang 01 reversible reactions 2

    1. 1. REVERSIBLE REACTIONS
    2. 2. REVERSIBLE REACTIONS What happens to gas when temperature changes?  T   V  T   P
    3. 3. REVERSIBLE REACTIONS <ul><li>When the system is heated: </li></ul><ul><ul><li>N 2 O 4(g) + energy  2 NO 2(g) </li></ul></ul><ul><li>When the system is cooled: </li></ul><ul><li>2 NO 2(g)  N 2 O 4(g) + energy </li></ul><ul><li>brown gas = NO 2 </li></ul><ul><li>colourless gas = N 2 O 4 </li></ul>
    4. 4. REVERSIBLE REACTIONS Both reactions are occurring simultaneously in a closed system at all times. The reaction: N 2 O 4(g) + energy <==> 2 NO 2(g) is described as reversible.
    5. 5. REVERSIBLE REACTIONS Only when both reactions are occurring at the same rate and no changes can be observed, a chemical equilibrium has been reached.
    6. 6. REVERSIBLE REACTIONS Factors to Reach Equilibrium <ul><li>closed system </li></ul><ul><li>simultaneous opposing reactions occurring at the same rate </li></ul><ul><li>equilibrium was reached by starting with reactants or products </li></ul><ul><li>temperature is constant </li></ul>
    7. 7. REVERSIBLE REACTIONS Describe what is happening when the temperature is changed? 2 NO 2(g) <==> N 2 O 4(g) + energy
    8. 8. REVERSIBLE REACTIONS <ul><li>Equilibrium DOES NOT mean the same concentrations of products and reactants. </li></ul><ul><ul><li>only that the rxn rates are equal </li></ul></ul><ul><li>Reaction rates change because of temperature change. </li></ul><ul><ul><li>equilibrium rxn rates are different at different temperatures </li></ul></ul>
    9. 9. REVERSIBLE REACTIONS When chemicals are reacted, there are 3 possible outcomes: 1. reaction goes to completion 2. reaction does not occur at all 3. reaction achieves equilibrium
    10. 10. REVERSIBLE REACTIONS Factors Determining Rxn Occurrence <ul><li>Rxns favour energy release (-  H) </li></ul><ul><li>Rxns favour a result of increased entropy (+  S) </li></ul><ul><li>When both of these statements are true, the reaction tends to completion. </li></ul>
    11. 11. REVERSIBLE REACTIONS Predicting Reaction Occurrence Example #1 Zn (s) + 2 HCl (aq)  ZnCl 2(aq) + H 2(g)  H = -152 kJ Enthalpy? Exothermic in forward direction Entropy? Favours forward direction Prediction? Will occur to completion
    12. 12. REVERSIBLE REACTIONS Predicting Reaction Occurrence Example #2 3 C (s) + 3 H 2(g)  C 3 H 6(g)  H = +20.4 kJ Enthalpy? Favours direction of reactants Entropy? Favours reactants Prediction? Both factors in same direction so reaction will not occur as written
    13. 13. REVERSIBLE REACTIONS Predicting Reaction Occurrence Example #3 2Pb(NO 3 ) 2(s)  2PbO (s) + 4NO 2(g) + O 2(g) Δ H = + 597 kJ Enthalpy? Favours reactants Entropy? Favours products Prediction? 2 factors are working against each other and reaction will reach equilibrium
    14. 14. REVERSIBLE REACTIONS Predicting Reaction Occurrence Example #4 The following reaction achieves equilibrium. 3 H 2(g) + N 2(g) <===> 2 NH 3(g)  H = positive or negative? + heat negative
    15. 15. REVERSIBLE REACTIONS Predicting Reaction Occurrence Example #5 PCl 5(g) <===> PCl 3(g) + Cl 2(g)  H = positive or negative? heat + positive
    16. 16. REVERSIBLE REACTIONS Predicting Reaction Occurrence Recall, some reactions require very large E A values. Therefore another factor in determining reaction occurrence. If no information on E A is given, assume sufficient energy is available.

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