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. REVERSIBLE REACTIONS Only when both reactions are occurring at the same rate and no changes can be observed, a chemical equilibrium has been reached.
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
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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. 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. 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. 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. REVERSIBLE REACTIONS Predicting Reaction Occurrence Example #5 PCl 5(g) <===> PCl 3(g) + Cl 2(g) H = positive or negative? heat + positive
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.