1797f_PChem Revision Notes 2

1797f_PChem Revision Notes 2 - Raffles Junior College H2...

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Pg 1 I. Chemical Equilibrium A Reversible Reactions and Dynamic Equilibrium 1. Irreversible reactions are reactions that take place in one direction almost exclusively. Eg: Burning of magnesium in air: 2Mg (s) + O 2 (g) 2MgO (s) Only the forward reaction takes place. 2. Reversible reactions are reactions that take place in both directions ( ) at comparable rates. Eg: Rxn of SO 2 with O 2 : 2 SO 2 (g) + O 2 (g) 2 SO 3 (g) Both the forward and backward reaction takes place simultaneously. A + B C A + B C 3. For reversible reactions, the system can reach a stage in which the rate of forward reaction = rate of backward reaction so that no macroscopic changes are observed. The system is said to be at equilibrium. Since microscopic changes are still occurring, the system is said to be in dynamic equilibrium. (For dynamic equilibrium, rate of forward reaction = rate of backward reaction 0) 4. Equilibrium is only achieved in a closed system. An open system which allows matter to escape or enter cannot reach equilibrium unless there is absolutely no gain or loss of materials from or to the surroundings. 5. Once equilibrium is reached, the system will remain in this state, until conditions affecting rate of forward/ backward reaction is altered. (eg, temperature, pressure, concentration of reactant/product) B Law of Chemical Equilibrium and Equilibrium Constants 1. Reaction Quotient (or mass action expression) (a) For a reversible reaction represented by a A + b B c C + d D, Reaction Quotient, Q c = b a d c ] B [ ] A [ ] D [ ] C [ [X] is concentration (in mol dm - 3 )of X at time t (b) From time = 0 till time = t eqm , the reaction quotient changes as the concentrations of reactants and products change until equilibrium is reached. Raffles Junior College H2 Chemistry 2008/9 Physical Chemistry Revision Part 2 time Conc. /mol dm -3 x t eqm [B] [C] [B] [A] [C] t c time Concentration /moldm -3 [A]
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Pg 2 2. Law of Chemical Equilibrium (or law of mass action) k f (a) For a reversible reaction represented by a A + b B c C + d D, k b At time = t, Rate of forward reaction, R f = k f [A] t a [B] t b Rate of backward reaction, R b = k b [C] t c [D] t d At equilibrium, R f = R b k f [A] a [B] b = k b [C] c [D] d (subscript ‘eqm’ is left out for convenience) Hence, reaction quotient, Q c = b a d c ] B [ ] A [ ] D [ ] C [ = b f k k = K c , Equilibrium constant (b) Relationship between, Q c , K c and equilibrium (c) Equilibrium constant is a constant at a fixed temperature. Hence, the value of equilibrium constant will not change unless temperature changes . The effect of temperature on the value of equilibrium constant is shown in the table below: Δ H of forward reaction >0 (ie endothermic) = 0 <0 (ie exothermic) Effect of K c when temperature K c No change K c Effect of K c when temperature K c No change K c C Types of Equilibrium Constants Equilibrium Constant Reactions Example Expression & Units
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1797f_PChem Revision Notes 2 - Raffles Junior College H2...

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