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# 7 Equilibria

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### 7 Equilibria

1. 1. Equilibria
2. 2. <ul><li>Conditions that apply to all equilibria </li></ul><ul><li>Equilibria can only be set up in a closed system </li></ul><ul><li>Equilibrium has been reached when the properties of the system do not change with time </li></ul><ul><li>Equilibrium can be approached from either direction </li></ul><ul><li>Equilibrium is a dynamic process. It occurs when the rate of two opposing processes are the same </li></ul><ul><li>A dynamic equilibrium is one where rate of the forward and reverse reactions are equal and the concentration of each species is constant. </li></ul>
3. 3. <ul><li>Le Chatalier’s Principle </li></ul><ul><li>In any equilibrium when a change is made to some external factor the change in the position of the equilibrium is such as to tend to change the external factor in the opposite direction </li></ul><ul><li>i.e. If you add more product you’ll make more reactant etc </li></ul>
4. 4. <ul><li>A reversible reaction can be simple e.g. </li></ul><ul><li>H 2 O (l)  H 2 O (g)‏ </li></ul><ul><li>Or more complicated </li></ul><ul><li>C 2 H 5 OH + CH 3 CO 2 H  CH 3 CO 2 C 2 H 5 + H 2 O </li></ul><ul><li>ethanol ethanoic acid ethyl ethanoate water </li></ul><ul><li>If we mix ethanol and ethanoic acid we will end up with a mixture containing ethanol and ethanoic acid plus ethyl ethanoate and water. </li></ul><ul><li>If we mix ethyl ethanoate and water we will end up with exactly the same mixture! </li></ul>
5. 5. <ul><li>If several experiments are done to measure the concentrations of both reactants and products in an equilibrium at the same temperature the following ratio is always found </li></ul><ul><li>For an equilibrium </li></ul><ul><li>A + B  C + D </li></ul><ul><li>C x D is a constant (no units they cancel)‏ </li></ul><ul><li>A x B </li></ul><ul><li>This ratio is called the equilibrium constant K c (constant in terms of concentration)‏ </li></ul><ul><li>For another equilibrium A + B  C </li></ul><ul><li>K c = C units mol/L = mol -1 L -1 </li></ul><ul><li>A x B (mol/L) 2 </li></ul><ul><li>Note that if a solid is present it is not included in the expression for K c as it is a constant </li></ul>
6. 6. <ul><li>Position of an equilibrium </li></ul><ul><li>A reaction that has reached equilibrium may contain more reactants than products, more products than reactants, or comparable amounts of both. </li></ul><ul><li>The size of the equilibrium constant gives an indication of the composition of the equilibrium mixture. </li></ul><ul><li>If K c > 1 then there will be more products than reactants. We say that the position of the equilibrium is to the right. </li></ul><ul><li>If K c <1 then there will be more reactants than products. We say that the position of the equilibrium is to the left. </li></ul><ul><li>Reactions where K c is > 10 10 we regard as going to completion. </li></ul><ul><li>Reactions where K c is < 10 -10 we regard as not taking place at all </li></ul>
7. 7. <ul><li>A mixture of 1 mol hydrogen and 1 mol iodine are reacted together and reach equilibrium at constant temperature 1.56 mols of HI are present at equilibrium. Find K c </li></ul><ul><li>H 2 + I 2  2HI </li></ul><ul><li>K c = [ HI] x [HI] = [HI] 2 </li></ul><ul><li>[H 2 ] x [I 2 ] [H 2 ][I 2 ] </li></ul>
8. 8. <ul><li>If amount of H 2 reacted is x then at equilibrium concentrations are </li></ul><ul><li>H 2 + I 2  2HI </li></ul><ul><li>1 – x 1 – x 2x </li></ul><ul><li>2x = 1.56 </li></ul><ul><li>x = 0.78 </li></ul><ul><li>H 2 + I 2  2HI </li></ul><ul><li>1-0.78 1- 0.78 1.56 </li></ul><ul><li>0.22 0.22 1.56 </li></ul><ul><li>And K c = ( 1.56) 2 = 50.3 (no units – they cancel) </li></ul><ul><li>0.22 x 0.22 </li></ul>
9. 9. Partial Pressure Where P A = partial pressure of component A etc <ul><ul><li>Total pressure = P A + P B + P C + ...... </li></ul></ul>
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