The Equilibrium Condition

1. The Equilibrium Condition • Equilibrium – the exact balancing of two processes, one of which is the opposite of the other Equilibrium Animation

2. The Equilibrium Condition • Chemical equilibrium – a dynamic state where the concentrations of all reactants and products remain constant

3. Reversible Reactions • Some reactions do not go to completion as we have assumed • They may be reversible – a reaction in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously • Forward: 2SO2(g) + O2(g)→ 2SO3(g) • Reverse: 2SO2(g) + O2(g)← 2SO3(g)

4. Reversible Reactions • The two equations can be combined into one, by using a double arrow, which tells us that it is a reversible reaction: 2SO2(g) + O2(g)↔ 2SO3(g) • A chemical equilibrium occurs, and no net change occurs in the actual amounts of the components of the system.

5. Reversible Reactions • Even though the rates of the forward and reverse are equal, the concentrations of components on both sides may not be equal • An equlibrium position may be shown: A B or A B 1% 99% 99% 1% • Note the emphasis of the arrows direction • It depends on which side is favored; almost all reactions are reversible to some extent

6. Imagine you squeeze a balloon on one side. Air moves toward the other side of the balloon causing the it to bulge. As a result, the pressure is reduced on the side where you are squeezing. The air moves in a way that relieves the stress caused by increased pressure.

7. Le Chatelier’s Principle • The French chemist Henri Le Chatelier (1850-1936) studied how the equilibrium position shifts as a result of changing conditions • Le Chatelier’s principle: If stress is applied to a system in equilibrium, the system changes in a way that relieves the stress

8. Le Chatelier’s Principle • What items did he consider to be stress on the equilibrium? • Concentration • Temperature • Pressure • Concentration – adding more reactant produces more product, and removing the product as it forms will produce more product Each of these will now be discussed in detail

9. Le Chatelier’s Principle • Effect of a Change in Concentration If the concentration of a reactant is increased, the reaction will shift in a way that reduces it’s concentration.

10. Le Chatelier’s Principle • increasing the temperature causes the equilibrium position to shift in the direction that absorbs heat • Exothermic reaction – produces heat (heat is a product) • Adding energy shifts the equilibrium to the left (away from the heat term). • Endothermic reaction – absorbs energy (heat is a reactant) • Adding energy shifts the equilibrium to the right (away from the heat term). • Effect of a Change in Temperature

11. Le Chatelier’s Principle • If heat is one of the products (just like a chemical), it is part of the equilibrium • so cooling an exothermic reaction will produce more product, and heating it would shift the reaction to the reactant side of the equilibrium: C + O2(g)→ CO2(g) + 393.5 kJ

12. Le Châtelier’s Principle; Temperature Effect Chemistry; The Science in Context;by Thomas R. Gilbert, Rein V. Kirss, and Geoffrey Davies, Norton Publisher, 2004, p 764

13. Le Chatelier’s Principle • Pressure – changes in pressure will only effect gaseous equilibria • Increasing the pressure will usually favor the direction that has fewer molecules N2(g) + 3H2(g)↔ 2NH3(g) • For every two molecules of ammonia made, four molecules of reactant are used up – this equilibrium shifts to the right with an increase in pressure

14. Le Chatelier’s Principle • Increasing the Pressure (Decreasing the volume) • Effect of a Change in Pressure (Volume) • The system shifts in the direction that gives the fewest number of gas molecules.

15. Le Châtelier’s Principle • Changes in Volume and Pressure • (Only a factor with gases) A (g) + B (g) C (g) Change Shifts the Equilibrium Increase pressure Side with fewest moles of gas Decrease pressure Side with most moles of gas Increase volume Side with most moles of gas Decrease volume Side with fewest moles of gas 14.5

16. Equilibrium Constants: Keq • Chemists generally express the position of equilibrium in terms of numerical values, not just percent • These values relate to the amounts (Molarity) of reactants and products at equilibrium • This is called the equilibrium constant, and abbreviated Keq

17. Equilibrium Constants • consider this reaction (the capital letters are the chemical, and the lower case letters are the balancing coefficient): aA + bB  cC + dD • The equilibrium constant (Keq) is the ratio of product concentration to the reactant concentration at equilibrium, with each concentration raised to a power (which is the balancing coefficient).

18. Equilibrium Constants • consider this reaction: aA + bB  cC + dD • Thus, the “equilibrium constantexpression” has this general form: [C]c x [D]d [A]a x [B]b (brackets: [ ] = molarity concentration) Note that Keq has no units on the answer; it is only a number because it is a ratio Keq =

19. Equilibrium Constants • the equilibrium constants provide valuable information, such as whether products or reactants are favored: if Keq > 1, products favored at equilibrium if Keq < 1, reactants favored at equilibrium