Presentation subtitle K,Q & LeChatelier

K is CONSTANT At any temperature. Temperature affects rate. The equilibrium concentrations don’t have to be the same only K. Equilibrium position is a set of concentrations at equilibrium. There are an unlimited number.

The units for K Are determined by the various powers and units of concentrations. They depend on the reaction.

What Does the Value of K Mean? If K>>1, the reaction is product-favored; product predominates at equilibrium.

What Does the Value of K Mean? If K>>1, the reaction is product-favored; product predominates at equilibrium. If K<<1, the reaction is reactant-favored; reactant predominates at equilibrium. If K<<1, the reaction is reactant-favored; reactant predominates at equilibrium.

© 2009, Prentice-Hall, Inc. The Reaction Quotient (Q) Q gives the same ratio the equilibrium expression gives, but for a system that is not at equilibrium. To calculate Q, one substitutes the initial concentrations on reactants and products into the equilibrium expression.

© 2009, Prentice-Hall, Inc. If Q = K, the system is at equilibrium.

© 2009, Prentice-Hall, Inc. If Q > K, there is too much product, and the equilibrium shifts to the left.

© 2009, Prentice-Hall, Inc. If Q < K, there is too much reactant, and the equilibrium shifts to the right.

Le Chatelier’s Principle Le Chatelier’s principle: If stress is applied to a system in equilibrium, the system changes in a way that relieves the stress

Le Chatelier’s Principle Concentration – adding more reactant produces more product, and removing the product as it forms will produce more product

Adding product makes Q>K Removing reactant makes Q>K Adding reactant makes Q<K Removing product makes Q<K Determine the effect on Q, will tell you the direction of shift

Le Chatelier’s Principle Temperature – increasing the temperature causes the equilibrium position to shift in the direction that absorbs heat 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 + O 2(g) → CO 2(g) kJ

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 N 2(g) + 3H 2(g) ↔ 2NH 3(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

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