Reversible Reactions u Some reactions may be reversible
Reversible Reactions u. Some reactions may be reversible – the conversion of reactants to products and the conversion of products to reactants occur simultaneously u. Forward: 2 SO 2(g) + O 2(g) → 2 SO 3(g) u. Reverse: 2 SO 2(g) + O 2(g) ← 2 SO 3(g)
Reversible Reactions u. The two equations can be combined into one, by using a double arrow, 2 SO 2(g) + O 2(g) ↔ 2 SO 3(g) ü A chemical equilibrium occurs
Le Chatelier’s Principle u. Le Chatelier’s principle: If stress is applied to a system in equilibrium, the system changes in a way that relieves the stress u. ONLY changed by gasses!
Le Chatelier’s Principle 1) Concentration 2) Temperature 3) Pressure • Concentration – adding to one side pushes equilibrium to the other side. Taking away from one side then favors that side. • Adding reactants makes more products
Le Chatelier’s Principle • Temperature – increasing the temperature causes the equilibrium position to shift away from the side with heat on it • Decreasing temperature will favor the side with heat on it. • N 2(g) + 3 H 2(g) ↔ 2 NH 3(g) + heat • If temp increases, the equilibrium shifts left and produces more reactants while using some products.
Le Chatelier’s Principle • Pressure – changes in pressure will only effect gaseous substances • Increasing the pressure will usually favor the direction that has fewer molecules N 2(g) + 3 H 2(g) ↔ 2 NH 3(g) If pressure increases the reaction favors the right because there are only 2 atoms, vs 4 on the left.
Equilibrium Constants: Keq • Chemists generally express the position of equilibrium in terms of numerical values –These values relate to the amounts (Molarity) of reactants and products at equilibrium –This is called the equilibrium constant, and abbreviated Keq
Equilibrium Constants • consider this reaction (the capital letters are the chemical, and the lower case letters are the balancing coefficient): a. A + b. B c. C + d. D – The equilibrium constant (Keq) is the ratio of product concentration to the reactant concentration at equilibrium, with each concentration raised to a power
Equilibrium Constants • consider this reaction: a. A + b. B c. C + d. D – Thus, the “equilibrium constant expression” has this general form: [C]c [D]d x Keq = [A]a x [B]b Note that Keq has no units on the answer; it is only a number because it is a ratio (brackets: [ ] = molarity concentration)
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 – if Keq = 1, the amounts of product and reactants will be approximately equal
Summarize u How do the 3 types of stresses affect the equilibrium of an equation?
- Slides: 11