Mr. Bouyer
Day 1 | Day 2 | Lab | Skills Test

Write the equilibrium constant expression for a reversible chemical reaction. Use the equilibrium constand to determine the amount of product formed during a reaction. Combine Le Chatelier's Principle and the equilibrium constant expression to calculate the affects of changes in temperature, pressure, and concentration on a reversible reaction.

Example: H₂ + I₂ 2HI

The rate expression for the forward reaction is: rate = k_f [H₂] [I₂]
The rate expression for the reverse reaction is: rate = k_r [HI]²
At equilibrium, the rates of these two reactions are equal: k_f [H₂] [I₂] = k_r [HI]²

Since both k_f and k_r are constants, the ratio ^k_f / _kr is also a constant.

This new constant is the equilibrium constant, K_eq

Equilibrium constant expression:

• In the hypothetical balanced equation, aA + bB cC + dD, coefficients are represented by small letters and chemical symbols are represented by capital letters.
  The equilibrium constant expression for the equation is:

  Keq

  The exponents are the same as the coefficients in the balanced equation.

• Interpreting K_eq
  • If K_eq is much greater than 1, the reaction will approach completion.
  • If K_eq is much less than 1, equilibrium is established before much product is formed.
  • Each chemical reaction has a unique K_eq for every temperature.

Test Your Concept Understanding: Ammonia vapor combines with oxygen to produce water vapor and nitrogen. Write a balanced equation for this reaction at equilibrium. Write the expression for the equilibrium constant. At a given temperature, the concentration of each substance in the reaction is 1.0M. What is the Keq for that temperature? Calculate the Keq if [NH3] is 2.5M, [O2] 3.0M, [H2O] is 3.5M, and [N2] is 4.0M Based on your answer from D, does the reaction go to completion? At a given temperature, the following gaseous reaction CO + H2O H2 + CO2 produces the following concentrations: CO = 0.2M, H2O = 0.5M, H2 = 0.32M, and CO2 = 0.42M. Determine the Keq at this temperature. Does the reaction go to completion?

Use this webtest to practice equilibrium calculations.

Day 2

Le Chatelier's Principle:

The conditions affecting equilibrium are temperature, pressure, and concentration (of reactants and products). If a system is in equilibrium and some condition changes, the system will shift to restore equilibrium.

Concentration - If the concentration of either of the reactants is increased, the number of collisions between reactant particles will increase. The result is an increase of the reaction rate toward the right. As the concentration of products increases, the rate of the reverse reaction will also increase. However, the net result result to the system as a whole is to shift the reaction toward the right, producing more product.

Pressure - Pressure has an effect only on gases in a reaction. In most gaseous reactions, reactants and products are under the same conditions. An increase in pressure will move the particles closer together, increasing the concentration of all particles. How this affects the equilibrium depends on the number of moles of reactants and the number of moles of products. An increase in pressure always drives the reaction in the direction of the smaller number of moles of gas.

• In the reaction H_{2 (g)} + Cl_{2 (g)} 2HCl _(g) there are two moles of reactant gases and two moles of product gases. A change in the pressure on this reaction will speed up the reaction in both directions, but not shift the equilibrium.
• In the reaction N_{2 (g)} + 3H_{2 (g)} 2NH_{3 (g)} there are four moles of reactant gases and two moles of product gases. A change in the pressure on this reaction will shift the equilibrium to the right.

Temperature - Both the forward and reverse reactions are speeded up by an increase in temperature. Since the equilibrium constant itself is changed by a temperature chance, it is difficult to predict the direction an equilibrium will shift. One easy way is to consider energy as either a reactant or product. A reaction always moves away from the energy to reduce the stress.

• In the reaction N_{2 (g)} + 3H_{2 (g)} 2NH_{3 (g)} + energy - considering energy as a product increases the concentration of the products. The equilibrium shifts to the left to reduce this stress.

Optimum conditions: the conditions that produce the highest product yield.

In-class Assignment 232:
This assignment must be turned in by the end of class today to receive credit.
Scoring criteria

1. For the reaction: 2NO_(g) + H_2(g) N₂O_(g) + H₂O_(g) + energy
   1. Write the equilibrium constant expression for the reaction.
   2. What effect would an increase in temperature have on the reaction rate?
   3. Assume the reaction is a single-step reaction. What would be the effect on the reaction rate if the [H₂] were doubled?
   4. Assume the reaction is a single-step reaction. What would be the effect on the reaction rate if the pressure on the reaction were dooubled?
   5. Assume the reaction is reversible and at equilibrium. What shifts in the equilibrium of this reaction are suggested by Le Chatelier's Principle?