Consider the following equilibrium

Question

4NH3 + 3O2 <--> 2N2 + 6H2O

∆H= -1531 kJ

a) State whether the concentration of the reactants and products would increase, decrease,or remain constant when:

1) the temperature of the system is decreased

2) 2 mol of nitrogen are added to the system

b) Highly toxic gas with the formula COCl2

CO(g)+Cl2(g)⇌COCl2(g)

The equilibrium constant is Kc for this reaction at 400°C is 1.20 x 10^3. Predict the direction of the reaction when a mixture contains 0.012 mol of CO, 0.025 mol Cl2 and 0.03 mol COCl2 in 200mL container,

Answer 1

You are given the equation:

4NH3 + 3O2 <------> 2N2 + +6H2O

delta H = -1531 KJ

a} State whether the concentration of the reactants and products would increase, decrease or remain the same.

1) This is an exothermic reaction, meaning energy is released when products are formed as evidenced by the delta H being negative. If the temperature of the system decreases in an exothermic reaction there is a decrease in the amount of heat (energy) released when the products are formed. According to Le Chatelier's Principle the reaction will shift to remove the stress on the system, in this case the reaction will shift to the right (forward toward the products) raising the product concentration and decreasing the reactant concentration to raise the temperature and bring system back to equilibrium.

2) when 2 moles of N2 are added to the system you are adding additional product lifting the reaction away from its equilibrium state, and again Le Chatelier's Principle dictates that the system will respond to relieve the stress. The system will subsequently react by shifting left (toward the reactants) and increase the reactant concentration and decrease the product concentration and bring the reaction back to equilibrium.

B) You have the highly toxic gas COCl2 with the reaction:

CO(g) + Cl2(g)<-------> COCl2(g)

Its Kc at 400 Deg. C is 1.2 x 103, predict the direction of the reaction when a mixture in a 200 ml container contains - CO 0.012 mol, Cl2 0.025 mol and COCl2 0.03 mol.

In this question you have initial moles of gas in a specified volume. To determine which direction the reaction shifts to you need to use the reaction quotient or Q which uses the initial concentrations of products and reactants from which, after calculating can you compare Q to the Kc which will inform you whether the reaction is in equilibrium, greater in product, or greater in reactant that will dictate which direction the reaction will shift to. The equation for determining Q is exactly as how you would set up an an equilibrium equation with Q = {products}/{reactants raised to the power of their coefficients.

If Q greater than the Kc the reaction will shift to the left toward the reactants.

If Q is less than the Kc the reaction will shift to the right forward toward the products.

If Q is = to the Kc the reaction is in equilibrium.

Q = {COCl2}/{CO}{Cl2}

Concentrations of each gas within the mixture need to be calculated as they are given in moles with the volume the gases are contained in at 200 ml.

Molarity for CO = 0.012 mol/0.2L = 0.06 M

Molarity for Cl2 = 0.025mol/0.2L = 0.125 M

Molarity for COCl2 = 0.03 mol/0.2L = 0.15 M

Q = 0.15/(0.06)(0.125)

Q = 20

Kc = 1200

Q is less than Kc therefore the reaction will shift to the right (forward) toward product.