Chemical Equilibrium

1. Chemical Equilibrium

2. Reaction Types • So far this year we have been writing chemical formulas as completion reaction. • In other words, the reaction completely goes to products. • We represented this by using  in the chemical equation. • Example: S8 + 8O2  8SO2

3. Reversible Reactions • However, many of the reactions we study are actually reversible. • Reversible reactions are chemical reactions in which the products re-form the original reactants. • Eventually, a chemical equilibrium is formed or a state of balance where the rateforward reaction equals the rate in reverse.

5. Chemical equilibria are dynamic • In other words, even though it looks like the reaction has stopped, the substances are still reacting. • However, for every new product that was made, an old product turned back into reactant. • Note that this does not mean that products and reactants need to be equal in concentration.

6. That is why we have the EquilibriumConstant • A number that relates the concentrations of starting materials to products of a reversible reaction at a given temperature. • Symbolized by Keq • They do not have units • Remember, concentrations are expressed in molarity (mol/L)

7. Keq can show whether a reaction is favorable • If Keq equals a large number, then there will be more products than reactants. (bigger arrow pointing to the right) • Reactions that produce more products than reactants are known as favorable. • If Keq equals a small number, then there will be more reactants than products. (bigger arrow pointing to the left) • If the value is 1, then it is 50:50

8. How do we determine if Keq is small or large? • We need to write the Keq formula (expression) for the reaction • Remember, these constants only apply to equations in equilibrium • Step 1 – Write the balancedequation for the reaction.

9. Step 2 – Write the equilibrium expression. • Place the product concentrations in the numerator and the reactant concentration in the denominator. • All will be multiplied • This is done by writing each ion, compound, etc, in brackets • Leave out any solids or pure liquids • This includes water if dealing with an aqueous solution

10. Step 3 • Complete the equilibrium expression • This is done by raising each concentration by its coefficient from the balanced equation. • Examples CaCO3(s) + 2H3O+(aq) Ca 2+(aq) + CO2(g) + 3H2O(l) N2(g) + O2(g) 2NO(g)

11. We can then use these expressions to solve problems. Let’s look at page 506; problems 1 and 2.

12. Solubility Product Constant, Ksp • A special equilibrium constant • These are equilibrium constants for the dissolution of slightly soluble salts • It is for describing the saturation of a salt in solution. • The expression is written in the same way as the other equilibrium constants. • Remember, solids and pure liquids are not included in the expression.

13. Examples • The Ksp for silver carbonate is 8.4x10-12 at 298. The concentration of carbonate ions in a saturated solution is 1.28 x 10-4 M. What is the concentration of silver ions? • The ionic substance T3U2 ionizes to form T2+ and U3- ions. The solubility of T3U2 is 3.77 x 10-20 mol/L. What is the value of the solubility-product constant?

14. Before we work on some problems in Groups, all groups to a board Part 1) Methane, CH4 and water, H2O, react to make carbon monoxide, CO, and hydrogen, H2. All are gases. Write the balanced equation. Part 2) Write the equilibrium expression. Part 3) Suppose you were given the concentrations: H2 = 1.56 M, CH4 = 3.70 x 10-2 M, H2O = 0.827, and the Keq = 3.112 x 102. What is the concentration of CO?

15. Homework • Page 522-523: 17 (pick 4 of the eight to complete), 27, 28, 31, 35