1 Reversible Reactions & Equilibrium reversible reaction A reversible reaction is one which can be made to go in either forward or reverse direction. Only the order of steps has changed.

2 That is, the chemical mechanism is the same for reactants --> products or products --> reactants. Only the timing of steps is in reverse order.

3 irreversiblecan not be made to occur An irreversible reaction can not be made to occur in either the direction products --> reactants or reactants --> products by changing the order of reaction steps by the same mechanism merely by changing the order of reaction steps.

4 We must be careful to distinguish between: * “reversing a process” as a net result, and * a reversible process. Reversing a process undoes the result by whatever means needed.

5 A reversible process occurs by the same mechanism or steps in both directions: only the order of events in time has changed. IMPORTANT IDEA !!!!!

6 A common reversible process: water freezes, ice melts This is a physical change: H 2 O remains H 2 O. However, we can turn water into ice and ice into water merely by removing or adding heat from the H 2 O.

7 An irreversible process: dropping a carton of eggs, thus breaking them. Reversing the results: buying a new carton of eggs. You can reverse the result but not by same mechanism!

8 equilibrium A reversible reaction reaches equilibrium when: rates of the forward and reverse reactions are equalthe rates of the forward and reverse reactions are equal. As a result: concentrations of reactants and products no longer changethe concentrations of reactants and products no longer change over time.

9 Consider the reversible reaction: H 2 O + CO H 2 + CO 2. Start with only H 2 O and CO: reaction begins to produce H 2 and CO. Reaction slows down over time as H 2 O and CO used up:

10 time [ ] concentration of CO & H 2 O at start of forward reaction

11 But consider reverse reaction: CO 2 + H 2 CO + H 2 O. At start of forward reaction, there was no CO 2 & H 2. Over time, the concentrations of CO 2 and H 2 increase, and so does the rate of the reverse reaction:

12 time [ ] concentration of CO 2 & H 2 at start of forward reaction

13 When concentrations no longer changing, reaction at equilibrium:time [ ] CO 2 + H 2 CO + H 2 O

14 at equilibrium concentrations are not changing anymore Notice: at equilibrium the concentrations are not necessarily equal, but not changing anymore. Reaction continues in both directions, but at equal rates to maintain constant concentrations.

15 What are the concentrations of products and reactants once equilibrium is reached? What is the “balance point”? The equilibrium constant expresses these relationships.

16 General reversible reaction abcd a A + b B c C + d D A,B,C,D are chemical compounds; a,b,c,d a,b,c,d are # moles of each compound in balanced equation is symbol for reversible reaction

17 For abcd a A + b B c C + d D Equilibrium constant is K eq = product concentrations cd [C] c [D] d ab [A] a [B] b reactant concentrations =

18 Thus, for the reaction CO + H 2 O CO 2 + H 2, K eq = [CO 2 ] [H 2 ] [CO] [H 2 O]

19 Equilibrium constants: * are numerical values determined experimentally; * are constant at constant temperature; * change with changing temperature.

20 Size of K eq : large, >>1 favors products small, <<1 favors reactants near to or = 1, does not favor either products or reactants.

21 What happens when a reaction at equilibrium is disturbed? Le Chatelier’s Principle Le Chatelier’s Principle says: When a system at equilibrium is acted upon by an external stress, the system acts to counterbalance or relieve the stress.

22 Here, a “system” means the chemical reaction mixture. An “external stress” means: * adding or removing reactant * adding or removing product * heating or cooling the reaction

23 Example: CO + H 2 O CO 2 + H 2. If we: *Add more CO, then: reaction moves to right -- reaction moves to right -- creates more CO 2 & H 2 to relieve excess CO. This also uses up more H 2 O.

24 CO + H 2 O CO 2 + H 2. If we: * add more CO 2, then: reaction moves to left -- attempts to use up extra CO 2 by making more CO and H 2 O. This also uses up more H 2.

25 CO + H 2 O CO 2 + H 2. If we: * remove H 2, then: reaction moves to the right -- tries to make more H 2 to replace what was lost. This uses up CO and H 2 O and also makes more CO 2.

26 CO + H 2 O CO 2 + H 2. exothermic heat is a product. This is an exothermic reaction: this means heat is a product. If we heat an exothermic equilibrium reaction, then: the reaction is driven to left, towards reactants.

27 endothermic heat is a reactant For endothermic reactions, heat is added; heat is a reactant. If we heat an endothermic equilibrium reaction, then: the reaction is driven in direction of forming products, to the right.

28 Adding a catalyst speeds up an equilibrium reaction, but does not change the value of K eq = the position of balance. With a catalyst, the reaction reaches the point of not changing sooner or faster than without a catalyst.