Chemical Equilibrium Reversible Reactions Mr. Shields Regents Chemistry U13 L01

Reversible Reactions What do we mean when we say something is And what does it mean in terms of chemical reactions? For example if the following chemical reaction Occurred what would be the reverse reaction? 2H2 + O2  2 H2O + Energy (exothermic rxn) Energy + 2H2O  2H2 + O2 (endothermic rxn) Heat would be released in the 1st rxn and Elec. could provide the energy to drive the 2nd

Many reactions in chemistry can proceed in either Direction depending upon the conditions applied. These reversible rxns are designated with either a double split arrow Or double tipped arrow  The former is more common but we’ll use also use From time to time the 2nd Format ( ) So our previous example would be written … 2H2 + O2  2 H2O + Energy

Equilibrium Now let’s discuss what we mean when we use the Word “Equilibrium”. For example, what does it mean when you say … “I got dizzy all of a sudden and lost my equilibrium”? You might re-explain it as follows: I was balanced on my feet and thus standing straight. Then I got dizzy which caused my center of balance to Shift driving me in a direction away from “upright”

At the point in time when we had not yet lost our Balance we stood straight. Forces worked to Overcome gravity so we did not fall one way or another . As soon as these forces became unbalanced our Body began moving in a particular direction. Equilibrium only occurs when all opposing forces are… Precisely balanced.

Solution Equilibrium Recall our earlier discussions of solution ... What is the definition of a saturated sol’n ? “a saturated sol’n is one in which the solvent can not dissolve any more solute” If we add more solute to the solution It should fall to the bottom and just lay there. Right? Wrong! The process is not static!

So what is exactly happening? Excess solute goes into sol’n At a rate equal to solute Coming out of sol’n Solute (s)  Solute (aq) = rate1 Solute (aq)  Solute (s) = rate2 When rate1 = rate2 the system is said to be in Equilibrium Notice that even though solute is going into sol’n it is also Coming out of sol’n at an equal rate. So… The overall concentration of solute in sol’n does not change.

Equilibrium in the gas phase Now Consider the following equilibrium reaction: N2O4 (g) + heat  2NO2 (g) (at a given T) At first the conc. Of Gas is all N2O4 (a) As temp inc. the conc. Of NO2 begins to inc. & N2O4 dec. (b) With time an new equilibrium is estab- lished between NO2 N2O4 (c) T1 T2

The reaction continues until there is a new equilibrium Established between the concentration of N2O4 and NO2 at the new temperature NOTE THAT THIS DOES NOT IMPLY THEIR CONC. NEEDS TO BE EQUAL % Conc. N2O4 NO2 100 0 85 15 75 25 71 29 68 32 66 34 All N2O4 at T1 2/3 N2O4 at equilibrium At equilibrium the Concentration of Both reactant and Product remains constant

At the same time the concentrations of reactants And products are reaching equilibrium the Rate of Change of reactant and product concentration is approaching one another Rate 1: N2O4  2NO2 Rate 2: 2 NO2  N2O4 Rate 1 = Rate 2 At equilibrium the Rate of the forward Reaction equals the Rate of the reverse reaction The rates are Balanced! Note: the rate of change of rxn 1 and rxn 2 Do not have to be the same at any time EXCEPT when they are at equilibrium