Chapter 13. The Concept of Equilibrium. A System at Equilibrium. The Concept of Equilibrium. Chemical Equilibrium. N 2 O 4 (g) 2 NO 2 (g)

Transcription

1 PowerPoint to accompany The Concept of Equilibrium Chapter 13 Chemical Equilibrium Figure 13.1 Chemical equilibrium occurs when a reaction and its reverse reaction proceed at the same rate. The Concept of Equilibrium A System at Equilibrium Figure 13.2 (b) As a system approaches equilibrium, both the forward and reverse reactions are occurring. At equilibrium, the forward and reverse reactions are proceeding at the same rate. Once equilibrium is achieved, the amount of each reactant and product remains constant. Figure 13.2 (a)

2 Depicting Equilibrium In a system at equilibrium, both the forward and reverse reactions are being carried out; as a result, we write its equation with a double arrow. The Equilibrium Constant Forward reaction Rate law Rate = k f [N 2 O 4 ] The Equilibrium Constant Consider the following general equilibrium reaction: aa + bb Equilibrium constant K c = = dd + ee [products] [reactants] [D] d [E] e [A] a [B] b The Equilibrium Constant Because pressure is proportional to concentration for gases in a closed system, the equilibrium expression can also be written: K p = (P C) c (P D ) d (P A ) a (P B ) b The constant expression depends only on the stoichiometry of the reaction, not on its mechanism.

3 Relationship between K c and K p From the ideal gas law we know that PV = nrt Rearranging it, we get P = n V RT Relationship between K c and K p Plugging this into the expression for K p for each substance, the relationship between K c and K p becomes: where, K p = K c (RT) n n = (moles of gaseous product) (moles of gaseous reactant) Equilibrium Can Be Reached from Either Direction Equilibrium Can Be Reached from Either Direction As you can see, the ratio of 2 ] 2 to [N 2 O 4 ] remains constant (at this temperature) regardless of the initial concentrations of NO 2 and N 2 O 4. This is the data from the last two trials from the table on the previous slide. Figure 13.5

4 Equilibrium Can Be Reached from Either Direction What Does the Value of K Mean? Figure 13.3 If K >> 1, the reaction favours the product, i.e. the product predominates at equilibrium. It does not matter whether we start with N 2 and H 2 or whether we start with NH 3. We will have the same proportions of all three substances at equilibrium. Figure 13.6 If K << 1, the reaction favours the reactant, i.e. the reactant predominates at equilibrium. Manipulating Equilibrium Constants The equilibrium constant of a reaction in the reverse reaction is the reciprocal of the equilibrium constant of the forward reaction. K c = 2 ] 2 [N 2 O 4 ] = at 100C 1 K c = [N 2O 4 ] = 2 ] = 4.72 at 100C Manipulating Equilibrium Constants The equilibrium constant of a reaction that has been multiplied by a number is the equilibrium constant raised to the power equal to that number. K c = 2 ] 2 [N 2 O 4 ] = at 100C K c = 2 ] 2 N 4 [N = (0.212) 2 2 O 4 (g) 4 NO 2 (g) at 100C 2 O 4 ] 2

5 Manipulating Equilibrium Constants The equilibrium constant for a net reaction made up of two or more steps is the product of the equilibrium constants for the individual steps. The Reaction Quotient (Q) To calculate Q, one substitutes the initial concentrations on reactants and products into the equilibrium expression. Q gives the same ratio the equilibrium expression gives, but for a system that is not at equilibrium. If Q = K The system is at equilibrium. If Q > K There is too much product and the equilibrium shifts to the left. Figure 13.8 Figure 13.8

6 If Q < K Le Châtelier s Principle There is too much reactant and the equilibrium shifts to the right. If a system at equilibrium is disturbed by a change in temperature, pressure, or the concentration of one of the components, the system will shift its equilibrium position so as to counteract the effect of the disturbance. Figure 13.8 Change in Reactant or Product Concentrations The transformation of nitrogen and hydrogen into ammonia (NH 3 ) is of tremendous significance in agriculture, where ammonia-based fertilisers are of utmost importance. Figure 13.4 Change in Reactant or Product Concentrations Figure 13.4 Figure E.g. If H 2 is added to the system, N 2 will be consumed and the two reagents will form more NH 3. Adding a substance will shift the reaction so as to re-establish the equilibrium by consuming part of that substance.

7 The Effects of Volume and Pressure Changes The Effect of Changes in Temperature At constant temperature, reducing the volume of a gaseous equilibrium mixture, i.e. increasing the pressure, causes the system to shift in the direction that reduces the number of moles of gas (in an effort to absorb the increased pressure). Co(H 2 O) 6 2+ (aq) + 4 Cl (aq) CoCl 4 (aq) + 6 H 2 O (l) Conversely, increasing the volume, i.e. reducing the pressure, causes a shift in the direction that produces more gas molecules. Figure The Effect of Catalysts Figure Catalysts increase the rate of both the forward and reverse reactions but do not change the composition of the equilibrium mixture.