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At equilibrium, the concentrations of the reactants and products no longer change with time
For equilibrium to occur, neither reactants, nor products can escape from the system
At equilibrium, a particular ratio of concentration terms equals a constant.
Q: at equilibrium are concentrations of reactants and products equal?
The equilibrium condition is reached from either direction ( makes no difference whether we
start with reactants, with products or with combination of the mixture) at equilibrium, at a given
temperature, the ratio equals a specific value
Q: What does the magnitude of equilibrium constant tell us about the relative concentrations of
the reactants and products in an equilibrium mixture?
Very small Kc indicates that the equilibrium mixture contains mostly reactants.
It is forward and reverse rates that are equal at equilibrium, not the concentrations.
The magnitude of K indicates the extent to which the reaction proceeds
PREDICTING THE DIRECTION OF REACTION
For the formation of NH3(g) K= 0.105 at 427OC, suppose we place 2.00 mol of H2(g), 1.00 mol of
N2(g) and 2.00 mol of NH3(g) in a 1.00L container
Q=
0.500
To reach equilibrium the must decrease from the starting value to equilibrium value.
That can happen only if [NH3] decreases
And [H2] and [N2] increase -> thus the reaction proceeds to equilibrium by forming N2 and H2
from NH3
This approach can be formalized by defining quantity called reaction quotient: Q
Q is a number obtained by substituting reactant and product concentrations at any point
during a reaction into and equilibrium constant expression.
Q=K The reaction quotient equals the equilibrium constant only if the system is at equilibrium
Q>K The concentration of products is to large and that of the reactants to small. The reaction
proceeds from right to left to approach equilibrium
Q<K The concentration of products is to small, and that of the reactants to large. The reaction
achieves equilibrium by forming more products, it proceeds from left to right.
SAMPLE EXERCISE 1 Predicting the direction of approach to equilibrium
At 448oC the equilibrium constant Kc for the reaction H2 + I2 2HI
is 50.5. Predict in which direction the reaction proceeds to reach equilibrium if we start with
2.0x10-2 mol of HI, 1.0x10-2 mol H2, and 3.0x10-2 mol I2 in a 2.0 L container.
SAMPLE EXERCISE 2 Calculating equilibrium concentrations
SAMPLE EXERCISE 3 Calculating equilibrium concentrations from initial concentrations
A 1.000L flask is filled with 1.000 mol of H2 and 2.000 mol of I2 at 448 oC. The value of the equilibrium
constant Kc for the reaction H2 + I22HI is 50.5. What are the equilibrium concentrations of H2, I2 and HI
in moles per liter?