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Chemical Equilibrium: Basic Concepts. I. What is equilibrium?. Consider the reaction for the formation of ammonia from nitrogen and hydrogen. . Chemical Equilibrium: Basic Concepts.
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I. What is equilibrium?
Consider the reaction for the formation of ammonia from nitrogen and hydrogen.
What happens when one mole of nitrogen and three moles of hydrogen, the amounts shown in the equation, are placed in a closed reaction vessel at 723 K?
The nitrogen and hydrogen begin to react.
The concentrations of the reactants (H2 and N2) decrease at first while the concentration of the product (NH3) increases.
Then, before the reactants are used up, all concentrations become constant.
The reactants, H2 and N2, are consumed in the reaction, so their concentrations gradually decrease.
After a period of time, however, the concentrations of H2, N2, and NH3 no longer change.
When a reaction results in almost complete conversion of reactants to products, chemists say that the reaction “goes to completion”.
But most reactions, including the ammonia-forming reaction, do not go to completion. They appear to stop.
The reason is that these reactions are reversible.
A reversible reaction is one that can occur in both the forward and the reverse directions.
Chemists combine these two equations into a single equation that uses a double arrow to show that both reactions occur.
When you read the equation, the reactants in the forward reaction are on the left.
In the reverse reaction, the reactants are on the right.
In the forward reaction, hydrogen and nitrogen combine to form the product ammonia.
In the reverse reaction, ammonia decomposes into the products hydrogen and nitrogen.
The reaction begins at a definite, initial rate; no ammonia is present so only the forward reaction can occur.
As hydrogen and nitrogen combine to form ammonia, their concentrations decrease.
The rate of a reaction depends upon the concentration of the reactants.
The decrease in the concentration of the reactants causes the rate of the forward reaction to decrease.
As soon as ammonia is present, the reverse reaction can occur, slowly at first, but at an increasing rate as the concentration of ammonia increases.
As the reaction proceeds, the rate of the forward reaction continues to decrease and the rate of the reverse reaction continues to increase until the two rates are equal.
At that point, ammonia is being produced as fast as it is being decomposed, so the concentrations of nitrogen, hydrogen, and ammonia remain constant.
The system has reached a state of balance or equilibrium.
The word equilibrium means that opposing processes are in balance.
Chemical equilibrium is a state in which the forward and reverse reactions balance each other because they take place at equal rates.
Rateforward reaction = Ratereversereaction
You can recognize that the ammonia-forming reaction reaches a state of chemical equilibrium because its chemical equation is written with a double arrow like this.
At equilibrium, the concentrations of reactants and products are constant.
However, that does not mean that the amounts or concentrations of reactants and products are equal.
II. Equilibrium Expressions and Constants
Some chemical reactions systems have little tendency to react, and others go readily to completion.
In between these two extremes are the majority of reactions that reach a state of equilibrium with varying amounts of reactants unconsumed.
If the reactants are not consumed, then not all the product predicted by the balanced chemical equation will be produced.
According to the equation for the ammonia-producing reaction, two moles of ammonia should be produced when one mole of nitrogen and three moles of hydrogen react.
Because the reaction reaches a state of equilibrium, however, fewer than two moles of ammonia will actually be obtained.
Chemists need to be able to predict the yield of a reaction.
In 1864, the Norwegian chemists Cato Maximilian Guldberg and Peter Waage proposed the law of chemical equilibrium, which states that at a given temperature, a chemical system may reach a state in which a particular ratio of reactant and product concentrations has a constant value.
For example, the general equation for a reaction at equilibrium can be written as follows.
A and B are the reactants; C and D the products.*
The coefficients in the balanced equation are a, b, c, and d.
If the law of chemical equilibrium is applied to this reaction, the following ratio is obtained.
This ratio is called the equilibrium constant expression.
The square brackets indicate the molarity of the reactants and products at equilibrium in mol/L.
The equilibrium constant, Keq, is the numerical value of the ratio of product concentrations to reactant concentrations, with each concentration raised to the power corresponding to its coefficient in the balanced equation.
The value of Keq is constant only at a specified temperature.
Keq > 1: More products than reactants at equilibrium.
Keq < 1: More reactants than products at equilibrium.
1. How would you write the equilibrium constant expression for this reaction in which hydrogen and iodine react to form hydrogen iodide?
Keq for this homogeneous equilibrium at 731 K is 49.7.
Note that 49.7 has no units. In writing equilibrium constant expressions, it’s customary to omit units.
2. Write the equilibrium constant expression for the reaction in which ammonia gas is produced from hydrogen and nitrogen.
Determining the Value of Equilibrium Constants
When equilibrium is established, the concentration of each substance can be determined experimentally.
Although an equilibrium system has only one value for Keq at a particular temperature, it has an unlimited number of equilibrium positions.
Equilibrium positions depend upon the initial concentrations of the reactants and products.
N2(g) + 3H2(g) 2NH3(g) at 500 C
Calculating the value of the Equilibrium Constant when given the equilibrium concentrations.
EX: Calculate the value of Keq for the rxn
given concentration data at equilibrium: [NH3] = 0.933 mol/L, [N2] = 0.533 mol/L, [H2] = 1.600 mol/L.
b) Plug in given concentrations for reactants and productsand solve for K
E.g. 4SO2(g) + O2(g) 2SO3(g)
[SO2 ] = 2.00M [SO2 ] = 1.50M
[O2 ] = 1.50M [O2 ] = 1.25M
[SO3 ] = 3.00M [SO3] = 3.50M
Equilibrium constant for Experiment 1 =
[SO2 ] = 0.500M [SO2 ] = 0.590M
[O2 ] = 0.00M [O2 ] = 0.045M
[SO2 ] = 0.350M [SO3 ] = 0.260M
Equilibrium constant for Experiment 2 =
For example, consider the equilibrium system
Factors Affecting Chemical Equilibrium
Le Châtelier’s principlestates that if a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress.
1. If an additional amount of reactant (NO or Br2) is added to the system, the equilibrium will shift to the right, that is, more product (NOBr) will be formed.
Conversely, adding more NOBr to the system will result in a shift to the left, forming more NO and Br2.
2. The removal of a reactant or product also results in a shift in the equilibrium.
Removing a reactant causes the equilibrium to shift to the left, forming more reactants.
Removing the product causes a shift to the right, forming more product.
Suppose the volume of the reaction vessel for the system below is decreased, resulting in an increase in pressure.
3. Le Châtelier’s principle also applies to changes in the volume of a reaction vessel containing gases in an equilibrium system.
The equilibrium will shift to relieve the stress of increased pressure.
In this case, the shift will be to the right because three moles of reactant gas combine to form only two moles of product gas.
If the volume of the reaction vessel was increased, the equilibrium would shift to the left, and more of the reactants would be formed.
Increase volume = shift to side with more moles of gas
Note that changing the volume of the reaction vessel causes no shift in the equilibrium when the number of moles of product gas equals the number of moles of reactant gas; an example is the equilibrium
Decrease volume = shift to side with less moles of gas
Even though equilibrium may shift to the right or left in response to a change in concentration or volume, the value of the equilibrium constant remains the same.
A change in temperature, however, alters both the equilibrium position and the value of Keq.
For example, consider the thermochemical equation for the reversible formation of hydrogen chloride gas from its elements.
The forward reaction releases heat, so you can consider heat as a product in the forward reaction and a reactant in the reverse reaction.
Raising the temperature of this system requires the addition of heat, which shifts the equilibrium to the left and reduces the concentration of hydrogen chloride.
Thus, the value of Keq decreases.
Lowering the temperature of the system means that heat is removed, so the equilibrium relieves the stress by shifting to the right, increasing both the concentration of hydrogen chloride and Keq.