15.1.1 The Concept of Equilibrium

Question 1

The Concept of Equilibrium

Answer 1

• Chemical equilibrium is a type of dynamic equilibrium in which concentrations of products and reactants remain constant.
• If a chemical equilibrium is disrupted, a new equilibrium will be established in which the overall ratios of products to reactants (raised to their respective powers) are equal.
• The equilibrium constant (K) reflects the direction of overall balance in a chemical system.

Question 2

note

Answer 2

• In chemical equilibrium, the rate that the products are formed equals the rate that products degenerate to reform reactants. As a result, the concentrations of products and reactants remain constant.
• For example, hydrogen gas reacts with iodine to form hydrogen iodide. In time, equilibrium is established between the reactants and the products, and the rate of conversion of hydrogen iodide to hydrogen and iodine equals the rate of hydrogen iodide production.
• The direction of equilibrium (whether it favors the reactants or products) may be determined by examining the partial pressures of each gaseous component of the reaction.
• The graph on the left shows the change in partial pressures of the three components over time. After a certain amount of time passes, the reaction reaches equilibrium and the partial pressures of all three components level out.
• The graph on the left shows the forward and the reverse rates of the reaction. The rate of hydrogen iodide production is initially high, but steadily decreases over time. The rate of the reverse reaction is initially low (because there is hardly any product to convert back to hydrogen and iodine), but steadily increases over time. At equilibrium, the forward and reverse rates are equal.
• When more of one of the components is added to the reaction, the system will shift and reach a new equilibrium. At the new equilibrium point, the reaction rates of the products and reactants will again be equal.
• When a reaction goes from one state of equilibrium to another, the values for each of the partial pressures in the second equilibrium will be different than the partial pressures of the first equilibrium, but the ratio of products to reactants (raised to their respective powers) will remain constant

Question 3

For a particular endothermic reaction, heat is required for the reactant molecules to overcome the activation-energy barrier and to react to form the product molecules. Which statement about this reaction is correct?

Answer 3

Increasing the temperature of the reaction increases the value of the equilibrium constant

Question 4

For a reaction A+B -> AB, the value of the equilibrium constant is less than 1. Therefore, which of the following statements is correct?

Answer 4

Equilibrium lies toward the formation of the reactants.

Question 5

In the following reaction PCl5 dissociates to give PCl3 and Cl2
PCl5 (g) -> PCl3(g) + Cl2(g)
The partial pressures of PCl5, PCl3, and Cl2 are 0.875 atm, 0.463 atm, and 1.98 atm, respectively. The value of the equilibrium constant is ______

Answer 5

1.05

Question 6

Consider the following reaction
N2(g) + 3H2(g) -> 2NH3 (g)
At the start of the reaction 0.071 mol/L of N2, 0.0092 mol/L of H2, and 0.00018 mol/L of NH3 are put in an enclosed vessel at 375 degrees C. If the equilibrium constant for the reaction is 1.2, which of the following statements is NOT correct?

Answer 6

The reaction is at equilibrium and therefore, no reaction will occur.

Question 7

In a forward reaction, a new product is formed from the reactants. In a backward reaction, the products are converted back to the original reactants. At equilibrium, which of the following statements is correct?

Answer 7

The rate of the forward reaction is equal to the rate of the backward reaction.

Question 8

H2 (hydrogen) reacts with I2 (iodine) to given HI (hydrogen iodide)
H2 + I2 -> 2HI
At equilibrium the partial pressure of H2, I2, and HI are PH, PI, and PHI. The equilibrium constant can be calculated by which of the following expressions?

Answer 8

PHI2 / PH2PI2

Question 9

x moles of hydrogen are added to y moles of iodine, and z moles of hydrogen iodide are formed. What is the value of the equilibrium constant?

Answer 9

z ^2 / [(x − z/2) (y − z/2)]

Question 10

Is the following statement true or false?

The value of the equilibrium constant can be negative.

Answer 10

false

Question 11

A reaction reaches equilibrium in four hours. The equilibrium constant for this reaction is K1. A catalyst is added to this reaction, the reaction rate increases, and equilibrium is reached in one minute. The equilibrium constant for this reaction is K2. How are K1 and K2 related?

Answer 11

K1 = K2

Question 12

Is the following statement true or false?

If the value of the equilibrium constant is greater than 1, then the equilibrium position lies toward the formation of the products.

Answer 12

true