MU #1 Some chemical and physical changes can reach equilibrium. At equilibrium the rate of the forward reaction equals the rate of the reverse reaction. The measurable quantities of reactants and products remain constant at equilibrium.

Example

Water in a closed system, at constant temperature, is in a dynamic equilibrium.

H₂O(l) --> H₂O(g) forward reaction

H₂O(g) --> H₂O(l) reverse reaction

At equilibrium H₂O(l) <--> H₂O(g)

Example

In a saturated solution of KCl(aq), at constant temperature, with excess KCl(s) the following reactions occur.

KCl(s) --> K⁺(aq) + Cl^-(aq) forward reaction (dissolution)

K⁺(aq) + Cl^-(aq) --> KCl(s) reverse reaction (crystallization)

At equilibrium KCl(s) <--> K⁺(aq) + Cl^-(aq)   

TEST YOUR UNDERSTANDING

1/07

21 Which statement must be true for any chemical reaction at equilibrium?

(1) The concentration of the products is greater than the concentration of the reactants.

(2) The concentration of the products is less than the concentration of the reactants.

(3) The concentration of the products and the concentration of the reactants are equal.

(4) The concentration of the products and the concentration of the reactants are constant.

43 Which balanced equation represents a phase equilibrium?

(1) H₂(g) + I₂(g) <-->2HI(g)     (2) 2NO₂(g) <-->N₂O₄(g)      (3) Cl₂(g) <-->Cl₂(l)     (4) 3O₂(g) <--> 2O₃(g)

6/07

15 Given the equation representing a phase change at equilibrium: C₂H₅OH(l)<-->C₂H₅OH(g)

Which statement is true?

(1) The forward process proceeds faster than the reverse process. (2) The reverse process proceeds faster than the forward process.

(3) The forward and reverse processes proceed at the same rate.    (4) The forward and reverse processes both stop.

15 Which statement must be true when solution equilibrium occurs?

(1) The solution is at STP.                                                            (2) The solution is supersaturated.

(3) The concentration of the solution remains constant.        (4) The masses of the dissolved solute and the undissolved solute are equal.

2002-6

Given the equilibrium reaction at STP: N₂O₄(g) <--> 2 NO₂(g) Which statement correctly describes this system?

(1) The forward and reverse reaction rates are equal. (3) The concentrations of N₂O₄ and NO₂ are equal.

(2) The forward and reverse reaction rates are both increasing. (4) The concentrations of N₂O₄ and NO₂ are both increasing.

2002-8

Which statement correctly describes a chemical reaction at equilibrium?

(1) The concentrations of the products and reactants are equal.

(2) The concentrations of the products and reactants are constant.

(3) The rate of the forward reaction is less than the rate of the reverse reaction.

(4) The rate of the forward reaction is greater than the rate of the reverse reaction.

2003-1

Which type or types of change, if any, can reach equilibrium?

(1) a chemical change, only (2) a physical change, only (3) both a chemical and a physical change (4) neither a chemical nor a physical change

2003-8

22 Given the reaction:

              H₂O

AgCl(s) <---->Ag ⁺(aq) + Cl ^- (aq) Once equilibrium is reached, which statement is accurate?

    (1) The concentration of Ag ⁺ (aq) is greater than the concentration of Cl ^- (aq).    (3) The rates of the forward and reverse reactions are equal.
    (2) The AgCl( s) will be completely consumed.                                                              (4) The entropy of the forward reaction will continue to decrease.

1/04

17 Given the reaction at equilibrium: H₂ (g) + Br₂ (g) --> 2 HBr(g) The rate of the forward reaction is
        (1) greater than the rate of the reverse reaction         (2) less than the rate of the reverse reaction

        (3) equal to the rate of the reverse reaction                 (4) independent of the rate of the reverse reaction

6/04

20 Which statement about a system at equilibrium is true?
        (1) The forward reaction rate is less than the reverse reaction rate.     (2) The forward reaction rate is greater than the reverse reaction rate.
        (3) The forward reaction rate is equal to the reverse reaction rate.       (4) The forward reaction rate stops and the reverse reaction rate continues.

42 A solution that is at equilibrium must be (1) concentrated (3) saturated (2) dilute (4) unsaturated

6/05

18 Which statement must be true about a chemical system at equilibrium? 

     (1) The forward and reverse reactions stop. 

     (2) The concentration of reactants and products are equal. 

     (3) The rate of the forward reaction is equal to the rate of the reverse reaction. 

     (4) The number of moles of reactants is equal to the number of moles of product. 

6/06

8/06

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