Chemical Equilibrium
Chemistry · 11th Class (Intermediate Part 1) · BISE Lahore
Tests for this chapter
Tests run in your chosen medium — the sample below shows both.
Chemical Equilibrium Test
10 questions · Chemical Equilibrium × 10
Full Book Test
20 questions · Periodic Table and Periodic Properties × 1, Atomic Structure × 1, Chemical Bonding × 2, Stoichiometry × 1, States and Phases of Matter × 1, Chemical Energetics × 2, Reaction Kinetics × 1, Chemical Equilibrium × 1, Acid-Base Chemistry × 1, Electrochemistry × 2, Hydrocarbons × 2, Nitrogen and Sulfur × 1, Halogens × 1, Atmosphere × 1, Basic Separation Techniques × 1, Lab Safety and Practical Skills × 1
Sample questions
Showing 10 of 70 questions from this chapter. The full bank is in the chapter test below.
- 1
Which term describes a reaction in which the reactants are completely converted into products and the reaction stops when the limiting reactant is used up?
- A.Reversible reaction
- B.Dynamic reaction
- C.Irreversible reaction (correct answer)
- D.Equilibrium reaction
Why: An irreversible reaction goes to completion: the reactants are used up and the reaction then stops. A reversible reaction never stops because it continues in both directions. - 2
What does the use of a double arrow (⇌) in a chemical equation indicate?
- A.The reaction is reversible and can proceed in both directions (correct answer)
- B.The reaction is exothermic in the forward direction
- C.A catalyst is needed for the reaction to occur
- D.The reaction goes to completion in the forward direction
Why: The double arrow shows that the reactants form products and the products re-form reactants at the same time, so the change is reversible and can end in equilibrium. - 3
The state of a reversible reaction at which the composition of the reaction mixture does not change and the forward and reverse rates are equal is called
- A.the irreversible state
- B.the static state
- C.the limiting state
- D.chemical equilibrium (correct answer)
Why: Chemical equilibrium is reached when the forward and reverse reactions occur at equal rates, so the concentrations of all species stay constant with time. - 4
Which of the following is a microscopic event?
- A.Evolution of a gas from the reaction mixture
- B.Breaking and forming of bonds between molecules (correct answer)
- C.Change in the colour of the reaction mixture
- D.Formation of a precipitate in the solution
Why: Microscopic events such as collisions and bond breaking or bond making cannot be seen with the naked eye. Gas evolution, colour change and precipitation are macroscopic events. - 5
Which of the following can be observed with the naked eye during a chemical reaction?
- A.Collisions between the reactant molecules
- B.Rearrangement of atoms within a molecule
- C.Loss or gain of electrons by an ion
- D.Change in the colour of the reaction mixture (correct answer)
Why: A change in colour is a macroscopic property and is directly observable. Collisions, atomic rearrangement and electron transfer are microscopic events that cannot be seen. - 6
As a reversible reaction approaches equilibrium, the rate of the forward reaction decreases mainly because
- A.the reverse reaction removes the products as fast as they form
- B.the concentrations of the reactants fall as they are consumed (correct answer)
- C.the activation energy of the forward reaction increases with time
- D.the temperature of the reacting system falls steadily
Why: The forward rate depends on the concentrations of the reactants; as they are used up, fewer collisions occur per second and the forward rate falls until it equals the reverse rate. - 7
At chemical equilibrium the concentrations of reactants and products remain constant because
- A.the rates of the forward and reverse reactions have become equal (correct answer)
- B.both the forward and reverse reactions have stopped completely
- C.the closed vessel prevents collisions between molecules
- D.the forward reaction has used up all the limiting reactant
Why: Equilibrium is dynamic: both reactions continue, but at equal rates, so the amounts of reactants and products formed and consumed balance and the concentrations stay constant. - 8
How are the macroscopic changes observed in an equilibrium system explained at the microscopic level?
- A.By the escape of particles from the closed vessel
- B.By a change in the total mass of the reacting system
- C.By collisions between particles and the breaking of bonds (correct answer)
- D.By the absorption of heat from an outside source
Why: Macroscopic changes are the sum of many simultaneous microscopic events. Collisions between particles, together with bond breaking and bond forming, change the forward and reverse rates and so alter the observable properties. - 9
A liquid and its vapour are in dynamic equilibrium inside a closed container. Which statement about this state is correct?
- A.Evaporation stops as soon as equilibrium is reached
- B.The vapour pressure keeps on increasing with time
- C.The rate of condensation becomes zero at equilibrium
- D.The rate of evaporation equals the rate of condensation (correct answer)
Why: In a closed container evaporation and condensation continue, but at equal rates, so the amount of vapour and hence the vapour pressure stays constant at a fixed temperature. - 10
A liquid in a closed vessel is at equilibrium with its vapour. When the temperature is raised, the new state of dynamic equilibrium has
- A.a higher rate of evaporation but the same equilibrium vapour pressure
- B.the same vapour pressure but a lower rate of evaporation
- C.a higher rate of evaporation and a higher equilibrium vapour pressure (correct answer)
- D.a higher rate of condensation but a lower equilibrium vapour pressure
Why: A higher temperature gives the molecules more average kinetic energy, so more escape per second and the equilibrium vapour pressure rises; condensation speeds up too and equals evaporation at the new pressure.