Chemical Equilibria

1. Build the sentence describing dynamic equilibrium: at equilibrium ...

Answer: the forward and backward rates are equal

At dynamic equilibrium the forward and backward rates are equal.

2. Which statements are true of a system at dynamic equilibrium? (Select all that apply.)

• The forward and backward rates are equalcorrect
• Concentrations of reactants and products stay constantcorrect
• Both the forward and backward reactions are still occurringcorrect
• All the reactants have been used up

At equilibrium the forward and backward rates are equal, concentrations stay constant, and both reactions keep occurring.

3. Once equilibrium is reached, both the forward and backward reactions completely stop.

Answer: False

Equilibrium is dynamic: both reactions continue at equal rates, so nothing actually stops.

4. Dynamic equilibrium can only be reached in a ____ system, where nothing can enter or leave.

• closedcorrect
• open
• heated
• acidic

A closed system stops reactants and products escaping, allowing the forward and backward rates to become equal.

5. At dynamic equilibrium, how does the rate of the forward reaction compare with the rate of the backward reaction?

• They are equalcorrect
• The forward rate is much faster
• The backward rate is much faster
• Both rates fall to zero

At dynamic equilibrium the forward and backward reactions occur at exactly the same rate.

6. 🧂 The picture shows a saturated salt solution with some solid still on the bottom. At this point, how do the rates of dissolving and crystallising compare?

• They are equal - a dynamic equilibriumcorrect
• Dissolving is faster
• Crystallising is faster
• Both have stopped completely

In a saturated solution the solid dissolves and crystallises at equal rates, a dynamic equilibrium, so the amount dissolved stays constant.

7. For H2(g) + I2(g) <=> 2HI(g), an equilibrium mixture has [H2] = 0.5, [I2] = 0.5 and [HI] = 2.0 mol dm^-3. What is the value of Kc?

Answer: 16 (no units)

Kc = [HI]^2 / ([H2][I2]) = 2.0^2 / (0.5 x 0.5) = 4 / 0.25 = 16, and the units cancel.

8. Build the statement describing when a saturated solution stops dissolving more solid.

Answer: the ionic product equals the solubility product

No more solid dissolves once the ionic product of the dissolved ions equals the solubility product, Ksp.

9. For H2(g) + I2(g) <=> 2HI(g), Kc has no units because there are equal numbers of moles of gas on each side.

Answer: True

True: with 2 moles of gas on each side the concentration units cancel exactly, so Kc is dimensionless.

10. The only factor that changes the value of the equilibrium constant Kc for a given reaction is ____.

• temperaturecorrect
• pressure
• the catalyst used
• the concentration of reactants

Only a change in temperature alters the value of Kc; changing concentration, pressure or adding a catalyst does not.

11. Match each equilibrium term to its correct meaning.

Answer: Kc = Equilibrium constant using concentrations; Kp = Equilibrium constant using partial pressures; Mole fraction = Moles of a gas divided by total moles; Partial pressure = Mole fraction multiplied by total pressure

Kc uses concentrations and Kp uses partial pressures; a partial pressure equals mole fraction times total pressure.

12. A reaction has a very large equilibrium constant, Kc. What does this tell you about the equilibrium mixture?

• It contains mostly productscorrect
• It contains mostly reactants
• It contains equal amounts of reactants and products
• No reaction has taken place

A large Kc means the position of equilibrium lies well to the right, so the mixture contains mostly products.

13. Adding a catalyst increases the equilibrium yield of product.

Answer: False

A catalyst speeds up the forward and reverse reactions equally, so equilibrium is reached faster but yield and Kc are unchanged.

14. Increasing the concentration of a reactant shifts the position of equilibrium to the ____ (towards the products).

• rightcorrect
• left
• middle
• top

Adding reactant makes the system oppose the change by moving right, using up the extra reactant to form more product.

15. What does Le Chatelier's principle state?

• The equilibrium shifts to oppose any change made to the systemcorrect
• The equilibrium always shifts to make more product
• The equilibrium never responds to changes
• The reaction rate always doubles when heated

Le Chatelier's principle says that if a change is made to a system at equilibrium, the position of equilibrium shifts to oppose that change.

16. Sort each concentration change by the direction it pushes the equilibrium.

Answer: Add more reactant = Shifts towards products; Remove a product = Shifts towards products; Add more product = Shifts towards reactants; Remove a reactant = Shifts towards reactants

The system opposes the change: adding reactant or removing product shifts it towards products, while adding product or removing reactant shifts it towards reactants.

17. Lowering the temperature favours the exothermic direction of a reversible reaction.

Answer: True

Cooling the system makes it shift in the exothermic direction to release heat and oppose the temperature drop.

18. Three of these changes leave the value of Kc unchanged. Which is the odd one out that actually changes Kc?

• Raising the temperaturecorrect
• Adding a catalyst
• Increasing the pressure
• Adding more reactant

Only a change of temperature alters the value of Kc; catalysts, pressure and adding reactant only shift the position.

19. In a reversible reaction, the products can react together to reform the original reactants.

Answer: True

This is exactly what makes a reaction reversible: it can run both forwards and backwards.

20. How is a reversible reaction shown in a chemical equation?

• Two half-arrows pointing in opposite directionscorrect
• A single arrow pointing right only
• A plus sign between the two sides
• An ordinary equals sign

A reversible reaction is written with two half-arrows pointing in opposite directions, showing it can go forwards and backwards.

21. Heated ammonium chloride decomposes into ammonia and hydrogen chloride, which recombine on cooling. This shows the reaction is ____.

• reversiblecorrect
• irreversible
• explosive
• neutral

Because the products can reform the original ammonium chloride, the decomposition of ammonium chloride is a reversible reaction.

22. Match each substance to its appearance.

Answer: Hydrated copper(II) sulfate = Blue crystals; Anhydrous copper(II) sulfate = White powder; Hydrated cobalt(II) chloride = Pink solid; Anhydrous cobalt(II) chloride = Blue solid

Hydrated salts are often coloured while their anhydrous forms differ; these colour changes are reversible by adding or removing water.

23. Which of these changes is NOT reversible?

• Burning wood in aircorrect
• Heating hydrated copper(II) sulfate
• Adding water to anhydrous copper(II) sulfate
• Heating solid ammonium chloride

Burning wood is an irreversible reaction; the others can be reversed by adding or removing water or heat.

24. Which of these are features of a reversible reaction? Select all that apply.

• It can proceed in both the forward and backward directionscorrect
• It can reach a dynamic equilibrium in a closed systemcorrect
• It always goes fully to completion
• The products can react to reform the reactantscorrect

Reversible reactions run both ways, their products can reform reactants, and in a closed system they reach dynamic equilibrium rather than going to completion.

25. Build the true statement about the effect of pressure in the Haber process.

Answer: High pressure increases the yield of ammonia

Higher pressure shifts the equilibrium toward the 2 moles of ammonia from 4 moles of reactant gas, raising the yield.

26. The hydrogen used in the Haber process is mainly obtained from ____ (methane).

• natural gascorrect
• the air
• iron ore
• salt

Hydrogen for the Haber process comes chiefly from natural gas, which is largely methane.

27. What is the approximate temperature used in the Haber process, in degrees C?

Answer: 450 degrees C

The Haber process typically runs at about 450 degrees C, a compromise between yield and reaction rate.

28. Which equation correctly represents the Haber process?

• N2 + 3H2 <=> 2NH3correct
• N2 + H2 <=> NH3
• 2N2 + 3H2 <=> 2NH3
• N2 + 2H2 <=> 2NH3

The Haber process is N2 + 3H2 <=> 2NH3, a reversible reaction between nitrogen and hydrogen.

29. Put the industrial steps of the Haber process in order.

Answer: Purified nitrogen and hydrogen are pumped into the reactor -> The gas mixture passes over the iron catalyst -> Ammonia is cooled and condensed to a liquid -> Unreacted nitrogen and hydrogen are recycled

Purified nitrogen and hydrogen are pumped in, passed over the iron catalyst, the ammonia is condensed off, and the unreacted gases are recycled.

30. Put the main stages of the Contact process for making sulfuric acid in order.

Answer: Burn sulfur in air to make sulfur dioxide -> Purify the gas mixture -> Pass SO2 and O2 over a V2O5 catalyst at about 450 C to form SO3 -> Dissolve the SO3 in concentrated sulfuric acid to make oleum -> Add water to the oleum to make concentrated sulfuric acid

Sulfur is burned to SO2, the gases are purified, SO2 and O2 form SO3 over V2O5, SO3 is absorbed into acid to make oleum, then water is added.