How Far? The Extent Of Chemical Change

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Notes de leçon

Dynamic Equilibrium

In a reversible reaction, products can react to reform the original reactants; shown by the ⇌ symbol.
Dynamic equilibrium is reached in a closed system when the rate of the forward reaction equals the rate of the backward reaction.
At equilibrium, the concentrations of reactants and products remain constant, but they are not necessarily equal.
Macroscopic properties such as colour and density remain constant because they depend on concentration.
Equilibrium can be reached starting from either reactants or products.
For reactions involving gases, equilibrium can only be reached in a closed system; reactions entirely in solution can reach equilibrium in open flasks.
Physical equilibria also exist, e.g. C₂H₅OH(l) ⇌ C₂H₅OH(g) in a sealed bottle.

A reversible reaction at equilibrium

The Equilibrium Law and Kc Expression

For the general reaction aA + bB ⇌ cC + dD, the equilibrium constant expression is: Kc = [C]^c[D]^d / [A]^a[B]^b.
Square brackets represent equilibrium concentrations in mol dm⁻³; round brackets are incorrect and lose marks.
Solids are ignored in equilibrium constant expressions (e.g. Ag(s) is omitted).
The equilibrium constant, Kc, is specific to a given equation and depends on temperature.
For the reverse reaction, K' = 1/Kc (the reciprocal).
If an equation is multiplied by a factor n, the equilibrium constant is raised to the power n.
Example: N₂(g) + 3H₂(g) ⇌ 2NH₃(g) gives Kc = [NH₃]² / ([N₂][H₂]³).

Interpreting the Magnitude of K

If K << 1, the reaction hardly proceeds; equilibrium lies far to the left and the mixture contains mostly reactants.
If K < 1, the reaction favours the reactants; equilibrium lies to the left.
If K = 1, there are significant amounts of both reactants and products; equilibrium is balanced.
If K > 1, the reaction favours the products; equilibrium lies to the right.
If K >> 1, the reaction goes almost to completion; equilibrium lies far to the right with mostly products present.
Stronger acids have a higher K value because they dissociate more, shifting equilibrium towards products.

Le Chatelier's Principle

Le Chatelier's principle: if a change is made to a system at dynamic equilibrium, the position of equilibrium moves to minimise the change.
The position of equilibrium refers to the relative amounts of products and reactants in the mixture.
Changes that can affect equilibrium include temperature, pressure, and concentration.
A catalyst increases the rate of both forward and reverse reactions equally; it has no effect on the position of equilibrium or the value of Kc.
Concentration changes: increasing a reactant shifts equilibrium right; decreasing a reactant shifts it left; increasing a product shifts it left; decreasing a product shifts it right.
Changes in concentration do not change the value of Kc, provided temperature remains constant.

Equilibrium and the Haber process

Effects of Pressure and Temperature on Equilibrium

Pressure changes only affect reactions involving gases; if the number of gas molecules is the same on both sides, pressure has no effect.
Increasing pressure shifts equilibrium towards the side with fewer gas molecules to reduce the pressure.
Decreasing pressure shifts equilibrium towards the side with more gas molecules to increase the pressure.
Changes in pressure do not change the value of Kc.
Increasing temperature shifts equilibrium in the endothermic direction (absorbs energy).
Decreasing temperature shifts equilibrium in the exothermic direction (releases energy).
Temperature changes do change the value of Kc: for an endothermic reaction, Kc increases with increasing temperature; for an exothermic reaction, Kc decreases with increasing temperature.

The Reaction Quotient, Q (HL)

The reaction quotient, Q, uses the same expression as Kc but with concentrations that may not be at equilibrium.
If Q = Kc, the reaction is at equilibrium and no net reaction occurs.
If Q < Kc, the reaction proceeds to the right, favouring products.
If Q > Kc, the reaction proceeds to the left, favouring reactants.
Q is not a fixed value; it can be measured at any time, whereas Kc is constant at a given temperature.
Comparing Q and Kc allows prediction of the direction in which a reaction will shift to reach equilibrium.

Equilibrium Calculations (HL)

Concentration is calculated using: concentration (mol dm⁻³) = amount (mol) / volume (dm³).
ICE tables (Initial, Change, Equilibrium) are used when initial and equilibrium concentrations of all species are not given.
Use the stoichiometric ratios from the balanced equation to determine changes in moles or concentration.
For reactions with the same number of concentration terms in numerator and denominator, volume cancels and Kc can be calculated directly from moles.
When Kc < 10⁻³, the reaction lies far to the left; assume the equilibrium concentration of reactant ≈ initial concentration.
In such cases, state the approximation and justify it because Kc is small.
Always give the final answer to the appropriate number of significant figures.

Equilibrium Constant and Gibbs Energy (HL)

The equilibrium constant is related to Gibbs energy change by: ΔG° = –RT lnK.
R = 8.31 J K⁻¹ mol⁻¹; T is in Kelvin; ΔG° is usually in kJ mol⁻¹ (convert to J mol⁻¹ for calculations).
If K > 1, ΔG° < 0 (negative); products are favoured and the reaction is feasible/spontaneous.
If K = 1, ΔG° = 0; neither side is favoured.
If K < 1, ΔG° > 0 (positive); reactants are favoured.
As ΔG° becomes more negative, the forward reaction is favoured more and K increases.
The equation can be rearranged to: ln K = –ΔG° / RT, allowing calculation of K from ΔG°.

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1.Which statement correctly describes the concentrations of reactants and products in a system at dynamic equilibrium?
Easy
- AThe concentrations of reactants and products are constant.
- BThe concentrations of reactants and products are equal.
- CThe concentration of reactants is always greater than that of products.
- DThe concentrations of reactants and products are zero.
2.For the reaction N2(g) + 3H2(g) ⇌ 2NH3(g), which is the correct equilibrium constant expression?
Easy
- AKc = [NH3]² / ([N2][H2]³)
- BKc = ([N2][H2]³) / [NH3]²
- CKc = [NH3]² / ([N2] + [H2]³)
- DKc = [NH3] / ([N2][H2])
3.For the reaction 2SO2(g) + O2(g) ⇌ 2SO3(g), the equilibrium constant Kc is 0.282 at temperature T. If the reaction quotient Q is calculated to be 0.5, in which direction will the reaction proceed?
Medium
- ATo the left, towards reactants.
- BTo the right, towards products.
- CThe reaction is already at equilibrium.
- DThe direction cannot be determined without more information.
4.For the reaction 2NH3(g) + CO2(g) ⇌ CO(NH2)2(g) + H2O(g), ΔH < 0. What is the effect of increasing the temperature on the equilibrium constant Kc?
Medium
- AKc decreases.
- BKc increases.
- CKc remains the same.
- DKc becomes zero.
5.For the reaction H2(g) + I2(g) ⇌ 2HI(g) with ΔH < 0, what happens to the value of Kc when the temperature is increased from 761 K?
Medium
- AKc decreases.
- BKc increases.
- CKc remains unchanged.
- DKc becomes zero.
6.For the reaction H2(g) + I2(g) ⇌ 2HI(g), the equilibrium constant Kc is 48.52. What is the value of Kc for the reaction ½H2(g) + ½I2(g) ⇌ HI(g) at the same temperature?
Medium
- A6.97
- B48.52
- C97.04
- D24.26
7.What is the effect of adding a catalyst to a reaction at equilibrium?
Easy
- AIt increases the rate of both forward and reverse reactions equally, so the position of equilibrium remains unchanged.
- BIt shifts the equilibrium to the right, increasing the yield of products.
- CIt shifts the equilibrium to the left, increasing the yield of reactants.
- DIt increases the value of the equilibrium constant Kc.
8.For the reaction 2SO3(g) ⇌ 2SO2(g) + O2(g) with ΔH = +196 kJ mol⁻¹, what is the effect on the yield of SO2 if the concentration of SO3 is increased?
Medium
- AThe yield of SO2 increases.
- BThe yield of SO2 decreases.
- CThe yield of SO2 remains the same.
- DThe equilibrium shifts to the left.

Plus de questions

9.For the reaction N2O4(g) ⇌ 2NO2(g), what is the correct expression for Kc?
Medium
- AKc = [NO2]² / [N2O4]
- BKc = [N2O4] / [NO2]²
- CKc = [NO2] / [N2O4]
- DKc = [NO2]² / [N2O4]²
10.For the reaction N2(g) + 3H2(g) ⇌ 2NH3(g) with ΔH = -92.6 kJ, what is the effect of increasing the volume of the container on the position of equilibrium?
Medium
- AThe equilibrium shifts to the left, towards reactants.
- BThe equilibrium shifts to the right, towards products.
- CThe equilibrium position remains unchanged.
- DThe equilibrium constant Kc changes.
11.For the reaction 2SO2(g) + O2(g) ⇌ 2SO3(g), what is the correct equilibrium constant expression?
Medium
- AKc = [SO3]² / ([SO2]² [O2])
- BKc = ([SO2]² [O2]) / [SO3]²
- CKc = [SO3]² / ([SO2]² + [O2])
- DKc = [SO3] / ([SO2][O2])
12.For the reaction Ag+(aq) + Fe2+(aq) ⇌ Ag(s) + Fe3+(aq), which is the correct equilibrium constant expression?
Medium
- AKc = [Fe3+] / ([Fe2+][Ag+])
- BKc = ([Fe2+][Ag+]) / [Fe3+]
- CKc = [Fe3+] / ([Fe2+][Ag+][Ag])
- DKc = [Fe3+][Ag] / ([Fe2+][Ag+])
13.Which of the following are characteristics of a system at dynamic equilibrium? (select all that apply)
Easy
- AThe rate of the forward reaction equals the rate of the backward reaction
- BThe concentrations of reactants and products are equal
- CThe concentrations of reactants and products remain constant
- DThe reaction has stopped completely
- EThe system is closed so no matter escapes
14.At dynamic equilibrium, the concentrations of reactants and products are equal.
Easy
True or false?
15.For the reaction N2(g) + 3H2(g) ⇌ 2NH3(g), Kc = 2.6 × 10^−18 at a certain temperature. What does this value tell you about the equilibrium mixture?
Medium
- AThe mixture contains mostly reactants and the equilibrium lies far to the left
- BThe mixture contains mostly products and the equilibrium lies far to the right
- CThe reaction has reached completion
- DThe reaction is not at equilibrium
16.For the reaction 2SO2(g) + O2(g) ⇌ 2SO3(g), Kc = 5.0 × 10¹³ at a given temperature. Which statement is correct?
Medium
- AThe equilibrium lies far to the right and the mixture contains mostly products
- BThe equilibrium lies far to the left and the mixture contains mostly reactants
- CThe reaction hardly proceeds
- DThe value of Kc will change if the pressure is increased
17.The equilibrium constant for the reaction 2NO2(g) + F2(g) ⇌ 2NO2F(g) is 7.1 × 10³². What is the value of the equilibrium constant for the reverse reaction at the same temperature?
Medium
- A1.4 × 10^−33
- B−7.1 × 10³²
- C7.1 × 10³²
- D1.4 × 10³³
18.Match each change to a system at equilibrium with the resulting shift in the position of equilibrium.
Medium
- Increase in concentration of a reactant
- Decrease in concentration of a product
- Increase in total pressure (fewer gas moles on right)
- Increase in temperature for an exothermic forward reaction
- Shifts right
- Shifts left
19.Place the steps for calculating Kc from equilibrium concentrations in the correct order.
Medium
- Write the balanced chemical equation
- Write the equilibrium constant expression
- Substitute the equilibrium concentrations into the expression
- Calculate the value of Kc
20.For the reaction CH3COOH(l) + C2H5OH(l) ⇌ CH3COOC2H5(l) + H2O(l), which statements about the equilibrium constant expression are correct? (select all that apply)
Medium
- AKc = [CH3COOC2H5][H2O] / ([CH3COOH][C2H5OH])
- BThe volume of the container cancels out in the expression
- CWater is excluded from the expression because it is a liquid
- DKc has units of mol dm⁻³
- EKc = [CH3COOH][C2H5OH] / ([CH3COOC2H5][H2O])
21.Adding a catalyst to a reaction at equilibrium increases the value of the equilibrium constant Kc.
Easy
True or false?
22.For the reaction 2SO2(g) + O2(g) ⇌ 2SO3(g), at 600°C the equilibrium concentrations are [SO3] = 0.093 mol dm⁻³, [SO2] = 0.100 mol dm⁻³ and [O2] = 0.200 mol dm⁻³. Calculate Kc to 3 significant figures.
Medium
- A4.32
- B0.216
- C9.30
- D2.16
23.For the reaction N2O4(g) ⇌ 2NO2(g), which change will shift the equilibrium to the right?
Medium
- AIncreasing the pressure by decreasing the volume
- BIncreasing the concentration of NO2
- CDecreasing the concentration of N2O4
- DAdding a catalyst
24.For the reaction N2(g) + 3H2(g) ⇌ 2NH3(g) with ΔH = −92.6 kJ mol⁻¹, which changes will increase the equilibrium yield of ammonia? (select all that apply)
Hard
- AIncreasing the pressure
- BDecreasing the temperature
- CAdding a catalyst
- DRemoving ammonia as it forms
- EIncreasing the volume of the container
25.Match each value of K to the correct description of the equilibrium mixture.
Medium
- K >> 1
- K << 1
- K = 1
- K = 5.86
- Reaction goes almost to completion, mostly products
- Reaction hardly proceeds, mostly reactants
- Significant amounts of both reactants and products
- Equilibrium lies to the right, products favoured
26.For the reaction COI2(g) ⇌ CO(g) + I2(g), Kc = 5.1 × 10⁻² at 298 K. A reaction mixture has [COI2] = 0.150 mol dm⁻³, [CO] = 0.025 mol dm⁻³ and [I2] = 0.025 mol dm⁻³. In which direction will the reaction proceed to reach equilibrium?
Medium
- ATo the right, because Q < Kc
- BTo the left, because Q > Kc
- CIt is already at equilibrium because Q = Kc
- DTo the right, because Q > Kc
27.A reaction has Kc = 2 × 10⁻⁹ mol dm⁻³ at 298 K. What is the value of Kc for the reverse reaction at the same temperature?
Medium
- A5 × 10⁸ mol⁻¹ dm³
- B2 × 10⁹ mol⁻¹ dm³
- C−2 × 10⁻⁹ mol dm⁻³
- D4 × 10⁻¹⁸ mol² dm⁻⁶
28.At 761 K, the equilibrium constant for H2(g) + I2(g) ⇌ 2HI(g) is 48.52. Using ΔG° = −RT lnK with R = 8.31 J K⁻¹ mol⁻¹, what is the value of ΔG° in kJ mol⁻¹?
Medium
- A−24.5 kJ mol⁻¹
- B+24.5 kJ mol⁻¹
- C−2.45 kJ mol⁻¹
- D−245 kJ mol⁻¹
29.For the reaction N2O4(g) ⇌ 2NO2(g), ΔH = +58 kJ mol⁻¹, which changes will increase the equilibrium concentration of NO2? (select all that apply)
Hard
- AIncreasing the temperature
- BIncreasing the total pressure by decreasing the volume
- CAdding more N2O4
- DAdding a catalyst
- ERemoving NO2 as it forms
30.For the reaction 2SO2(g) + O2(g) ⇌ 2SO3(g), Kc >> 1 at 200 °C and 100 kPa. What does this tell you about the equilibrium mixture?
Medium
- AThe equilibrium mixture contains mostly products, and the reaction goes almost to completion.
- BThe equilibrium mixture contains mostly reactants, and the reaction hardly proceeds.
- CThe equilibrium mixture contains equal amounts of reactants and products.
- DThe reaction is not feasible at these conditions.
31.The Contact Process uses 450 °C and 200 kPa for the reaction 2SO2(g) + O2(g) ⇌ 2SO3(g), ΔH = −198 kJ mol⁻¹. Why is a temperature of 450 °C used rather than a lower temperature?
Medium
- AA lower temperature would give a higher yield but too slow a rate, so a compromise temperature is used.
- BA lower temperature would give a lower yield and a slower rate.
- CA higher temperature increases the value of Kc, giving a higher yield.
- DA higher temperature is needed to keep the catalyst active indefinitely.
32.For a reaction at equilibrium, adding a catalyst increases the value of the equilibrium constant Kc.
Easy
True or false?
33.For the reaction 2NO2(g) + F2(g) ⇌ 2NO2F(g), Kc = 7.1 × 10³². What is the value of Kc for the reverse reaction 2NO2F(g) ⇌ 2NO2(g) + F2(g) at the same temperature?
Medium
- A1.41 × 10⁻³³
- B7.1 × 10⁻³²
- C−7.1 × 10³²
- D3.55 × 10³²
34.For the reaction 2SO2(g) + O2(g) ⇌ 2SO3(g), ΔH = −198 kJ mol⁻¹, which change will increase both the equilibrium yield of SO3 and the value of Kc?
Medium
- ADecreasing the temperature
- BIncreasing the temperature
- CIncreasing the volume of the container
- DAdding a catalyst
35.For a reaction at equilibrium, which statements about the effect of a change in pressure are correct? (select all that apply)
Hard
- AIncreasing pressure shifts equilibrium to the side with fewer gas molecules.
- BDecreasing pressure shifts equilibrium to the side with more gas molecules.
- CChanging pressure changes the value of Kc.
- DPressure changes have no effect if the number of gas molecules is the same on both sides.
- EPressure changes only affect reactions involving gases.
36.Match each change to a system at equilibrium with the resulting effect on the value of Kc.
Medium
- Increasing temperature for an exothermic reaction
- Increasing temperature for an endothermic reaction
- Adding a catalyst
- Changing the pressure
- Kc decreases
- Kc increases
- Kc unchanged
37.What is the effect on the value of Kc if the temperature is increased from 761 K?
Medium
- AKc decreases because the forward reaction is exothermic.
- BKc increases because the forward reaction is exothermic.
- CKc stays the same because temperature does not affect Kc.
- DKc decreases because the forward reaction is endothermic.
38.What happens to the value of Kc when a catalyst is added?
Medium
- AIt remains unchanged.
- BIt increases.
- CIt decreases.
- DIt first increases then decreases.
39.For the reaction CH3COOH(l) + C2H5OH(l) ⇌ CH3COOC2H5(l) + H2O(l), at equilibrium 500 cm³ of the mixture contains 0.235 mol ethanoic acid, 0.035 mol ethanol, 0.182 mol ethyl ethanoate and 0.182 mol water. What is the value of Kc?
Medium
- A4.03
- B0.248
- C5.20
- D1.00
40.For the reaction N2(g) + 3H2(g) ⇌ 2NH3(g), ΔH = −92.6 kJ mol⁻¹, which set of conditions would give the highest equilibrium yield of ammonia?
Hard
- AHigh pressure and low temperature
- BLow pressure and high temperature
- CHigh pressure and high temperature
- DLow pressure and low temperature
41.For the reaction H2(g) + I2(g) ⇌ 2HI(g), Kc = 48.52 at 761 K. Calculate the value of Kc for the reaction ½H2(g) + ½I2(g) ⇌ HI(g) at the same temperature. Give your answer to 2 decimal places.
Medium
42.Complete the statement about the relationship between Q and K.
Easy
If Q < K, the reaction proceeds to the ____ to reach equilibrium. If Q > K, the reaction proceeds to the ____.
43.For the reaction N2O4(g) ⇌ 2NO2(g), at equilibrium [N2O4] = 0.0647 mol dm⁻³ and [NO2] = 0.0206 mol dm⁻³. Calculate Kc to 3 significant figures.
Medium
44.Complete the relationship between Gibbs energy and the equilibrium constant.
Medium
The equation linking Gibbs energy change and the equilibrium constant is ΔG = ____. If K > 1, then ΔG is ____.
45.For the reaction CH3COOH(l) + C2H5OH(l) ⇌ CH3COOC2H5(l) + H2O(l), ΔG° = −4.38 kJ mol⁻¹ at 25°C. Using R = 8.31 J K⁻¹ mol⁻¹, calculate the value of Kc. Give your answer to 2 decimal places.
Hard
46.For the reaction COI2(g) ⇌ CO(g) + I2(g), calculate the value of the reaction quotient Q for a mixture with [COI2] = 0.012 mol dm⁻³, [CO] = 0.050 mol dm⁻³ and [I2] = 0.050 mol dm⁻³. Give your answer to 2 significant figures.
Medium
47.At 761 K, the equilibrium concentrations for H2(g) + I2(g) ⇌ 2HI(g) are [H2] = 8.72 × 10⁻⁴ mol dm⁻³, [I2] = 2.72 × 10⁻³ mol dm⁻³ and [HI] = 1.04 × 10⁻² mol dm⁻³. Calculate Kc to 3 significant figures.
Medium
48.Complete the sentence about the relationship between Q and K.
Medium
If Q < K, the reaction proceeds to the ____ to reach equilibrium. If Q > K, the reaction proceeds to the ____ to reach equilibrium. If Q = K, the reaction is at ____.
49.Ethyl ethanoate is hydrolysed: CH3COOC2H5(l) + H2O(l) ⇌ CH3COOH(l) + C2H5OH(l). 0.1000 mol of ethyl ethanoate is added to 0.1000 mol of water and the mixture made up to 1 dm³. At equilibrium 0.0654 mol of water are present. Calculate Kc to 3 significant figures.
Hard
50.State Le Chatelier's principle.
Hard
51.Explain the effect of increasing the temperature on the value of Kc for an exothermic reaction.
Hard
52.The equilibrium constant for a reaction is 5.1 × 10^−2 at 298 K. A reaction mixture has [COI2] = 0.012 mol dm⁻³, [CO] = 0.050 mol dm⁻³ and [I2] = 0.050 mol dm⁻³. Calculate Q and determine the direction in which the reaction will proceed to reach equilibrium.
Hard
53.At 35 °C, the standard free energy change for the esterification of methanoic acid with methanol is −3.79 kJ mol⁻¹. Calculate the value of Kc for this reaction to 2 decimal places. (R = 8.31 J K⁻¹ mol⁻¹)
Hard
54.At 450 K, the equilibrium constant for the decomposition of ammonia, 2NH3(g) ⇌ 3H2(g) + N2(g), is 3.85 × 10⁻⁴. A reaction is set up at 450 K with an initial concentration of ammonia of 0.20 mol dm⁻³. Calculate the equilibrium concentration of hydrogen, stating any approximation you make.
Hard
55.For the reaction H2(g) + I2(g) ⇌ 2HI(g), Kc = 48.52 at 761 K. Calculate the value of Kc for the reaction ½H2(g) + ½I2(g) ⇌ HI(g) at the same temperature. Give your answer to 2 decimal places.
Medium
- A5.97
- B7.97
- C8.97
- D6.97
56.For the reaction N2O4(g) ⇌ 2NO2(g), at equilibrium [N2O4] = 0.0647 mol dm⁻³ and [NO2] = 0.0206 mol dm⁻³. Calculate Kc to 3 significant figures.
Medium
- A2.0066 mol dm⁻³
- B-0.9934 mol dm⁻³
- C1.0066 mol dm⁻³
- D0.00656 mol dm⁻³
57.For the reaction CH3COOH(l) + C2H5OH(l) ⇌ CH3COOC2H5(l) + H2O(l), ΔG° = −4.38 kJ mol⁻¹ at 25°C. Using R = 8.31 J K⁻¹ mol⁻¹, calculate the value of Kc. Give your answer to 2 decimal places.
Hard
- A4.86
- B7.86
- C6.86
- D5.86
58.For the reaction COI2(g) ⇌ CO(g) + I2(g), calculate the value of the reaction quotient Q for a mixture with [COI2] = 0.012 mol dm⁻³, [CO] = 0.050 mol dm⁻³ and [I2] = 0.050 mol dm⁻³. Give your answer to 2 significant figures.
Medium
- A0.21 mol dm⁻³
- B2.21 mol dm⁻³
- C1.21 mol dm⁻³
- D-0.79 mol dm⁻³
59.At 761 K, the equilibrium concentrations for H2(g) + I2(g) ⇌ 2HI(g) are [H2] = 8.72 × 10⁻⁴ mol dm⁻³, [I2] = 2.72 × 10⁻³ mol dm⁻³ and [HI] = 1.04 × 10⁻² mol dm⁻³. Calculate Kc to 3 significant figures.
Medium
- A50.2
- B54.8
- C41
- D45.6
60.Ethyl ethanoate is hydrolysed: CH3COOC2H5(l) + H2O(l) ⇌ CH3COOH(l) + C2H5OH(l). 0.1000 mol of ethyl ethanoate is added to 0.1000 mol of water and the mixture made up to 1 dm³. At equilibrium 0.0654 mol of water are present. Calculate Kc to 3 significant figures.
Hard
- A-0.72
- B1.28
- C0.28
- D2.28

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