What is a dynamic equilibrium ?

Dynamic equilibrium exists in a closed system when the forward and reverse reactions occur at equal rates, so the concentrations of reactants and products remain constant. It requires a closed system — equilibrium cannot be established if products escape.

True or False? At dynamic equilibrium, the concentrations of reactants and products are equal.

False. At dynamic equilibrium, the concentrations of reactants and products are constant , not necessarily equal. The forward and reverse reactions continue at the same rate.

A reversible reaction uses the symbol .......... to show that both forward and backward reactions can occur. Equilibrium can only be reached in a .......... system where reactants and products cannot .......... .

A reversible reaction uses the symbol ⇌ to show that both forward and backward reactions can occur. Equilibrium can only be reached in a closed system where reactants and products cannot escape .

What is a closed system and why is it necessary for dynamic equilibrium?

A closed system is one where no matter is exchanged with the surroundings, but energy can still be transferred. It is necessary because if products escaped, the reverse reaction could not occur and equilibrium would never be established.

As a reversible reaction approaches equilibrium from reactants, the rate of the .......... reaction decreases while the rate of the .......... reaction increases, until both rates are .......... .

As a reversible reaction approaches equilibrium from reactants, the rate of the forward reaction decreases while the rate of the reverse reaction increases, until both rates are equal .

True or False? In an open system involving gases, dynamic equilibrium cannot be established.

True. In an open system, gaseous products escape the reaction mixture, so the reverse reaction cannot occur at the same rate as the forward reaction. The reaction will proceed to completion instead.

State Le Chatelier's principle .

A standard enthalpy change of hydration is the enthalpy change when one mole of gaseous ions is dissolved in water to form an infinitely dilute solution under standard conditions (298 K, 100 kPa).

If the concentration of a reactant is increased, the equilibrium shifts to the .......... to oppose the change, producing more .......... . If a product is removed, the equilibrium shifts to the .......... to replace it.

If the concentration of a reactant is increased, the equilibrium shifts to the right to oppose the change, producing more products . If a product is removed, the equilibrium shifts to the right to replace it.

True or False? Increasing pressure always shifts the equilibrium to the right.

False. Increasing pressure shifts equilibrium towards the side with fewer moles of gas . If there are more moles of gas on the right, equilibrium shifts left . If both sides have equal moles of gas, pressure has no effect.

How does increasing temperature affect the position of equilibrium in an exothermic forward reaction?

Increasing temperature shifts the equilibrium to the left (towards reactants). The system opposes the temperature increase by favouring the endothermic (reverse) reaction, which absorbs heat.

A catalyst .......... the position of equilibrium. It speeds up both the .......... and .......... reactions equally, so equilibrium is reached .......... , but the equilibrium composition is unchanged.

A catalyst does not change the position of equilibrium. It speeds up both the forward and reverse reactions equally, so equilibrium is reached faster , but the equilibrium composition is unchanged.

True or False? Adding water (diluting) an equilibrium mixture of aqueous ions always shifts the position of equilibrium.

False. Dilution reduces the concentration of all aqueous species equally. If the ratio of reactants to products is unchanged, the position of equilibrium does not shift.

For the reaction N 2 O 4 (g) ⇌ 2NO 2 (g), predict the effect of increasing pressure on the equilibrium position.

The equilibrium shifts to the left (towards N 2 O 4 ). There are fewer moles of gas on the left (1 mol vs 2 mol), so the system reduces pressure by forming more N 2 O 4 .

What is the equilibrium constant K c ?

An enthalpy change is the heat energy transferred at constant pressure during a chemical or physical process. It is measured in kJ mol -1 and is negative for exothermic reactions and positive for endothermic reactions.

For the reaction aA + bB ⇌ cC + dD, the expression for K c is: K c = ( [C] c × [D] d ) ÷ ( .......... × .......... ) Species present as .......... are excluded from the expression.

K c = ( [C] c × [D] d ) ÷ ( [A] a × [B] b ) Species present as solids are excluded from the expression.

True or False? The equilibrium constant K c changes if more reactant is added to the system at constant temperature.

False. K c is constant at a given temperature . Adding reactant shifts the equilibrium position but does not change the value of K c . Only a change in temperature alters K c .

Write the K c expression for: N 2 (g) + 3H 2 (g) ⇌ 2NH 3 (g)

K c = [NH 3 (g)] 2 ÷ ( [N 2 (g)] × [H 2 (g)] 3 ) Product concentrations appear in the numerator; reactant concentrations in the denominator, each raised to their stoichiometric coefficients.

In the K c expression for Ag + (aq) + Fe 2+ (aq) ⇌ Ag (s) + Fe 3+ (aq), the concentration of .......... is excluded because it is a .......... . The expression becomes K c = .......... ÷ .......... .

The concentration of Ag is excluded because it is a solid . The expression becomes K c = [Fe 3+ (aq)] ÷ ([Fe 2+ (aq)] × [Ag + (aq)]) .

True or False? The units of K c are always mol dm -3 .

False. The units of K c depend on the form of the equilibrium expression. If the number of moles of products equals the number of moles of reactants in the expression, the units cancel out and K c is dimensionless.

How do you calculate the concentration of a species from moles and volume in a K c calculation?

concentration (mol dm -3 ) = moles (mol) ÷ volume (dm 3 ) Convert cm 3 to dm 3 by dividing by 1000 before substituting into the K c expression.

In a K c calculation, when given initial and equilibrium concentrations but not product concentrations, use an .......... , .......... , .......... (ICE) table to determine the equilibrium concentration of the products from the molar ratios of the balanced equation.

In a K c calculation, when given initial and equilibrium concentrations but not product concentrations, use an initial , change , equilibrium (ICE) table to determine the equilibrium concentration of the products from the molar ratios of the balanced equation.

True or False? If the units of concentration cancel in a K c expression, K c has no units.

True. When the total stoichiometric coefficients on both sides of the equation are equal, all concentration units cancel and K c is dimensionless (no units).

In a K c calculation, the number of significant figures in your answer should match what?

The answer should be given to the same number of significant figures as the least precise data value given in the question.

For the following reaction: CH 3 COOH (l) + C 2 H 5 OH (l) ⇌ CH 3 COOC 2 H 5 (l) + H 2 O (l) [CH 3 COOH] = 0.470 mol dm -3 [C 2 H 5 OH] = 0.070 mol dm -3 [CH 3 COOC 2 H 5 ] = 0.364 mol dm -3 [H 2 O] = 0.364 mol dm -3 K c = .......... (no units)

K c = (0.364 × 0.364) ÷ (0.470 × 0.070) = 4.0 (no units). All concentration units cancel because there are equal moles of products and reactants.

True or False? In a K c calculation, you can use moles directly without converting to concentration, as long as both sides of the expression use the same volume.

False. You must always use concentrations (mol dm -3 ) in the K c expression, not raw moles. The volume only cancels if the stoichiometric coefficients balance on both sides.

What is the step-by-step approach to calculate K c when given equilibrium moles and total volume?

Convert moles to concentrations: c = n ÷ V. Write the K c expression from the balanced equation. Substitute equilibrium concentrations. Calculate K c and determine units.

Define the equilibrium constant ( K c )

Equilibrium constant ( K c ) is a value that expresses the ratio of the concentrations of products to reactants at equilibrium, with each concentration raised to the power of its stoichiometric coefficient. Its value is constant at a fixed temperature.

How do you deduce the units of K c when concentration terms do not fully cancel?

Substitute mol dm -3 for each concentration term in the K c expression and cancel units algebraically. Example: N 2 (g) + 3H 2 (g) ⇌ 2NH 3 (g) K c = [NH 3 ] 2 / ([N 2 ][H 2 ] 3 ) Units = (mol dm -3 ) 2 / ((mol dm -3 )(mol dm -3 ) 3 ) = mol -2 dm 6

For N 2 (g) + 3H 2 (g) ⇌ 2NH 3 (g), starting with [N 2 ] = 0.500 mol dm -3 and [H 2 ] = 1.500 mol dm -3 , the equilibrium concentration of NH 3 is 0.300 mol dm -3 . Calculate K c .

Set up an ICE table (mol dm -3 ): N 2 H 2 NH 3 I 0.500 1.500 0 C −0.150 −0.450 +0.300 E 0.350 1.050 0.300 K c = (0.300) 2 / (0.350 × (1.050) 3 ) = 0.0900 / 0.405 = 0.222 mol -2 dm 6

What does the equilibrium constant K c tell you about an equilibrium reaction?

The equilibrium constant K c gives the ratio of product concentrations to reactant concentrations at equilibrium, each raised to the power of their stoichiometric coefficients. A large K c means the equilibrium lies to the right (favouring products). A small K c means the equilibrium lies to the left (favouring reactants).

True or False? Adding a catalyst to an equilibrium mixture increases the value of K c .

False. A catalyst speeds up both the forward and reverse reactions equally, so the ratio of [products] to [reactants] at equilibrium is unchanged and K c remains the same.

For an exothermic equilibrium reaction, increasing the temperature causes K c to .......... , because the equilibrium shifts to the .......... to absorb the extra energy.

For an exothermic equilibrium reaction, increasing the temperature causes K c to decrease , because the equilibrium shifts to the left to absorb the extra energy.

Which single factor changes the value of K c for an equilibrium reaction?

Temperature is the only factor that changes the value of K c . Changes in concentration, pressure, and the presence of a catalyst all shift the position of equilibrium but leave K c unchanged.

What is meant by a ' compromise temperature ' in an industrial equilibrium process?

A standard enthalpy change of solution is the enthalpy change when one mole of a solute dissolves completely in excess solvent to form an infinitely dilute solution under standard conditions (298 K, 100 kPa).

In the Haber process, N 2 (g) + 3H 2 (g) ⇌ 2NH 3 (g), the compromise conditions are a pressure of .......... atm and a temperature of .......... .

In the Haber process, N 2 (g) + 3H 2 (g) ⇌ 2NH 3 (g), the compromise conditions are a pressure of 200 atm and a temperature of 400–450 °C .

True or False? In the Contact process, very high pressures are required to obtain a good yield of SO 3 .

False. The reaction is already at a position well to the right because K c is very large, so the process is carried out at approximately 1 atm. Higher pressures would be unnecessary and economically unjustified.

Why is ammonia continuously removed from the Haber process reaction vessel?

Removing ammonia by condensing it to a liquid lowers the product concentration. This causes the equilibrium to shift to the right to replace the ammonia, producing more product from the remaining nitrogen and hydrogen.

A LevelChemistryAQAFlashcardsPhysical ChemistryChemical Equilibria, Le Chatelier’s Principle & Kc

Chemical Equilibria, Le Chatelier’s Principle & Kc (AQA A Level Chemistry): Flashcards

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What is a dynamic equilibrium?

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What is a dynamic equilibrium?
Dynamic equilibrium exists in a closed system when the forward and reverse reactions occur at equal rates, so the concentrations of reactants and products remain constant.
It requires a closed system — equilibrium cannot be established if products escape.
True or False?
At dynamic equilibrium, the concentrations of reactants and products are equal.
False.
At dynamic equilibrium, the concentrations of reactants and products are constant, not necessarily equal. The forward and reverse reactions continue at the same rate.
A reversible reaction uses the symbol .......... to show that both forward and backward reactions can occur. Equilibrium can only be reached in a .......... system where reactants and products cannot .......... .
A reversible reaction uses the symbol ⇌ to show that both forward and backward reactions can occur. Equilibrium can only be reached in a closed system where reactants and products cannot escape.
What is a closed system and why is it necessary for dynamic equilibrium?
A closed system is one where no matter is exchanged with the surroundings, but energy can still be transferred.
It is necessary because if products escaped, the reverse reaction could not occur and equilibrium would never be established.
As a reversible reaction approaches equilibrium from reactants, the rate of the .......... reaction decreases while the rate of the .......... reaction increases, until both rates are .......... .
As a reversible reaction approaches equilibrium from reactants, the rate of the forward reaction decreases while the rate of the reverse reaction increases, until both rates are equal.
True or False?
In an open system involving gases, dynamic equilibrium cannot be established.
True.
In an open system, gaseous products escape the reaction mixture, so the reverse reaction cannot occur at the same rate as the forward reaction. The reaction will proceed to completion instead.
State Le Chatelier's principle.
A standard enthalpy change of hydration is the enthalpy change when one mole of gaseous ions is dissolved in water to form an infinitely dilute solution under standard conditions (298 K, 100 kPa).
If the concentration of a reactant is increased, the equilibrium shifts to the .......... to oppose the change, producing more .......... . If a product is removed, the equilibrium shifts to the .......... to replace it.
If the concentration of a reactant is increased, the equilibrium shifts to the right to oppose the change, producing more products. If a product is removed, the equilibrium shifts to the right to replace it.
True or False?
Increasing pressure always shifts the equilibrium to the right.
False.
Increasing pressure shifts equilibrium towards the side with fewer moles of gas. If there are more moles of gas on the right, equilibrium shifts left. If both sides have equal moles of gas, pressure has no effect.
How does increasing temperature affect the position of equilibrium in an exothermic forward reaction?
Increasing temperature shifts the equilibrium to the left (towards reactants).
The system opposes the temperature increase by favouring the endothermic (reverse) reaction, which absorbs heat.
A catalyst .......... the position of equilibrium. It speeds up both the .......... and .......... reactions equally, so equilibrium is reached .......... , but the equilibrium composition is unchanged.
A catalyst does not change the position of equilibrium. It speeds up both the forward and reverse reactions equally, so equilibrium is reached faster, but the equilibrium composition is unchanged.
True or False?
Adding water (diluting) an equilibrium mixture of aqueous ions always shifts the position of equilibrium.
False.
Dilution reduces the concentration of all aqueous species equally. If the ratio of reactants to products is unchanged, the position of equilibrium does not shift.
For the reaction N₂O₄ (g) ⇌ 2NO₂ (g), predict the effect of increasing pressure on the equilibrium position.
The equilibrium shifts to the left (towards N₂O₄).
There are fewer moles of gas on the left (1 mol vs 2 mol), so the system reduces pressure by forming more N₂O₄.
What is the equilibrium constant K_c?
An enthalpy change is the heat energy transferred at constant pressure during a chemical or physical process. It is measured in kJ mol^-1 and is negative for exothermic reactions and positive for endothermic reactions.
For the reaction aA + bB ⇌ cC + dD, the expression for K_c is:
K_c = ( [C]^c × [D]^d ) ÷ ( .......... × .......... )
Species present as .......... are excluded from the expression.
K_c = ( [C]^c × [D]^d ) ÷ ( [A]^a × [B]^b )
Species present as solids are excluded from the expression.
True or False?
The equilibrium constant K_c changes if more reactant is added to the system at constant temperature.
False.
K_c is constant at a given temperature. Adding reactant shifts the equilibrium position but does not change the value of K_c. Only a change in temperature alters K_c.
Write the K_c expression for: N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)
K_c = [NH₃ (g)]² ÷ ( [N₂ (g)] × [H₂ (g)]³ )
Product concentrations appear in the numerator; reactant concentrations in the denominator, each raised to their stoichiometric coefficients.
In the K_c expression for Ag⁺ (aq) + Fe²⁺ (aq) ⇌ Ag (s) + Fe³⁺ (aq), the concentration of .......... is excluded because it is a .......... .
The expression becomes K_c = .......... ÷ .......... .
The concentration of Ag is excluded because it is a solid.
The expression becomes K_c = [Fe³⁺ (aq)] ÷ ([Fe²⁺ (aq)] × [Ag⁺ (aq)]).
True or False?
The units of K_c are always mol dm^-3.
False.
The units of K_c depend on the form of the equilibrium expression. If the number of moles of products equals the number of moles of reactants in the expression, the units cancel out and K_c is dimensionless.
How do you calculate the concentration of a species from moles and volume in a K_c calculation?
concentration (mol dm^-3) = moles (mol) ÷ volume (dm³)
Convert cm³ to dm³ by dividing by 1000 before substituting into the K_c expression.
In a K_c calculation, when given initial and equilibrium concentrations but not product concentrations, use an .........., .........., .......... (ICE) table to determine the equilibrium concentration of the products from the molar ratios of the balanced equation.
In a K_c calculation, when given initial and equilibrium concentrations but not product concentrations, use an initial, change, equilibrium (ICE) table to determine the equilibrium concentration of the products from the molar ratios of the balanced equation.
True or False?
If the units of concentration cancel in a K_c expression, K_c has no units.
True.
When the total stoichiometric coefficients on both sides of the equation are equal, all concentration units cancel and K_c is dimensionless (no units).
In a K_c calculation, the number of significant figures in your answer should match what?
The answer should be given to the same number of significant figures as the least precise data value given in the question.
For the following reaction:
CH₃COOH (l) + C₂H₅OH (l) ⇌ CH₃COOC₂H₅ (l) + H₂O (l)
[CH₃COOH] = 0.470 mol dm^-3
[C₂H₅OH] = 0.070 mol dm^-3
[CH₃COOC₂H₅] = 0.364 mol dm^-3
[H₂O] = 0.364 mol dm^-3
K_c = .......... (no units)
K_c = (0.364 × 0.364) ÷ (0.470 × 0.070) = 4.0 (no units).
All concentration units cancel because there are equal moles of products and reactants.
True or False?
In a K_c calculation, you can use moles directly without converting to concentration, as long as both sides of the expression use the same volume.
False.
You must always use concentrations (mol dm^-3) in the K_c expression, not raw moles. The volume only cancels if the stoichiometric coefficients balance on both sides.
What is the step-by-step approach to calculate K_c when given equilibrium moles and total volume?
1. Convert moles to concentrations: c = n ÷ V.
2. Write the K_c expression from the balanced equation.
3. Substitute equilibrium concentrations.
4. Calculate K_c and determine units.
Define the equilibrium constant (K_c)
Equilibrium constant (K_c) is a value that expresses the ratio of the concentrations of products to reactants at equilibrium, with each concentration raised to the power of its stoichiometric coefficient. Its value is constant at a fixed temperature.
How do you deduce the units of K_c when concentration terms do not fully cancel?
Substitute mol dm^-3 for each concentration term in the K_c expression and cancel units algebraically.
Example: N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)
K_c = [NH₃]² / ([N₂][H₂]³)
Units = (mol dm^-3)² / ((mol dm^-3)(mol dm^-3)³) = mol^-2 dm⁶
For N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g), starting with [N₂] = 0.500 mol dm^-3 and [H₂] = 1.500 mol dm^-3, the equilibrium concentration of NH₃ is 0.300 mol dm^-3. Calculate K_c.
Set up an ICE table (mol dm^-3):
N₂
H₂
NH₃
I
0.500
1.500
0
C
−0.150
−0.450
+0.300
E
0.350
1.050
0.300
K_c = (0.300)² / (0.350 × (1.050)³) = 0.0900 / 0.405 = 0.222 mol^-2 dm⁶
What does the equilibrium constant K_c tell you about an equilibrium reaction?
The equilibrium constant K_c gives the ratio of product concentrations to reactant concentrations at equilibrium, each raised to the power of their stoichiometric coefficients.
A large K_c means the equilibrium lies to the right (favouring products).
A small K_c means the equilibrium lies to the left (favouring reactants).
True or False?
Adding a catalyst to an equilibrium mixture increases the value of K_c.
False.
A catalyst speeds up both the forward and reverse reactions equally, so the ratio of [products] to [reactants] at equilibrium is unchanged and K_c remains the same.
For an exothermic equilibrium reaction, increasing the temperature causes K_c to .........., because the equilibrium shifts to the .......... to absorb the extra energy.
For an exothermic equilibrium reaction, increasing the temperature causes K_c to decrease, because the equilibrium shifts to the left to absorb the extra energy.
Which single factor changes the value of K_c for an equilibrium reaction?
Temperature is the only factor that changes the value of K_c.
Changes in concentration, pressure, and the presence of a catalyst all shift the position of equilibrium but leave K_c unchanged.
What is meant by a 'compromise temperature' in an industrial equilibrium process?
A standard enthalpy change of solution is the enthalpy change when one mole of a solute dissolves completely in excess solvent to form an infinitely dilute solution under standard conditions (298 K, 100 kPa).
In the Haber process, N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g), the compromise conditions are a pressure of .......... atm and a temperature of .......... .
In the Haber process, N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g), the compromise conditions are a pressure of 200 atm and a temperature of 400–450 °C.
True or False?
In the Contact process, very high pressures are required to obtain a good yield of SO₃.
False.
The reaction is already at a position well to the right because K_c is very large, so the process is carried out at approximately 1 atm. Higher pressures would be unnecessary and economically unjustified.
Why is ammonia continuously removed from the Haber process reaction vessel?
Removing ammonia by condensing it to a liquid lowers the product concentration.
This causes the equilibrium to shift to the right to replace the ammonia, producing more product from the remaining nitrogen and hydrogen.

	N₂	H₂	NH₃
I	0.500	1.500	0
C	−0.150	−0.450	+0.300
E	0.350	1.050	0.300

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