A mole fraction of a substance is the fraction of the total number of moles in a mixture that is due to that substance. It is calculated as moles of substance divided by total moles.

Kp is the equilibrium constant expressed in terms of the partial pressures of gaseous reactants and products at equilibrium. Like Kc, its value only changes when temperature changes.

Define Le Chatelier's principle

Le Chatelier's principle states that if a change is imposed on a system in dynamic equilibrium , the position of equilibrium shifts to counteract that change. It is used to predict how equilibria respond to changes in temperature, pressure, or concentration.

Why is the concept of partial pressure important in equilibria involving gases?

Partial pressure replaces concentration in the equilibrium expression for gaseous systems, allowing the equilibrium constant Kp to be calculated. Each gas exerts a pressure proportional to its mole fraction of the total pressure.

Why are solids and liquids omitted from Kp expressions?

Solids and liquids do not have a meaningful partial pressure and their concentrations remain effectively constant during the reaction. Including them would make the equilibrium expression incorrect.

What happens to the value of Kc when temperature is increased for a reaction that is exothermic in the forward direction?

Kc decreases , because the equilibrium shifts to the left to oppose the temperature rise. This reduces the ratio of product concentrations to reactant concentrations .

The partial pressure of a gas equals its .......... multiplied by the .......... pressure.

The partial pressure of a gas equals its mole fraction multiplied by the total pressure.

To calculate Kp, you must first find the .......... fractions of each gas, then their .......... pressures, and finally substitute into the Kp expression.

To calculate Kp, you must first find the mole fractions of each gas, then their partial pressures, and finally substitute into the Kp expression.

True or False? Adding a catalyst to a gaseous equilibrium increases the value of Kp.

False. A catalyst speeds up both the forward and reverse reactions equally, so the ratio of partial pressures at equilibrium is unchanged and Kp remains the same.

True or False? The partial pressures of all gases in a mixture must sum to the total pressure.

True. This is Dalton's Law : the total pressure of a gas mixture equals the sum of the partial pressures of each individual gas.

True or False? The value of Kp changes when the pressure of a gaseous equilibrium is increased.

False. Kp is unaffected by changes in pressure . A change in pressure shifts the position of equilibrium to restore the value of Kp, which itself remains constant.

Explain why a catalyst has no effect on the position of equilibrium once equilibrium is reached.

A catalyst increases the rate of both the forward and reverse reactions by equal amounts, so the equilibrium ratio of products to reactants is not disturbed. The only effect is that equilibrium is reached faster .

Define partial pressure

A partial pressure of a gas is the pressure it would exert if it occupied the container alone. It equals the mole fraction of that gas multiplied by the total pressure of the mixture.

Define equilibrium expression (Kc)

An equilibrium expression for Kc is the ratio of the equilibrium concentrations of products to reactants, with each concentration raised to the power of its stoichiometric coefficient . For aA + bB ⇌ cC + dD: Kc = [C] c [D] d / [A] a [B] b .

For an endothermic reaction, increasing the temperature causes Kc to .......... and the equilibrium position to shift to the .......... .

For an endothermic reaction, increasing the temperature causes Kc to increase and the equilibrium position to shift to the right .

What information is needed to calculate the partial pressure of a gas in a mixture?

You need the total pressure in the container and the mole fraction of the gas. Multiply these two values together to obtain the partial pressure.

Write the Kp expression for N 2 (g) + 3H 2 (g) ⇌ 2NH 3 (g).

Kp = p(NH 3 ) 2 / (p(N 2 ) x p(H 2 ) 3 ). The partial pressure of each gas is raised to its stoichiometric coefficient from the balanced equation.

True or False? Increasing the pressure of a gaseous equilibrium changes the value of Kp.

False. Changes in pressure shift the position of equilibrium to a new state that restores the original value of Kp, which itself remains constant.

Explain why the mole fractions of all gases in a mixture must add up to 1.

Each mole fraction represents that gas's share of the total moles present, so all shares together account for 100% of the mixture. If they do not sum to 1, at least one mole fraction has been calculated incorrectly.

Explain why Kp only changes in value when temperature changes, not when pressure or concentration changes.

A change in pressure or concentration shifts the position of equilibrium so that the ratio of partial pressures is restored to its original value. Only a temperature change alters the energy of the system, producing a new ratio of partial pressures and a new value of Kp.

Explain why only a change in temperature alters the value of an equilibrium constant.

Temperature changes alter the energy of the system, changing the relative rates of the forward and reverse reactions and producing a new ratio of products to reactants. Changes in pressure or concentration are counteracted by a shift in the position of equilibrium that restores the original equilibrium constant.

Concentration (mol dm -3 ) = number of .......... ÷ .......... of solution (dm 3 ).

Concentration (mol dm -3 ) = number of moles ÷ volume of solution (dm 3 ).

What is the purpose of an ICE table in equilibrium calculations?

An ICE (Initial, Change, Equilibrium) table organises the concentrations of reactants and products before and after the reaction reaches equilibrium, using stoichiometric ratios to determine unknown equilibrium quantities.

What information is needed to find the equilibrium concentrations of all species using an ICE table?

You need the initial concentrations (or moles and volume) of all species, at least one equilibrium value , and the stoichiometric ratios from the balanced equation to calculate the change in each species.

A LevelChemistryOCRFlashcards5. Physical Chemistry & Transition ElementsEquilibria

Equilibria (OCR A Level Chemistry A): Flashcards

Exam code: H432

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Cards in this collection (24)

Define mole fraction
A mole fraction of a substance is the fraction of the total number of moles in a mixture that is due to that substance. It is calculated as moles of substance divided by total moles.
Define Kp
Kp is the equilibrium constant expressed in terms of the partial pressures of gaseous reactants and products at equilibrium. Like Kc, its value only changes when temperature changes.
Define Le Chatelier's principle
Le Chatelier's principle states that if a change is imposed on a system in dynamic equilibrium, the position of equilibrium shifts to counteract that change. It is used to predict how equilibria respond to changes in temperature, pressure, or concentration.
Why is the concept of partial pressure important in equilibria involving gases?
Partial pressure replaces concentration in the equilibrium expression for gaseous systems, allowing the equilibrium constant Kp to be calculated. Each gas exerts a pressure proportional to its mole fraction of the total pressure.
Why are solids and liquids omitted from Kp expressions?
Solids and liquids do not have a meaningful partial pressure and their concentrations remain effectively constant during the reaction. Including them would make the equilibrium expression incorrect.
What happens to the value of Kc when temperature is increased for a reaction that is exothermic in the forward direction?
Kc decreases, because the equilibrium shifts to the left to oppose the temperature rise. This reduces the ratio of product concentrations to reactant concentrations.
The partial pressure of a gas equals its .......... multiplied by the .......... pressure.
The partial pressure of a gas equals its mole fraction multiplied by the total pressure.
To calculate Kp, you must first find the .......... fractions of each gas, then their .......... pressures, and finally substitute into the Kp expression.
To calculate Kp, you must first find the mole fractions of each gas, then their partial pressures, and finally substitute into the Kp expression.
True or False?
Adding a catalyst to a gaseous equilibrium increases the value of Kp.
False.
A catalyst speeds up both the forward and reverse reactions equally, so the ratio of partial pressures at equilibrium is unchanged and Kp remains the same.
True or False?
The partial pressures of all gases in a mixture must sum to the total pressure.
True.
This is Dalton's Law: the total pressure of a gas mixture equals the sum of the partial pressures of each individual gas.
True or False?
The value of Kp changes when the pressure of a gaseous equilibrium is increased.
False.
Kp is unaffected by changes in pressure. A change in pressure shifts the position of equilibrium to restore the value of Kp, which itself remains constant.
Explain why a catalyst has no effect on the position of equilibrium once equilibrium is reached.
A catalyst increases the rate of both the forward and reverse reactions by equal amounts, so the equilibrium ratio of products to reactants is not disturbed. The only effect is that equilibrium is reached faster.
Define partial pressure
A partial pressure of a gas is the pressure it would exert if it occupied the container alone. It equals the mole fraction of that gas multiplied by the total pressure of the mixture.
Define equilibrium expression (Kc)
An equilibrium expression for Kc is the ratio of the equilibrium concentrations of products to reactants, with each concentration raised to the power of its stoichiometric coefficient. For aA + bB ⇌ cC + dD: Kc = [C]^c[D]^d / [A]^a[B]^b.
For an endothermic reaction, increasing the temperature causes Kc to .......... and the equilibrium position to shift to the .......... .
For an endothermic reaction, increasing the temperature causes Kc to increase and the equilibrium position to shift to the right.
What information is needed to calculate the partial pressure of a gas in a mixture?
You need the total pressure in the container and the mole fraction of the gas. Multiply these two values together to obtain the partial pressure.
Write the Kp expression for N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g).
Kp = p(NH₃)² / (p(N₂) x p(H₂)³). The partial pressure of each gas is raised to its stoichiometric coefficient from the balanced equation.
True or False?
Increasing the pressure of a gaseous equilibrium changes the value of Kp.
False.
Changes in pressure shift the position of equilibrium to a new state that restores the original value of Kp, which itself remains constant.
Explain why the mole fractions of all gases in a mixture must add up to 1.
Each mole fraction represents that gas's share of the total moles present, so all shares together account for 100% of the mixture. If they do not sum to 1, at least one mole fraction has been calculated incorrectly.
Explain why Kp only changes in value when temperature changes, not when pressure or concentration changes.
A change in pressure or concentration shifts the position of equilibrium so that the ratio of partial pressures is restored to its original value. Only a temperature change alters the energy of the system, producing a new ratio of partial pressures and a new value of Kp.
Explain why only a change in temperature alters the value of an equilibrium constant.
Temperature changes alter the energy of the system, changing the relative rates of the forward and reverse reactions and producing a new ratio of products to reactants. Changes in pressure or concentration are counteracted by a shift in the position of equilibrium that restores the original equilibrium constant.
Concentration (mol dm^-3) = number of .......... ÷ .......... of solution (dm³).
Concentration (mol dm^-3) = number of moles ÷ volume of solution (dm³).
What is the purpose of an ICE table in equilibrium calculations?
An ICE (Initial, Change, Equilibrium) table organises the concentrations of reactants and products before and after the reaction reaches equilibrium, using stoichiometric ratios to determine unknown equilibrium quantities.
What information is needed to find the equilibrium concentrations of all species using an ICE table?
You need the initial concentrations (or moles and volume) of all species, at least one equilibrium value, and the stoichiometric ratios from the balanced equation to calculate the change in each species.

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