What does K p represent in a gaseous equilibrium?

K p is the equilibrium constant for a gaseous reaction expressed in terms of the partial pressures of the reactants and products at equilibrium. It is constant at a given temperature.

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

K p = p 2 (NH 3 ) / (p(N 2 ) × p 3 (H 2 )) Units: kPa 2 / (kPa × kPa 3 ) = kPa -2

True or False? Solids and liquids are included in the K p expression.

False. Solids and liquids are omitted from the K p expression. Solids do not have a partial pressure; liquids have a constant vapour pressure. Their contribution is incorporated into the value of K p .

In a K p expression, square brackets .......... be used because they denote .......... . Instead, use the symbol .......... to denote partial pressure.

In a K p expression, square brackets must not be used because they denote concentration . Instead, use the symbol p (or pp) to denote partial pressure.

What is the only factor that changes the value of K p for a given reaction?

Temperature. Changes in pressure, concentration or the addition of a catalyst do not alter K p .

For the reaction N 2 O 4 (g) ⇌ 2NO 2 (g), what are the units of K p if pressure is measured in kPa?

K p = p 2 (NO 2 ) / p(N 2 O 4 ) Units = kPa 2 / kPa = kPa

What is the partial pressure of a gas in a mixture?

The 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.

The mole fraction of a gas = number of moles of that gas ÷ .......... . The partial pressure of a gas = mole fraction × .......... .

The mole fraction of a gas = number of moles of that gas ÷ total number of moles of all gases . The partial pressure of a gas = mole fraction × total pressure .

True or False? The sum of the mole fractions of all gases in a mixture always equals 1.

True. The sum of all mole fractions must equal 1. Similarly, the sum of all partial pressures must equal the total pressure — both are useful checks in K p calculations.

List the steps needed to calculate K p when given initial moles, an equilibrium mole of one gas and the total pressure.

Use an ICE table to find equilibrium moles of all gases. Calculate the total equilibrium moles. Calculate mole fractions. Calculate partial pressures (mole fraction × total pressure). Write the K p expression. Substitute and calculate K p , including units.

When substituting into a K p expression, each partial pressure must be raised to the power of its .......... coefficient. Mole fractions .......... be substituted directly — you must first convert them to .......... .

When substituting into a K p expression, each partial pressure must be raised to the power of its stoichiometric coefficient. Mole fractions must not be substituted directly — you must first convert them to partial pressures .

True or False? If the K p expression has equal numbers of mole terms on the top and bottom (e.g. H 2 + I 2 ⇌ 2HI), K p has no units.

True. When the total number of moles of gas is the same on both sides, the pressure units cancel and K p is dimensionless.

Why is K p preferred over K c for reactions involving gases?

Pressure is easier to measure directly for gases than concentration. K p uses partial pressures, which are readily obtained from total pressure and mole fractions.

How does increasing temperature affect K p for an exothermic forward reaction?

Increasing temperature shifts the equilibrium to the left (to counteract the added heat). The ratio of products to reactants decreases, so K p decreases .

For an endothermic forward reaction, increasing temperature shifts equilibrium to the .......... , meaning the ratio of products to reactants .......... and K p .......... .

For an endothermic forward reaction, increasing temperature shifts equilibrium to the right , meaning the ratio of products to reactants increases and K p increases .

True or False? Changing the pressure of a gaseous equilibrium changes the value of K p .

False. Pressure changes shift the position of equilibrium to a new position that restores K p , but the value of K p itself remains unchanged. Only temperature alters K p .

State Le Chatelier's Principle as applied to gaseous equilibria.

A dynamic equilibrium is the state reached in a closed system when the forward and reverse reactions proceed at equal rates, so that the concentrations of reactants and products remain constant over time.

What effect does adding a catalyst have on the value of K p and on the position of equilibrium?

A catalyst has no effect on K p or on the position of equilibrium. It speeds up both the forward and reverse reactions equally, causing equilibrium to be reached faster but not shifting where it lies.

Increasing pressure on a gaseous equilibrium shifts the position towards the side with .......... moles of gas. This change .......... affect the value of K p .

Increasing pressure on a gaseous equilibrium shifts the position towards the side with fewer moles of gas. This change does not affect the value of K p .

A LevelChemistryAQAFlashcardsAdvanced Physical ChemistryEquilibrium Constant (Kp) for Homogeneous Systems

Equilibrium Constant (Kp) for Homogeneous Systems (AQA A Level Chemistry): Flashcards

Exam code: 7405

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What does K_p represent in a gaseous equilibrium?

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What does K_p represent in a gaseous equilibrium?
K_p is the equilibrium constant for a gaseous reaction expressed in terms of the partial pressures of the reactants and products at equilibrium. It is constant at a given temperature.
Write the K_p expression for: N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)
K_p = p²(NH₃) / (p(N₂) × p³(H₂))
Units: kPa² / (kPa × kPa³) = kPa^-2
True or False?
Solids and liquids are included in the K_p expression.
False.
Solids and liquids are omitted from the K_p expression. Solids do not have a partial pressure; liquids have a constant vapour pressure. Their contribution is incorporated into the value of K_p.
In a K_p expression, square brackets .......... be used because they denote .......... . Instead, use the symbol .......... to denote partial pressure.
In a K_p expression, square brackets must not be used because they denote concentration. Instead, use the symbol p (or pp) to denote partial pressure.
What is the only factor that changes the value of K_p for a given reaction?
Temperature. Changes in pressure, concentration or the addition of a catalyst do not alter K_p.
True or False?
For the reaction:
2SO₂ (g) + O₂ (g) ⇌ 2SO₃ (g)
The K_p expression is K_p = $\frac{p^{2} (S O_{3})}{p^{2} (S O_{2}) \times p (O_{2})}$ and the units are kPa^-1.
True.
For the reaction:
2SO₂ (g) + O₂ (g) ⇌ 2SO₃ (g)
The K_p expression is K_p = $\frac{p^{2} (S O_{3})}{p^{2} (S O_{2}) \times p (O_{2})}$ and the units are kPa^-1.
For the reaction N₂O₄ (g) ⇌ 2NO₂ (g), what are the units of K_p if pressure is measured in kPa?
K_p = p²(NO₂) / p(N₂O₄)
Units = kPa² / kPa = kPa
What is the partial pressure of a gas in a mixture?
The 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.
The mole fraction of a gas = number of moles of that gas ÷ .......... . The partial pressure of a gas = mole fraction × .......... .
The mole fraction of a gas = number of moles of that gas ÷ total number of moles of all gases. The partial pressure of a gas = mole fraction × total pressure.
True or False?
The sum of the mole fractions of all gases in a mixture always equals 1.
True.
The sum of all mole fractions must equal 1. Similarly, the sum of all partial pressures must equal the total pressure — both are useful checks in K_p calculations.
List the steps needed to calculate K_p when given initial moles, an equilibrium mole of one gas and the total pressure.
1. Use an ICE table to find equilibrium moles of all gases.
2. Calculate the total equilibrium moles.
3. Calculate mole fractions.
4. Calculate partial pressures (mole fraction × total pressure).
5. Write the K_p expression.
6. Substitute and calculate K_p, including units.
When substituting into a K_p expression, each partial pressure must be raised to the power of its .......... coefficient. Mole fractions .......... be substituted directly — you must first convert them to .......... .
When substituting into a K_p expression, each partial pressure must be raised to the power of its stoichiometric coefficient. Mole fractions must not be substituted directly — you must first convert them to partial pressures.
True or False?
If the K_p expression has equal numbers of mole terms on the top and bottom (e.g. H₂ + I₂ ⇌ 2HI), K_p has no units.
True.
When the total number of moles of gas is the same on both sides, the pressure units cancel and K_p is dimensionless.
Why is K_p preferred over K_c for reactions involving gases?
Pressure is easier to measure directly for gases than concentration. K_p uses partial pressures, which are readily obtained from total pressure and mole fractions.
How does increasing temperature affect K_p for an exothermic forward reaction?
Increasing temperature shifts the equilibrium to the left (to counteract the added heat). The ratio of products to reactants decreases, so K_p decreases.
For an endothermic forward reaction, increasing temperature shifts equilibrium to the .......... , meaning the ratio of products to reactants .......... and K_p .......... .
For an endothermic forward reaction, increasing temperature shifts equilibrium to the right, meaning the ratio of products to reactants increases and K_p increases.
True or False?
Changing the pressure of a gaseous equilibrium changes the value of K_p.
False.
Pressure changes shift the position of equilibrium to a new position that restores K_p, but the value of K_p itself remains unchanged. Only temperature alters K_p.
State Le Chatelier's Principle as applied to gaseous equilibria.
A dynamic equilibrium is the state reached in a closed system when the forward and reverse reactions proceed at equal rates, so that the concentrations of reactants and products remain constant over time.
What effect does adding a catalyst have on the value of K_p and on the position of equilibrium?
A catalyst has no effect on K_p or on the position of equilibrium. It speeds up both the forward and reverse reactions equally, causing equilibrium to be reached faster but not shifting where it lies.
Increasing pressure on a gaseous equilibrium shifts the position towards the side with .......... moles of gas. This change .......... affect the value of K_p.
Increasing pressure on a gaseous equilibrium shifts the position towards the side with fewer moles of gas. This change does not affect the value of K_p.

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