Equilibrium II (Edexcel A Level Chemistry): Flashcards
Exam code: 9CHO
1/270Still learning
Know0
Define Kp.
Was this flashcard helpful?
Cards in this collection (27)
Define Kp.
Kp is the equilibrium constant expressed in terms of the partial pressures of reactants and products in a gaseous equilibrium. Its value is constant at constant temperature.
Which substances are excluded from a Kp expression?
Pure solids and pure liquids are excluded from Kp expressions. Only gaseous species appear in the expression.
Write the Kp expression for:
N2 (g) + 3H2 (g) ⇌ 2NH3 (g)
The Kp expression is:
Kp = (pNH3)2 ÷ (pN2 × (pH2)3)
The stoichiometric coefficients become the powers on each partial pressure term.
Define partial pressure.
Partial pressure (p) is the pressure that a gas in a mixture would exert if it alone occupied the same volume. It equals the mole fraction of the gas multiplied by the total pressure.
The partial pressure of a gas equals its .......... multiplied by the .......... pressure of the mixture.
The partial pressure of a gas equals its mole fraction multiplied by the total pressure of the mixture.
Write the Kp expression for:
CaCO3 (s) ⇌ CaO (s) + CO2 (g)
Kp = pCO2
CaCO3 and CaO are pure solids, so they are omitted from the Kp expression. Only the gaseous CO2 appears.
True or False?
The units of Kp are always Pa.
False.
The units of Kp depend on the form of the equilibrium expression. If the powers of the partial pressures cancel, Kp is unitless. Otherwise the units are Pan (or kPan), where n is the net power in the expression.
Define mole fraction.
Mole fraction (x) is the ratio of the number of moles of one component to the total number of moles in the mixture. It has no units.
What is the only factor that changes the value of Kp?
Temperature is the only factor that changes Kp. Changes in pressure, concentration, or the addition of a catalyst do not alter its value.
Define Kc.
Kc is the equilibrium constant expressed in terms of the molar concentrations of reactants and products at equilibrium. Its value is constant at constant temperature.
How do you calculate the concentration of a species from moles and volume?
Concentration (mol dm-3) = moles ÷ volume (dm3)
This step is always needed before substituting values into a Kc expression.
What is the purpose of an ICE table in equilibrium calculations?
An ICE table (Initial, Change, Equilibrium) tracks the molar quantities of reactants and products. It uses the stoichiometric ratio to determine equilibrium concentrations when only initial and one equilibrium value are given.
To calculate Kp from moles and total pressure, first calculate the .......... of each gas, then multiply by the .......... pressure to get partial pressures.
To calculate Kp from moles and total pressure, first calculate the mole fraction of each gas, then multiply by the total pressure to get partial pressures.
In the reaction 2SO2 (g) + O2 (g) ⇌ 2SO3 (g), what are the units of Kp if partial pressures are in Pa?
Kp = Pa2 ÷ (Pa2 × Pa) = Pa-1
The net power is –1, so the units are Pa-1.
True or False?
Kc always has units of mol dm-3.
False.
The units of Kc depend on the equilibrium expression. If the total powers in the numerator and denominator are equal, the units cancel and Kc is dimensionless.
In a Kp calculation, how do you find the mole fraction of a gas?
Mole fraction (x) = moles of that gas ÷ total moles of all gases in the mixture.
Partial pressure is then found by multiplying x by the total pressure P.
True or False?
If a reaction has reached equilibrium, adding more reactant will change the value of Kc.
False.
Adding more reactant shifts the position of equilibrium but does not change Kc. The equilibrium shifts to restore the same ratio of products to reactants. Only a change in temperature alters Kc.
Why is significant figure discipline important when reporting Kc or Kp?
The final answer should match the fewest significant figures used in the data given in the question.
Using more significant figures than justified by the data overstates the precision of the result.
What is the equilibrium position?
An equilibrium position is the relative concentrations (or partial pressures) of reactants and products at which a system is at equilibrium. It can shift in response to changes in concentration, pressure, or temperature, even when K is unchanged.
Which factor is the ONLY one that changes the numerical value of Kc or Kp?
Temperature is the only factor that changes Kc or Kp.
Changes in concentration, pressure, or the presence of a catalyst shift the position of equilibrium but leave the value of the equilibrium constant unchanged.
True or False?
A catalyst increases the value of Kc by speeding up the forward reaction more than the reverse reaction.
False.
A catalyst speeds up both the forward and reverse reactions equally. It does not change the value of Kc or the position of equilibrium — it only allows equilibrium to be reached faster.
How does increasing temperature affect Kp for an exothermic reaction?
For an exothermic forward reaction, increasing temperature shifts equilibrium to the left (towards reactants).
The ratio of products to reactants decreases, so Kp decreases.
For an exothermic reaction, decreasing temperature shifts equilibrium to the .........., causing Kp to .......... .
For an exothermic reaction, decreasing temperature shifts equilibrium to the right, causing Kp to increase.
How does increasing temperature affect Kc for an endothermic reaction?
For an endothermic forward reaction, increasing temperature shifts equilibrium to the right (towards products).
The ratio of products to reactants increases, so Kc increases.
True or False?
Increasing the pressure of a gaseous equilibrium changes the value of Kp.
False.
Increasing pressure shifts the position of equilibrium (towards the side with fewer moles of gas) but does not change the value of Kp. The system adjusts its partial pressures to restore the same Kp.
Why does changing concentration not change Kc?
Adding a reactant or product shifts the position of equilibrium, but the system re-equilibrates until the ratio of product concentrations to reactant concentrations (each raised to their stoichiometric powers) returns to its original value.
The value of Kc is therefore restored and remains unchanged.
For the reaction:
2A (g) + B (g) ⇌ 2C (g) ΔH = +6.5 kJ mol-1
What change increases the value of Kp?
Increasing the temperature increases Kp.
The forward reaction is endothermic (ΔH positive), so higher temperature favours product formation, raising the ratio of products to reactants and therefore increasing Kp.
Sign up to unlock flashcards
or