Equilibrium constant Kp for homogeneous systems

What is the total pressure in a reaction?

The sum of all the individual pressures (partial pressures) of gases.

How do you write the partial pressure of x in a Kp expression?

(p_x)

Reaction: A →← B + C

How would you work out the partial pressure of A when you know the total pressure and the partial pressure of C?

1) Products B and C are in a 1:1 ratio, as they are both products that means that the partial pressure of B is equal to the partial pressure of C.

2) Add up those two partial pressures and subtract them from the total pressure, that value is the partial pressure of A.

True or false? In a reaction at equilibrium you cannot equate the mole fractions on the reactants side to the products side as being the same.

True.

What does mole fraction mean?

The proportion of a specific gas in a gas mixture.

How do you calculate the mole fraction of a gas?

1) Work out the total moles of gas in the reaction mixture (either given the actual values or can use the balanced symbol equation for the reaction).

2) The moles of the gas over the total moles of gas in the reaction mixture is the mole fraction of the gas.

Give an equation for calculating the mole fraction of a gas.

Mole fraction of a gas = number of moles of gas / total number of moles of gas in the mixture

Give an equation for calculating partial pressure with mole fraction and total pressure.

Partial pressure of gas = mole fraction of gas x total pressure

How do reactions end up with more total moles in the end than were put into the reaction?

Substances can get broken down into smaller molecules, moles doesn’t work like mass, moles don’t depend on the size of the molecule just how many molecules there are so overall moles can increase in a reaction.

What does Kp stand for?

The gas equilibrium constant.

Reaction: aA (g) + bB (g) →← cC (g) + dD (g)

What would the Kp expression for this reaction be?

Kp = (p_C)^c (p_D)^d / (p_A)^a (p_B)^b

How do you work out Kp units?

The same way you work out Kc units but replace mol dm^-3 with kPa.

Give one factor that will affect the value of Kp.

Temperature.

True or false? The Kp value for a reaction will always be the same.

False. THE Kp VALUE FOR A REACTION WILL ONLY BE VALID AT ONE TEMPERATURE.

Why will changing temperature change the Kp value?

Changing temperatures changes pressures which will change the value of Kp.

Which reaction will be favoured if there is an increase in temperature?

The endothermic reaction (to oppose the increase in temperature).

Which reaction will be favoured if there is a decrease in temperature?

The exothermic reaction (to oppose the decrease in temperature).

True or false? The value of Kp is unaffected by any changes in pressure.

True.

True or false? Adding a catalyst to a reaction will decrease the value of Kp.

False. THE PRESENCE OF A CATALYST HAS NO EFFECT ON THE VALUE OF Kp.

How do you work out the change in moles in a reaction at equilibrium?

Use the balanced symbol equation for the reaction to get the mole ratio. Apply the mole ratio of the reaction to the change in moles, this gives you the change in moles.

How do you work out Kp for a reaction where all you know are the ICE moles.

1) Work out the mole fractions.

2) Replace a number for pressure (since you don’t have one) with P.

3) Substitute everything into the Kp expression.

4) P should cancel out meaning you can just use the mole fractions to work out Kp.

True or false? When you do not know the value of total pressure, so you replace it with P, you are only able to cancel out the Ps in the Kp expression if there are the same number of Ps on the numerator as on the denominator.

False. YOU ALWAYS CANCEL OUT P SO THAT YOU ARE JUST LEFT WITH THE MOLE FRACTIONS.