3.1.6 Chemical Equilibria, Le Chateliers Principle and Kc Flashcards
What is dynamic equilibrium?
It is the stage in a reversible reaction where the rate of the forward reaction is equal to the rate of the backward reaction in a closed system.
What does Le Chatelier’s Principle state?
It states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium shifts to counteract the change to reestablish an equilibrium.
What is the position of equilibrium?
It shows the concentrations of the reactants and products when the rates of the forward and reverse reactions are equal.
What is an enthalpy change?
The change in heat energy under a constant pressure.
If the bond enthalpy is negative, the reaction is __________.
Exothermic
If the bond enthalpy is positive, the reaction is ___________.
Endothermic.
What is the yield in dynamic equilibrium?
The higher the concentration of products on the favoured side of the reaction, the higher the percentage yield.
What effect does increasing/decreasing the pressure have on dynamic equilibrium?
Increase - equilibrium will shift to the side with fewer gas molecules.
Decrease - equilibrium will shift to the side with more gas molecules.
What effect does increasing/decreasing the temperature have on dynamic equilibrium?
Increase - equilibrium will shift in the endothermic direction.
Decrease - equilibrium will shift in the exothermic direction.
What effect does increasing/decreasing the concentration of a substance have on dynamic equilibrium?
Increase - equilibrium will shift to the side that uses up the added substance.
Decrease - equilibrium will shift to the side that creates more of the removed substance.
What is Kc?
An equilibrium constant (Kc) is a value that represents the relative concentrations of reactants and products in a chemical reaction at equilibrium.
How do we write the Kc expression?
In the reaction:
aA + bB ⇌ cC + dD
Kc = [C]^c [D]^d | products
÷ | ÷
. [A]^a [B]^b | reactants
[A] = concentration of A in moldm:3
a = number of moles of A
[ ] must be used
How do you work out Kc units?
• Work out the expression for Kc.
• Replace reactants/products with moldm^-3.
• Cancel out the top and bottom where possible.
• Whatever’s remaining = the units.
How do you find moles at equilibrium
• Create an ICE table (initial, change, equilibrium) with the products and reactants at the top.
• Write in the initial moles found at the start of the reaction.
• Write in any moles you’re told at
equilibrium.
• Find the change in moles.
• If you’re given the equilibrium moles of reactant, subtract the initial moles from the equilibrium moles.
• If you’re given the equilibrium moles of product, add the initial moles to the equilibrium moles.
• Apply the change in moles using molar ratios across the other unknowns.
How do you calculate Kc values?
• Construct the Kc expression using molar ratios as the indices.
• Insert the given concentrations into the equation .
• Rearrange if needed for the unknown value.
• Find the unknown value (do any extra data handling if needed).
