Chemical Equilibria, Le Chatlier’s Principle and Kc

what is a reversible reaction? how are they represented?

when a reaction can proceed in both directions simultaneously, at the same rate, represented by this symbol:

if a reverse reaction is favoured, we will never run out of products, this helps explain why (if a reaction is reversible) the ? yield will always be less than the ? yield

this helps explain why the actual yield will always be less than the theoretical yield

what is rate of reaction?

how quickly reactants are turned to products in a reaction in a specific time period

rate of reaction graph? y and x axis?

starts at origin: vol. of reactants is high (rate is high), then it slows as reactants are used up (rate decreasing), when it levels off it shows the reaction is finished

how do the axis differ for a reversible reaction on a graph?

instead of volume on the y axis, we use concentration;

there are 2 curves, one for products and one for reactants

when forward reaction is favoured, how are the graph curves for each presented?

why do concentration of reactants not decrease to 0 when the forward reaction is favoured?

the increase in conc. of products, increases rate of the backwards reaction, but they will level off (happens until conc. becomes constant)

what is dynamic equilibrium?

where both forward reaction and backward reactions are occurring simultaneously, at the same rate, in a closed system with a fairly constant concentration

what is a closed system in terms of reactions?

no reactants are added, no products are lost

e.g. conc. of reactants and products remain the same

what is static equilibrium?

state where bodies are at rest

what is dynamic equilibrium?

state where bodies are moving at a constant velocity/rate

what does Le Chatelier's Principle state?

if an external condition is changed the equilibrium will shift to oppose the change

what is Le Chateliers principle used to work out?

how changing external conditions affect the position of equilibrium

do exothermic reactions make the system hotter or cooler?

hotter: energy is released

chemical energy -> heat energy = hotter

do endothermic reactions make the system hotter or cooler?

cooler: energy is taken in

heat energy -> chemical energy = colder

how would equilibrium oppose an increase in temperature when the forward reaction is endothermic and backwards is exothermic?

equilibrium position moves to oppose increase in temperature, by moving in the endothermic direction, increasing yield of products

does endothermic give a negative or positive enthalpy?

positive

does exothermic give a negative or positive enthalpy?

negative

how would equilibrium oppose increase in pressure?

equilibrium will shift to the side with less gas molecules to oppose increase in pressure

how does equilibrium oppose increase in concentration?

equilibrium moves to oppose the increase in conc., by moving in the other direction to use it up, decreasing conc.

what do square brackets [ ] mean?

concentration

Kc equation?

conc. of products/conc. of reactants

what is Kc?

equilibrium constant

acronym to remember how to find concentration for kc?

ICE

Initial moles

Change in moles

Equilibrium - amount of moles at equilibrium

concentration, moles and volume equation?

concentration = moles/volume

concentration, mass and volume equation?

concentration = mass/volume

indice rules for:

a) multiplication - a^b x a^c

b) division - a^b / a^c

c) brackets - (a^b)^c

d) power of 0 - a^0

e) dividing by number with an indice 1/a^b

a) add indices - a^2 x a^3 = a^5

b) subtract indices - a^2/a^3 = a^-1

c) times indices - (a^2)^3 = a^6

d) always equal 1 - 5^0 = 1

e) becomes negative - 1/a^3 = a^-3

true or false: Kc is affected by concentration and catalysts

false

why doesnt changing conc. affect Kc?

equilibrium constant, Kc, is the ratio of rate constants for the forward and reverse reactions.

Since concentration doesn't directly appear in the rate constants, it doesn't affect Kc.

Even if concentrations change due to volume adjustments or gas additions, the rate constants remain constant, preserving Kc

why doesnt a catalyst affect Kc?

the ratio of product concentrations to reactant concentrations ([products] to [reactants]) remains unchanged, so Kc remains constant

how is Kc affected by temperature?

as the equilibrium position changes it results in different concentrations of reactants and products,

in endothermic reactions: Kc increases

exothermic: Kc decreases