Chemical equilibrium calculations

when is chemical process gone to completion

until one of the reactants has been used up

what is a forward reaction

a reaction that continues until the reactants are completely used up

in the form of reactants to products and normal arrows (→)

what is a reverse/ reversible reaction

as the products form they are able to break down and reform the answers

in the form of ⇌ 

what does the equilibrium arrow imply

that both the forward and reverse reactions are occurring at the same time

when has a system reached equilibrium

when the rate of the forward reactions equals the rate of the backward reaction

what happens to the amount of reactants and products during equilibrium

they remain constant as the continue going but at the same rate

how can an equilibrium system be recognised

by the presence of both reactants and products

when a chemical equilibrium occurs what can be established

* reaction is dynamic (moving forward and backwards)
* the rates of forward and backward reactions are equal
* react and products are always present
* equilibrium can be approached from either side
* the concentration of reactants and products remains constant

reaction quotient formula

what is reaction quotient

it measures the relative amount of products and reactants present during a reaction at a particular point in time

example of reaction quotient formula with a reversible reaction

pg. 245

when does Q = Kc

when a system is at equilibrium

as the concentration becomes constant and the ratio becomes fixed at any given temperature this is when Kc occurs as the equilibrium has become constant

equilibrium constant formula

what is the unit for equilibrium constant

mol L-1

key points for equilibrium constant

* no solids or liquids are involved in the expression
* products always over reactants
* when their is a number in front of the compound, write it as an indices outside of the bracket

what is Kc (equilibrium constant) affected by and not affected by

NOT:

* changes in concentration
* changes in pressure
* addition of a catalyst

IS:

* change in temperature

what is an equilibrium constant useful for

determining how far a reaction has proceeded

when does Kc favour products

if Kc is greater than 1 at equilibrium, there will be more products than reactants (favour products)

when does Kc demonstrate the reaction has gone to completion

if Kc is very large and is greater than 10 to the power of 20 (1e+20)

when does Kc favour reactants

if Kc is less than 1 at equilibrium, there will be less products than there are reactants (favour reactants)

when has Kc demonstrated the reaction has not occurred

if Kc is very small and is less than 10 to the power of -20 (1e-20)

what does Kc give no information about

the rate of reaction (how fast the reaction proceeds)

difference between Kc and Q

Kc is used for a system at equilibrium, whereas Q can be calculated for any system

steps to calculating an equilibrium constant

1. put into Kc form (product/reactant)
2. substitute the equilibrium concentrations into the expression
3. use calculator to calculate the answer

put a photo example pg. 247

steps to calculating the concentration at equilibrium

1. substitute the value of Kc and the equilibrium concentration of known compounds
2. rearrange expression to make the missing concentration the subject
3. then calculate (don’t forget to square root if needed)

put photo example pg. 247

at equilibrium what is the ratio of the reactants and products like

the ratio is constant for a given temperature during equilibrium

what is the ratio of reactants and products prior to equilibrium

although both reactants and products will be present in the system, the ratio between reactants and products will not be the same for Kc

how to determine if a system is at equilibrium

the measured concentrations can be substituted in the Q expression and the results can be compared to Kc

as if Q and Kc are not equal, the system is not at equilibrium

steps to determining whether a system is at equilibrium

1. substitute all concentrations of the compounds into the Q formula (same one as Kc)

2. calculate the value of Q from the equation and compare to Kc value given

3. if the values are not the same the system is not at equilibrium

4. state if the value is higher, lower or equal

what does the size/extent of Kc indicate

• if it is more than 1 → favours the product (more products produced), moves to the right, forwards reaction

• if it is less than 1 → favours the reactants (more reactants produced), moves to the left, backwards reaction

• around 1 → equilibrium

  the larger the value of Kc, the larger the product

what to do if there aren’t all values of Kc

rearrange values

what does the value of Kc demonstrate

how long the reaction has left

what is kC

the ratio of products over reactants

what does it mean if the value of Kc increases

it indicates that the ratio of product to reactants increases

what does it mean by the extent of a reaction in terms of what the value of Kc demonstrates

• how many products and reactants

• what side the equilibrium favours

use the value and size of Kc to describe