chapter 19- equilibrium constants

what is Kc

the terms used for equilibrium concentration

how is Kc written

[products]/[reactants]

the larger the Kc value….

the further equilibrium lies towards the products

how are Kc units calculated

• write all the units out

• cancel out from the top and bottom

more units on the top

all of the signs stay the same

more units on the bottom

the signs on the units flip (eg from moldm^-3 to dm³mol^-1)

two types of equilibria

homgenous

heterogenous

what is a homogenous equilibrium

contains equilibrium species that all have the same state or phase (eg, are all gaseous)

what is a heterogenous equilibrium

contain equilibrium species that have different states or phases

important to remeber about hetergeous equilibria

• solids and liquids are omitted because they are essentially constant

what are the only states used in Kc

gaseous and aqueous

how could you experimentally find Kc in a lab

• combine ethanoic acid and ethanol (known mols) and a HCl catalyst and add a stopper

• add HCl to a seperate conical flask and a stopper for a control

• allow both to reach equilibrium

• titrate the mixture with a known conc of NaOH

• titrate the HCl with a known conc of NaOH

• determines the amount of acid catalyst added

what is Kp

used for equilbria involving gases

why is Kp used instead of Kc for gases

pressure is easier to determine than concentration

what is a mole fraction

the proportion by volume to the total volume of gases in a gas mixture

equation for mole fraction

number of moles of A/ total number of moles

what is a partial pressure

the contribution that the gas makes towards the total pressure

partial pressure equation

partial pressure of A= mole fraction of A x total pressure

how do you write Kp

with a p in front of each compound

appropriate units for Kp

KPa, Pa, atm

which states can be used for Kp

ONLY gases

K=1

equilibrium lies exactly between the products and the reactants

K=100

equilibrium lies far towards the products

K= 1×10^-2

equilibrium lies far towards the reactants

how does a temperature rise affect exothermic reactions

the equilibrium constant will decrease so the equilibrium yield decreases

why does a increase in temp shift exothermic left

Kp decreases and so the system is no longer at equilibrium

the ratio is now greater than Kp (so lies towards the products)

• reactant partial pressure must increase

• product partial pressure must decrease

• equilibrium will now shift left

how does a temperature rise affect endothermic reactions

the equilibrium constant will increase so the equilibrium yield increase

why does a increase in temp shift endothermic right

Kp increases and so the system is no longer at equilibrium

the ratio is now less than Kp ( and so lies towards the reactants)

• the partial pressure of products must increase

• the partial pressure of reactants must decrease

• equilibrium will now shift right

how do concentration changes affect equilibrium

• they change the value of Kc (if put into the equation)

• if it decreases Kc then product concentration must increase

• if it increases Kc then reactant concentration must increase

if there are fewer moles of gaseous products

• equilibrium shifts right

• ratio<K

• products increase, reactants decrease

if there are more moles of gaseous products

• equilibrium shifts left

• ratio>K

• products decrease, reactants increase

how does a catalyst affect equilibrium constant

• they affect the RATE

• catalysts speed the forward and reverse proportionately so doesnt change the equilbrium position

what methods can be used to measure equlibrium

• colour change

• pH probe

• titration

what is the relationship between concentration and its partial pressure

theyre proportional

how to find minimum feasible temp

enthalpy change/ entropy change