topic 11: equilibrium II
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what is the dynamic equilibrium
when forward and backward reactions are occurring at equal rates. the concentrations of reactants and products stays constant and the reaction is continuous
what is the general formula of Kc (use the equation mA + nB —> pC + qD
[ C]p [D]q - m,n,p,q are the stoichiometric balancing numbers - A,B,C,D stand for the chemical formula
what is the Kc
equilibrium constant
what is the unit used in Kc for the concentration
mol dm-3
what is the equation to work out the moles of reactants at equilibrium
moles of reactant at equilibrium = initial moles – moles reacted
what is the equation to work out the moles of products at equilibrium
moles of product at equilibrium = initial moles + moles formed
CH3CO2H +CH3CH2OH <—-> CH3CO2CH2CH3 + H2O PRACTICAL: part 1 - preparing the equilibrium mixture
• use burettes to prepare a mixture in boiling tube of carboxylic acid, alcohol, and dilute sulfuric acid • swirl and bung tube. leave the mixture to reach equilibrium for one week
CH3CO2H +CH3CH2OH <—-> CH3CO2CH2CH3 + H2O PRACTICAL: part 2 - titrating the equilibrium mixture
• rinse a 250 cm3 volumetric flask with distilled water. use a funnel to transfer the contents of the boiling tube into the flask. rinse the boiling tube with water and add the washings to the volumetric flask • use distilled water to make up the solution in the volumetric flask to exactly 250 cm3. stopper the flask, then invert and shake the contents thoroughly • use the pipette to transfer 25.0 cm3 of the diluted equilibrium mixture to a 250 cm3 conical flask • add 3 or 4 drops of phenolphthalein indicator to the conical flask • set up the burette with sodium hydroxide solution • add the sodium hydroxide solution from the burette until the mixture in the conical flask just turns pink. record this burette reading in your table • repeat the titration until you obtain a minimum of two concordant titres
PRACTICAL: why does the pink colour of the phenolphthalein in the titration fade after the end-point of the titration has been reached
because the addition of sodium hydroxide may make the equilibrium shift towards the reactants
PRACTICAL: what does the sodium hydroxide react with
the sulfuric acid catalyst and any unreacted carboxylic acid in the equilibrium mixture
what is the partial pressure of a gas in a mixture
the pressure that the gas would have if it alone occupied the volume occupied by the whole mixture
how do you work out the mole fraction of a gas
number of moles of a gas/ total number of moles of all gases
who do you work out the partial pressure of 1 gas
= mole fraction x total pressure
what is the unit of pressure
atm
work out the expression of Kp for the equation N2(g) + 3H2(g) <—> 2 NH3(g)
p(N2) p(H2)^3
the substances in what state should be included in the Kp expression
only gases
what is the Kc value if the reaction doesn’t happen
Kc < 10^-10
what is the Kc value if reactants are predominate in an equilibrium
Kc is approximately 0.1
what is the Kc value when equal amounts of reactants and products
Kc = 1
what is the Kc value when products are predominate in equilibrium
Kc is approximately 10
what is the Kc value when the reaction goes too completion
Kc > 10^10
as ΔStotal increases,, what happens to the equilibrium constant
increases
what is the equation of ΔS
ΔS = R lnK
what is the effect of temperature on position of equilibrium and Kp
Both the position of equilibrium and the value of Kc or Kp will change it temperature is altered - if temperature is increased the reaction will shift to oppose the change and move in the backwards endothermic direction - the position of equilibrium shifts left - the value of Kc gets smaller as there are fewer products