equilibria

what does forward reaction look like on graph?

initially reactants used up quickly, but then slow as their concentration drops
- downward slope

what does backward reaction look like on graph?

initially reactants reformed slowly but then speed up as concentration of products increases

dynamic equilibrium?

rate of forward reaction = rate of backward reaction
in a closed system, with constant concentration of products + reactant

le chateliers principle?

if a reaction is subjected to a change in pressure, temp or conc, the position ion equilibrium will shift to counteract the change

homogenous equilibria?

all reactants + products in the same phase

what happens when one side’s conc. is increased?

system is in dynamic equilibrium
increase in conc. causes equilibrium to shift to counteract the change
equilibrium shifts to other side to reduce the conc. of the increased side

what happens when pressure is increased?

equilibrium shifts to try reduce it
shifts to side with fewest no. gas particles to reduce pressure

what happens when temp increases?

equilibrium shifts to try decrease temp
shifts in the endothermic direction (backward)

effect of catalyst on equilibrium?

no effect
speeds up rate of forward + backward reaction equally
so speedy up rate at which equilibrium is reached, but has no effect on yield

what is the equation to form ethanol?

C2H4 + H20 ⇌ C2H5OH

what is the standard enthalpy of formation for ethanol in gas phase?

-46KJ/jmol

what conditions is ethanol formed under?

60 atmospheres
300^oC
phosphoric acid catalyst

why is a low temp used to make ethanol?

forward reaction is exothermic, so a low temp allows equilibrium to shift to the right
producing more ethanol

what is the bad thing about using a low temp for formation of ethanol?

low temp = slow rate
300^oC is a compromise between yield and rate

why is a high pressure used?

causes equilibrium to shift to the right, so more ethanol is produced
as there are less moles on the right
increasing rate

what is bad about using a high pressure for formation of ethanol?

more expensive as thicker, more robust pipes required
so 60 atmospheres is a compromise between yield and cost

how is Kc calculated?

product of concentration of products each to the power of their molar values/that of reactants

what are molar values?

big number in front of species in an equation

how to calculate units of the Kc value of an equation?

write out the units of each value on top + bottom (write each out according to molar number eg. value², is written twice)
and cancel out like terms

acronym to calculate Kc?

ICE (for no. moles)

what affects Kc value?

temp; value only valid for one temp
changing temp changes equilibrium concs. which changes Kc

how is Kc value increased?

when temp change shifts equilibrium to the right

how is Kc value decreased?

when temp change shifts equilibrium to the left

what doesn’t affect Kc?

concentration
catalysts