3.8 Equilibrium Constants

Question 1

dynamic equil

Answer 1

dyn equil occurs in a reversible rctn when the reate of forward rctn equals the rate of backwards rctn, at equil the concs on rctntns & prods stay constant

Question 2

le Chatelier’s principle

Answer 2

when a rctn at equil is subjected to a change in conc remp or pressure, the position of equil will move to counteract the change

Question 3

diff bet Kp & Kc

Answer 3

Kc & Kp are both equil constants
Kc found using molar concs in calc whereas Kp uses partial pressures, if you are dealing w a rctn in which all sustances are gaseous easier to use Kp

Question 4

deduce an eqn for Kc

Answer 4

Kc = [prod]^mol/[rctnt]^mol

Question 5

why does Kc change when temp of a reversible rctn in a closed system is changed?

Answer 5

The Kc value is only valid for a certain temp, when the temp changes the position of equil shifts so the equil concs of the products & rctnts changes leading to change in Kc

Question 6

consider a reversible rctn where the forward rctn is endothermic, how will increasing temp affect Kc??

Answer 6

increased temp favours forward rctn, position of equil shifts towards prod, this means conc of prod will increase and conc of rctnts decrease leading to an increase in Kc as numerator on fraction is larger in calc

Question 7

consider a reversible rctn where forward rctn is exothermic how will increasing temp affect Kc??

Answer 7

increased temp favours backwards rctn, position of equil move towards rctnts meaning conc of rctnts increase and conc of prod decrease leading to a decrease in Kc as denominator of Kc calc is larger

Question 8

explain why a compromised temp of 450C is used in Haber proc?? N2(g) + 3H2(g) -><- 2NH3(g) deltaH=-46.2kJmol-1

Answer 8

since deltaH is -ve the forward rctn exothermic so low temps req to shift equil towards prods to increase prod yield, but temp cannot be too low as as this will lead to very slow rate of rctn so compromised temp used so forward rctn favoured but rate of rctn is also relatively fast

Question 9

how does changing conc of rctnt/ prod affect value of Kc

Answer 9

changing conc of a rectnt =/ prod no effect on Kc

Question 10

how does a catalyst affect value of Kc??

Answer 10

A catalyst has no effect on Kc, The catalyst will speed up the forward & backwards rctn at the same rate so the ratio of prod & rctns will remain the same

Question 11

consider a reversible rctn where the backwards rctn is exothermic, how could u increase value of Kc??

Answer 11

Kc will increase if the conc of prod increases the forward rctn is endothermic & so to favour this direction & hence increase the conc of prod the temp needs to be increased

Question 12

define partial pressure and total pressure in relation to a mixture of gases

Answer 12

partial pressure-> the pressure exerted by an individual gas in a mixture
total pressure-> sum of all partial pressures of the individual gases in the mixture

Question 13

define mole fraction

Answer 13

proportion of a gas mixture that is made up of a particular gas

Question 14

give the formula to calc mole fraction

Answer 14

mole fraction of gas X= number of moles of gas X/total number of moles of gas in mixture

Question 15

give formula to calc partial pressure of a gas??

Answer 15

partial pressure of gas X = (mole fraction of gas X) x (total pressure of mixture)

Question 16

units for partial pressure??(Kp)

Answer 16

kPa

Question 17

eqn for Kp of following gaseuos eqn: A + 3B -><- D + 2E

Answer 17

Kp= (pD)(pE)^2/ (pA) (pB)^3

Question 18

why does Kp change when the temp of a closed system reversibe rctn is changed??

Answer 18

Kp value is only valid for a certian temp when temp changes, position of equil shifts changing the mole fractions of gases present, this affects their partial pressures so Kp changes

Question 19

consider a reversible rctn where forward rctn exothermic, how will decreasing temp affect Kp??

Answer 19

if temp decreased forward exothermic rctn favoured position of equil move towards prod, meaning molar frac & partial press of prod increase & molar frac and partial press of rctnts dcrease leading to an increase in Kp as in calc for Kp the numerator is larger

Question 20

how will Kp be affected if temp is increased in an exothermic rctn??

Answer 20

forward rctn exothermic, if temp increased the equil shifts to left(in endothermic direction) t counteract change meaning less prod formed so partial press of prod decreases while partial press of rcnts increase causing Kp to decrease

Question 21

significance of the magnitude of an equil constant??

Answer 21

magnitude of the equil constant indicates where the position of equil lies, larger the value of Kp or Kc the further towards the right(towards prod) the equil position lies