The Equilibrium Constant

Question 1

what is a homogeneous system

Answer 1

a system in which all components are in the same phase

Question 2

what does them being in the same phase mean

Answer 2

they are all in one and the same state of matter (gas / liquid etc)

Question 3

what is a heterogeneous system

Answer 3

a system in which at least two different phases are present

Question 4

what phase would H2(g) + I2(g) = 2HI(g) be in

Answer 4

gas phase

Question 5

what phases would CaCO3(s) = CaO(s) + CO2(g) be in

Answer 5

gas and solid phases

Question 6

what is the symbol for the equilibrium constant

Answer 6

Kc

Question 7

if an equilibrium mixture contains substances A B C D related by the equation aA + bB = cC + dD, what would the equation for its equilibrium constant look like

Answer 7

Kc = ([C]^c [D]^d) / ([A]^a [B]^b)

Question 8

what are the two important rules you need to know when writing out equilibrium constant equations

Answer 8

• the products go on the top

- the coefficients of the balanced (stoichiometric) equation are the powers of the relative substances

Question 9

what numerical value would actually be substituted in the for this equilibrium constant equation

Answer 9

the concentration of the substances (mol dm-3)

Question 10

what is this general equation known as

Answer 10

equilibrium law

Question 11

what is important to remember about the concentrations of the substances youre placing in the equation

Answer 11

they are the concentrations of each substance after the system has reached equilibrium

Question 12

Kc is essentially a ratio of concentrations, but why could it have units and why can they change

Answer 12

• because they depend on the powers (coefficients of the balanced equation) on the substances
• if there were no powers then the units would cancel out to just give a ratio
• but if you had uneven powers on the top or bottom, you would have units
• the difference between the powers on the top and bottom determine the units

Question 13

what would the equilibrium constant equation be for H2(g) + I2(g) = 2HI(g)

Answer 13

Kc = [HI(g)]^2 / ([H2(g)] [I2(g)]

Question 14

if that reaction had a Kc of 50, what would the value of Kc be for 2HI(g) = H2(g) + I2(g)

Answer 14

• if Kc for 2HI / H2 * I2 = 50, you can say 2HI = 50 and H2 * I2 = 1
• so if they have swapped places, it is now H2 I2 on top and 2HI at the bottom
• so the value would be 1 / 50 = 0.02

Question 15

what would the value of Kc be for 1/2 H2 + 1/2 I2 = HI

Answer 15

• all the powers are half of the original equation
• so the whole Kc has just been rooted by 2
• so the root of 50 = 7.07

Question 16

what is different about the Kc equations for heterogeneous systems and why

Answer 16

• you dont include substances that dont have concentrations
• like water and solids
• basically just dont put them in the equation

Question 17

what is the Kc equation of H2O(l) = H2O(g)

Answer 17

Kc = [H2O(g)]

Question 18

why dot you include water and solids into these

Answer 18

because their concentrations are inherently constant for any temp

Question 19

if you had Kc = [N2O4(g)] / [NO2(g)]^2, what would be the effect of increasing the concentration of nitrogen dioxide

Answer 19

• the denominator would increase

- so Kc would decrease

Question 20

where would the equilibrium have to shift if you wanted to restore Kc’s value and why

Answer 20

• it would have to shift to the right
• because the equation for this reaction is 2NO2 = N2O4
• and shifting it to the right would increase the rate of reaction of the right
• therefore increasing the concentration of N2O4 and decreasing NO2’s
• leading to an increase in the numerator and denominator of the Kc equation
• therefore increasing its value