1.
(a)
The diagram below shows the effect of temperature and pressure on the
equilibrium yield of the product in a gaseous equilibrium.
10 MPa
30 MPa
50 MPa
Yield/%
Temperature
(i)
Use the diagram to deduce whether the forward reaction involves an
increase or a decrease in the number of moles of gas. Explain your answer.
Change in number of moles .............................................................................
Explanation ......................................................................................................
..........................................................................................................................
.
..........................................................................................................................
.
(ii)
Use the diagram to deduce whether the forward reaction is exothermic or
endothermic.
Explain your answer.
The forward reaction is
....................................................................................
Explanation ......................................................................................................
..........................................................................................................................
.
..........................................................................................................................
.
(6)
(b)
When a 0.218 mol sample of hydrogen iodide was heated in a flask of volume V
dm3, the following equilibrium was established at 700 K.
2HI(g)
H2(g) + I2(g)
The equilibrium mixture was found to contain 0.023 mol of hydrogen.
(i)
Calculate the number of moles of iodine and the number of moles of
hydrogen iodide in the equilibrium mixture.
Number of moles of
iodine..................................................................................
Number of moles of hydrogen
iodide.................................................................
..........................................................................................................................
.
(ii)
Write an expression for Kc for the equilibrium.
..........................................................................................................................
.
..........................................................................................................................
.
(iii)
State why the volume of the flask need not be known when calculating a
value for Kc.
..........................................................................................................................
.
..........................................................................................................................
.
(iv)
Calculate the value of Kc at 700 K.
..........................................................................................................................
.
..........................................................................................................................
.
..........................................................................................................................
.
..........................................................................................................................
.
(v)
Calculate the value of Kc at 700 K for the equilibrium
H2(g) + I2(g)
2HI(g)
..........................................................................................................................
.
..........................................................................................................................
.
(7)
(Total 13 marks)
2.
Acid X reacts with methanol to form ester Y according to the following equation.
CH2 COOH
CH2 COOH
acid X
+ 2 CH3 OH
CH2 COOCH3
CH2 COOCH3
+
2H2 O
H = –15 kJ mol –1
ester Y
A mixture of 0.25 mol of X and 0.34 mol of methanol was left to reach equilibrium in the
presence of a small amount of concentrated sulphuric acid. The equilibrium mixture thus
formed contained 0.13 mol of Y in a total volume of V dm3.
(a)
Using X to represent the acid and Y to represent the ester, write an expression for
the equilibrium constant, Kc, for this reaction.
(1)
(b)
Calculate the number of moles of X, the number of moles of methanol and the
number of moles of water in the equilibrium mixture.
Moles of X ..............................................................................................................
Moles of methanol ...................................................................................................
Moles of water .........................................................................................................
(3)
(c)
State why the volume V need not be known in calculating the value of Kc for the
reaction.
................................................................................................................................
................................................................................................................................
(1)
(d)
Calculate the value of Kc for this reaction and deduce its units.
Calculation .............................................................................................................
................................................................................................................................
................................................................................................................................
................................................................................................................................
Units of Kc ..............................................................................................................
................................................................................................................................
(3)
(e)
State the effect, if any, of increasing the temperature on the value of Kc
................................................................................................................................
(1)
(Total 9 marks)
3.
As a first step in the manufacture of nitric acid it has been suggested that nitrogen
monoxide, NO, can be formed from nitrogen and oxygen in a reversible reaction.
(a)
Write an equation for this reaction and deduce an expression for the equilibrium
constant, Kc
Equation......................................................................................................................
.
Kc ................................................................................................................................
(2)
(b)
The sketch graph below shows how the value of Kc for this reaction changes with
temperature.
Kc
Temperature
Use this graph to deduce whether the reaction is exothermic or endothermic.
Explain your answer.
.....................................................................................................................................
.....................................................................................................................................
(2)
(c)
The value of Kc for this reaction is 1 × 10–5 at 1500 K.
Explain the significance of this value for an industrial chemist interested in
manufacturing nitrogen monoxide by the direct combination of the elements.
.....................................................................................................................................
.....................................................................................................................................
(2)
(d)
When cooled, nitrogen monoxide reacts with oxygen to form gaseous nitrogen
dioxide, NO2, in a reversible reaction.
(i)
Write an equation for this reaction.
..........................................................................................................................
.
(ii)
State how an increase in pressure would change the position of the
equilibrium and the value of the equilibrium constant for this reaction.
Change in equilibrium
position.........................................................................
Change in equilibrium constant........................................................................
(3)
(Total 9 marks)
1.
(a)
(b)
(i)
Increase (if wrong no further marks in part (i)
higher P gives lower yield or moves to left
Eqm shifts to reduce P or eqm favours side with fewer moles
(ii)
Endothermic if wrong no further marks in part (ii)
increase T increases yield or moves to right
Eqm shifts to reduce T or eqm favours endothermic direction
(i)
Moles of iodine = 0.023
Moles of HI
= 0.172
If × 2 missed, max 1 in part (iv)
If wrong no marks in (i)
(ii)
Kc =
1
(iii)
V cancels in Kc expression
or no moles same on top and bottom of expression
or total moles reactants = moles products,
i.e. total no of moles does not change
1
(iv)
[H 2 ][I 2 ]
must be square brackets (penalise once in paper)
[HI]2
– if round, penalise but mark on in (iv)
if Kc wrong, no marks in (iv) either but mark on from a minor
slip in formula
2
Kc = (0.023 )
(0.172 ) 2
1
1
1
1
1
1
1
1
1
= 0.0179 or 1.79 × 10–2
(v)
Conseq on (i)
Allow 0.018 or 1.8 × 10–2
1
Kc = 55.9 or 56
1
Conseq i.e. (answer to (iv))–1
[13]
2.
(a)
Kc =
[Y][H 2 O]2
[X] [CH 3 OH]2
(1)
if Kc expression wrong lose units mark in (e) also
must be [ ]
1
(b)
(c)
Moles of X: 0.25 - 0.13 = 0.12 (1)
Moles of methanol: 0.34 - 0.26 = 0.08 (1)
Moles of water: 0.26 (1)
3
Equal no. of moles on each side of equation (1)
OR V cancels out (provided not incorrectly qualified)
1
(d)
 0.13  0.26 



 V  V 
2
 0.12  0.08 



 V  V 
= 11(.4) (1)
2
Calculation: Kc =
(1)
Can score all 3 conseq on (b) and (c)
If different values from (c) used allow units only
(conseq on correct Kc)
Units of Kc: none (1)
but lose this mark if Kc is wrong even if none given
3
(e)
decrease (1)
1
[9]
3.
(a)
Equation
Kc
N2 + O2 2NO (1)
[NO]2 / [N2] [O2] (1)
2
Kc
(b)
Temperature
(c)
(d)
As temperature increases Kc increases (or yield increases) (1)
Hence reaction endothermic (1)
2
The product yield is very small (1)
Yield does not justify cost of producing high temp (1)
2
(i)
2NO + O2
2NO2 (1)
(ii)
Change in equilibrium position
Change in equilibrium constant
Displaced to the right (1)
No change (1)
3
[9]