CHEMISTRY 2.6
Level Two
Credits: Five
INSTRUCTIONS
Answer ALL questions.
Show working for all calculations and provide answers to three significant figures.
© New Zealand Institute of Chemistry 2006

2
You should spend about 50 minutes on this assessment
20 mL of 1 mol L
–1
HNO
3
is added to a lump of CaCO
3
so that the solid is completely covered. The equation for the reaction is
CaCO
3
(s) + 2HNO
3
(aq) Ca(NO
3
)
2
(aq) + CO
2
(g) + H
2
O(l) a . Complete the table below by describing the effect that each of the following changes would have on the rate of production of CO
2
and explaining your answers with reference to collisions between particles.
Change Effect on CO
2 production
Explanation i) Deceasing the temperature ii) Using 40 mL of 1 mol L
–1
HNO
3 instead of 20 mL iii) Using 20 mL of 2 mol L
HNO
3 instead of 1 mol L
–1
–1 b . During the reaction heat energy is released. i) Complete an energy diagram for the reaction, labelling the reactants, products, ∆ H and the activation energy.
Energy
Reaction coordinate
TURN OVER

3 ii) Circle the word which best describes this reaction:
Exothermic
Justify your choice.
Endothermic
______________________________________________________________________________
______________________________________________________________________________ iii) A catalyst is added to a reaction to increase its rate. Describe how a catalyst would cause this effect.
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
An equilibrium exists between the brown gas NO
2
and the colourless gas N
2
O
4
. a . A chemical equilibrium is referred to as a dynamic equilibrium. What is meant by the term
'dynamic equilibrium'?
______________________________________________________________________________
______________________________________________________________________________ b . The reaction is represented by the following equation
N
2
O
4
(g)
(Colourless gas)
2NO
2
(g)
∆
H = + 58 kJ mol
–1
(Brown gas) i) Describe what changes, if any, would be seen if a stoppered flask containing the two gases at equilibrium is placed in ice and give a reason for your answer.
Observation: ____________________________________________________________________
______________________________________________________________________________
Reason: ________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
TURN OVER

4 ii) Describe what changes, if any, would be seen if the pressure is increased and give a reason for your answer.
Observation: ____________________________________________________________________
______________________________________________________________________________
Reason: ________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________ iii) Write an equilibrium constant expression ( K c
) for the reaction.
______________________________________________________________________________ iv) The K c
of the equilibrium system is 0.90 at 120 ºC.
I) Circle the change that would occur in the K c
value if pressure is increased.
Increase No change
Give a reason for your answer
Decrease
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
The reaction of aluminium with iron (III) oxide is a highly exothermic reaction used in the process of welding steel.
2Al(s) + Fe
2
O
3
(s) Al
2
O
3
(s) + 2Fe(s) a . In a small scale demonstration of the reaction, 4.2 g of aluminium was reacted with excess iron III oxide and 137.6 kJ of energy was released.
Calculate is ∆ r
H for the reaction. M (Al) = 27 g mol
–1 b . Calculate the mass of aluminium oxide is produced when 89 kJ of heat energy is given off.
Give your answer to three significant figures. M (Al) = 27 g mol
–1
; M (O) = 16 g mol
–1
TURN OVER

5
Complete the following table:
K w = 1.00

10
–14
[H
3
O + ]
(i)
3.5

10
–2
mol L
–1
(iii)
[OH
–
]
1.0

10
–6
mol L
–1
(v) (vi)
(ii)
(iv) pH
9.5
a.
Circle the numbers of the equations below that represent acid-base reactions. i) ii)
HCl + H
2
O
NH
4
+ + OH
–
Cl
–
+ H
3
O
+
NH
3
+ H
2
O iii) iv)
Ba(NO
3
)
2
+ Na
2
CO
3
H
3
O
+
+ OH
–
BaCO
2H
2
O
3
+ 2NaNO
3 b.
For one of the equations you have circled above complete the table below by identifying both conjugate acid-base pairs and the acid and the base of each. acid-base pair acid base c.
(i) Write the definition of an acid used in answering a and b .
______________________________________________________________________________
TURN OVER

6
(ii) Discuss the acid-base behaviour of the hydrogen carbonate ions and the water molecules in the following reactions.
HCO
3
–
+ H
2
O
HCO
3
–
+ H
2
O
H
2
CO
3
+ OH
CO
3
-
2–
+ H
3
O
+
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________ d) Complete the equations below and state whether the resulting solutions are acidic or basic.
NH
3
+ H
2
O
NH
4
+
+ H
2
O
___________________________
___________________________
ACIDIC
ACIDIC
BASIC
BASIC
Nitric acid, HNO
3
, is a strong acid. Ethanoic acid, CH
3
COOH, is a weak acid. a) Write the definition of a weak acid.
______________________________________________________________________________ b) Describe a simple laboratory test that would distinguish between 0.1 mol L
–1
solutions of
HNO
3
and CH
3
COOH.
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________ c) Compare the composition of 0.1 mol L
–1
solutions of HNO
3
and CH
3
COOH in terms of the species present and their relative concentrations. Include equations in your discussion.
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________