CHEMISTRY 2.6 An assessment for AS90310 Describe principles of chemical reactivity

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CHEMISTRY 2.6

An assessment for AS90310

Level Two

Describe principles of chemical reactivity

Answer Credits: Five

INSTRUCTIONS 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

QUESTION ONE:

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

a

. CaCO 3 (s) + 2HNO 3 (aq) Ca(NO 3 ) 2 (aq) + CO 2 (g) + H 2 O(l) 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 –1 HNO 3 instead of 1 mol L –1

b

. During the reaction heat energy is released. i) Complete an energy diagram for the reaction, labelling the reactants, products, ∆ the activation energy.

H

and 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. ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________

QUESTION TWO

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) (Brown gas) ∆

H

= + 58 kJ mol –1 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

______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________

b

.

QUESTION THREE:

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 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

QUESTION FOUR:

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

QUESTION FIVE

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

b.

iii) iv) Ba(NO 3 ) 2 + Na 2 CO 3 H 3 O + + OH – BaCO 2H 2 O 3 + 2NaNO 3 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 NH 3 4 + H + + H 2 2 O O ___________________________

ACIDIC

___________________________

ACIDIC BASIC BASIC

QUESTION SIX:

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. ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________

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