AS LEVEL CHEMISTRY

ASSESSMENT POINT 2
PAPER 2 - TOPICS 2 AND 4
Answer all questions
Max 80 marks

	
Name		……………………………………………………………..

Mark		……../80	……....%	Grade ………

1.	
  Hydrated calcium nitrate can be represented by the formula Ca(NO3)2.xH2O where x is an integer.
A 6.04 g sample of Ca(NO3)2.xH2O contains 1.84 g of water of crystallisation.
Use this information to calculate a value for x.
Show your working.
......................................................................................................................
......................................................................................................................
......................................................................................................................
......................................................................................................................
......................................................................................................................
......................................................................................................................
(3)
(Total 3 marks)

2.	  Calcium hydroxide is slightly soluble in water at room temperature. As the temperature rises, the solubility decreases. When the maximum amount of solid has dissolved at a particular temperature the solution is said to be saturated.
In an experiment, the solubility of calcium hydroxide was measured over a range of temperatures. The results are shown in the graph.

(a)	Use data from the graph to calculate the concentration, in mol dm−3, of a saturated solution of calcium hydroxide at 30 °C. Give your answer to 3 significant figures.
Show your working.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
(3)
(b)     You are given a sample of saturated calcium hydroxide solution. Outline the practical steps that you would take to determine the solubility of calcium hydroxide in this solution.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
(3)
(Total 6 marks)

3.   (a)     A student investigated the acid content of a different crater-lake solution. The student used a 50.0 cm3 burette to measure out different volumes of this crater-lake solution. Each volume of crater-lake solution was titrated with a 0.100 mol dm–3 sodium hydroxide solution. Each titration was repeated. The results are shown below.
 
 	Volume of crater-lake solution / cm3	10.0	20.0	30.0	40.0	50.0	
 	Volume of sodium hydroxide solution / cm3	Experiment 1	5.85	17.00	20.00	26.50	32.45	
 		Experiment 2	6.15	13.00	19.90	26.50	32.55	
 	Average titre / cm3	6.00	15.00	19.95	26.50	32.50	

(i)      On the graph paper below, plot a graph of average titre (y-axis) against volume of crater-lake solution. Both axes must start at zero.
 
(3)
(ii)     Draw a line of best fit on the graph.
(1)
(iii)    Use the graph to determine the titre that the student would have obtained using a 25.0 cm3 sample of crater-lake solution.
...............................................................................................................
(1)
(iv)    Excluding any anomalous points, which average titre value would you expect to be the least accurate value? Give one reason for your choice.
Least accurate average titre ................................................................
Reason .................................................................................................
...............................................................................................................
(2)

(b)     Another 100 cm3 sample of crater-lake solution was reacted with an excess of powdered limestone. The gas produced was collected in a gas syringe. The equation for the reaction between the sulfuric(IV) acid in the crater-lake solution and the calcium carbonate in the powdered limestone is shown below.
H2SO3  +  CaCO3   CaSO3  +  H2O  +  CO2
The volume of gas collected from the reaction of the sulfuric(IV) acid in 100 cm3 of crater-lake solution with an excess of powdered limestone was 81.0 cm3 at 298 K and 1.00 × 105 Pa.
(i)      State the ideal gas equation.
...............................................................................................................
(1)
(ii)     Use the ideal gas equation to calculate the amount, in moles, of carbon dioxide formed. 
Show your working. 

	...............................................................................................................
...............................................................................................................
...............................................................................................................
...............................................................................................................
(3)
(iii)    Use the equation for the reaction and your answer from part (b)(ii) to calculate the minimum mass of calcium carbonate needed to neutralise the sulfuric(IV) acid in 1.00 dm3 of crater-lake solution. 
Show your working.
(If you could not complete the calculation in part (b)(ii) assume that the amount of carbon dioxide is 1.25 × 10–2 mol. This is not the correct value.)
...............................................................................................................
...............................................................................................................
...............................................................................................................
...............................................................................................................
(3)
(iv)    The percentage by mass of calcium carbonate in the powdered limestone was 95.0%.
Calculate the minimum mass of this powdered limestone needed to neutralise the sulfuric(IV) acid in 1.00 dm3 of this crater-lake solution.
...............................................................................................................
...............................................................................................................
(2)
(v)     Give one reason, other than cost, why limestone rather than solid sodium hydroxide is often used to neutralise acidity in lakes.
...............................................................................................................
...............................................................................................................
(1)
(Total 17 marks)

4.	When heated, iron(III) nitrate (Mr = 241.8) is converted into iron(III) oxide, nitrogen dioxide and oxygen.
4Fe(NO3)3(s)     2Fe2O3(s)    +    12NO2(g)    +    3O2(g)
A 2.16 g sample of iron(III) nitrate was completely converted into the products shown.
(a)     (i)      Calculate the amount, in moles, of iron(III) nitrate in the 2.16 g sample.
Give your answer to 3 significant figures.
...............................................................................................................
...............................................................................................................
(1)
(ii)     Calculate the amount, in moles, of oxygen gas produced in this reaction.
...............................................................................................................
...............................................................................................................
(1)
(iii)     Calculate the volume, in m3, of nitrogen dioxide gas at 293 °C and 100 kPa produced from 2.16 g of iron(III) nitrate.
The gas constant is R = 8.31 JK–1 mol–1.
(If you have been unable to obtain an answer to part (i), you may assume the number of moles of iron(III) nitrate is 0.00642. This is not the correct answer.)
...............................................................................................................
...............................................................................................................
...............................................................................................................
...............................................................................................................
...............................................................................................................
...............................................................................................................
...............................................................................................................
 (4)
(c)     Suggest why the iron(III) oxide obtained is pure.
Assume a complete reaction.
........................................................................................................................
........................................................................................................................
(1)
(Total 7 marks)

5.     (a)     Draw the shape of a molecule of BeCl2 and the shape of a molecule of Cl2O. Show any lone pairs of electrons on the central atom. Name the shape of each molecule.
                     BeCl2                                                                Cl2O

Name of shape ................................ Name of shape ..................................
(4)
(b)     The equation for the reaction between magnesium hydroxide and hydrochloric acid is shown below.
Mg(OH)2(s) + 2HCl(aq) → MgCl2(aq) + 2H2O(l)
Calculate the volume, in cm3, of 1.00 mol dm–3 hydrochloric acid required to react completely with 1.00 g of magnesium hydroxide.
......................................................................................................................
......................................................................................................................
......................................................................................................................
......................................................................................................................
......................................................................................................................
......................................................................................................................
(4)
(Total 8 marks)
 
6.	Hydrazine (N2H4) decomposes in an exothermic reaction. Hydrazine also reacts exothermically with hydrogen peroxide when used as a rocket fuel.
(a)     Write an equation for the decomposition of hydrazine into ammonia and nitrogen only.
........................................................................................................................
(1)
(b)     State the meaning of the term mean bond enthalpy.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
(2)

(c)     Some mean bond enthalpies are given in the table.
 
 	 	N–H	N–N	N≡N	O–H	O–O	
 	Mean bond enthalpy / kJ mol−1	388	163	944	463	146	

Use these data to calculate the enthalpy change for the gas-phase reaction between hydrazine and hydrogen peroxide.

........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
(3)
(Total 6 marks)

7.    (a)    Define the term standard enthalpy of formation, ∆Hfο
......................................................................................................................
......................................................................................................................
......................................................................................................................
(3)
(b)     Use the data in the table to calculate the standard enthalpy of formation of liquid methylbenzene, C7H8
 
Substance	C(s)	H2(g)	C7H8(l)	
Standard enthalpy of combustion, ∆Hcο /kJ mol–1	–394	–286	–3909	

7C(s) + 4H2(g) → C7H8(l)
......................................................................................................................
......................................................................................................................
......................................................................................................................
(3)

(c)     An experiment was carried out to determine a value for the enthalpy of combustion of liquid methylbenzene using the apparatus shown in the diagram.
 
Burning 2.5 g of methylbenzene caused the temperature of 250 g of water to rise by 60°C. Use this information to calculate a value for the enthalpy of combustion of methylbenzene, C7H8
(The specific heat capacity of water is 4.18 J K–1 g–1. Ignore the heat capacity of the container.)
......................................................................................................................
......................................................................................................................
......................................................................................................................
......................................................................................................................
(4)
(d)     A 25.0 cm3 sample of 2.00 mol dm–3 hydrochloric acid was mixed with 50.0 cm3 of a 1.00 mol dm–3 solution of sodium hydroxide. Both solutions were initially at 18.0 °C.
After mixing, the temperature of the final solution was 26.5°C.
Use this information to calculate a value for the standard enthalpy change for the following reaction.
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
In your calculation, assume that the density of the final solution is 1.00 g cm–3 and that its specific heat capacity is the same as that of water. (Ignore the heat capacity of the container.)
......................................................................................................................
......................................................................................................................
......................................................................................................................
......................................................................................................................
(4)
(e)     Give one reason why your answer to part (d) has a much smaller experimental error than your answer to part (c).
......................................................................................................................
......................................................................................................................
(1)
(Total 15 marks)

8.    (a)     The table below contains some mean bond enthalpy data.
 
Bond	H–O	O–O	O=O	
Mean bond enthalpy/kJ mol–1	463	146	496	

The bonding in hydrogen peroxide, H2O2, can be represented by H–O–O–H. Use these data to calculate the enthalpy change for the following reaction.
H2O2(g) → H2O2(g) + O2(g)
......................................................................................................................
......................................................................................................................
......................................................................................................................
......................................................................................................................
(3)
(b)     The standard enthalpy of formation, ΔHf for methane, is –74.9 kJ mol–1. Write an equation, including state symbols, for the reaction to which this enthalpy change applies.
......................................................................................................................
(2)

(c)     The enthalpy changes for the formation of atomic hydrogen and atomic carbon from their respective elements in their standard states are as follows.
H2(g) → H(g)      ΔH = +218 kJ mol–1
C(s) → C(g)           ΔH = +715 kJ mol–1
(i)      By reference to its structure, suggest why a large amount of heat energy is required to produce free carbon atoms from solid carbon.
.............................................................................................................
.............................................................................................................
(ii)     Parts (b) and (c) give enthalpy data for the formation of CH4(g), H(g) and C(g).
Use these data and Hess’s Law to calculate the value of the enthalpy change for the following reaction.
CH4(g) → C(g) + 4H(g)
.............................................................................................................
.............................................................................................................
.............................................................................................................
.............................................................................................................
.............................................................................................................
(iii)    Use your answer from part (c)(ii) to calculate a value for the mean bond enthalpy of a C–H bond in methane.
.............................................................................................................
(5)
(Total 10 marks)

9.      Barium can be extracted from barium oxide (BaO) in a process using aluminium.
A mixture of powdered barium oxide and powdered aluminium is heated strongly.
The equation for this extraction process is shown below.
3BaO(s) + 2Al(s) → 3Ba(s) + Al2O3(s)
Some standard enthalpies of formation are given in the table below.
 
Substance	BaO(s)	Al2O3(s)	
∆H  f ο / kJ mol–1	–558	–1669	
(a)     (i)     State why the standard enthalpy of formation of barium and that of aluminium are both zero.
.............................................................................................................
(1)
(ii)    Use the data to calculate the standard enthalpy change for the reaction shown by the equation above.
.............................................................................................................
.............................................................................................................
.............................................................................................................
.............................................................................................................
.............................................................................................................
(3)
        (iii)      Suggest the major reason why this method of extracting barium is expensive.
.............................................................................................................
(1)
 (Total 5 marks)

9.	Using the information below, answer this question.
Fe2O3(s) + 3H2(g) → 2Fe(s) + 3H2O(g)   ΔH= +96 kJ mol−1
 
 	 	Fe2O3(s)	H2(g)	Fe(s)	
 	ΔH / kJ mol−1	−822.0	       0	        0	
The standard enthalpy of formation of steam is
A       +286 kJ mol−1
B       +242 kJ mol−1
C       −242 KJ mol−1
D       −286 kJ mol−1
(Total 1 mark)

11.	On complete combustion, 0.0150 mol of an organic acid produced 735 cm3 of carbon dioxide (measured at 101 kPa and 298 K). The same amount of acid required 15.0 cm3 of 2.00 M sodium hydroxide solution for neutralisation. Which one of the following could be the formula of the acid?
A       HCOOH
B       CH3COOH
C       HOOCCOH
D       HOOCCH2CH2COOH
(Total 1 mark)

11.	The data below refer to the industrial production of nitric acid from ammonia.
Reaction 1    4NH3(g) + 5O2(g)   4NO(g) + 6H2O(g)              ∆H = −909 kJ mol−1
Reaction 2    2NO(g) + O2(g)   2NO2(g)                                 ∆H = −115 kJ mol−1
The direct oxidation of ammonia to nitrogen dioxide can be represented by the equation
4NH3(g) + 7O2(g) → 4NO2(g) + 6H2O(g)
for which the standard enthalpy change, in kJ mol−1, is
A       −1139
B       −1024
C       −794
D       −679
(Total 1 mark)