Cambridge IGCSE Chemistry - 0620 - Supplement Paper 4 2009 Summer Zone 2

Some grass is crushed and mixed with the solvent, propanone. The colour pigments are extracted to give a deep green solution.
(a) (i) Draw a labelled diagram to describe how you could show that there is more than one coloured pigment in the green solution. [3]
(ii) Given a pure sample of chlorophyll, how could you show that the green solution from the grass contained chlorophyll? .............................................................................................................................................................................................................................................................................................................................................................................................................................. [2]
(b) Explain the role of chlorophyll in green plants. .............................................................................................................................................................................................................................................................................................................................................................................................................................. [3]

The results of experiments on electrolysis using inert electrodes are given in the table. Complete the table; the first line has been completed as an example.
[Table_1]

Table_1:
electrolyte | change at negative electrode | change at positive electrode | change to electrolyte
---|---|---|---
molten lead(II) bromide | lead formed | bromine formed | used up
.................................. | lithium formed | chlorine formed | used up
dilute aqueous sodium chloride | .................................. | .................................. | ..................................
.................................. | aqueous copper(II) sulfate | .................................. | ..................................
hydrogen formed | bromine formed | potassium hydroxide formed

The following is a list of the electron distributions of atoms of unknown elements.
[Table_1: element - electron distribution]

(a) Choose an element from the list for each of the following descriptions.
(i) It is a noble gas. ..............................................................
(ii) It is a soft metal with a low density. ..............................................................
(iii) It can form a covalent compound with element A. ..............................................................
(iv) It has a giant covalent structure similar to diamond. ..............................................................
(v) It is a diatomic gas with molecules of the type $X_2$. .............................................................. [5]

(b) Elements C and A can form an ionic compound.
(i) Draw a diagram that shows the formula of this compound, the charges on the ions and the arrangement of the valency electrons around the negative ion. Use $\text{o}$ to represent an electron from an atom of C. Use $\text{x}$ to represent an electron from an atom of A. [3]
(ii) Predict \textbf{two} properties of this compound. .............................................................. .............................................................. .............................................................. [2]

The reactivity series of metals given below contains both familiar and unfamiliar elements. For most of the unfamiliar elements, which are marked *, their common oxidation states are given.

[Table_1]

Choose metal(s) from the above list to answer the following questions.

(i) Which two metals would not react with dilute hydrochloric acid?
...................................................................................................................................[2]
(ii) Which two unfamiliar metals (*) would react with cold water?
...................................................................................................................................[2]
(iii) What is the oxidation state of barium?
...................................................................................................................................[1]
(iv) Name an unfamiliar metal (*) whose oxide cannot be reduced by carbon.
...................................................................................................................................[1]
(v) Why should you be able to predict that metals such as iron and chromium have more than one oxidation state?
...................................................................................................................................[1]

(a) A preparation of the insoluble salt iron fluoride is described below.
To 15 cm³ of aqueous iron(III) chloride, 45 cm³ of aqueous sodium fluoride is added. The concentration of both solutions is 1.00 mol / dm³. The mixture is filtered and the precipitate washed with distilled water. Finally, the precipitate is heated in an oven.
(i) Complete the equation.
Fe³⁺ + ..........F⁻ ⟶ ............ [2]
(ii) Why is the volume of sodium fluoride solution three times that of the iron(III) chloride solution?
.................................................................................................................................
................................................................................................................................. [1]
(iii) Why is the mixture washed with distilled water?
.................................................................................................................................
................................................................................................................................. [1]
(iv) Why is the solid heated?
.................................................................................................................................
................................................................................................................................. [1]

(b) The formulae of insoluble compounds can be found by precipitation reactions.
To 18.0 cm³ of an aqueous solution of the nitrate of metal T was added 2.0 cm³ of aqueous sodium phosphate, Na₃PO₄. The concentration of both solutions was 1.00 mol/dm³. When the precipitate had settled, its height was measured.

The experiment was repeated using different volumes of the phosphate solution. The results are shown on the following graph.

What is the formula of the phosphate of metal T? Give your reasoning.
.................................................................................................................................
.................................................................................................................................
.................................................................................................................................
................................................................................................................................. [3]

Ammonia is manufactured by the Haber process.

$\text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g)$ the forward reaction is exothermic

(a) (i) Name the raw materials from which nitrogen and hydrogen are obtained.
nitrogen from ............................................................... [1]
hydrogen from ............................................................ [1]

(ii) Name the catalyst used in this process.
.......................................................................... [1]

(iii) What is the most important use of ammonia?
........................................................................... [1]

(b) The following graph shows how the percentage of ammonia in the equilibrium mixture changes with pressure.



(i) Explain the term _equilibrium_.
.................................................................
.................................................................
.................................................................
................................................................. [2]

(ii) How does the percentage of ammonia vary with pressure?
.................................................................... [1]

(c) (i) Sketch a graph which shows how the percentage of ammonia in the equilibrium mixture varies with temperature.

[1]

(ii) Explain why the graph has the shape shown.
.................................................................
.................................................................
................................................................. [2]

(a) Bond energy is the amount of energy, in kJ, that must be supplied (endothermic) to break one mole of a bond.

[Table_1]

Use the above data to show that the following reaction is exothermic.

$$\text{H-H} + \text{F-F} \rightarrow \text{2H-F}$$

(b) They react with water to form acidic solutions.

$$\text{HCl} + \text{H}_2\text{O} \rightleftharpoons \text{H}_3\text{O}^+ + \text{Cl}^-$$
$$\text{HF} + \text{H}_2\text{O} \rightleftharpoons \text{H}_3\text{O}^+ + \text{F}^-$$

(i) Explain why water behaves as a base in both of these reactions.
..................................................................................
..................................................................................
[2]

(ii) At equilibrium, only 1% of the hydrogen chloride exists as molecules, the rest has formed ions. In the other equilibrium, 97% of the hydrogen fluoride exists as molecules, only 3% has formed ions.

What does this tell you about the strength of each acid?
..................................................................................
..................................................................................
[2]

(iii) How would the pH of these two solutions differ?
..................................................................................
[1]

Lactic acid can be made from corn starch.
!(image_1_url)
It polymerises to form the polymer, polylactic acid (PLA ) which is biodegradable.
(a) Suggest two advantages that PLA has compared with a polymer made from petroleum.
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................................................................................................................................................................. [2]
(b) The structure of PLA is given below.
!(image_2_url)
(i) What type of compound contains the group that is circled?
................................................................................................................................................................. [1]
(ii) Complete the following sentence.
Lactic acid molecules can form this group because they contain both an .................................................... group and an .................................................group. [2]
(iii) Is the formation of PLA, an addition or condensation polymerisation? Give a reason for your choice.
.................................................................................................................................................................
.................................................................................................................................................................
................................................................................................................................................................. [2]
(c) When lactic acid is heated, acrylic acid is formed.
!(image_3_url)
(i) Complete the word equation for the action of heat on lactic acid.
lactic acid $\rightarrow$ ............................................... + ............................................... [1]
(ii) Describe a test that would distinguish between lactic acid and acrylic acid.
_test_
.................................................................................................................................................................
_result for lactic acid_ .................................................................................................................................................................
_result for acrylic acid_ ................................................................................................................................................................. [3]
(iii) Describe a test, other than using an indicator, which would show that both chemicals contain an acid group.
_test_
.................................................................................................................................................................
_result_
.................................................................................................................................................................
................................................................................................................................................................. [2]

Quantities of chemicals, expressed in moles, can be used to find the formula of a compound, to establish an equation and to determine reacting masses.

(a) A compound contains 72% magnesium and 28% nitrogen. What is its empirical formula?
.....................................................................................................................................................
.....................................................................................................................................................
.....................................................................................................................................................
..................................................................................................................................................... [2]

(b) A compound contains only aluminium and carbon. 0.03 moles of this compound reacted with excess water to form 0.12 moles of Al(OH)_3 and 0.09 moles of CH_4.
Write a balanced equation for this reaction.
.....................................................................................................................................................
.....................................................................................................................................................
.....................................................................................................................................................
..................................................................................................................................................... [2]

(c) 0.08 moles of silicon reacts with 7.2 g of fluorine.
Si + 2F_2 \rightarrow SiF_4
(i) Which one is the limiting reagent? Explain your choice.
.....................................................................................................................................................
.....................................................................................................................................................
.....................................................................................................................................................
..................................................................................................................................................... [3]
(ii) How many moles of SiF_4 are formed?
..................................................................................................................................................... [1]