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Secondary 3 Chemistry Semestral Assessment 2 (End of Year) Paper 4

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TuitionGoWhere Practice Paper - Chemistry Secondary 3

TuitionGoWhere Exam Practice (AI)

Subject: Chemistry
Level: Secondary 3 (Express)
Paper: SA2 Practice Paper (Version 4 of 5)
Duration: 1 hour 15 minutes
Total Marks: 50

Name: ________________________
Class: ________________________
Date: ________________________

Instructions to Candidates

Write your Name, Class, and Date in the spaces provided.
Answer all questions.
Write your answers in the spaces provided on the question paper.
The number of marks is given in brackets [ ] at the end of each question or part question.
You may use a calculator.

Section A: Structured Questions (30 Marks)

Answer all questions in this section.

1. The table below shows the pH values of four different soil samples collected from a farm.

Soil Sample	pH Value
A	4.5
B	7.0
C	8.2
D	5.8

(a) Which soil sample is neutral?
........................................................................................................................................ [1]

(b) Soil sample A is too acidic for most crops. Name a common solid compound that can be added to this soil to raise its pH.
........................................................................................................................................ [1]

(c) Explain why the compound you named in (b) is preferred over sodium hydroxide for treating soil.
........................................................................................................................................
........................................................................................................................................ [1]

2. A student investigates the reaction between dilute sulfuric acid and excess zinc granules. The equation for the reaction is:

$Zn(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2(g)$

The student measures the volume of hydrogen gas produced every 30 seconds.

(a) Describe a suitable method to collect and measure the volume of hydrogen gas produced in this experiment.
........................................................................................................................................
........................................................................................................................................ [2]

(b) The student repeats the experiment using the same mass of zinc but with a higher concentration of sulfuric acid. All other conditions remain constant.
Sketch the expected result on the grid below, labeling the new curve as B and the original curve as A.

Image pending generation for this question.

Graph space

(c) Explain, in terms of particle collision theory, why the initial rate of reaction is higher in experiment B.
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................ [2]

3. Ammonium nitrate ( $NH_4NO_3$ ) is a widely used fertilizer.

(a) Name the two reactants required to produce ammonium nitrate via a neutralization reaction.

...........................................................................................................................
........................................................................................................................... [1]

(b) Ammonium nitrate is a salt. Describe the method used to prepare a pure, dry sample of ammonium nitrate crystals from the reactants named in (a). Include the key steps in your answer.
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................ [3]

4. Substance X is an oxide of an element. It has the following properties:

It is a white solid.
It reacts with hydrochloric acid to form a salt and water.
It reacts with aqueous sodium hydroxide to form a salt and water.

(a) What term is used to describe oxides that react with both acids and bases?
........................................................................................................................................ [1]

(b) Identify Substance X.
........................................................................................................................................ [1]

(c) Write a balanced chemical equation for the reaction between Substance X and hydrochloric acid.
........................................................................................................................................ [2]

5. A solution of ethanoic acid ( $CH_3COOH$ ) has a pH of 3. A solution of hydrochloric acid ( $HCl$ ) of the same concentration has a pH of 1.

(a) Explain the difference in pH values between the two acids.
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................ [2]

(b) Both acids react with magnesium ribbon. State one similarity and one difference in the observations when excess magnesium is added to equal volumes of these two acids.
Similarity: ...................................................................................................................
Difference: .................................................................................................................. [2]

6. Barium sulfate is an insoluble salt.

(a) Describe how you would prepare a pure, dry sample of barium sulfate starting from aqueous barium nitrate and aqueous sodium sulfate.
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................ [3]

(b) Write the ionic equation for this precipitation reaction, including state symbols.
........................................................................................................................................ [2]

7. The diagram below represents the electronic structure of a molecule of nitrogen trifluoride ( $NF_3$ ).

Image pending generation for this question.

(Diagram shows Nitrogen atom bonded to three Fluorine atoms. Nitrogen has 5 valence electrons, Fluorine has 7.)

(a) Draw the dot-and-cross diagram for $NF_3$ , showing only the outer shell electrons. Use dots for Nitrogen electrons and crosses for Fluorine electrons.

Drawing space

[2]

(b) Nitrogen trifluoride is a gas at room temperature. Explain why it has a low boiling point, referring to its structure and bonding.
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................ [2]

8. Copper(II) carbonate reacts with dilute nitric acid.

(a) Write the balanced chemical equation for this reaction, including state symbols.
........................................................................................................................................ [2]

(b) Describe the observations made during this reaction.
........................................................................................................................................
........................................................................................................................................ [2]

9. A student titrates 25.0 cm³ of 0.10 mol/dm³ sodium hydroxide solution against dilute hydrochloric acid. The equation is:

$NaOH(aq) + HCl(aq) \rightarrow NaCl(aq) + H_2O(l)$

(a) Name a suitable indicator for this titration and state the color change at the endpoint.
Indicator: .....................................................................................................................
Color Change: ............................................................................................................ [2]

(b) The student finds that 20.0 cm³ of hydrochloric acid is required to neutralize the sodium hydroxide. Calculate the concentration of the hydrochloric acid in mol/dm³.

[Space for Working]

Concentration = ______________________ mol/dm³ [3]

10. The table below lists some properties of three substances: Diamond, Graphite, and Silicon(IV) oxide.

Substance	Structure	Electrical Conductivity
Diamond	Giant Covalent	Non-conductor
Graphite	Giant Covalent	Conductor
Silicon(IV) oxide	Giant Covalent	Non-conductor

(a) Explain why diamond is very hard.
........................................................................................................................................
........................................................................................................................................ [2]

(b) Explain why graphite conducts electricity, whereas diamond does not.
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................ [2]

Section B: Free-Response Questions (20 Marks)

Answer all questions in this section.

11. Iron(II) sulfate crystals ( $FeSO_4 \cdot 7H_2O$ ) can be prepared by reacting excess iron filings with warm dilute sulfuric acid.

(a) Why is excess iron used in this preparation?
........................................................................................................................................ [1]

(b) After the reaction is complete, the mixture is filtered.
(i) What is removed by filtration?
................................................................................................................................ [1]
(ii) What is present in the filtrate?
................................................................................................................................ [1]

(c) Describe the subsequent steps required to obtain pure, dry crystals of iron(II) sulfate from the filtrate.
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................ [3]

(d) Iron(II) sulfate crystals are pale green. When strongly heated, they lose their water of crystallization and turn white.
(i) What is the term for water that is chemically bound in a crystal structure?
................................................................................................................................ [1]
(ii) Write the equation for this thermal decomposition.
................................................................................................................................ [2]

12. Hydrochloric acid is a strong acid, while ethanoic acid is a weak acid.

(a) Define the term strong acid in terms of ionization.
........................................................................................................................................
........................................................................................................................................ [1]

(b) A solution of hydrochloric acid has a concentration of 0.1 mol/dm³.
(i) Calculate the number of moles of HCl in 25.0 cm³ of this solution.

[Space for Working]

Moles = ______________________ [2]

(ii) This solution is neutralized by 25.0 cm³ of 0.1 mol/dm³ sodium hydroxide.
If the experiment is repeated using 0.1 mol/dm³ ethanoic acid instead of hydrochloric acid, will the volume of sodium hydroxide required for neutralization be greater than, less than, or equal to 25.0 cm³? Explain your answer.

Volume: ......................................................................................................................
Explanation: ..............................................................................................................
........................................................................................................................................
........................................................................................................................................ [3]

(c) Ethanoic acid reacts with ethanol in the presence of concentrated sulfuric acid to form an ester.
(i) Name the ester formed.
................................................................................................................................ [1]
(ii) State one characteristic physical property of esters.
................................................................................................................................ [1]

13. Zinc oxide is an amphoteric oxide.

(a) Explain what is meant by the term amphoteric.
........................................................................................................................................
........................................................................................................................................ [1]

(b) Write balanced chemical equations for the reactions of zinc oxide with:
(i) Dilute hydrochloric acid.
................................................................................................................................ [2]
(ii) Aqueous sodium hydroxide. (Note: The product is sodium zincate, $Na_2ZnO_2$ ).
................................................................................................................................ [2]

(c) A student is given two white powders: Zinc oxide and Magnesium oxide.
Describe a chemical test, using a named reagent, that can distinguish between these two oxides. State the expected observations for each.

Reagent: ....................................................................................................................
Observation with Zinc Oxide: ...................................................................................
Observation with Magnesium Oxide: ....................................................................... [3]

End of Paper

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Answers

TuitionGoWhere Practice Paper - Chemistry Secondary 3

Answer Key & Marking Scheme SA2 Practice Paper (Version 4 of 5)

Section A: Structured Questions

1. (a) B [1] (b) Calcium oxide / Calcium hydroxide / Calcium carbonate / Limestone / Slaked lime / Quicklime [1] Note: Do not accept Sodium hydroxide or Potassium hydroxide. (c) Sodium hydroxide is a strong alkali / highly corrosive / causes soil pH to rise too rapidly / damages plants. Calcium compounds are milder / less soluble / act slowly. [1]

2. (a) Use a gas syringe connected to the reaction flask via a delivery tube. OR Use an inverted measuring cylinder/burette filled with water in a trough, connected via delivery tube. [2] 1 mark for apparatus, 1 mark for connection/collection method. (b) Curve B should start steeper than A (higher gradient) and plateau at the same volume (60 cm³). [2] 1 mark for steeper initial slope, 1 mark for same final volume. (c) Higher concentration means more acid particles per unit volume. This leads to a higher frequency of effective collisions between zinc and hydrogen ions. [2] 1 mark for more particles/collisions, 1 mark for frequency/rate link.

3. (a) Ammonia (solution) and Nitric acid. [1] Must have both correct. (b) 1. Titrate ammonia solution with nitric acid using an indicator to find the exact volume for neutralization. [1] 2. Repeat the titration without indicator using the exact volumes determined. [1] 3. Heat the resulting solution to evaporate some water (until saturation point/crystallization point). [1] 4. Allow to cool and crystallize. Filter, wash with cold distilled water, and dry between filter papers. [1] Max 3 marks. Accept "Titration method" description.

4. (a) Amphoteric [1] (b) Aluminum oxide ( $Al_2O_3$ ) OR Zinc oxide ( $ZnO$ ) [1] (c) $Al_2O_3 + 6HCl \rightarrow 2AlCl_3 + 3H_2O$ OR $ZnO + 2HCl \rightarrow ZnCl_2 + H_2O$ [2] 1 mark for correct formulae, 1 mark for balancing.

5. (a) Hydrochloric acid is a strong acid and ionizes completely in water, producing a high concentration of $H^+$ ions. Ethanoic acid is a weak acid and ionizes only partially, producing a lower concentration of $H^+$ ions. [2] 1 mark for complete vs partial ionization, 1 mark for $H^+$ concentration link. (b) Similarity: Effervescence / Bubbles of gas produced / Magnesium dissolves. [1] Difference: The reaction with hydrochloric acid is faster / more vigorous / exothermic (hotter) than with ethanoic acid. [1]

6. (a) 1. Mix aqueous barium nitrate and aqueous sodium sulfate in a beaker. [1] 2. Filter the mixture to collect the precipitate (residue). [1] 3. Wash the residue with distilled water to remove soluble impurities. [1] 4. Dry the residue in an oven or between filter papers. [1] Max 3 marks. (b) $Ba^{2+}(aq) + SO_4^{2-}(aq) \rightarrow BaSO_4(s)$ [2] 1 mark for correct ions and state symbols, 1 mark for correct product and balancing.

7. (a) Diagram: Nitrogen in center with 3 single bonds to Fluorine. Nitrogen has one lone pair. Each Fluorine has 3 lone pairs. Shared pairs should show one dot and one cross. [2] 1 mark for correct bonding pairs, 1 mark for correct lone pairs on N and F. (b) $NF_3$ has a simple molecular structure. The molecules are held together by weak intermolecular forces (van der Waals forces). Little energy is required to overcome these forces. [2] 1 mark for weak intermolecular forces, 1 mark for low energy requirement.

8. (a) $CuCO_3(s) + 2HNO_3(aq) \rightarrow Cu(NO_3)_2(aq) + H_2O(l) + CO_2(g)$ [2] 1 mark for correct formulae, 1 mark for balancing and states. (b) Green solid dissolves. Blue solution forms. Effervescence / bubbles of gas produced. [2] 1 mark for color change/solution, 1 mark for gas.

9. (a) Methyl orange (Red to Yellow/Orange) OR Phenolphthalein (Pink to Colorless). [2] 1 mark for indicator, 1 mark for correct color change. (b) Moles of NaOH = $0.10 \times \frac{25.0}{1000} = 0.0025$ mol. [1] Mole ratio NaOH : HCl is 1 : 1. Moles of HCl = 0.0025 mol. [1] Concentration of HCl = $\frac{0.0025}{\frac{20.0}{1000}} = \frac{0.0025}{0.020} = 0.125$ mol/dm³. [1]

10. (a) Diamond has a giant covalent structure. Each carbon atom is covalently bonded to four other carbon atoms in a rigid tetrahedral arrangement. Strong covalent bonds extend throughout the structure, requiring much energy to break. [2] 1 mark for structure/bonding description, 1 mark for energy/strength link. (b) In graphite, each carbon atom is bonded to three others, leaving one delocalized electron per atom. These delocalized electrons can move through the structure and carry charge. In diamond, all four valence electrons are used in bonding, so there are no free/delocalized electrons. [2] 1 mark for delocalized electrons in graphite, 1 mark for absence in diamond.

Section B: Free-Response Questions

11. (a) To ensure all the sulfuric acid reacts / to prevent the formation of iron(III) sulfate / to ensure only iron(II) sulfate is formed. [1] (b) (i) Excess iron filings. [1] (ii) Iron(II) sulfate solution / Filtrate contains dissolved $FeSO_4$ . [1] (c) 1. Heat the filtrate to evaporate water until saturated (crystallization point). [1] 2. Allow the solution to cool slowly to form crystals. [1] 3. Filter to collect crystals. [1] 4. Wash with a little cold distilled water and dry. [1] Max 3 marks. (d) (i) Water of crystallization. [1] (ii) $FeSO_4 \cdot 7H_2O(s) \rightarrow FeSO_4(s) + 7H_2O(g)$ [2] 1 mark for correct formulae, 1 mark for balancing/states.

12. (a) A strong acid is an acid that dissociates/ionizes completely in water to produce hydrogen ions. [1] (b) (i) Moles = $0.1 \times \frac{25.0}{1000} = 0.0025$ mol. [2] 1 mark for conversion, 1 mark for answer. (ii) Volume: Equal to 25.0 cm³. [1] Explanation: Neutralization depends on the total number of moles of acid available. Although ethanoic acid is weak and partially ionized, as $H^+$ ions are removed by $OH^-$ , the equilibrium shifts to ionize more acid until all ethanoic acid molecules have reacted. Since the concentration and volume are the same, the total moles of acid are the same. [2] 1 mark for "same moles/stoichiometry", 1 mark for equilibrium shift explanation. (c) (i) Ethyl ethanoate. [1] (ii) Sweet / fruity smell. [1]

13. (a) An amphoteric substance is one that can act as both an acid and a base (reacts with both acids and bases). [1] (b) (i) $ZnO + 2HCl \rightarrow ZnCl_2 + H_2O$ [2] (ii) $ZnO + 2NaOH \rightarrow Na_2ZnO_2 + H_2O$ [2] 1 mark for formulae, 1 mark for balancing. (c) Reagent: Aqueous Sodium Hydroxide (or Aqueous Ammonia). [1] Observation with Zinc Oxide: The white solid dissolves to form a colorless solution. [1] Observation with Magnesium Oxide: The white solid does not dissolve / no reaction. [1] Alternative: Use Acid. Both dissolve, so this is NOT a distinguishing test unless followed by NaOH test on the resulting solution. The NaOH test on the oxide directly is preferred for simplicity.