SECONDARY FOUR SCIENCE(CHEMISTRY) EXERCISE 3
1. Liquid sodium reacts with hydrogen gas to give an ionic solid, sodium hydride (NaH).
(a) By using oxidation numbers, explain why this is a redox reaction.
(b) Draw a dot and cross diagram to show the electronic structure of sodium hydride. You only need to show the valence electrons.
(c) Explain why sodium hydride conducts electricity when dissolved in aqueous solution.
2. Argon is often used in light bulbs.
(a) Give one property of argon that makes it suitable for this application.
(b) How many valence electrons does one atom of argon have?
3. Zinc can be obtained from different ore forms. The extraction of zinc typically involves two processes. The first process involves the decomposition of zinc carbonate. The second process involves the reduction of the zinc oxide with charcoal (a form of carbon).
(a) Write two balanced equations of how zinc can be extracted from zinc carbonate.
(b) What type of oxide is zinc oxide?
(c) What gas is produced when zinc reacts with hydrochloric acid?
4. Air is a mixture of gases.
(a) Nine people are trapped in a lift. Describe the changes in the composition of air in the lift after one hour.
(b) With which experimental method do you extract pure nitrogen gas from an air sample?
5. A barium salt, BaFeO4, contains iron in its highest oxidation state.
(a) The cation in this salt is Ba2+. Deduce the formula of the anion in this salt.
(b) Calculate the oxidation state of iron in this salt.
(c) A redox reaction will occur when BaFeO4 is mixed with potassium iodide. State the product that iodide ion is oxidized or reduced to.
6. Aluminium hydroxide is often used as an antacid.
(a) Assuming that the stomach contains hydrochloric acid only, write the balanced chemical equation of the antacid that contains aluminium hydroxide reacting with the contents of the stomach.
(b) When aqueous ammonia is added dropwise followed by in excess to a solution of the antacid, describe the observations expected.
7. Aqueous potassium nitrate can be prepared by titrating aqueous potassium hydroxide with dilute nitric acid.
(a) Write a balanced equation for the reaction between aqueous potassium hydroxide and dilute nitric acid.
(b) Name the two pieces of apparatus used to measure accurately the volumes of aqueous potassium hydroxide and dilute nitric acid.
(c) 40 grams of potassium hydroxide is placed in 100 cm3 of water. Calculate the concentration in mol/dm3 of potassium hydroxide used in the titration.
8. Pentane belongs to the homologous series of alkanes.
(a) Draw the full structural formula of one isomer of pentane.
(b) Pentane reacts with chlorine in the presence of sunlight.
a. Name the type of reaction which takes place.
b. Draw the full structural formula of one organic product formed in the reaction.
9. The table lists the melting points of aluminium fluoride and phosphorus trifluoride.
Compound Chemical formula Melting point / oC
Aluminium fluoride 1291
Phosphorus trifluoride -151
(a) Write down the chemical formula of aluminium fluoride and phosphorus trifluoride.
(b) What type of bonding are present in aluminium fluoride and phosphorus trifluoride?
(c) Explain in terms of bonding and structure why the two compounds have such different melting points.
(d) Draw a dot and cross diagram to show the bonding in phosphorus trifluoride. You need to show only the outer electrons.
10. Ethanol combines with oxygen in two different processes, namely combustion and oxidation.
(a) Construct a balanced chemical equation, with state symbols, for each process.
(b) Draw the full structural formula of an ethanol molecule.
(c) Glucose can be fermented to produce ethanol and a gas. Write down a balanced chemical equation for the reaction.
11. Draw 3 repeat units of the polymer when propene is polymerized to form poly(propene).
Short answers to Exercise 3
1(a) 2Na + H2 - > 2NaH
Oxidation number of Na changes from 0 to +1. it is oxidized.
Oxidation number of H changes from 0 to -1. it is reduced.
Therefore, this is a redox reaction.
(b This is an ionic compound. Draw Na+ and H-. H- ion has two electrons and Na+ ion has 8 valence electrons.
(c) In aqueous solution, sodium hydride consists of mobile ions which can act as charge carriers to conduct electricity.
2.(a) Argon is inert and unreactive. Light bulbs need an inert atmosphere.
(b) Argon has 8 valence electrons.
3(a) ZnCO3 - > ZnO + CO2
ZnO + C - > Zn + CO
(b amphoteric oxide
(c) hydrogen gas.
4.(a) The percentage composition of oxygen in the air inside the lift at the start is 21%. As the nine people consume oxygen and breath out carbon dioxide, the percentage composition of oxygen in the air inside the lift will decrease. The amount of carbon dioxide will increase.
(b by fractional distillation of air.
5(a) FeO4 charge : 2-
(b +6
(c) iodide is oxidized to iodine.
6(a) Al(OH)3 + 3HCl à AlCl3 + 3H2O
(b White precipitate insoluble in excess aqueous ammonia.
7. (a) KOH + HNO3 - > KNO3 + H2O
(b pipette and burette
(c) Change mass of KOH into no. of moles. Change volume into dm3.
Concentration = no. of moles / volume
8(a) 2-methylbutane (draw it)
(b Type of reaction : substitution
Chloropentane. (Draw it)
9(a) AlF3 and PF3
(b ionic bonding and covalent bonding
(c) Strong ionic bonds in AlF3 and weak VdW forces in PF3.
(d) PF3. P is the central atom and it has 2 electrons which are not bonded (lone pair).
10. C2H5OH + O2 - > CO2 + H2O (balance equation)
C2H5OH + [O] - > CH3COOH + H2O (balance equation)
(b Answer is in the textbook. Chapter : Alcohols
(c C6H12O6 - > CO2 + C2H5OH (balance equation)
11. Similar to ethene forming poly(ethene) .
1. Liquid sodium reacts with hydrogen gas to give an ionic solid, sodium hydride (NaH).
(a) By using oxidation numbers, explain why this is a redox reaction.
(b) Draw a dot and cross diagram to show the electronic structure of sodium hydride. You only need to show the valence electrons.
(c) Explain why sodium hydride conducts electricity when dissolved in aqueous solution.
2. Argon is often used in light bulbs.
(a) Give one property of argon that makes it suitable for this application.
(b) How many valence electrons does one atom of argon have?
3. Zinc can be obtained from different ore forms. The extraction of zinc typically involves two processes. The first process involves the decomposition of zinc carbonate. The second process involves the reduction of the zinc oxide with charcoal (a form of carbon).
(a) Write two balanced equations of how zinc can be extracted from zinc carbonate.
(b) What type of oxide is zinc oxide?
(c) What gas is produced when zinc reacts with hydrochloric acid?
4. Air is a mixture of gases.
(a) Nine people are trapped in a lift. Describe the changes in the composition of air in the lift after one hour.
(b) With which experimental method do you extract pure nitrogen gas from an air sample?
5. A barium salt, BaFeO4, contains iron in its highest oxidation state.
(a) The cation in this salt is Ba2+. Deduce the formula of the anion in this salt.
(b) Calculate the oxidation state of iron in this salt.
(c) A redox reaction will occur when BaFeO4 is mixed with potassium iodide. State the product that iodide ion is oxidized or reduced to.
6. Aluminium hydroxide is often used as an antacid.
(a) Assuming that the stomach contains hydrochloric acid only, write the balanced chemical equation of the antacid that contains aluminium hydroxide reacting with the contents of the stomach.
(b) When aqueous ammonia is added dropwise followed by in excess to a solution of the antacid, describe the observations expected.
7. Aqueous potassium nitrate can be prepared by titrating aqueous potassium hydroxide with dilute nitric acid.
(a) Write a balanced equation for the reaction between aqueous potassium hydroxide and dilute nitric acid.
(b) Name the two pieces of apparatus used to measure accurately the volumes of aqueous potassium hydroxide and dilute nitric acid.
(c) 40 grams of potassium hydroxide is placed in 100 cm3 of water. Calculate the concentration in mol/dm3 of potassium hydroxide used in the titration.
8. Pentane belongs to the homologous series of alkanes.
(a) Draw the full structural formula of one isomer of pentane.
(b) Pentane reacts with chlorine in the presence of sunlight.
a. Name the type of reaction which takes place.
b. Draw the full structural formula of one organic product formed in the reaction.
9. The table lists the melting points of aluminium fluoride and phosphorus trifluoride.
Compound Chemical formula Melting point / oC
Aluminium fluoride 1291
Phosphorus trifluoride -151
(a) Write down the chemical formula of aluminium fluoride and phosphorus trifluoride.
(b) What type of bonding are present in aluminium fluoride and phosphorus trifluoride?
(c) Explain in terms of bonding and structure why the two compounds have such different melting points.
(d) Draw a dot and cross diagram to show the bonding in phosphorus trifluoride. You need to show only the outer electrons.
10. Ethanol combines with oxygen in two different processes, namely combustion and oxidation.
(a) Construct a balanced chemical equation, with state symbols, for each process.
(b) Draw the full structural formula of an ethanol molecule.
(c) Glucose can be fermented to produce ethanol and a gas. Write down a balanced chemical equation for the reaction.
11. Draw 3 repeat units of the polymer when propene is polymerized to form poly(propene).
Short answers to Exercise 3
1(a) 2Na + H2 - > 2NaH
Oxidation number of Na changes from 0 to +1. it is oxidized.
Oxidation number of H changes from 0 to -1. it is reduced.
Therefore, this is a redox reaction.
(b This is an ionic compound. Draw Na+ and H-. H- ion has two electrons and Na+ ion has 8 valence electrons.
(c) In aqueous solution, sodium hydride consists of mobile ions which can act as charge carriers to conduct electricity.
2.(a) Argon is inert and unreactive. Light bulbs need an inert atmosphere.
(b) Argon has 8 valence electrons.
3(a) ZnCO3 - > ZnO + CO2
ZnO + C - > Zn + CO
(b amphoteric oxide
(c) hydrogen gas.
4.(a) The percentage composition of oxygen in the air inside the lift at the start is 21%. As the nine people consume oxygen and breath out carbon dioxide, the percentage composition of oxygen in the air inside the lift will decrease. The amount of carbon dioxide will increase.
(b by fractional distillation of air.
5(a) FeO4 charge : 2-
(b +6
(c) iodide is oxidized to iodine.
6(a) Al(OH)3 + 3HCl à AlCl3 + 3H2O
(b White precipitate insoluble in excess aqueous ammonia.
7. (a) KOH + HNO3 - > KNO3 + H2O
(b pipette and burette
(c) Change mass of KOH into no. of moles. Change volume into dm3.
Concentration = no. of moles / volume
8(a) 2-methylbutane (draw it)
(b Type of reaction : substitution
Chloropentane. (Draw it)
9(a) AlF3 and PF3
(b ionic bonding and covalent bonding
(c) Strong ionic bonds in AlF3 and weak VdW forces in PF3.
(d) PF3. P is the central atom and it has 2 electrons which are not bonded (lone pair).
10. C2H5OH + O2 - > CO2 + H2O (balance equation)
C2H5OH + [O] - > CH3COOH + H2O (balance equation)
(b Answer is in the textbook. Chapter : Alcohols
(c C6H12O6 - > CO2 + C2H5OH (balance equation)
11. Similar to ethene forming poly(ethene) .
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