Topic – 6.1
A list of chemical and physical processes, A to H, is shown.
A combustion
B diffusion
C melting
D neutralisation
E photosynthesis
F reversible reaction
G roasting
H thermal decomposition
Answer the following questions about processes A to H.
Each letter may be used once, more than once or not at all.
(a) State which of the processes A to H happens when an acid reacts with an alkali.
(b) State which of the processes A to H reaches a position of equilibrium.
(c) State which of the processes A to H involves particles changing from fixed positions to being mobile, but still touching.
(d) State which two of the processes A to H are physical changes.
(e) State which of the processes A to H is caused by gas particles colliding with each other.
▶️ Answer/Explanation
(a) D (neutralisation)
Neutralisation is the process that occurs when an acid reacts with an alkali to form a salt and water.
(b) F (reversible reaction)
Reversible reactions can reach a position of equilibrium where the forward and reverse reactions occur at the same rate.
(c) C (melting)
Melting involves particles changing from fixed positions in a solid to being mobile (but still touching) in a liquid state.
(d) B (diffusion) and C (melting)
These are physical changes because they don’t involve the formation of new substances. Diffusion is the movement of particles, and melting is a change of state.
(e) B (diffusion)
Diffusion is caused by gas particles colliding with each other and moving from areas of high concentration to low concentration.
Topic – 2.2
This question is about atomic structure and the Periodic Table.
(a) Define the term nucleon number.
(b) State the connection between the number of occupied electron shells in an atom and the period number of that element.
(c) Write the electronic configuration of the following atom and ion.
\(^{28}_{14}Si\) ………………………………………..
\(^{37}_{17}Cl^-\) ………………………………………
(d) Complete Table 2.1.
Table 2.1
(e) A sample of thallium, Tl, contains two isotopes, \(^{203}Tl\) and \(^{205}Tl\).
(i) Define the term isotopes.
(ii) The relative abundance of \(^{203}Tl:^{205}Tl\) is in the ratio 3:7.
Calculate the relative atomic mass of thallium in the sample to one decimal place.
relative atomic mass = ……
(iii) Suggest why these two isotopes have identical chemical properties.
▶️ Answer/Explanation
(a) The nucleon number is the total number of protons and neutrons in the nucleus of an atom.
(b) The period number of an element equals the number of occupied electron shells in its atom.
(c) For \(^{28}_{14}Si\): 2,8,4
For \(^{37}_{17}Cl^-\): 2,8,8
The silicon atom has 14 electrons arranged in shells as 2 in the first, 8 in the second, and 4 in the third. The chloride ion has gained one extra electron (17+1=18) arranged as 2,8,8.
(d) Completed table:
| atom or ion | number of protons | number of neutrons | number of electrons |
|---|---|---|---|
| \(^{23}_{11}Na\) | 11 | 12 | 11 |
| \(^{19}_{9}F^-\) | 9 | 10 | 10 |
| \(^{69}_{31}Ga^{3+}\) | 31 | 38 | 28 |
For sodium: neutrons = 23-11 = 12, electrons = protons = 11.
For fluoride ion: neutrons = 19-9 = 10.
The last row is for a gallium ion (Ga³⁺) with 31 protons, 38 neutrons, and 28 electrons (31-3=28).
(e) (i) Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
(ii) Relative atomic mass calculation:
(3 × 203) + (7 × 205) = 609 + 1435 = 2044
2044 ÷ 10 = 204.4
(iii) The two isotopes have identical chemical properties because they have the same number and arrangement of electrons, which determines chemical behavior.
Topic – 7.3
Copper(II) sulfate has the formula CuSO4. Aqueous copper(II) sulfate is a blue solution.
A sample of aqueous copper(II) sulfate is made by adding excess copper(II) oxide, CuO, to hot dilute sulfuric acid, H2SO4.
(a) Complete the symbol equation for this reaction. Include state symbols.
CuO(……) + H2SO4(……) → CuSO4(……) + …… (l)
(b) State one observation which shows that copper(II) oxide is added in excess.
(c) Describe how aqueous copper(II) sulfate can be separated from the reaction mixture.
(d) Crystals of hydrated copper(II) sulfate can be obtained from aqueous copper(II) sulfate by crystallisation.
(i) State what is meant by the term hydrated.
(ii) Write the formula of hydrated copper(II) sulfate.
(iii) Describe how this crystallisation is done.
(e) Aqueous copper(II) sulfate undergoes electrolysis using graphite electrodes.
(i) State why aqueous copper(II) sulfate conducts electricity.
(ii) Give two reasons why the electrodes are made of graphite.
(iii) Describe how the appearance of the electrolyte changes during the electrolysis of aqueous copper(II) sulfate.
(iv) Describe what is seen at the cathode during the electrolysis of aqueous copper(II) sulfate.
(v) Write the ionic half-equation for the reaction at the anode.
(vi) State two differences seen if the electrolysis is repeated using copper electrodes instead of graphite electrodes.
▶️ Answer/Explanation
(a) CuO(s) + H2SO4(aq) → CuSO4(aq) + H2O(l)
Copper(II) oxide is a solid (s), sulfuric acid is aqueous (aq), copper(II) sulfate solution is aqueous (aq), and water is liquid (l).
(b) No more solid dissolves / some solid remains undissolved.
When copper(II) oxide is in excess, there will be some solid left at the bottom of the container that doesn’t dissolve.
(c) Filtration
The excess solid copper(II) oxide can be separated from the aqueous copper(II) sulfate solution by filtration using filter paper.
(d)(i) Substance that is chemically combined with water.
Hydrated compounds have water molecules incorporated into their crystal structure.
(d)(ii) CuSO4•5H2O
Copper(II) sulfate forms a pentahydrate with 5 water molecules per formula unit.
(d)(iii) Heat the solution until saturation point is reached (when crystals start to form at the solution’s surface), then allow it to cool slowly.
The heating evaporates some water, increasing concentration until saturation. Cooling allows crystals to form as solubility decreases with temperature.
(e)(i) It contains mobile ions.
The Cu2+ and SO42- ions in solution are free to move and carry electric charge.
(e)(ii) 1. Conducts electricity
2. Inert (does not react with the electrolyte)
Graphite is used because it’s a good conductor and doesn’t participate in the reaction, serving only as an electrode.
(e)(iii) The blue color becomes lighter/fades.
As copper ions are deposited at the cathode, their concentration in solution decreases, making the color less intense.
(e)(iv) Pink solid (copper metal) forms.
Copper ions (Cu2+) gain electrons at the cathode to form copper metal which appears pinkish-brown.
(e)(v) 4OH– → 2H2O + O2 + 4e–
Hydroxide ions from water are preferentially discharged at the anode, producing oxygen gas and water.
(e)(vi) 1. The color of the solution remains constant
2. The anode dissolves instead of producing gas bubbles
With copper electrodes: copper dissolves at the anode (Cu → Cu2+ + 2e–) and deposits at the cathode, maintaining ion concentration and color. No oxygen is produced.
Topic – 6.4
When magnesium nitrate is heated strongly, magnesium oxide is formed.
(a) The equation for this reaction is shown.
\[ 2Mg(NO_3)_2 \rightarrow 2MgO + 4NO_2 + O_2 \]
(i) State the change in oxidation number of nitrogen, N, in this reaction.
from ………………………. to ……………………….
(ii) Identify the element which is oxidised in this reaction.
(iii) Calculate the volume of NO2 gas, at r.t.p., formed when 7.40 g of Mg(NO3)2 is heated.
Use the following steps.
● Calculate the Mr of Mg(NO3)2.
…………………………
● Calculate the number of moles of Mg(NO3)2 used.
………………………… mol
● Determine the number of moles of NO2 formed.
………………………… mol
● Calculate the volume of NO2 gas, in cm3, at r.t.p.
………………………… cm3
(b) Magnesium oxide, MgO, is an ionic compound.
Complete the dot-and-cross diagram in Fig. 4.1 of the ions in magnesium oxide.
Give the charges on each of the ions.
(c) Oxygen is a covalent molecule.
Complete the dot-and-cross diagram in Fig. 4.2 of a molecule of oxygen. The inner shells have been drawn.
▶️ Answer/Explanation
(a)(i) from +5 to +4
In Mg(NO3)2, nitrogen has an oxidation number of +5 (each oxygen is -2, and the three oxygens make -6, so nitrogen must be +5 to balance). In NO2, nitrogen has an oxidation number of +4 (each oxygen is -2, two oxygens make -4, so nitrogen is +4).
(a)(ii) oxygen
The oxidation number of oxygen changes from -2 in NO3– to 0 in O2, indicating oxidation.
(a)(iii)
1. Mr of Mg(NO3)2 = 24 + (14 + 48)×2 = 148
2. Moles of Mg(NO3)2 = 7.40 g / 148 g/mol = 0.0500 mol
3. From the equation, 2 moles Mg(NO3)2 produce 4 moles NO2, so 0.0500 mol produces 0.100 mol NO2
4. Volume of NO2 = 0.100 mol × 24,000 cm3/mol = 2,400 cm3
(b)
1. Mg ion should show 8 electrons (2 in inner shell, 8 in outer shell) with a 2+ charge
2. O ion should show 8 electrons (2 in inner shell, 8 in outer shell) with a 2- charge
3. The diagram should show Mg2+ and O2- ions with their respective electron configurations
(c)
1. Oxygen molecule should show a double bond between the two oxygen atoms
2. Each oxygen atom should have 4 additional electrons (2 lone pairs)
3. The diagram should show O=O with 4 non-bonding electrons on each oxygen
Topic – 4.2
Hydrogen is the first element of the Periodic Table.
(a) Hydrogen is used in fuel cells to produce electricity in vehicles.
(i) Name the substance which combines with hydrogen in a fuel cell.
(ii) Give one advantage and one disadvantage of using fuel cells instead of gasoline in vehicle engines.
(b) Hydrogen gas can be made from petroleum by a two-step procedure.
step 1 Petroleum is separated into different components.
step 2 Large molecules obtained in step 1 are converted into smaller molecules including hydrogen gas.
(i) Name the process used in step 1.
(ii) Name the process used in step 2.
(c) Organic compounds contain hydrogen atoms.
Calculate the number of hydrogen atoms in 44.0 g of the ester methyl propanoate, CH3CH2COOCH3.
One mole of CH3CH2COOCH3 contains \(6.02 \times 10^{23}\) molecules.
Give your answer in standard form.
(d) For each of the homologous series shown, name a member that contains six hydrogen atoms.
– alkanes ……
– alkenes ……
– alcohols ……
– carboxylic acids ……
(e) Unsaturated alkenes are converted into saturated alkanes by reaction with hydrogen gas.
(i) State why alkenes and alkanes are hydrocarbons.
(ii) State why alkenes are unsaturated.
(iii) Name the catalyst needed to convert alkenes into alkanes.
(iv) Explain why the conversion of alkenes into alkanes is an addition reaction.
▶️ Answer/Explanation
(a)(i) oxygen
In fuel cells, hydrogen combines with oxygen to produce electricity, with water as the only byproduct.
(a)(ii) Advantage: Water is the only product (no pollution) / more efficient
Disadvantage: Hydrogen needs to be stored at high pressure / fewer hydrogen filling stations available
Fuel cells are cleaner but face infrastructure challenges for hydrogen storage and distribution.
(b)(i) fractional distillation
Petroleum is separated into its components based on their different boiling points.
(b)(ii) cracking
Large hydrocarbon molecules are broken down into smaller ones, including hydrogen gas.
(c) \(2.408 \times 10^{24}\) hydrogen atoms
Calculation steps:
1. Molar mass of CH3CH2COOCH3 = 88 g/mol
2. Moles of ester = 44.0 g / 88 g/mol = 0.5 mol
3. Each molecule has 8 H atoms (count the subscripts)
4. Total H atoms = 0.5 mol × 8 × \(6.02 \times 10^{23}\) = \(2.408 \times 10^{24}\)
(d)
– alkanes: ethane (C2H6)
– alkenes: propene (C3H6)
– alcohols: ethanol (C2H5OH)
– carboxylic acids: propanoic acid (C2H5COOH)
(e)(i) They contain carbon and hydrogen atoms only.
Hydrocarbons are compounds consisting exclusively of carbon and hydrogen.
(e)(ii) They have a carbon-carbon double bond.
The presence of double bonds makes alkenes unsaturated as they can add more hydrogen atoms.
(e)(iii) nickel
Nickel is commonly used as a catalyst for hydrogenation reactions.
(e)(iv) Only one product is formed when hydrogen adds across the double bond.
In addition reactions, atoms are added to the double bond without any byproducts being formed.
Topic – 11.8
Natural polyamides are polymers made from amino acid monomers.
(a) State the type of polymerisation reaction that occurs when natural polyamides form.
(b) State the term given to natural polyamides.
(c) An amino acid is represented as shown in Fig. 6.1.
Complete Fig. 6.2 to show the general structure of an amino acid.
Show all of the atoms and all of the bonds in the functional groups.
(d) Three different amino acids are represented as shown in Fig. 6.3.
Complete the diagram in Fig. 6.4 to show the part of the structure of the natural polyamide that forms when the three amino acids, A, B and C, combine. Show all of the atoms and all of the bonds in the linkages.
(e) A mixture of the three amino acids, A, B and C, can be separated and the amino acids identified using paper chromatography.
Complete the equation for \( R_f \).
\[R_f =\]
(f) A sample of the mixture of the three amino acids, A, B and C, is placed onto the baseline and a chromatogram is allowed to develop as shown in Fig. 6.5.
The finished chromatogram is shown in Fig. 6.6.
The amino acids, A, B and C, are colourless. Water is used as the solvent.
(i) Explain why the baseline is drawn in pencil.
(ii) State the type of substance used to make the colourless amino acids visible on the chromatogram in Fig. 6.6.
(iii) Explain why in Fig. 6.6 only two spots are seen from the mixture of three amino acids.
(iv) Suggest how the experiment can be changed to separate all three amino acids.
▶️ Answer/Explanation
(a) condensation
Natural polyamides form through condensation polymerization where amino acids join together, releasing water molecules in the process.
(b) proteins
Natural polyamides are commonly known as proteins, which are essential biological macromolecules made from amino acid monomers.
(c) The general structure of an amino acid is:
This shows the amino group (NH2), carboxyl group (COOH), hydrogen atom, and R group (variable side chain) all connected to a central carbon atom.
(d) The polymer structure would show:
This shows the amide linkages (peptide bonds) between the amino acids, with the R groups extending from the central carbon atoms.
(e) \[R_f = \frac{\text{distance travelled by substance}}{\text{distance travelled by solvent}}\]
The Rf value is a ratio used in chromatography to identify substances based on how far they travel compared to the solvent front.
(f)(i) pencil is insoluble (in solvent)
Pencil marks won’t dissolve in the solvent and interfere with the chromatography results, unlike ink which might run.
(f)(ii) locating agent
A chemical like ninhydrin is used to react with amino acids and produce colored spots that can be seen.
(f)(iii) same \( R_f \) value
Two of the amino acids must have identical or very similar Rf values in this solvent system, causing them to appear as a single spot.
(f)(iv) use a different solvent
Changing the solvent system would alter the relative solubilities and migration rates of the amino acids, potentially separating all three.