Topic – 2.5
The formulae of the molecules A to I are shown in Table 1.1.
Table 1.1
Answer the following questions about the molecules, A to I. Each letter may be used once, more than once or not at all.
State which of the molecules A to I:
(a) is an element with a triple bond
(b) is a product of photosynthesis
(c) is used as a fuel
(d) turns limewater milky
(e) undergoes a substitution reaction with alkanes
(f) is a colourless liquid at r.t.p.
(g) is unsaturated
(h) is 21% of clean, dry air
(i) is a reactant in the Haber process.
▶️ Answer/Explanation
(a) G (\( N_2 \))
Nitrogen (\( N_2 \)) is an element with a triple bond between the two nitrogen atoms.
(b) D (\( CO_2 \))
Carbon dioxide is a product of photosynthesis, along with oxygen.
(c) B (\( C_2H_5OH \))
Ethanol (\( C_2H_5OH \)) is commonly used as a fuel, especially in biofuel applications.
(d) D (\( CO_2 \))
Carbon dioxide turns limewater milky due to the formation of calcium carbonate precipitate.
(e) E (\( Cl_2 \))
Chlorine undergoes substitution reactions with alkanes, replacing hydrogen atoms.
(f) B (\( C_2H_5OH \))
Ethanol is a colourless liquid at room temperature and pressure (r.t.p.).
(g) A (\( C_2H_4 \))
Ethene (\( C_2H_4 \)) is unsaturated as it contains a carbon-carbon double bond.
(h) H (\( O_2 \))
Oxygen makes up approximately 21% of clean, dry air.
(i) G (\( N_2 \))
Nitrogen is one of the reactants in the Haber process for ammonia production.
Topic – 4.1
Aluminium is manufactured by the electrolysis of aluminium oxide.
(a) State the name of the main ore of aluminium.
(b) Name the substance mixed with aluminium oxide to reduce the operating temperature of the process.
(c) Explain why the molten mixture in (b) conducts electricity.
(d) Table 2.1 contains some information about the processes which take place at the anode and the cathode.
Table 2.1
(i) Complete Table 2.1:
– Write the number of electrons needed to balance the ionic half-equation for the reaction at the anode.
– Write the ionic half-equation for the reaction at the cathode.
(ii) State why the process at the anode is an oxidation.
(iii) Oxygen is formed at the anode.
Explain why the main gas given off at the anode is carbon dioxide and not oxygen.
(e) State why aluminium is used in food containers.
(f) Aluminium reacts with fluorine to form the ionic compound aluminium fluoride.
Complete the dot-and-cross diagram in Fig. 2.1 of the ions in aluminium fluoride.
Give the charges on the ions.
▶️ Answer/Explanation
(a) bauxite
Bauxite is the primary ore from which aluminium is extracted, containing aluminium oxide along with various impurities.
(b) cryolite
Cryolite (Na3AlF6) is mixed with aluminium oxide to lower its melting point from about 2072°C to around 950°C, making the electrolysis process more energy-efficient.
(c) (it has) mobile ions
The molten mixture contains free-moving ions (Al3+ and O2-) which can carry electric charge through the solution, enabling electrical conduction.
(d)(i)
Anode: 4 electrons (\( 2O^{2-} \rightarrow O_2 + 4e^- \))
Cathode: \( Al^{3+} + 3e^- \rightarrow Al \)
The anode equation shows oxygen ions losing electrons to form oxygen gas. The cathode equation shows aluminium ions gaining electrons to form aluminium metal.
(d)(ii) electrons are lost (from oxide ions)
Oxidation is defined as the loss of electrons. At the anode, oxide ions (O2-) lose electrons to form oxygen gas.
(d)(iii)
1. The anode is made of carbon/graphite
2. The carbon anode reacts with the oxygen produced, forming carbon dioxide
Instead of oxygen gas being released, it reacts with the carbon electrodes to form CO2, which is why we observe carbon dioxide rather than oxygen at the anode.
(e) aluminium is resistant to corrosion
Aluminium forms a protective oxide layer that prevents further reaction, making it ideal for food containers where corrosion resistance is important.
(f)
Dot-and-cross diagram should show:
– Aluminium ion (Al3+) with empty outer shell
– Three fluoride ions (F–) each with 8 electrons (7 dots and 1 cross)
Charges: Al3+ and F–
Aluminium fluoride is ionic with Al transferring 3 electrons (one to each F atom) to form Al3+ and F– ions. The diagram should clearly show this electron transfer.
Topic – 2.5
Sulfur forms two chlorides, P and Q. Chloride P has the formula \( S_2Cl_2 \). Chloride Q has the formula \( SCl_2 \).
(a) Both chlorides are covalently bonded and have low melting points.
Suggest, in terms of attraction between particles, why these chlorides have low melting points.
(b) Chloride P, \( S_2Cl_2 \), forms when sulfur reacts with chlorine.
Write the symbol equation for this reaction.
(c) Complete the dot-and-cross diagram in Fig. 3.1 of a molecule of chloride Q, \( SCl_2 \).
Show outer electrons only.
(d) Chloride P is converted to chloride Q by reaction with chlorine in a closed system. The reversible reaction reaches an equilibrium.
The forward reaction is exothermic.
Suggest two changes to the conditions which will result in a decrease in the concentration of chloride Q at equilibrium.
(e) The rate of the forward reaction in (d) is determined by collision theory.
The rate of reaction depends upon two factors:
- the frequency of collisions between particles
- the proportion of collisions which have energy greater than or equal to the activation energy.
(i) Define the term activation energy.
(ii) Give the symbol for activation energy.
(iii) Complete Table 3.1 to show the effect, if any, when the conditions are changed.
Use only the words increases, decreases or no change.
(f) The reaction of chloride P with chlorine is a redox reaction.
The oxidation number of Cl in chloride P and chloride Q is -1.
Use oxidation numbers to explain why:
- sulfur is oxidised in the forward reaction
- chlorine is oxidised in the reverse reaction.
▶️ Answer/Explanation
(a) These chlorides have low melting points because:
- The attraction between molecules is weak (intermolecular forces)
- They are simple covalent molecules with weak van der Waals’ forces between molecules
(b) The symbol equation is: \( 2S + Cl_2 \rightarrow S_2Cl_2 \)
(c) The dot-and-cross diagram for \( SCl_2 \) should show:
- Sulfur with one bonding pair (dot-cross) with each chlorine
- Sulfur with four non-bonding electrons (dots)
- Each chlorine with six non-bonding electrons (crosses)
(d) To decrease concentration of Q at equilibrium:
- Decrease concentration of \( S_2Cl_2 \) (P) and/or \( Cl_2 \)
- Increase temperature (since forward reaction is exothermic)
(e)(i) Activation energy is the minimum energy that colliding particles must have to react.
(e)(ii) The symbol for activation energy is \( E_a \).
(e)(iii) Table completion:
- Increasing chlorine concentration: increases frequency of collisions, no change to proportion with sufficient energy
- Increasing temperature: increases both frequency and proportion with sufficient energy
- Adding a catalyst: no change to frequency, increases proportion with sufficient energy (by lowering \( E_a \))
(f) Redox explanation:
- Sulfur is oxidized in forward reaction because its oxidation number increases from +1 in \( S_2Cl_2 \) to +2 in \( SCl_2 \)
- Chlorine is oxidized in reverse reaction because its oxidation number increases from -1 in \( SCl_2 \) to 0 in \( Cl_2 \)
In the forward reaction, sulfur’s oxidation state increases (oxidation) while chlorine’s decreases (reduction). The reverse is true for the reverse reaction.
Topic – 7.3
Silver bromide, AgBr, is made when aqueous silver ethanoate, CH3COOAg, is added to aqueous sodium bromide, NaBr.
The equation for the reaction is shown in equation 1.
equation 1
CH3COOAg + NaBr → CH3COONa + AgBr
The method includes the following steps.
step 1 Add 200.0 cm3 of 0.0500 mol/dm3 CH3COOAg to a beaker. This volume contains 0.0100 mol of Ag+ ions.
step 2 Add 50.0 cm3 of aqueous NaBr. This volume contains 0.0100 mol of Br– ions. A precipitate forms.
step 3 Filter the mixture.
step 4 Dry the solid residue until all the water is removed.
step 5 Record the mass of the dry residue.
(a) Complete the ionic equation for the reaction by adding the missing state symbols.
\[ \text{Ag}^+(……) + \text{Br}^-(……) \rightarrow \text{AgBr}(……) \]
(b) Name a different aqueous silver salt which could be used in step 1.
(c) Use the information in step 2 to calculate the concentration of aqueous NaBr.
concentration = …… mol/dm3
(d) State the colour of the precipitate which forms in step 2.
(e) Use the information in step 1, step 2 and equation 1 to determine the number of moles of AgBr formed. Use this value to calculate the mass of AgBr formed.
number of moles of AgBr = ……
mass of AgBr = …… g
(f) Name the salt dissolved in the filtrate in step 3.
(g) The recorded mass of the dry residue in step 5 is greater than the mass calculated in (e) because a step is missing from the procedure.
(i) Suggest the missing step.
(ii) Name the substance responsible for the greater mass of the dry residue.
(h) Barium sulfate can be made by the same method but with different aqueous solutions.
(i) Suggest two aqueous solutions which can be added together to make barium sulfate.
(ii) Write the balanced symbol equation for this reaction.
▶️ Answer/Explanation
(a) \[ \text{Ag}^+(aq) + \text{Br}^-(aq) \rightarrow \text{AgBr}(s) \]
The silver ions and bromide ions are in aqueous solution before reacting, and silver bromide forms a solid precipitate.
(b) silver nitrate
Silver nitrate is another common soluble silver salt that could provide Ag+ ions for the reaction.
(c) 0.200 mol/dm3
Calculation: concentration = moles/volume = 0.0100 mol / (50.0 cm3/1000) = 0.200 mol/dm3
(d) cream
Silver bromide forms a cream-colored precipitate, distinct from the white of silver chloride or yellow of silver iodide.
(e) number of moles of AgBr = 0.0100
mass of AgBr = 1.88 g
Calculation: The limiting reactant is either Ag+ or Br–, both at 0.0100 mol. Molar mass of AgBr = 107.9 (Ag) + 79.9 (Br) = 187.8 g/mol. Mass = 0.0100 mol × 187.8 g/mol = 1.878 g ≈ 1.88 g.
(f) sodium ethanoate
CH3COONa is the soluble byproduct of the reaction that remains in the filtrate.
(g)(i) rinsing of residue
The missing step is washing the precipitate to remove any soluble impurities.
(g)(ii) (crystals of) sodium ethanoate
The extra mass comes from sodium ethanoate that wasn’t washed away, remaining with the precipitate.
(h)(i) barium chloride and sodium sulfate
Any soluble barium salt (e.g., barium chloride, barium nitrate) can react with any soluble sulfate (e.g., sodium sulfate, sulfuric acid) to form barium sulfate precipitate.
(h)(ii) BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaCl(aq)
This is one possible balanced equation for the precipitation reaction forming barium sulfate.
Topic – 11.5
Alkenes are manufactured by cracking larger alkane molecules.
(a) State the source of the large alkane molecules used in cracking.
(b) State two conditions needed for cracking large alkane molecules.
(c) When one molecule of dodecane, \( C_{12}H_{26} \), is cracked, three molecules of but-1-ene and one other product are formed.
(i) Use molecular formulae to complete the symbol equation for this reaction.
\( C_{12}H_{26} \rightarrow \) ………………………………… + …………………………………
(ii) Suggest the type of chemical reaction which happens during cracking.
(d) Propene will undergo polymerisation.
(i) Suggest the name of the polymer formed from propene.
(ii) Draw part of this polymer molecule to show three repeat units.
(iii) State the type of polymerisation propene undergoes.
▶️ Answer/Explanation
(a) petroleum
The large alkane molecules used in cracking come from petroleum, which is the crude oil extracted from the earth. Petroleum contains a mixture of hydrocarbons of varying chain lengths.
(b) 1. high temperature
2. catalyst
Cracking requires two main conditions: high temperature (typically around 400-700°C) to provide the necessary energy to break the carbon-carbon bonds, and a catalyst (usually silica or alumina) to speed up the reaction by lowering the activation energy.
(c)(i) \( C_{12}H_{26} \rightarrow 3C_4H_8 + H_2 \)
When dodecane (\( C_{12}H_{26} \)) cracks to form three but-1-ene (\( C_4H_8 \)) molecules, the remaining two hydrogen atoms form a hydrogen gas (\( H_2 \)) molecule to balance the equation.
(c)(ii) thermal decomposition
Cracking is a type of thermal decomposition reaction where large hydrocarbon molecules are broken down into smaller molecules by heat energy.
(d)(i) poly(propene)
The polymer formed from propene is called poly(propene) or polypropylene, where many propene monomers join together.
(d)(ii)
Structure showing three repeat units of poly(propene):
\[ \text{–[CH}_2\text{–CH(CH}_3\text{)]–[CH}_2\text{–CH(CH}_3\text{)]–[CH}_2\text{–CH(CH}_3\text{)]–} \]
The diagram should show a chain of six carbon atoms with methyl groups (\( CH_3 \)) attached to every other carbon, representing three repeat units of the polymer.
(d)(iii) addition
Propene undergoes addition polymerisation, where the double bond in the propene monomer opens up to form single bonds with adjacent monomers, without the elimination of any small molecules.
Topic – 11.8
Polyamides and polyesters are polymers.
Polyamides can occur naturally or can be manufactured.
(a) Part of the structure of a polyamide is shown in Fig. 6.1.
(i) On Fig. 6.1, draw a circle around one amide linkage.
(ii) Complete Fig. 6.2 to show the structures of the two monomers needed to make the polymer in Fig. 6.1.
Show all of the atoms and all of the bonds in the functional groups.
(iii) Name the other product formed in this polymerisation.
(iv) State the term given to natural polyamides.
(v) Name the type of monomers which are used to make natural polyamides.
(vi) One of the monomers which forms part of a natural polyamide has three carbon atoms.
Complete Fig. 6.3 to show the displayed formula of this monomer.
(b) PET is a polyester.
(i) Name the two types of monomer molecules needed to make polyesters.
(ii) Draw part of the structure of PET which shows two repeat units.
Show all of the atoms and all of the bonds in the linkages.
▶️ Answer/Explanation
(a)(i) One amide linkage circled (the -CO-NH- group)
(a)(ii) The two monomers are:
1. A dicarboxylic acid with structure HOOC-R-COOH (where R is the carbon chain)
2. A diamine with structure H₂N-R’-NH₂ (where R’ is the carbon chain)
Both must show all atoms and bonds in the functional groups.
(a)(iii) Water (H₂O) is the other product formed in this condensation polymerisation.
(a)(iv) Natural polyamides are called proteins.
(a)(v) The monomers used to make natural polyamides are amino acids.
(a)(vi) The displayed formula should show 2-aminopropanoic acid:
H₂N-CH(CH₃)-COOH or fully displayed with all bonds shown.
(b)(i) The two types of monomers needed to make polyesters are:
1. Dicarboxylic acids
2. Diols
(b)(ii)
The structure of PET showing two repeat units should include:
– The ester linkage (-COO-) between units
– The benzene rings in the main chain
– The continuation bonds at both ends to show it’s part of a larger polymer
Example: [-CO-C₆H₄-CO-O-CH₂-CH₂-O-] repeated twice with proper bonds shown