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.
(a) The diagram shows the apparatus used in the production of electrical energy in a simple cell.
The zinc electrode dissolves in the electrolyte forming Zn2+(aq) ions.
(i) Draw an arrow on the diagram to show the direction of electron flow.
(ii) Write the ionic half-equation for the reaction that occurs when the zinc electrode dissolves.
(b) The reading on the voltmeter can be increased if either zinc or iron is replaced by another metal.
(i) Name a metal that can replace zinc and increase the reading on the voltmeter.
(ii) Name a metal that can replace iron and increase the reading on the voltmeter.
(c) Fuel cells are used to generate electricity.
(i) Name the reactants in a fuel cell.
(ii) Name the waste product of a fuel cell.
(d) Electricity can be used to break down aqueous or molten ionic compounds.
(i) Name the process which uses electricity to break down aqueous or molten ionic compounds.
(ii) Explain why the ionic compound needs to be aqueous or molten.
(e) Brine is concentrated aqueous sodium chloride.
(i) Name three substances which are manufactured by passing electricity through brine.
(ii) Name a different substance formed when molten sodium chloride is used instead of concentrated aqueous sodium chloride.
▶️ Answer/Explanation
(a)(i) Ans: Arrow from Zn to Fe
Electrons flow from the more reactive metal (Zn) to the less reactive metal (Fe) in the external circuit.
(a)(ii) Ans: Zn → Zn2+ + 2e–
Zinc loses two electrons to form Zn2+ ions, which dissolve in the electrolyte.
(b)(i) Ans: Magnesium (or any metal above Zn in reactivity series)
A more reactive metal increases the potential difference, as it donates electrons more readily.
(b)(ii) Ans: Copper (or any metal below Fe in reactivity series)
A less reactive metal increases the voltage because the difference in reactivity between the two electrodes is greater.
(c)(i) Ans: Hydrogen and Oxygen
These gases react in the fuel cell to produce electricity and water.
(c)(ii) Ans: Water
The only byproduct of the reaction between hydrogen and oxygen in a fuel cell is water.
(d)(i) Ans: Electrolysis
This process uses electric current to decompose ionic compounds into their elements.
(d)(ii) Ans: Mobile ions are required for conduction
In solid state, ions are fixed in a lattice, but in molten/aqueous state, they can move and carry charge.
(e)(i) Ans: Hydrogen, Chlorine, Sodium Hydroxide
These are the three main products of brine electrolysis (chlor-alkali process).
(e)(ii) Ans: Sodium
Molten NaCl electrolysis produces sodium metal at the cathode instead of hydrogen.