Question
This question is about alkanes and alkenes.
(a) Short-chain alkanes and alkenes can be formed from long-chain alkanes in a chemical reaction.
(i) Name the type of chemical reaction which forms short‑chain alkanes and alkenes from long‑chain alkanes.[1]
(ii) Decane has 10 carbon atoms. It forms ethane and ethene as the only products in this type of chemical reaction.
Write the chemical equation for this reaction.[3]
(b) Ethane reacts with chlorine at room temperature to form chloroethane, C2H5Cl, and one other product.
(i) Name the other product formed.[1]
(ii) State the condition needed for this reaction to take place.[1]
(c) Ethene reacts with chlorine at room temperature to form dichloroethane, C2H4Cl2.
C2H4 + Cl2 → C2H4Cl2
(i) State why this is an addition reaction.[1]
(ii) The chemical equation for this reaction can be represented as shown.
The energy change for the reaction is –180kJ/mol.
Use the bond energies in the table to calculate the bond energy of a C–Cl bond, in kJ/mol.
Use the following steps.
step 1 Calculate the energy needed to break bonds.
energy needed to break bonds = kJ
step 2 Use your answer in step 1 and the energy change for the reaction to determine the energy released when bonds are formed.
energy released when bonds form = kJ
step 3 Use your answer in step 2 and bond energy values to determine the energy of a C–Cl bond.
bond energy of a C–Cl bond = kJ/mol
[4][Total: 11]
Answer/Explanation
Ans:
5(a)(i) cracking
5(a)(ii) C10H22 → 4C2H4 + C2H6
C10H22 as only reactant
formulae of ethene and ethane as only products
correct equation
5(b)(i) hydrogen chloride
5(b)(ii) ultraviolet light
5(c)(i) (only) one product is formed
5(c)(ii) M1 Bond energy in breaking bonds
= [(4 × 410) + 610 + 240] = 2490 (kJ / mol)
M2 Use of total E change to find bond energy of C2H4Cl2
= M1 + 180 = 2490 + 180 = 2670 (kJ / mol)
M3 Determination of total C–Cl bond energy
= M2 – [(4 × 410) + 350] = 2670 – 1990 = 680 (kJ / mol)
M4 Determination of each C–Cl bond energy
= M3 / 2 = 680 / 2 = 340 (kJ / mol)
Question
Many organic compounds contain carbon, hydrogen and oxygen only.
(a) An organic compound V has the following composition by mass.
C, 48.65%; H, 8.11%; O, 43.24%
Calculate the empirical formula of compound V.
empirical formula = …………………………
(b) Compound W has the empirical formula CH4O and a relative molecular mass of 32.
Calculate the molecular formula of compound W.
molecular formula = …………………………
(c) Compounds X and Y have the same general formula.
X and Y are both carboxylic acids.
Compound X has the molecular formula \(C_2H_4O_2\).
Compound Y has the molecular formula \(C_4H_8O_2\).
(i) Deduce the general formula of compounds X and Y.
(ii) Draw the structure of compound Y. Show all of the atoms and all of the bonds.
Name compound Y.
name …………………………………………………………………………………………………………………..
(iii) Give the name used to describe a ‘family’ of similar compounds with the same general
formula, similar chemical properties and the same functional group.
(d) Propene is an unsaturated hydrocarbon. The formula of propene is shown.
\(CH_3CH=CH_2\)
(i) State the colour change observed when propene is added to aqueous bromine.
from ……………………………………………………. to ……………………………………………………
(ii) Propene can be produced by cracking long chain alkanes.
Pentadecane, \(C_{15}H_{32}\), is cracked to produce an alkane and propene in a 1:2 molar ratio.
Complete the chemical equation for this reaction.
\(C_{15}H_{32}\) → ………………………………… + …………………………………
(iii) Propene can be converted into poly(propene).
Name the type of polymerisation that occurs when propene is converted into poly(propene).
(iv) Complete the diagram to show a section of poly(propene).
Answer/Explanation
Answer:
(a) 48.65 / 12 8.11 / 1 43.24 / 16 (1)
OR evaluation
4.05 8.11 2.7(0)
divide all by smallest
OR 1.5 : 3 : 1
OR
6 : 3 : 2 (1)
\(C_3H_6O_2\) (1) ALLOW symbols in any order
(b) ( \(M_r\) of \(CH_4O\) = 32)
\(CH_4O\) (1)
(c) (i) \(C_nH_{2n}O_2\)
OR
\(C_nH_{2n+1} COOH\)
(ii) butanoic acid (1)
fully displayed carboxylic acid group (1)
correct structure of butanoic acid showing all atoms and bonds (1)
(iii) homologous series
(d) (i) brown to colourless
(ii) \(C_9H_{20}\) (1)
\(2C_3H_6\) (1)
(iii) addition
(iv) 
any one repeat unit (1)
both repeat units fully correct (1)
Question
Alkenes and carboxylic acids are both families of similar compounds with similar chemical properties.
Alkenes and carboxylic acids have different reactions.
(a) State the term used for a ‘family’ of similar compounds.
(b) State the general formula of alkenes.
(c) The structure of but-2-ene is shown.
(i) But-2-ene reacts with aqueous bromine in an addition reaction.
Describe the colour change seen when but-2-ene is added to aqueous bromine.
from ……………………………………………………. to ……………………………………………………
(ii) State what is meant by the term addition reaction.
(iii) Write the chemical equation for the reaction between but-2-ene and bromine.
(iv) But-2-ene forms a polymer.
Suggest the name of the polymer formed from but-2-ene.
(v) Name and draw a structural isomer of but-2-ene.
Show all of the atoms and all of the bonds.
name …………………………………………………………………………………………………………………..
structure
(d) Butanoic acid, \(CH_3CH_2CH_2COOH\), is a carboxylic acid.
(i) Deduce the empirical formula of butanoic acid.
(ii) Complete the chemical equation for the reaction of butanoic acid and sodium carbonate,
\(Na_2CO_3\).
\(2CH_3CH_2CH_2COOH + Na_2CO_3\) → ……………………………… + …………… + ……………
(iii) Butanoic acid reacts with methanol to form an organic compound and water.
● Name the organic compound formed.
● Draw the structure of the organic compound formed.
Show all of the atoms and all of the bonds.
Answer/Explanation
Answer:
(a) homologous series
(b) \(C_nH_{2n}\)
(c) (i) orange to colourless
(ii) (only) one product is formed
(iii) \(C_4H_8 + Br_2 → C_4H_8Br_2\)
\(C_4H_8Br_2\) (1)
equation fully correct (1)
(iv) (poly) but-2-ene
(v) but-1-ene (1)
structure of but-1-ene
(d) (i) \(C_2H_4O\)
(ii) \(2C_3H_7COOH + Na_2CO_3 → 2C_3H_7COONa + H_2O + CO_2\)
\(C_3H_7COONa\) (1)
equation fully correct (1)
(iii) methyl butanoate (1)
ester link (1)
rest of structure (1)