(a) Write the molecular formula of X. (i) What type of reaction is represented by the equation? (ii) Consider the following reaction equation: \( \mathrm{C_...

The equation in (a)(ii) is the catalytic cracking of dodecane, \(C_{12}H_{26} \to X + C_8H_{18}\). Balancing atoms: carbon \(12-8=4\); hydrogen \(26-18=8\). Therefore X is butene, \(C_4H_8\), an alkene.

(a) Molecular formula of X

\[ X = C_4H_8 \]

(a)(i) Type of reaction

It is catalytic cracking (thermal decomposition of a long-chain alkane into a shorter alkane and an alkene).

(a)(iii) Structures of two isomers of X (\(C_4H_8\))

Two chain/positional isomers of butene are drawn below, showing every bond and hydrogen atom:

(a)(iv) Names of the isomers

Isomer A is but-1-ene (\(CH_2=CH{-}CH_2{-}CH_3\)) and Isomer B is but-2-ene (\(CH_3{-}CH=CH{-}CH_3\)).

(a)(v) Reaction of X with hydrogen

Butene undergoes catalytic addition (hydrogenation) with hydrogen to give butane:

\[ C_4H_{8} + H_2 \xrightarrow{\;Ni\;} C_4H_{10} \]

(b) One test for fats

Translucent (grease) spot test: rub a small quantity of the substance onto a piece of filter paper and allow it to dry (warm gently if solid). A permanent translucent (oily) spot that does not disappear on drying confirms the presence of fat. (Alternatively, adding Sudan III reagent gives a red-stained oily layer, confirming fat.)

(c) Mass of tetraoxosulphate (VI) acid produced

Equation: \(2SO_{2(g)} + O_{2(g)} + 2H_2O_{(l)} \to 2H_2SO_{4(aq)}\)

Molar mass of \(H_2SO_4 = (2\times1.0) + 32.0 + (4\times16.0) = 2 + 32 + 64 = 98\ \text{g mol}^{-1}\).

From the equation, \(1\) mole of \(O_2\) produces \(2\) moles of \(H_2SO_4\).

\[ 1.5\ \text{mol } O_2 \Rightarrow 2 \times 1.5 = 3\ \text{mol } H_2SO_4 \]

\[ \text{Mass} = \text{moles} \times \text{molar mass} = 3 \times 98 = \mathbf{294\ g} \]

(d)(i) Enthalpy changes in order of increasing magnitude

\[ \Delta H_2 < \Delta H_1 < \Delta H_3 \]

(d)(ii) Explanation

The enthalpy of neutralisation depends on the energy released when \(H^+\) and \(OH^-\) ions combine to form water; the more completely the acid and base ionise, the more \(H^+\) and \(OH^-\) are available and the greater the heat evolved. \(\Delta H_3\) (strong acid HCl + strong base NaOH) is largest because both are fully ionised. \(\Delta H_1\) (weak acid \(CH_3COOH\) + strong base NaOH) is smaller because some energy is absorbed to ionise the weak acid. \(\Delta H_2\) (weak acid + weak base \(NH_4OH\)) is smallest because energy is absorbed to ionise both the weak acid and the weak base.

(e)(i) Substance(s) that can conduct electricity

\(Cu_{(s)}\) only, because it is a metal with a sea of mobile (delocalised) electrons.

(e)(ii) Substance(s) soluble in water

\(BeCl_2\), \(NaH_{(s)}\) and \(HF\) dissolve in water (they are ionic/polar and interact with the polar water molecules), whereas the non-polar covalent \(CCl_4\) is insoluble.