Fuels, alkanes and alkenes — GCSE Chemistry practice questions

Question 1

Two unlabelled bottles in a school prep room contain hexane and hexene. A few drops of aqueous bromine are shaken with a sample from each bottle.

Which row shows the colour of each mixture at the end?

Accepted Answer

C: orange · colourless. Hexene is an alkene: bromine adds across its C=C bond, so the orange colour disappears and the mixture becomes colourless. Hexane is a saturated alkane and does not react, so the bromine stays orange.

Question 2

Fig. 1 shows the displayed formula of a hydrocarbon.

How many covalent bonds are there in one molecule of this hydrocarbon?

Accepted Answer

C: 16. The molecule is C₅H₁₂. Its five carbon atoms are joined by 4 C–C bonds, and each of the 12 hydrogen atoms makes one C–H bond, so there are 4 + 12 = 16 covalent bonds.

Question 3

Methane, CH₄, is mixed with an excess of chlorine and left in ultraviolet light. Hydrogen atoms in the methane are replaced by chlorine atoms, one at a time.

How many different organic products (compounds that contain carbon) can form?

Accepted Answer

C: 4. With excess chlorine the substitution continues: CH₃Cl, CH₂Cl₂, CHCl₃ and CCl₄ can all form, so there are 4 organic products. The other product of every step, HCl, contains no carbon, so it is not counted.

	with hexane	with hexene
A	colourless	colourless
B	colourless	orange
C	orange	colourless
D	orange	orange