Other important addition reactions of alkenes

3 marks · Normal

Ethene is hydrogenated in industry to form ethane by adding hydrogen gas. Calculate the mass of ethane produced when 4.2 g of ethene reacts completely with excess hydrogen. (Relative atomic masses: C = 12, H = 1)

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6.0 g of ethane g (±0.1)

Also accept: 5.9 g of ethane; 6.02 g of ethane

Common mistakes: Not using mole ratio correctly; Calculation for only 1 mol ethene instead of mass given

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Marking points

M1 · 1 mark · Calculates moles of ethene using relative formula mass (C2H4 = 28).

M2 · 1 mark · Uses correct 1:1 mole ratio from balanced equation to find moles of ethane. (follow-through)

A1 · 1 mark · Calculates correct final mass of ethane to 2 significant figures, in grams. (follow-through)

Worked solution

Step 1: Calculate relative formula masses:

Step 2: Calculate moles of ethene:

Step 3: According to the equation, 1 mol ethene produces 1 mol ethane, so moles of ethane = 0.15 mol.

Step 4: Mass of ethane produced:

(Correction: If an error is found in this working, recalculate with the molarity or correct masses.)

Final answer: 4.5 g of ethane.