Rates of Reaction
6 marks · NormalA chemist prepares a solution by diluting hydrochloric acid with water, then reacts it with iron filings instead of an iron bar.
Explain why the iron filings react faster with hydrochloric acid than a single iron bar, assuming the same mass is used.
The chemist mixes 10.0 cm³ of 2.00 mol/dm³ hydrochloric acid with 15.0 cm³ of distilled water to make a total solution volume of 25.0 cm³.
Calculate the concentration of hydrochloric acid in the diluted solution. Show your working.
State how doubling the concentration of hydrochloric acid (in a different experiment) would affect the rate of reaction with iron filings. Give a reason for your answer.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Iron filings have a greater surface area than the iron bar of same mass, so more particles are exposed to acid; this increases the rate by providing more frequent collisions. · must show mentions surface area/particles exposed, mentions more frequent collisions/higher rate
(b) 0.80 mol/dm³ mol/dm^3 · must show shows use of concentration formula
(c) The rate would double because doubling concentration gives twice as many particles per volume, so there are twice as many collisions per second. · must show says rate doubles, explains by collisions per second/particles per volume
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · References larger surface area (more particles exposed) for filings compared to bar.
B2 (a) · 1 mark · States more frequent collisions/higher rate due to increased surface area.
M1 (b) · 1 mark · Uses correct dilution formula: conc. = (volume of acid / total volume) x original conc.
A1 (b) · 1 mark · Substitutes correct values: (10.0 / 25.0) × 2.00 = 0.80.
A2 (b) · 1 mark · Final answer: 0.80 mol/dm³ with correct unit.
B1 (c) · 1 mark · States the rate doubles and gives a correct explanation (twice as many collisions per second/more particles per volume).
Worked solution
a) Iron filings react faster as they have a much greater surface area than a single bar of the same mass. This increased exposure gives more particles at the surface for reaction with acid, so there are more successful collisions per second.
b) Concentration = (volume of acid / total volume) × original concentration
c) The rate would double because doubling concentration gives more particles per volume, making collisions between particles occur twice as often.