Studyledge

Rates of Reaction: Question 4

Syllabus 6.2

Structured Core 6 marks

A school science technician prepares nitrogen gas for other experiments by warming a mixture of ammonium chloride solution and sodium nitrite solution, which react together: NH4Cl(aq)+NaNO2(aq)→NaCl(aq)+N2(g)+2H2O(l)\text{NH}_4\text{Cl}(aq) + \text{NaNO}_2(aq) \rightarrow \text{NaCl}(aq) + \text{N}_2(g) + 2\text{H}_2\text{O}(l)

The technician carries out two trials, each using the same total number of moles of ammonium chloride, and keeping every other condition the same:

  • Trial 1: the ammonium chloride is dissolved in a larger volume of water, making a more dilute solution.
  • Trial 2: the ammonium chloride is dissolved in a smaller volume of water, making a more concentrated solution.

The volume of nitrogen gas collected in a gas syringe is recorded every 30 seconds.

Time / s 0 30 60 90 120 150 180
Volume of N2 in trial 1 / cm3 0 12 20 26 30 32 32
Volume of N2 in trial 2 / cm3 0 20 28 31 32 32 32

(a) State the factor that is being investigated by comparing trial 1 and trial 2. [1]

(b) Calculate the average rate of reaction in trial 1 between t=0t=0 and t=60t=60 seconds. Give the units of your answer. [2]

(c) Both trials eventually produce the same final volume of gas, 32 cm332\text{ cm}^3. Using the fact (given above) that both trials use the same total number of moles of ammonium chloride, state whether it is the rate of reaction or the total amount of gas produced that is changed by altering the concentration of a solution, and explain your answer. [2]

(d) State what the shape of both graphs at t=150 st=150\text{ s} tells you about each reaction at this time. [1]

Show worked solution Hide worked solution

Worked solution

Part (a): Identifying the factor investigated

Everything about the two trials is kept the same except how the same amount of ammonium chloride was dissolved: a larger volume of water in trial 1 (dilute) and a smaller volume in trial 2 (concentrated). The factor being investigated is therefore the concentration of the ammonium chloride solution.

Part (b): Calculating the average rate in trial 1

Between t=0t=0 and t=60t=60 seconds, the volume of gas collected in trial 1 rises from 00 to 20 cm320\text{ cm}^3:

average rate=change in volumechange in time=20−060−0=2060\text{average rate} = \frac{\text{change in volume}}{\text{change in time}} = \frac{20-0}{60-0} = \frac{20}{60}

average rate=0.33 cm3/s  (to 2 s.f.)\text{average rate} = 0.33\text{ cm}^3\text{/s} \; \text{(to 2 s.f.)}

Part (c): Why both trials reach the same final volume

Even though trial 2 reacts faster (it is more concentrated), the question tells us both trials start with the same total number of moles of ammonium chloride. Using a smaller volume of water in trial 2 only changes how tightly packed those moles are (the concentration); it does not change how many moles of ammonium chloride are present.

This shows that changing the concentration of a solution changes the rate of a reaction. It does not change the total amount of product that can ever be formed. Since both trials use the same total amount of ammonium chloride, exactly the same maximum volume of nitrogen gas, 32 cm332\text{ cm}^3, is produced in each case; only how quickly that volume is reached is different.

Part (d): Interpreting the flat sections of the graphs

By t=150 st=150\text{ s}, the readings for both trial 1 and trial 2 have stopped increasing. The graphs are flat (horizontal). A flat graph means the volume of gas is no longer changing, so no more nitrogen gas is being produced. This tells us that both reactions have finished by t=150 st=150\text{ s}: all of the ammonium chloride available has already reacted in each trial.

Final answers

  • (a) The concentration of the ammonium chloride solution.
  • (b) 0.33\boxed{0.33} cm3/s (2 s.f.).
  • (c) Concentration changes the rate, not the total amount of gas: both trials use the same total moles of ammonium chloride, so they produce the same maximum volume of gas.
  • (d) Both reactions have finished (stopped producing gas) by t=150 st=150\text{ s}.