IB Chemistry HL Reactivity 2: How much, how fast and how far? questions

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Question 1

Paper 2Medium7 marks

A student prepares magnesium hydroxide by mixing aqueous magnesium chloride with aqueous sodium hydroxide. The reaction is

\ceMgCl2(aq)+2NaOH(aq)−>Mg(OH)2(s)+2NaCl(aq)\ce{MgCl2(aq) + 2NaOH(aq) -> Mg(OH)2(s) + 2NaCl(aq)}

Figure 1 shows the quantities used and the mass of solid collected after filtration and drying.

MgCl₂(aq) aliquotn(MgCl₂) = 7.50 × 10⁻³ molNaOH(aq)V = 30.0 cm³c = 0.400 mol dm⁻³mixfilter, thendrycollected solidmeasured mass =0.325 g

Use relative atomic masses from the data booklet where required.

(a)

Calculate the amount, in mol, of NaOH used. Give your answer to three significant figures.

[1]
(b)

Using your answer to (a) and Figure 1, determine the limiting reactant and calculate the theoretical mass, in g, of \ceMg(OH)2\ce{Mg(OH)2} Show your working.

[3]
(c)

Calculate the percentage yield of \ceMg(OH)2\ce{Mg(OH)2} to three significant figures.

[1]
(d)

Explain one reason why an experimental yield could be lower than the theoretical yield, and one reason why its apparent value could be higher.

[2]

Question 2

Paper 2Medium6 marks

Nitric oxide is oxidized by oxygen to form nitrogen dioxide:

2NO(g) + O₂(g) → 2NO₂(g)

Initial rates were measured at 298 K.

Trial[NO] / mol dm⁻³[O₂] / mol dm⁻³Initial rate / mol dm⁻³ s⁻¹
10.02000.01002.40 × 10⁻⁴
20.04000.01009.60 × 10⁻⁴
30.02000.02004.80 × 10⁻⁴

At 308 K, an experiment with [NO] = 0.0200 mol dm⁻³ and [O₂] = 0.0100 mol dm⁻³ gave an initial rate of 3.60 × 10⁻⁴ mol dm⁻³ s⁻¹. Assume the rate equation is unchanged between these temperatures.

(a)

Determine the order of the reaction with respect to NO and O₂.

[2]
(b)

Write the rate equation for the reaction.

[1]
(c)

Calculate the rate constant, k, at 298 K, including its units.

[2]
(d)

State which temperature has the larger value of k, using the measurements to justify your answer.

[1]

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