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SL Paper 2

Electrons are arranged in energy levels around the nucleus of an atom.

The diagram represents possible electron energy levels in a hydrogen atom.

Explain why the first ionization energy of calcium is greater than that of potassium.

[2]

All models have limitations. Suggest two limitations to this model of the electron energy levels.

[2]

b(i).

Draw an arrow, labelled X, to represent the electron transition for the ionization of a hydrogen atom in the ground state.

[1]

b(ii).

Draw an arrow, labelled Z, to represent the lowest energy electron transition in the visible spectrum.

[1]

b(iii).

When heated in air, magnesium ribbon reacts with oxygen to form magnesium oxide.

The reaction in (a)(i) was carried out in a crucible with a lid and the following data was recorded:

Mass of crucible and lid = 47.372 ±0.001 g

Mass of crucible, lid and magnesium ribbon before heating = 53.726 ±0.001 g

Mass of crucible, lid and product after heating = 56.941 ±0.001 g

When magnesium is burnt in air, some of it reacts with nitrogen to form magnesium nitride according to the equation:

3 Mg (s) + N₂(g) → Mg₃N₂(s)

The presence of magnesium nitride can be demonstrated by adding water to the product. It is hydrolysed to form magnesium hydroxide and ammonia.

Most nitride ions are ¹⁴N^3–.

Write a balanced equation for the reaction that occurs.

[1]

a(i).

State the block of the periodic table in which magnesium is located.

[1]

a(ii).

Identify a metal, in the same period as magnesium, that does not form a basic oxide.

[1]

a(iii).

Calculate the amount of magnesium, in mol, that was used.

[1]

b(i).

Determine the percentage uncertainty of the mass of product after heating.

[2]

b(ii).

Assume the reaction in (a)(i) is the only one occurring and it goes to completion, but some product has been lost from the crucible. Deduce the percentage yield of magnesium oxide in the crucible.

[2]

b(iii).

Evaluate whether this, rather than the loss of product, could explain the yield found in (b)(iii).

[1]

c(i).

Suggest an explanation, other than product being lost from the crucible or reacting with nitrogen, that could explain the yield found in (b)(iii).

[1]

c(ii).

Calculate coefficients that balance the equation for the following reaction.

__ Mg₃N₂(s) + __ H₂O (l) → __ Mg(OH)₂(s) + __ NH₃(aq)

[1]

d(i).

Determine the oxidation state of nitrogen in Mg₃N₂ and in NH₃.

[1]

d(ii).

Deduce, giving reasons, whether the reaction of magnesium nitride with water is an acid–base reaction, a redox reaction, neither or both.

[2]

d(iii).

State the number of subatomic particles in this ion.

[1]

e(i).

Some nitride ions are ¹⁵N^3–. State the term that describes the relationship between ¹⁴N^3– and ¹⁵N^3–.

[1]

e(ii).

The nitride ion and the magnesium ion are isoelectronic (they have the same electron configuration). Determine, giving a reason, which has the greater ionic radius.

[1]

e(iii).

Suggest two reasons why atoms are no longer regarded as the indivisible units of matter.

[2]

State the types of bonding in magnesium, oxygen and magnesium oxide, and how the valence electrons produce these types of bonding.

[4]

This question is about peroxides.

Hydrogen peroxide decomposes to water and oxygen when a catalyst such as potassium iodide, KI, is added.

2H₂O₂ (aq) $\xrightarrow{{{\text{KI (aq)}}}}$ O₂ (g) + 2H₂O (l)

Suggest why many chemicals, including hydrogen peroxide, are kept in brown bottles instead of clear colourless bottles.

[1]

In a laboratory experiment solutions of potassium iodide and hydrogen peroxide were mixed and the volume of oxygen generated was recorded. The volume was adjusted to 0 at t = 0.

The data for the first trial is given below.

Plot a graph on the axes below and from it determine the average rate of formation of oxygen gas in cm³ O₂ (g) s⁻¹.

Average rate of reaction:

[3]

b(i).

Additional experiments were carried out at an elevated temperature. On the axes below, sketch Maxwell–Boltzmann energy distribution curves at two temperatures T₁ and T₂, where T₂ > T₁.

[2]

b(ii).

Apart from a greater frequency of collisions, explain, by annotating your graphs in (b)(ii), why an increased temperature causes the rate of reaction to increase.

[2]

b(iii).

MnO₂ is another possible catalyst for the reaction. State the IUPAC name for MnO₂.

[1]

b(iv).

Comment on why peracetic acid, CH₃COOOH, is always sold in solution with ethanoic acid and hydrogen peroxide.

H₂O₂ (aq) + CH₃COOH (aq) $\rightleftharpoons$ CH₃COOOH (aq) + H₂O (l)

[1]

Sodium percarbonate, 2Na₂CO₃•3H₂O₂, is an adduct of sodium carbonate and hydrogen peroxide and is used as a cleaning agent.

M_r (2Na₂CO₃•3H₂O₂) = 314.04

Calculate the percentage by mass of hydrogen peroxide in sodium percarbonate, giving your answer to two decimal places.

[2]

Rhenium, Re, was the last element with a stable isotope to be isolated.

One chloride of rhenium has the empirical formula ReCl₃.

Before its isolation, scientists predicted the existence of rhenium and some of its properties.

Suggest the basis of these predictions.

[2]

Describe how the relative reactivity of rhenium, compared to silver, zinc, and copper, can be established using pieces of rhenium and solutions of these metal sulfates.

[2]

State the name of this compound, applying IUPAC rules.

[1]

c(i).

Calculate the percentage, by mass, of rhenium in ReCl₃.

[2]

c(ii).

Copper forms two chlorides, copper(I) chloride and copper(II) chloride.

An electrolysis cell was assembled using graphite electrodes and connected as shown.

State the electron configuration of the Cu⁺ ion.

[1]

a(i).

Copper(II) chloride is used as a catalyst in the production of chlorine from hydrogen chloride.

4HCl (g) + O₂ (g) → 2Cl₂ (g) + 2H₂O (g)

Calculate the standard enthalpy change, ΔH^θ, in kJ, for this reaction, using section 12 of the data booklet.

[2]

a(ii).

The diagram shows the Maxwell–Boltzmann distribution and potential energy profile for the reaction without a catalyst.

Annotate both charts to show the activation energy for the catalysed reaction, using the label E_{a (cat)}.

[2]

a(iii).

Explain how the catalyst increases the rate of the reaction.

[2]

a(iv).

Solid copper(II) chloride absorbs moisture from the atmosphere to form a hydrate of formula CuCl₂• $x$ H₂O.

A student heated a sample of hydrated copper(II) chloride, in order to determine the value of $x$ . The following results were obtained:

Mass of crucible = 16.221 g
Initial mass of crucible and hydrated copper(II) chloride = 18.360 g
Final mass of crucible and anhydrous copper(II) chloride = 17.917 g

Determine the value of $x$ .

[3]

State how current is conducted through the wires and through the electrolyte.

Wires:

Electrolyte:

[2]

c(i).

Write the half-equation for the formation of gas bubbles at electrode 1.

[1]

c(ii).