Shapes of molecules and ions

This question is about trends in the periodic table.

The graph shows the melting points for Period 3 elements from sodium to silicon.

Complete the graph to show approximate melting points for P, S, C l and Ar.

Explain why silicon has a higher melting point than aluminium.

Use ideas about structure and bonding in your answer.

The table shows the first ionisation energies for the elements in Period 3.

Write an equation to represent the first ionisation energy of sulfur. Include state symbols.

The table shows that there is a general increase in first ionisation energies across Period 3, but that this trend is not followed by phosphorus to sulfur.

Explain why the first ionisation energy of sulfur is lower than the first ionisation energy of phosphorus.

A student adds an aqueous solution of bromine, Br₂(aq), to two test tubes.

The student adds aqueous sodium iodide, NaI(aq), to the Br₂(aq) in one test tube, and aqueous sodium chloride, NaCl (aq), to the Br₂(aq) in the other test tube.

Complete the table to show the student's observations.

Write ionic equation(s) for any reaction(s) that take place.

Ionic equation(s)........

* The hydrides of silicon, phosphorus and sulfur have simple molecular structures, but their molecules have different shapes.

Explain why molecules of SiH₄, PH₃ and H₂S have different shapes.

Your answer should include ‘dot-and-cross’ diagrams (outer shells only), the names of the shapes and an explanation of why the bond angles are different.

This question is about two oxides of sulfur: sulfur dioxide, SO₂, and sulfur trioxide, SO₃.

SO₃ decomposes to form SO₂ and O₂, as shown in Equilibrium 18.1.

2.25 moles of SO₃ is heated to 550 C in the presence of a catalyst and the resulting mixture allowed to reach equilibrium.

The equilibrium mixture contains 0.900 mol of SO₂ and the total pressure is 2.80 atm.

Calculate the numerical value for K_p for Equilibrium 18.1 under these conditions and state the units of K_p.

Give your answer to 3 significant figures.

The numerical values of K_p for Equilibrium 18.1 at temperatures T₁ and T₂ are shown below.

Explain why T₂ is a higher temperature than T₁.

Suggest how the value of K_p would change if the reaction was repeated with no catalyst added and the pressure of the system increased.

Tick (✓) one box in each row.

SO₂ and SO₃ both have molecules with sulfur in the centre and bond angles of approximately 120.

Explain why the bond angles in SO₃ are 120.

Explain why both SO₂ and SO₃ have polar bonds, but only SO₂ has polar molecules.

The structure of but-1-ene is shown below.

Which row has the correct shape around carbon atoms labelled x and y ?

This question is about the chemistry of compounds containing phosphorus.

Phosphorus forms several acids including H₃PO₄ and H₃PO₃.

H₃PO₄ is a tribasic acid. The equilibria for the dissociations are shown below.

During the equilibria, H₂PO₄^– behaves both as an acid and as a base.

Explain this statement, using the equilibria 1, 2 and 3, as required.

In a H₃PO₃ molecule, the O atoms are covalently bonded to the P atom. The H atoms are bonded to the O atoms.

Draw the structure of a H₃PO₃ molecule, showing all the bonds.

On your diagram, add the values for the O–P–O and P–O–H bond angles.

The systematic name of H₃PO₄ is phosphoric(V) acid.

What is the systematic name of H₃PO₃?

Phosphine, PH₃, is a poisonous gas.

Phosphine reacts with oxygen gas to form phosphorus(V) oxide and water.

Write the equation for this reaction.

Aqueous silver nitrate, AgNO₃, is reduced by PH₃. The unbalanced equation is shown below.

Balance the equation and use oxidation numbers to explain why this is a redox reaction.

Explanation........

When phosphorus(V) chloride, PCl₅, and ammonium chloride are heated together, the compound P₃N₃Cl₆ is formed, together with HCl gas.

P₃N₃Cl₆ has a cyclic structure, like the Kekulé structure of benzene.

Write an equation for the reaction of PCl₅ and ammonium chloride to form P₃N₃Cl₆.

Calculate the percentage by mass of P in P₃N₃Cl₆.

Give your answer to 2 decimal places.

Suggest one example of evidence that could show that P₃N₃Cl₆ has a Kekulé structure rather than a delocalised structure.

In a molecule of P₃N₃Cl₆ all the N and Cl atoms are bonded to P atoms.

Suggest a possible structure for a molecule of P₃N₃Cl₆.

Which ion(s) contain(s) bond angles of approximately 120º?

1 CH₃COO^–

2 C₆H₅O^–

3 (CH₃)₃C⁺

A 1, 2 and 3

B Only 1 and 2

C Only 2 and 3

D Only 1

The three elements that, when in a molecule, can form hydrogen bonds with other suitable molecules.

An element that is a solid at RTP with a simple molecular lattice structure.

The two elements with atoms containing five unpaired electrons.

The element in Period 3 that exists in the solid state as a network of atoms bonded by strong covalent bonds.

The p-block element in Period 3 that forms a compound with fluorine with octahedral molecules.

The element that forms 1– ions most readily.

The element with an average atomic mass of 1.244 10^–22 g.

This question is about nitric acid, hydrochloric acid and sulfuric acid.

Dilute nitric acid reacts with aluminium oxide to form a solution of aluminium nitrate.

Write an equation for this reaction.

The solution contains nitrate ions, NO₃^–.

Draw a ‘dot-and-cross’ diagram for the NO₃^– ion.

Use a different symbol for the extra electron.

Balance the half-equation for the oxidation process in this reaction.

In the reduction process in this reaction, HNO₃ and H⁺ react together to form 2 oxides: X (M_r = 30) and Z (M_r = 18).

Determine the formulae of X and Z and write the half-equation for this reduction.

* Three reactions involving sulfuric acid are shown below.

Reaction 1 Dilute sulfuric acid is reacted with nickel(II) hydroxide to form a green solution.

The solvent is allowed to evaporate leaving hydrated crystals of compound D, with the percentage composition by mass: Ni, 22.33%; S, 12.20%; O, 60.87%; H, 4.60%.

Reaction 2 Concentrated sulfuric acid is reacted with hydrogen bromide, HBr, to form three products:

• an element which exists as diatomic molecules

• a gaseous compound E

• a liquid.

At RTP, 1.00 dm³ of compound E has a mass of 2.67 g.

Reaction 3 Concentrated sulfuric acid acts as a catalyst when 2-hydroxypropanoic acid reacts to form compound F (M_r = 144).

In this reaction, 2 mol of 2-hydroxypropanoic acid forms 1 mol of compound F and 2 mol of water.

Identify compounds D, E and F and construct equations for the reactions.

Show structures for any organic compounds.

These questions are from different areas of chemistry.

Ammonia, NH₃, and ammonium nitrate, NH₄NO₃, are compounds of nitrogen.

The boiling point of NH₃ is –33 °C.

The boiling point of NH₄NO₃ is 210 °C.

Explain why there is a large difference in boiling points.

Two students discuss the oxidation numbers in ammonium nitrate, NH₄NO₃.

One student claims that the two nitrogen atoms have the same oxidation number. The other student disagrees and claims that the nitrogen atoms have different oxidation numbers.

Explain with reasons which student is correct.

What are the percentage compositions of copper and zinc in the brass sample?

Calculate the relative atomic mass of zinc in the sample of brass.

Give your answer to 2 decimal places.

Fill in the table to predict the splitting patterns in the proton NMR spectrum of the

The table shows the chemical shifts for the peaks in the proton NMR spectrum at proton environments 2 and 3.

Proton environment	2	3
Chemical shift,	2.5 ppm	3.6 ppm

Suggest why the peaks for proton environments 2 and 3 have the chemical shifts which are shown in the table.

Glycine, H₂NCH₂COOH, is an -amino acid.

Glycine reacts with NaOH to form the salt H₂NCH₂COONa.

Glycine reacts with HCl to form the salt HOOCCH₂NH₃Cl.

The salts have different H–N–H bond angles.

State the different H–N–H bond angles and explain why they are different.

H₂NCH₂COONa H–N–H bond angle =........ °

HOOCCH₂NH₃Cl H–N–H bond angle =........ °

Glycine reacts with aqueous copper(II) ethanoate to form copper(II) glycinate, Cu(H₂NCH₂COO)₂, and ethanoic acid. Copper(II) glycinate is a complex which exists as two square planar isomers.

Write an equation for this reaction and draw the structures of the two square planar isomers of the complex Cu(H₂NCH₂COO)₂.

equation

Which statement(s) about a molecule of C l –N=O is/are correct?

1: It is a polar molecule.

2: It contains 6 lone pairs of electrons.

3: It has a bond angle of 180°.

A 1, 2 and 3

B Only 1 and 2

C Only 2 and 3

D Only 1

Your answer

Which compound contains a bond angle of approximately 120º?

A CH₃CH₂CN

B CH₃CH(OH)CH₃

C CH₃COOCH₂CH₂CH₃

D CH₃CH₂CH₂NH₂

This question is about two different types of acid found in organic compounds, carboxylic acids and sulfonic acids, as shown in Fig. 6.1.

Complete Table 6.1 to predict bond angles a and b and name the shapes which makes these bond angles in the functional groups of carboxylic acids and sulfonic acids.

Ethanoic acid, CH₃COOH, and methanesulfonic acid, CH₃SO₂OH, are both monobasic acids. The pK_a values are shown in the table.

A student suggests that 1.0 mol dm⁻³ CH₃SO₂OH should have a lower pH value than 1.0 mol dm⁻³ CH₃COOH.

Write an equation, showing conjugate acid–base pairs, for the equilibrium of CH₃SO₂OH with water and explain, with reasons, whether the student is correct.

Label the conjugate acid–base pairs: A1, B1 and A2, B2.

Carboxylic acids and sulfonic acids both form esters.

Sulfonic acid esters can be hydrolysed by aqueous alkali. The equation shows the alkaline hydrolysis of a sulfonic acid ester.

CH₃SO₂OCH₃ + OH⁻ → CH₃SO₂O⁻ + CH₃OH

In the 3 boxes below, add curly arrows to show the mechanism for this reaction.

In the first box, the hydroxide ion acts as a nucleophile.

This question is about copper(II) sulfate, CuSO₄, and sodium thiosulfate, Na₂S₂O₃.

* Calculate ∆_solH(CuSO₄(s)) for reaction 5.2 and determine the enthalpy change of reaction 5.1, ∆_rH.

Assume that the specific heat capacity, c, of the solution is the same as for water.

Show your working, including an energy cycle linking the enthalpy changes.

The standard enthalpy change of reaction, ∆_r , and the standard free energy change, ∆ for converting anhydrous sodium thiosulfate to hydrated sodium thiosulfate are shown below.

Standard entropies are given in the table.

Determine the standard entropy, , of anhydrous sodium thiosulfate, Na₂S₂O₃(s).

Give your answer to 3 significant figures.

Sodium thiosulfate contains the thiosulfate ion, S₂O₃²⁻. The displayed formula of S₂O₃²⁻ can be shown as below.

Predict the O–S–S bond angle and name of the shape of the thiosulfate ion.

Bond angle........

Name of shape........

In some of its reactions, the thiosulfate ion forms the tetrathionate ion, S₄O₆²⁻.

The S₄O₆²⁻ ion is a ‘dimer’ of S₂O₃²⁻.

Draw a displayed formula for the S₄O₆²⁻ ion.

Which molecule is not planar?

A C₂H₄

B C₂H₆

C H₂CO

D HCN

Select an element from the first three periods that fits each of the following descriptions.

The element that forms a 1– ion with the same electron configuration as helium.

The element with the highest first ionisation energy.

The element in Period 3 which has the successive ionisation energies shown below.

The element which forms a compound with fluorine that has octahedral molecules.

An element which reacts with water to form an acidic solution.

The element X, which forms a compound with hydrogen, XH₃, with a molar mass of 34.0 g mol^–1.

An element which forms a compound with hydrogen in which the element has an oxidation number of –4.

The element which has a density of 1.33 × 10^–3 g cm^–3 at room temperature and pressure.

Table 1.1 shows some properties of Period 3 chlorides.

Explain the properties shown in Table 1.1 in terms of bonding and structure.

Four atoms, 1–4, are labelled in the structure below.

Which atom has a trigonal planar arrangement of bonds around it?

A Atom 1

B Atom 2

C Atom 3

D Atom 4