Covalent bonding

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 metal hydrides, lithium aluminium hydride, LiAl H₄, and sodium borohydride, NaBH₄. Both hydrides are used as reducing agents in organic chemistry.

What is the systematic name of the alcohol C₆H₅CH₂OH?

Calculate the mass of LiAl H₄ that the student should use in Step 2 and the percentage yield of pure C₆H₅CH₂OH obtained in Step 4.

Give the mass of LiAl H₄ and the percentage yield of pure C₆H₅CH₂OH to 3 significant figures.

Percentage yield of pure C₆H₅CH₂OH =........ %

In the preparation, LiAl H₄ behaves as a nucleophile.

What is meant by the term nucleophile ?

The mechanism for Step 2 in the reduction of C₆H₅COOC₂H₅ is shown below. In the mechanism, LiAl H₄ can be represented as H^–.

Complete the mechanism by adding curly arrows to the three boxes.

Suggest why the student heated the mixture from Step 2 with dilute aqueous acid in Step 3.

NaBH₄ contains Na⁺ and BH₄^– ions.

Draw a ‘dot‑and‑cross’ diagram for NaBH₄.

Use a different symbol for the electrons from each element.

Show outer electrons only.

NaBH₄/ O₂ cells are being investigated as potential fuel cells.

• The positive electrode contains O₂.

• The negative electrode contains NaBH₄.

The reaction at the positive electrode and the overall cell reaction for a NaBH₄/ O₂ fuel cell are shown below.

Overall cell reaction: NaBH₄ + 2O₂ NaBO₂ + 2H₂O E = 1.64 V

What is the standard electrode potential of the negative electrode in this cell?

Construct the equation for the reaction at the negative electrode of this cell.

All organic compounds have covalent bonds.

What is the electrostatic attraction in a covalent bond between?

A A shared pair of electrons and the nuclei of the bonded atoms

B Cations and delocalised electrons

C Oppositely charged ions

D Two molecules

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.

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

This question is about acids and bases found in the home.

Ethanoic acid, CH₃COOH, is the acid present in vinegar.

A student carries out an experiment to determine the pK_a value of CH₃COOH.

• The concentration of CH₃COOH in the vinegar is 0.870 mol dm^–3.

• The pH of the vinegar is 2.41.

Write the expression for the acid dissociation constant, K_a, of CH₃COOH.

Calculate the pK_a value of CH₃COOH.

Give your answer to two decimal places.

Determine the percentage dissociation of ethanoic acid in the vinegar.

Give your answer to three significant figures.

Many solid drain cleaners are based on sodium hydroxide, NaOH.

• A student dissolves 1.26 g of a drain cleaner in water and makes up the solution to 100.0 cm³.

• The student measures the pH of this solution as 13.48.

Determine the percentage, by mass, of NaOH in the drain cleaner.

Give your answer to three significant figures.

Sodium carbonate, Na₂CO₃, is a base used in washing soda.

Na₂CO₃ contains the carbonate ion, CO₃^2–, shown below.

Draw the ‘ dot-and-cross ’ diagram for the carbonate ion.

Show outer electrons only and use different symbols for electrons from C and O, and any ‘extra’ electrons.