Shapes of molecules & ions

This question is about the halogens and some of their compounds.

Explain why chlorine has a lower boiling point than bromine.

Bromine reacts with fluorine to form BrF₃

Determine the oxidation state of bromine and of fluorine in BrF₃

Draw the shape of a BrF₃ molecule.

Include any lone pairs of electrons that influence the shape.

Explain why the BrF₃ molecule has the shape you have drawn.

Shape

Explanation of shape

Solid sodium bromide reacts with concentrated sulfuric acid.

Two reactions occur

• an acid-base reaction

• a redox reaction.

Give an equation and an observation for each reaction.

Equation for acid-base reaction

Observation for acid-base reaction

Equation for redox reaction

Observation for redox reaction

Iodide ions reduce copper(II) ions in aqueous solution to form a precipitate of copper(I) iodide.

Give an equation for this redox reaction.

Which species has a tetrahedral shape?

The oxides P₄O₁₀ and SO₂ both react with water to form acid molecules.

Which row gives the correct shape around the central atom for each acid molecule

This question is about chlorine.

Give an equation to show how chlorine forms an acidic solution in water.

Give an equation for the reaction between chlorine and cold, dilute aqueous sodium hydroxide.

In acidic conditions, ClO₃^– ions oxidise Cl^– ions to form Cl₂

Deduce a half-equation for the oxidation of Cl^– to Cl₂

Deduce a half-equation for the reduction of ClO₃^– to Cl₂

Deduce the overall equation for this reaction.

Half-equation for the oxidation of Cl^– to Cl₂

Half-equation for the reduction of ClO₃^– to Cl₂

Overall equation

Give the equation for the reaction of solid sodium chloride with concentrated sulfuric acid.

State the role of the chloride ions in this reaction.

Equation

Draw the shape of the Cl₃^– ion. Include any lone pairs of electrons that influence the shape.

Chlorine forms an ion with the Group 3 element thallium (T l).

State and explain the bond angle in TlCl₂⁺

Which substance does not have any bond angles of 120°?

This question is about complexes of the transition metal chromium.

State the meaning of the term transition metal complex.

Cr(PF₃)₆ is a complex of chromium that contains molecules of PF₃

The electron pair repulsion theory can be used to predict the shape of a PF₃ molecule.

Draw the shape of a PF₃ molecule. Include any lone pairs of electrons that influence the shape.

Name the shape.

Shape

Suggest why the oxidation state of chromium is zero in Cr(PF₃)₆

The compound [Cr(NH₃)₄Cl₂]Cl contains ammonia molecules.

Deduce the oxidation state of chromium in [Cr(NH₃)₄Cl₂]Cl

Name the type of bond between N and H in ammonia.

The compound [Cr(NH₃)₄Cl₂]Cl contains a complex ion that shows isomerism.

Draw the two isomers of the complex ion.

State the type of isomerism shown.

Isomer 1
Isomer 2

Type of isomerism

Complete the equation to show the formation of one complex that contains chromium in its +3 oxidation state.

The molar enthalpy of vaporisation (H_vap) of a liquid is the enthalpy change when one mole of liquid is converted to vapour at the boiling point of the liquid.

A student does an experiment to determine H_vap for water.

The student:

• places a large beaker on a balance

• pours 500 cm³ of water into the beaker

• uses a 2.4 kW heater to raise the temperature of the water to 100 C

• records the mass of the beaker and hot water

• uses the 2.4 kW heater to boil the water for 100 s

• records the mass of the beaker and remaining water.

The loss in mass is 103 g

Calculate H_vap for water.

Table 2 shows some data about three compounds that all contain the same number of electrons.

All three compounds in Table 2 are polar. Ethanol is the most polar and ethylamine is the least polar.

Explain why all three molecules are polar and why ethylamine is the least polar. In your answer refer to the shapes around, and relative electronegativities of, the most electronegative atoms.

Explain the trend in the boiling points of the three compounds. Refer to the intermolecular forces in all three compounds in your answer.

Which statement about the shapes of ions is not correct?

This question is about some elements in Group 7 and their compounds.

Chlorine is added to some drinking water supplies to decrease the risk of people suffering from diseases such as cholera.

State why the amount of chlorine added must be controlled.

Give an equation for the reaction of chlorine with water to form a solution containing two acids.

Explain, with reference to electrons, why this is a redox reaction.

Equation

A student bubbles chlorine gas through a solution of sodium iodide.

State the observation the student would make.

Give an ionic equation for the reaction.

Ionic equation

The student adds a few drops of concentrated sulfuric acid to a small amount of solid sodium iodide.

Two gaseous sulfur-containing products are formed.

Give an equation for the formation of each of these sulfur-containing products.

State the role of sulfuric acid in the formation of these products.

Equation 1

Equation 2

The student adds a few drops of acidified silver nitrate solution to a solution of an unknown impure sodium halide. The student observes bubbles of gas and a colourless solution. The student bubbles the gas through calcium hydroxide solution and a white precipitate forms.

Deduce the identity of the sodium halide.

Suggest the identity of the gas.

Give an ionic equation for the formation of this gas from the impurity.

Identity of sodium halide

Ionic equation

The ClF₂⁺ ion contains two different Group 7 elements.

Use your understanding of the electron pair repulsion theory to draw the shape of this ion.

Include any lone pairs of electrons that influence the shape.

Explain why the ion has the shape you have drawn.

Suggest a value for the bond angle in the ion.

Shape

Magnesium is used in the extraction of titanium from titanium(IV) chloride.

Give an equation for this reaction.

This question is about fuels.

The petrol fraction obtained from crude oil can be used as fuel in cars.

State the meaning of fraction, as used in the term petrol fraction.

Hexadecane (C₁₆H₃₄) can be cracked at high temperature to form petrol.

Complete the equation to show the cracking of one molecule of hexadecane to form hexane and cyclopentane only.

Give the name of a catalyst used in this cracking reaction.

Carbon dioxide is formed when petrol is burned. Carbon dioxide acts as a greenhouse gas when it absorbs infrared radiation.

Give a reason why carbon dioxide absorbs infrared radiation.

Compound Z (HOCH₂CH₂NH₂) can be used to remove carbon dioxide from the mixture of waste gases produced in some power stations.

Figure 1 shows part of a suggested mechanism for the reaction of Z with carbon dioxide.

Draw two curly arrows to complete the mechanism in Figure 1.

Name compound Z (HOCH₂CH₂NH₂)

Deduce the role of Z in step 2 of the mechanism.

HOCH₂CH₂NH₂ can be represented as XNH₂ [HOCH₂CH₂NH₃]⁺ can be represented as [XNH₃]⁺

Draw the shape of XNH₂ and of [XNH₃]⁺

State whether the H–N–H bond angle in XNH₂ is greater than, the same as, or smaller than that in [XNH₃]⁺

Explain your answer.

Shape of XNH₂ Shape of [XNH₃]⁺

Bioethanol is used as an alternative to fossil fuels.

This statement appeared on a website.

“The fact that bioethanol is a carbon-neutral fuel outweighs the environmental disadvantages of producing bioethanol.”

Evaluate this statement.

In your answer you should include:

• an outline of how bioethanol is produced

• relevant equations

• analysis of the environmental impacts.

Which species has a lone pair of electrons on the central atom?

In which molecule are all the atoms in the same plane?

The melting point of XeF₄ is higher than the melting point of PF₃

Explain why the melting points of these two compounds are different.

In your answer you should give the shape of each molecule, explain why each molecule has that shape and how the shape influences the forces that affect the melting point.

Which has a bond angle of 109.5°?

This question is about structure and bonding.

Draw a diagram to show the strongest type of interaction between two molecules of ethanol (C₂H₅OH) in the liquid phase.

Include all lone pairs and partial charges in your diagram.

Methoxymethane (CH₃OCH₃) is an isomer of ethanol.

Table 5 shows the boiling points of ethanol and methoxymethane.

In terms of the intermolecular forces involved, explain the difference in boiling points.

Draw the shape of the POCl₃ molecule and the shape of the ClF₄^– ion. Include any lone pairs of electrons that influence the shapes.

In a POCl₃ molecule the oxygen atom is attached to the phosphorus atom by a double bond that uses two electrons from phosphorus.

Name each shape.

Suggest a value for the bond angle in ClF₄ ̄

Shape of POCl₃ Shape of ClF₄ ̄

Name of shape of POCl₃

Name of shape of ClF₄ ̄

Bond angle in ClF₄ ̄ 11

Sodium thiosulfate reacts with dilute hydrochloric acid as shown.

Give the simplest ionic equation for this reaction.

The gas SO₂ is a pollutant.

State the property of SO₂ that causes pollution when it enters rivers.

Give an equation to show the reaction of SO₂ with water.

Draw a diagram to show the shape of a molecule of H₂O Include any lone pairs of electrons.

State the HOH bond angle.

Explain this shape and bond angle.

Diagram

The initial rate of the reaction between sodium thiosulfate and hydrochloric acid can be monitored by measuring the time taken for a fixed amount of sulfur to be produced.

Describe an experiment to investigate the effect of temperature on the initial rate of this reaction.

Include

• a brief outline of your method

• how you will measure the time taken for a fixed amount of sulfur to be formed

• how you will present your results in graphical form

• a sketch of the graph that you would expect.

This question is about sodium and some of its compounds.

Use your knowledge of structure and bonding to explain why sodium bromide has a melting point that is higher than that of sodium, and higher than that of sodium iodide.

When 250 mg of sodium were added to 500 cm³ of water at 25 °C a gas was produced.

Give an equation for the reaction that occurs. Calculate the volume, in cm³, of the gas formed at 101 kPa

The gas constant, R = 8.31 J K^–1 mol^–1

Calculate the concentration, in mol dm^–3, of sodium ions in the solution produced in the reaction in Question 08.2.

Sodium reacts with ammonia to form the compound NaNH₂ that contains the

Draw the shape of the NH₂^– ion. Include any lone pairs of electrons that influence the shape.

Predict the bond angle. Justify your prediction.

Shape

Which species has a shape that is influenced by the presence of one or more lone pairs of electrons around the central atom?