Electronegativity and bond polarity

Which molecule does not have a permanent dipole?

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 molecule has a permanent dipole?

This question is about Period 3 elements.

Figure 2 shows the second ionisation energies of some elements in Period 3.

Draw a cross (x) on Figure 2 to show the second ionisation energy of silicon.

Identify the element in Period 3, from sodium to argon, that has the highest second ionisation energy.

Give an equation, including state symbols, to show the process that occurs when the second ionisation energy of this element is measured.

If you were unable to identify the element you may use the symbol Q in your equation.

Equation

Explain why the atomic radius decreases across Period 3, from sodium to chlorine.

Identify the element in Period 3, from sodium to chlorine, that has the highest electronegativity.

Phosphorus burns in air to form phosphorus(V) oxide. Give an equation for this reaction.

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.

This question is about 2-bromopropane.

Define the term electronegativity.

Explain the polarity of the C–Br bond in 2-bromopropane.

Electronegativity

Draw the skeletal formula of the main organic species formed in the reaction between a large excess of 2-bromopropane and ammonia.

Give a use for the organic product.

Skeletal formula

This question is about periodicity, the Period 4 elements and their compounds.

State the meaning of the term periodicity.

Identify the element in Period 4 with the highest electronegativity value.

Identify the element in Period 4 with the largest atomic radius. Explain your answer.

The equations for two reactions of arsenic(III) oxide are shown.

Name the property of arsenic(III) oxide that describes its ability to react in these two

Complete the equation for the formation of arsenic hydride.

Which diagram shows the correct bonding and correct bond polarity in a molecule of oxygen difluoride?

In the early twentieth century the apparatus shown in the diagram was used to investigate atomic structure. When He²⁺ particles were fired at a thin sheet of gold, most of the particles were detected at point P.

Nitrobenzene reacts when heated with a mixture of concentrated nitric acid and concentrated sulfuric acid to form a mixture of three isomeric dinitrobenzenes.

Write an equation for the reaction of concentrated nitric acid with concentrated sulfuric acid to form the species that reacts with nitrobenzene.

Name and outline a mechanism for the reaction of this species with nitrobenzene to form 1,3-dinitrobenzene.

Name of mechanism

Mechanism

The dinitrobenzenes shown were investigated by thin layer chromatography

In an experiment, carried out in a fume cupboard, a concentrated solution of pure 1,4-dinitrobenzene was spotted on a TLC plate coated with a solid that contains polar bonds. Hexane was used as the solvent in a beaker with a lid.

The start line, drawn in pencil, the final position of the spot and the final solvent front are shown on the chromatogram in Figure 3

Use the chromatogram in Figure 3 to deduce the R_f value of 1,4-dinitrobenzene in this experiment.

Tick (✓) one box.

State in general terms what determines the distance travelled by a spot in TLC.

To obtain the chromatogram, the TLC plate was held by the edges and placed in the solvent in the beaker in the fume cupboard. The lid was then replaced on the beaker.

Give one other practical requirement when placing the plate in the beaker.

A second TLC experiment was carried out using 1,2-dinitrobenzene and 1,4-dinitrobenzene. An identical plate to that in Question 8.3 was used under the same conditions with the same solvent. In this experiment, the R_f value of 1,4-dinitrobenzene was found to be greater than that of 1,2-dinitrobenzene.

Deduce the relative polarities of the 1,2-dinitrobenzene and 1,4-dinitrobenzene and explain why 1,4-dinitrobenzene has the greater R_f value.

Relative polarities

Explanation

A third TLC experiment was carried out using 1,2-dinitrobenzene. An identical plate to that in Question 8.3 was used under the same conditions, but the solvent used contained a mixture of hexane and ethyl ethanoate.

A student stated that the R_f value of 1,2-dinitrobenzene in this third experiment would be greater than that of 1,2-dinitrobenzene in the experiment in Question 8.6

Is the student correct? Justify your answer.