Homework: Radiation, Radicals and Bonds worksheet (50 Marks) Name: _______________ Before starting this homework read Chemical Ideas 6.2 and 6.3. Add to your notes as necessary

Question 1 The gas NO2, like SO2, reacts in the atmosphere to form an acid. The mechanism for this is shown below:

step 1 O3 O2 + O

step 2 O + H2O 2OH

step 3 OH + NO2 HNO3

(i) What type of radiation is needed for step 1 to occur?

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(ii) In step 2, hydroxyl radicals, OH, are formed. What does the term radical tell you about the electron structure of OH?

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(iii) Name the acid formed

................................................................................................................................... [1] Question 2 Most of the chloromethane and bromomethane molecules that enter the troposphere are removed by reaction with hydroxyl radicals. Hydroxyl radicals form from water molecules in the troposphere.

(i) Draw a dot-cross diagram for the hydroxyl, OH, radical.

[2]

(ii) Ozone in the troposphere breaks down to form an oxygen atom and an oxygen molecule. Name the type of bond fission that occurs in this reaction.

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(iii) This bond breaking process requires energy. Name the type of radiation that is needed for this process and give the source of this energy.

Type.........................................................................................................................

Source..................................................................................................................... [2]

(iv) An oxygen atom reacts with a water molecule in the troposphere to produce hydroxyl radicals. Write the balanced chemical equation for this reaction.

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(v) Two of the steps in the chain reaction between hydroxyl radicals and chloromethane are shown below.

hv

CH3Cl + OH CH2Cl + H2O equation 1 CH2Cl + OH CH2ClOH equation 2

Classify the steps shown in equations 1 and 2 as being initiation, propagation or termination, as appropriate.

Equation 1...............................................................................................................

Equation 2.............................................................................................................. [2]

Question 3

Trifluorochloromethane, CF3Cl, is an example of a chlorofluorocarbon, CFC, which was commonly

used as a propellant in aerosols. Nowadays, CFCs have limited use because of the damage

caused to the ozone layer.

a. Draw a diagram to show the shape and bond angles of a molecule of CF3Cl. [2]

b. Suggest a property that made CF3Cl suitable as a propellant in an aerosol. ..................................................................................................................................... [1]

c. When CFCs are exposed to high energy ultraviolet radiation in the upper atmosphere, homolytic fission takes place to produce radicals. Explain what is meant by the terms homolytic and fission.

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d. Suggest which bond is most likely to be broken when CF3Cl is exposed to ultraviolet radiation. Explain your answer.

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e. Identify the two radicals most likely to be formed when CF3Cl is exposed to ultraviolet radiation. ...................................................... and ........................................................... [2]

Question 4

To change one mole of molecular HCl from the lowest vibrational energy level to the next requires 32.7kJ of energy.

a. How much energy in joules is needed to change one molecule of HCl from ground state to the next vibrational energy level?

enrgy J [2]

b. What frequency would be absorbed when one molecule of HCl absorbs energy in this way?

frequency Hz [2]

c. Which region of the electromagnetic spectrum is this frequency found? (See the electromagnetic spectrum in CI 6.1 p.119)

...................................................................................................................... [1]

d. Calculate the wavelength, in m, of this frequency. Give your answer to 3 significant figures. wavelength m [3]

Question 5 The reaction of carbon monoxide with hydroxyl radicals helps control atmospheric carbon monoxide concentrations. Hydroxyl radicals form by the breakdown of water molecules.

a. Name the type of bond breaking process that occurs to form hydroxyl radicals from water molecules.

...................................................................................................................................... [1]

b. The bond enthalpy for the OH bond is +464 kJmol-1. Calculate the energy, in Joules, needed to break a single OH bond. Avogadro constant, NA = 6.02 10

23 mol-1.

energy........................ J [2]

c. Calculate the frequency of radiation that is needed to break a single O-H bond. Give your answer to three significant figures. Planck constant, h = 6.63 x 10-34JHz-1.

frequency............................ Hz [3]

Question6

Forest fires produce small quantities of bromomethane, CH3Br. Bromomethane breaks down in

the stratosphere to produce bromine radicals.

a. Write an equation for the decomposition of bromomethane to produce bromine radicals.

...................................................................................................................................... [1]

b. Name the type of bond fission that is occurring here. ........................................................................................................................... [1]

c. The bond enthalpy of the CBr bond is +290 kJmol-1. Calculate the minimum energy

(in J) needed to break a single CBr bond. Avogadro constant, NA = 6.02 x 1023 mol-1.

minimum energy = .............................. J [2]

d. Calculate the minimum frequency of radiation needed to break a CBr bond. Planck constant, h = 6.63 x 10-34JHz-1.

minimum frequency = ........................... Hz [2]

e. How would the minimum frequency of radiation needed to break a CCl bond compare with that needed to break a CBr bond? Explain your answer.

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................................................................................................................................... [2]

Question 7

In the upper atmosphere, ozone molecules decompose as they absorb ultraviolet (UV) radiation,

as shown by the equation below:

O3(g) + h O2(g) + O(g)

Ozone absorbs harmful ultraviolet radiation that comes from the Sun. A molecule of O3(g) absorbs

a photon of energy with a frequency of 1.00 x 1015Hz.

a. How much energy (in J) does one ozone, O3(g), molecule absorb? minimum energy ........................ J [2]

b. Calculate the energy that would be absorbed by 1 mole of O3(g) molecules. Give your answer to 3 significant figures

energy ........................ kJmol-1 [3]

c. The minimum energy needed to break an oxygen-oxygen bond in ozone is 387kJmol-1. Does a photon with a frequency of 1.00 x 1015Hz have enough energy to break this bond? Explain your answer.

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