6.3/ OZ 3- Ozone Radical Reactions (Part 2)

Describe STEP 1 in the formation of ozone?

The O2 molecules in the stratosphere absorb UV radiation from the Sun with a wavelength of less than 240 nm-(shown by hv) high-energy to provide energy to break the bond

This electromagnetic radiation breaks the O2 molecule into two oxygen atoms/ radicals

This is called (photo)dissociation of dioxygen molecules

What types/amount of energy is needed for STEP 1 of the formation of ozone?

Lots/ the correct amount of energy is needed to break the bond (+498 kJmol-1)- provided by right frequency of ultraviolet radiation or an electric discharge

Give an equation for STEP 1 of the formation of ozone?

O2 + (electromagnetic radiation< 240nm/hv) 2O·

What happens to the oxygen radicals formed in STEP 1 of the ozone reaction?

These oxygen radicals can collide and react with and oxygen molecule, ozone or another oxygen atom

Describe STEP 2 of the formation of ozone

The O radicals then react with O2 molecule/ dioxygen molecule to form an ozone molecule O3 and heat

As the O atom is a radical it immediately reacts with other O2 molecules

What happens to the chemical energy released when O and O2 combine?

It is converted into kinetic energy of molecular motion (heat). This process generates heat which is absorbed by air molecules in the stratosphere raising its temperature.

Give the equation for STEP 2 of the formation of ozone

O2 + O · ---> O3 + heat

Where is ozone mostly formed?

Mainly in the upper regions of the stratosphere

What frequency/ wavelength of radiation do O3 molecules absorb, what effect does this create?

O3 molecules absorb UV with wavelengths between 240-310 nm in the 10.1 x 1014 to 14.0 x 1014 Hz region

This reaction is responsible for the vital screening effect of zone- it absorbs the radiation responsible for sunburn (UVB and UVC)

Which bond is broken in ozone when it absorbs the UV radiation?

The O-O bond

Give an equation for the reverse reaction to break down ozone, what type of reaction is this?

O3 + (radiation <310nm, hv) O2 + O·

This is a photodissociation reaction

Give the equilibrium reaction for the formation/ destruction of ozone

O· + O2 (reversible reaction arrow) O3 + (heat/UV radiation/ hv)

What does hv represent?

Represents a quantum of radiation

Describe how there is a stable balance of ozone formation and destruction, what happens to the concentration of the oxygen atoms/ molecules/ ozone?

This cycle repeats- The ozone layer is continually being destroyed and replaced as UV radiation hits the molecules this keeps the ozone layer in a stable balance- the reaction is reversible

As the reaction is in equilibrium the concentration of the oxygen atoms/ molecules/ ozone remain fairly constant (human activity disrupts this)

What state is reached with the destruction/formation of ozone?

Equilibrium is reached - a natural steady state is reached where ozone is being formed at the same rate that it is being broken down

What is the overall effect of ozone formation/ destruction, what does this process protect?

UV radiation penetrating the atmosphere is converted into heat without any net loss of ozone. This process protects living organisms from the harmful effects of high-energy UV radiation.

What does ozone smell of, where can this be smelt and why?

Ozone smells very sharp- can often be smelt around electric motors, photocopiers or ultraviolet lamps used to kill bacteria in food shops as the electric discharge makes some of the dioxygen molecule in the air dissociate into atoms which then react to make ozone

How does human activity affect ozone reactions?

When an oxygen atom/radical and an ozone molecule combine they form two O2 molecules (ending the cycle of ozone formation and removal)

O3 + O· ---> 2O2

But the removal rate is slow as the concentration of O radicals is low

Human activity can affect the concentration of O atom/ radicals so increasing the rate of ozone depletion

What can be sent into the stratosphere to measure ozone concentrations?

Helium-filled balloons

Where does some of the ozone in the troposphere come from?

Some of the ozone in the troposphere is formed in reactions taking place in photochemical smog

Oxygen atoms are produced by the action of sunlight on the pollutant gas nitrogen dioxide

Exam Question: One of the processes leading to the breakdown of ozone in the stratosphere can be represented by the following two equations.

NO(g) + O3(g) → NO2(g) + O2(g)

NO2(g) + O(g) → NO(g) + O2(g)

State and explain the role of the NO in this process (2 marks)

Catalyst (1 mark), as it is used and then remade/it is regenerated (1 mark)

Exam Question: Describe and explain how the concentration of ozone in the stratosphere is maintained. (2 marks)

O + O2 → O3OR O reacts with O2 to make ozone

OR the reaction is reversible

Rate of formation of ozone is the same as rate of decomposition

ALLOW O2 + O O3 OR O3 O2 + O

ALLOW is in equilibrium

OR in correct equation

OR has steady state condition

IGNORE other equations involving ozone

Exam Question:

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?

(ii) In step 2, hydroxyl radicals, OH, are formed. What does the term ‘radical’ tell you about the

electron structure of OH?

(iii) Give the oxidation states of nitrogen in the following compounds in step 3. NO2 ............ HNO3 ............

(iv) Use your answers to (iii) to decide what type of reaction is taking place in step 3. Explain your choice. [2]

[Total 6 marks]

(i) Ultraviolet / high frequency / low wavelength (1). 1

(ii) It has an unpaired / lone electron (1) do not allow ‘free’. 1

(iii) NO2 + 4 (1);HNO3 + 5(1).1 mark only if no + sign and numbers correct. 2

(iv) Oxidation / redox (1);(N) has lost an electron / oxidation state of (N) has increased / changed(watch for con) / AW (N may be inferred) ecf from (iii) (1). 2

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