A  The Atmosphere

 

A1 Synopsis

A study of chemical processes occurring in the atmosphere which have an important influence on life on Earth. The section focuses particularly on the ozone layer and the greenhouse effect, the way these are affected by human activities, and the role of chemistry in understanding and controlling the resulting problems. Main topics:

 

•  absorption of the Sun‘s radiation by matter, particularly by atmospheric gases;

 

•  ozone and its atmospheric chemistry;

 

• halogenoalkanes and their effect on the ozone layer;

 

• approaches to the control of the ozone layer depletion problem;


• origins of the greenhouse effect; greenhouse gases in the atmosphere;

 

• carbon dioxide and its role in the greenhouse effect and global warming;

 

• the global carbon cycle and ways of reducing atmospheric carbon dioxide.

 

 


 

A2 Learning outcomes

 

Candidates should be able to:

(a)     recall the gases present in the atmosphere, including some major pollutants

 

      (b) understand values for composition by volume measured in percentage

concentration and in parts per million (ppm);

 

      (c) recall that rotational, vibrational and electronic energies are quantised;

 

      (d) describe qualitatively the changes in rotational, vibrational and electronic

            energy  caused by the absorption of radiation of appropriate frequency;

 

(e)     use the equation E = hv to relate quantitatively the frequency and energy

             of  electromagnetic radiation;

 

(f)       use given data about absorption ranges to make predictions about the effect

       of  substances on radiation;

 

(g) recall the structure and reactivity of ozone and the way it is formed and destroyed in the stratosphere;

 

 

(h)

explain how ozone acts as a sunscreen in the stratosphere;

(i)

 

recall the factors that affect the rate of a chemical reaction and use collision theory to give simple explanations.

(j)

explain and use the terms: enthalpy profile, activation enthalpy;

(k)

 

use the concept of activation enthalpy to explain the qualitative effect of temperature changes on rate of reaction.

(l)

 

explain the role of catalysts in providing alternative routes of lower activation enthalpy;

 

(m)

 

distinguish between homolytic and heterolytic bond fission;

 

(n)

 

recall the formation, nature and reactivity of radicals;

(o)

 

explain the mechanism of a radical chain-reaction involving initiation, propagation and termination.

(p)

describe and explain the reaction of alkanes with halogens;

(q)

recognise members of the following homologous series: halogenoalkanes;

(r)

use systematic nomenclature to name halogenoalkanes;

(s)

explain and use the terms: hydrolysis, substitution, nucleophile, carbocation;

(t)

describe in outline the preparation of a halogenoalkane from an alcohol;

(u)

 

describe and explain the principle stages in the purification of an organic liquid product;

(v)

 

describe and explain the characteristic properties of halogenoalkanes, comparing fluoro-, chloro-, bromo- and iodo- compounds. The following aspects are to be considered:

 

(i)

boiling points,

 

(ii)

formation of radicals by interaction with ultraviolet radiation,

 

(iii)

nucleophilic substitution with water, hydroxide ions, ammonia;

(w)

 

explain the mechanism of nucleophilic substitution in halogenoalkanes (reference to SN1 and SN2 is not required);

 

(x)

 

use relative electronegativity values to predict bond polarity in a covalent bond;

(y)

 

relate reactivity of halogenoalkanes to bond enthalpy and bond polarity;

 

(z)

recall the nature and uses of chlorofluorocarbons (CFCs);

 

 

 

(aa) describe the chemical basis of the depletion of ozone in the stratosphere due to halogenoalkanes, in simple terms involving the formation of halogen atoms and the catalytic role of these atoms in ozone destruction;

(bb) outline a simple model to explain homogeneous catalysis in terms of the formation of intermediates;

(cc) discuss the relative advantages and disadvantages of replacement compounds for chlorofluorocarbons: hydrochlorofluorocarbons (HCFCs), hydrofluororocarbons (HFCs) and alkanes

(dd) relate the ”greenhouse effect‘ in the troposphere to the absorption characteristics of atmospheric gases;

 

(ee) discuss and evaluate different approaches to the control of global warming through  

        the control of carbon dioxide emissions; 

 

(ff)   explain and apply the term: dynamic equilibrium;

(gg) describe the physical and chemical changes occurring when carbon dioxide dissolves

         in water, and the associated equilibria;

 

(hh) use Le Chatelier's Principle to explain and predict, in a homogeneous reaction, the qualitative effects on the position of equilibrium of changes in the following conditions

(i)         concentration,

(ii)         temperature,

(iii)        pressure.