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8 Reviewing and reflecting

Figure 55 is a conceptual diagram that summarises this unit. Molecules are made of atoms, so it was with atoms, to the left of Figure 55, that we began. Early in Section 1 they acquired a structure with a positively charged nucleus surrounded by negatively charged electrons. To a chemist, the most important property of an atom is the
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7.5 Summary of Section 7

  1. The equilibrium constant of a reaction is fixed at any particular temperature. It depends only on the natures of the initial reactants and the final products; what happens as reactants change into products has no effect on the equilibrium constant or position of equilibrium.

  2. The rate of a chemical reaction is affected both by the temperature and by the pathway (reaction mechanism) through which reactants change into products. This pathway c
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7.4 Equilibrium positions and rates of reaction in this unit

Section 7 showed that if a reaction is to occur at a particular temperature, two conditions must be fulfilled: its equilibrium constant must be sufficiently large, and its rate sufficiently great. We finish by pointing out how this crucial distinction between the equilibrium constant and the rate reveals itself in Figure 52. The f
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7.3 Is the rate of reaction very slow?

If the equilibrium position is very favourable, then the reason why Reaction 8.1 fails to occur at 525 °C must be that its rate is very slow. Usually, a reasonable response would be to increase the temperature yet further, but the structure and economy of the car gives us little scope to do this. The alternative is to use a catalyst, which leaves the equilibrium constant unchanged, while speeding the reaction up.

Let us look at the changes that take place in the internal energy
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7.2 Is the equilibrium position unfavourable?

The first possibility is that the reaction system has been able to reach chemical equilibrium, but the equilibrium position is not favourable. How does this come about? If equilibrium has been reached, then the forward (left to right) and backward (right to left) reactions are occurring at equal rates. In such a case, we can emphasise the fact by writing the reaction with two opposed, half-headed arrows:

2NO(g) +
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7.1 Introduction

So far, we have concentrated on the electronic and spatial structures of chemical substances, but we have not said much about chemical reactions. Now we turn to the question of why chemical reactions happen. To remind you of the basic ideas, we shall concentrate on one particular reaction which occurs in the modern motor car.

Table 2 shows typical percentages of the main constituents of the exhaust gas that emerges from a modern car engine. The two most dangerous pollutants are carbon
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6.4 Summary of Section 6

  1. Molecules have a three-dimensional shape. Bulky irregularities in the shape of a molecule around a reactive site can exclude a potential reactant. Such effects are described as steric.

  2. A sufficient refinement of the molecular shape in the region of the reactive site can make that site specific to just one particular reactant. Many enzymes operate in this way.

  3. The shapes of simple molecules can be predicted using valence-she
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6.3 Valence-shell electron-pair repulsion theory

The theory of molecular shape that we have been working towards is called valence-shell electron-pair repulsion theory (VSEPR theory). When applied to molecules and ions of the typical elements, its success rate is high. Here is a stepwise procedure that you can follow when applying this theory. It is illustrated with the molecule XeF4 and the ion C1O3. Xenon tetrafluoride is one of the select band of noble gas compounds that were unknown before 1962
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5.2 Summary of Section 5

  1. The structural formulae of organic molecules can be divided into the carbon-hydrogen framework or skeleton, and the functional group(s). In the first approximation, the functional groups are the sites where reaction occurs, the framework remaining unreactive.

  2. This approximation works best when the framework consists of saturated carbon atoms.

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5.1 Molecular reactivity is concentrated at key sites

Reactivity is not spread evenly over a molecule; it tends to be concentrated at particular sites. The consequences of this idea are apparent in the chemistry of many elements. However, in organic chemistry, the idea has proved so valuable that it receives specific recognition through the concept of the functional group. Structure 6.1 shows the abbreviated structural formula of hexan-1-ol, an alcohol.


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4.6 Summary of Section 4

  1. The chemical formulae of many substances can be understood by arguing that their atoms attain noble gas structures by chemical combination.

  2. In ionic compounds, this is achieved by the transfer of electrons from one atom to another; in molecular substances, it happens through the sharing of electron pairs in covalent bonds. But in both cases, bonds between atoms consist of shared pairs of electrons. In covalent compounds the sharing is fairl
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4.5.4 Resonance structures

Gaseous oxygen occurs as O2 molecules. But ultraviolet light or an electric discharge converts some of the oxygen to ozone (Box 6). This has the molecular formula O3.

Box 6: Ozone is blue

Many people know that gaseous ozone in the stratosphere protects us from harmful sola
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4.5.2 Noble gas configurations under stress

It is remarkable how many molecules and ions of the typical elements can be represented by Lewis structures in which each atom has a noble gas shell structure. Nevertheless, many exceptions exist. According to the periodic trends summarised in Section 2, the highest fluorides of boron and phosphorus are BF3 and PF5. How
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4.5 More about covalent bonding

So far, the valencies in Table 1 have just been numbers that we use to predict the formulae of compounds. But in the case of covalent substances they can tell us more. In particular, they can tell us how the atoms are linked together in the molecule. This information is obtained from a two-dimensional drawing of the structural form
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1.2.1 Isotopes

All atoms of the same element have identical atomic numbers, and are chemically similar, but they may not be identical in other ways. Figure 2f shows copper. All copper atoms have atomic number 29: all their nuclei contain 29 protons. But they also contain uncharged particles called neutrons. In natural copper, the a
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6.2 Specific difficulties

Some students contend with physical difficulties in reading. Here is one:

And here is another being offered advice by a friend:

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2.8 Descriptive statistics

Scientists collect many different types of information, but sets of data may be very loosely classified into two different types. In the first type, so-called ‘repeated measurement’, an individual quantity is measured a number of times. An astronomer wanting to determine the light output of a star would take many measurements on a number of different nights to even out the effects of the various possible fluctuations in the atmosphere that are a cause of stars ‘twinkling’. In the seco
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Acknowledgements

The content acknowledged below is Proprietary (see terms and conditions) and is used under licence.

All materials included in this unit are derived from content originated at the Open University.


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1 Developing modelling skills

The main teaching text of this unit is provided in the workbook below. The answers to the exercises that you'll find throughout the workbook are given in the answer book. You can access it by clicking on the link under the workbook.

Click 'View document' to open the workbook (PDF, 0.2 MB).

Acknowledgements

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Unit Image

Wade_In_Tulsa, photos

All other materials included in thi
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