Zn 0 + Cu 2+ SO 4. Cu 0 + Zn 2+ SO Na + Cl. 2 Zn O.S.: 0 +2 Cu O.S.: +2 0

Transcription

1 Reduc&on and Oxida&on In addi&on to acid/base chemistry, another founda&on of chemistry is reduc+on/oxida+on (redox) chemistry. To understand redox reac&ons, we first have to understand the concept of the oxida+on state. There are a few simple rules for calcula&ng oxida&on states. To begin with, pure elements are always zero and monatomic ions have the same oxida&on state as their electric charge. Na O.S: Na 0 + Cl 0 2 2Na + Cl Cl O.S: 0 1 When the oxida&on state of an element (or compound) becomes more posi&ve, we say it is oxidised. When it becomes more nega&ve, we say it is reduced: Zn 0 + Cu 2+ SO 4 2 Cu 0 + Zn 2+ SO 4 2 Zn O.S.: 0 +2 Cu O.S.: +2 0 oxidised reduced We can see that oxida+on means losing electrons and reduc+on means gaining electrons. Electrons are never made or destroyed, so whenever one element (or compound) is oxidised another element (or compound) must be reduced. Elements or compounds that take electrons from other substances are known as oxidising agents because they oxidise something else. Elements or compounds that donate electrons to other substances are known as reducing agents because they reduce something else. It is important to remember that oxidising agents are themselves reduced and reducing agents are themselves oxidised by the reac&on. Note: Reac&ons where no elements change their oxida&on state are not redox reac&ons.

2 Calcula&ng Formal Oxida&on States What do we do for compounds that are not fully ionic? In this case, we use a set of rules to determine the formal oxida+on state of each element. A formal oxida&on state is not an electric charge, it is actually a way to keep track of the electrons in a reac&on. Remember that chemistry is all about the movement of electrons. In compounds: Oxygen is usually 2 Hydrogen is usually +1 Halogens are usually 1 Group 1 metals are always +1 Group 2 metals are always +2 (Carbon is usually +4, but the redox rules for organic chemistry are different) Most other elements can have different oxida&on states but they usually have their own favourite oxida&on states according to their group. An element cannot have an oxida&on state higher than the loss of all valence electrons, or lower than the filling of all valence sites. overall charge = 0 overall charge = 0 overall charge = 0 overall charge = 2 +1 x 2 2 x 1 H 2 O must be +3 1 x 3 PCl 3 must be +5 1 x 5 PCl 5 must be +6 2 x 4 SO 4 2

3 Balancing Redox Reac&ons We can use oxida&on states to balance chemical equa&ons (and we some&mes do things the other way and use chemical equa&ons to determine oxida&on states). Remember that when one substance is oxidised, another substance must be reduced and vice versa. We can balance these equa&ons because the changes in oxida&on states on both sides must also be balanced. The following example is between copper metal (Cu 0 ) and silver nitrate (Ag + NO 3 ). XCu 0 + YAg + NO 3 YAg 0 + XCu 2+ (NO 3 ) Y Cu: 0 +2 Ag: +1 0 We know the oxida&on states of the pure metals and the NO 3 anion because they are always the same and experiments quickly tell us the oxida&on state of the metallic ca&ons. It is now easy to balance the equa&on so X must equal 1 and Y must equal 2. Cu 0 + 2Ag + NO 3 2Ag 0 + Cu 2+ (NO 3 ) 2 Here s another example: XFe 2+ + YH 2 O 2 XFe YOH Fe: O: 1 2 At first sight, it might look like X = Y because Fe 2+ (iron) increases by one and oxygen decreases by one. But we have to remember that one mole of H 2 O 2 (hydrogen peroxide) makes two moles of OH (hydroxide) so we need two moles of Fe 2+ for every mole of H 2 O 2 : 2Fe 2+ + H 2 O 2 2Fe OH Note that in H 2 O 2, oxygen has an oxida&on state of 1 but it usually wants an oxida&on state of 2. This gives us a clue that hydrogen peroxide will take electrons from other compounds, i.e. it is a strong oxidising agent.

4 Formal Oxida&on States and Formal Charges. It is easy to get confused about the difference between formal oxida&on states and formal charges. This is because they are two different ways of simplifying the complex arrangement of electrons in a molecule. In some cases, both methods give the same answer but in others they give different answers. Formal oxida+on states Method was developed before formal charges. Is mainly for empirical comparisons. Atoms in molecules are not considered to share electrons. They are approximated to being ions for calcula&ng oxida&on states. Used mainly for balancing equa&ons or comparing reac&vity of elements and compounds. POCl 3, P O.S. = +5 this is a very high O.S. P will probably be reduced in reac&ons so it must be a good oxidising agent. PH 3, P O.S. = 3, this is a very low O.S. P will probably be oxidised in reac&ons so it must be a good reducing agent. Formal charges Method was developed afer formal oxida&on states. Is mainly for theore&cal use. Atoms in molecules share their electrons and shared electrons only count as ½ for calcula&ng formal charges. Used mainly for determining proper&es within a molecule, or the net charge on a molecular ion. Cl Cl Cl P O Cl + Cl P O Cl Explains many aspects of POCl 3 chemistry.

5 States of Mager In chemistry, there are three basic states of mager and some other special states: gas liquid solid Glass: Similar in apparent structure to a liquid but the par&cles do not move. Supercri+cal fluid: a hybrid of the gaseous and liquid states. Only occurs at very high temperatures and pressures.

6 The Gas Phase Gases are composed mostly of space with molecules moving about independently and randomly. This means they can be easily compressed up to a certain point. The pressure, volume and temperature of a gas are related proper&es. We can make a simple equa&on for a theore&cal system which 1) does not exchange energy with its surroundings and 2) does not have any form of interac&on between gas par&cles. This theore&cal system is called an ideal gas and the equa&on is known as the ideal gas equa+on. pv = nrt Where p= pressure, V = volume, n = moles of gas par&cles R = gas constant (note, there are three ways of expressing R, see p.415 of the textbook) and T = temperature. The ideal gas equa&on means that we can calculate either the pressure, volume or temperature of a gas if we know the other two values. An ideal gas is a purely theore&cal model but it actually works very well for many common applica&ons at room temperature and above and at atmospheric pressure or below.

7 Molar Volume and Par&al Pressures One thing we can learn from the ideal gas equa&on is that, if we keep pressure and temperature constant, the volume of a gas is directly propor&onal to the number of moles of that gas. This volume only depends on the number and not the type of par&cles involved because the differences in the size of the molecules is &ny compared with the spaces between them. At a pressure of 1 atmosphere (1 atm) and 273 K (0 C), 1 mole of gas occupies 22.4 litres. Conversely, if we keep the volume fixed, the pressure of a gas will depend on the number of par&cles of that gas. When we mix two or more different gases, the total volume or pressure of the mixture is the sum of the volumes/pressures of those gases. The pressure of each gas in a mixture is known as the par+al pressure of each gas. V total = V gas A + V gas B + V gas C P total = P gas A + P gas B + P gas C This means that if we have one mole of gas A and three moles of gas B in a container, with a total pressure of 1 atm, the par&al pressure of gas A must be 0.25 atm and that of gas B must be 0.75 atm. Alterna&vely, if we have n moles of gas X in a fixed- volume container at a pressure of 1 atm and add 2n moles of gas Z, the final pressure will be 3 atm.

8 Devia&ons from Ideal Behaviour Gas par&cles cannot really pass through each other without interac&ng but at pressures up to a ligle over atmospheric pressure and at temperatures at or above room temperature, the ideal gas law works pregy well. However, at high pressures or at very low temperatures, the collisions between gas par&cles become significant. At high pressures, there are too many collisions and at low temperatures, par&cles start s&cking to each other by agrac&ve forces. The rules for the behaviour of gases under these condi&ons are a ligle complicated and we will not cover them in this course.

9 The Liquid Phase As for gases, the par&cles in a liquid are free to move about. However, there are strong agrac&ve forces that make the par&cles s&ck to each other and this is why liquids will stay in an open- topped container. Because liquid par&cles are already in direct contact with each other, it is very difficult to compress a liquid. Liquid phase: agrac&ve interac&ons between molecules. Gas phase: no (or few) interac&ons between molecules.

10 Liquids Physical Proper&es Intermolecular interac&ons determine the physical proper&es of materials. Viscosity Viscosity: Viscosity refers to the ability of a liquid to flow. Liquids with a high viscosity flow very slowly and liquids with a low viscosity flow very easily. A high viscosity means that the molecules of the liquid are strongly agracted to each other. For instance, honey contains a very high concentra&on of sugar and sugar molecules have strong intermolecular aurac+ons. This agrac&on makes honey very viscous. Surface tension: Par&cles in the middle of a liquid are pulled equally from all direc&ons but par&cles at the surface are only pulled downwards. This makes the surface layer s&ff. Liquids that have strong intermolecular agrac&ons have a strong surface tension.

11 Liquids Physical Proper&es 2 Capillary Ac+on: If liquid par&cles are strongly agracted to the par&cles of a surface, they will creep along the surface. The difference between the liquid- liquid agrac&on and the liquid- surface agrac&on determines the shape of the meniscus. In the above example, the water molecules are strongly agracted to the glass wall of the tube and so water has a concave meniscus in glass. The mercury atoms (Hg), however, are more strongly agracted to themselves than to the glass and so Hg has a convex meniscus in glass.

12 Evapora&on Because par&cles randomly exchange energy, some&mes a par&cle at the surface of a liquid will collect enough energy to break its agrac&on to the rest of the liquid. It will then fly off on its own as a vapour par&cle. This process is called evapora+on. Evapora&on is a reversible process. The vapour par&cles can return to the liquid and lose their energy (and vice versa). Because the par&cles with above average energy leave the liquid, evapora&on causes a liquid to lose energy and cool down. Evapora&on occurs more quickly at lower par&al pressures of the vapour. This is because there are fewer vapour molecules returning to the liquid. Note: There is much confusion, even for scien&sts and engineers, about the terms gas and vapour. Some people use gas for substances that are gases at room temperature and atmospheric pressure and vapour for other substances in the gas- phase. Other people use vapour for gas- phase substances in equilibrium with the liquid phase (e.g. water vapour below 100 C) but gas for gas- phase substances that are not in equilibrium with the liquid phase (e.g. steam at over 100 C).

13 Vapour Pressure and Vola&lity The vapour that evaporates from a liquid adds to the pressure of any gas above the liquid. The maximum vapour pressure is fixed for a given temperature, which means that liquids will evaporate un&l this vapour pressure is reached but if the maximum is exceeded, some of the vapour will condense back into the liquid. Liquids that have a high vapour pressure, i.e. they evaporate easily, are said to be vola+le. Examples of liquids that are vola&le at room temperature are petrol (gasoline), alcohol (ethanol) and diethyl ether (a common laboratory solvent). Note that because vapour pressure depends on temperature, liquids that are not very vola&le at room temperature might become vola&le at high temperatures Vapour pressure depends only on temperature and not on the total pressure. This can cause dangerous situa&ons when storing liquids at high temperatures since their high vapour pressure can burst the container they are being stored in. Diethyl ether has a dangerously high vapour pressure in a laboratory on a summer s day and oils can achieve a dangerously high vapour pressure in a fire. This is why keeping liquid storage tanks cool is ofen a priority in industrial fires.

14 Boiling The boiling point of a liquid is the temperature at which all the liquid turns into a gas more specifically, it is the point where the vapour pressure of the liquid equals the total pressure of the surroundings. At this combina&on of temperature and pressure, there are enough high energy par&cles in the liquid to push back the rest of the liquid and form a bubble. Because the boiling point depends on the pressure of the surroundings, higher total pressures increase the boiling point and lower total pressures decrease the boiling point. A prac&cal example of this is that it is reportedly difficult to make good tea in the Himalayas because the mountains are so high, atmospheric pressure is low and water boils at only 90 C but tea is best made at near 100 C. Note that boiling is different to evapora&on evapora&on only occurs at the surface and the vapour is in equilibrium with the liquid but boiling occurs within the liquid (actually on surfaces under the liquid) and the gas molecules will not return to the liquid state un&l they reduce their temperature below the boiling point.

15 Dis&lla&on We can use the proper&es of boiling liquids to purify and separate liquids in a process called dis+lla+on. Dis&lla&on involves boiling a liquid, condensing the vapour using a cold surface and then collec&ng the liquid condensate in a separate vessel. We can use it to separate liquids from solids and to separate mixtures of liquids, as long as the liquids have sufficiently different boiling points.

16 Past Exam Ques&ons i) Balance the following equa&on: _NH 3(g) + _O 2(g) _NO (g) + _H 2 O (g) ( (g) means gas ) ii) What is the oxida&on state of nitrogen (N) in the two compounds? iii) What kind of reac&on is this? iv) This reac&on is conducted in a sealed 200L reac&on vessel at an ini&al pressure of 0.9 atm and a temperature of 560 C. Assuming the temperature and volume is kept constant over the course of the reac&on, what is the pressure in the reac&on vessel once the reac&on has completed? v) A ligle &me afer the reac&on has finished, the vessel is allowed to cool below 100 C, whereupon the pressure in the vessel suddenly drops significantly, even though no further reac&on occurs. What is the reason for this sudden drop in pressure? vi) What is the total number of molecules in the reac&on vessel at the end of the reac&on? (Use R = L atm K 1 mol 1 )

17 Answers C) i) Balance the following equa+on: 4NH 3(g) + 5O 2(g) 4NO (g) + 6H 2 O (g) ( (g) means gas ) ii) This reac&on is conducted in a sealed 200L reac&on vessel at an ini&al pressure of 0.9 atm and a temperature of 560 C. Assuming the temperature and volume is kept constant over the course of the reac&on, what is the pressure in the reac&on vessel once the reac&on has completed? 9 mol of gas becomes 10 mol of gas => pressure is 10mol/9mol x 0.9 atm = 1 atm iii) A ligle &me afer the reac&on has finished, the vessel is allowed to cool below 100 C, whereupon the pressure in the vessel suddenly drops significantly, even though no further reac&on occurs. What is the reason for this sudden drop in pressure? The H 2 O gas condenses into a liquid. iv)what is the total number of molecules in the reac&on vessel at the end of the reac&on? n = 1 atm x 200L/0.082 L atm K 1 mol 1 x 833 K = 2.9 mol => 1.8 x molecules

18 Bonus Exam Ques&ons A) In winter, when the temperature outside is significantly colder than the temperature inside, it is common to see water condensing on the inside of a window. In terms of vapour pressure, explain why this occurs. (B) i) The air pressure inside a plane is slightly lower than 100 kpa (1 atm) while the air pressure in a submarine is slightly higher. It is known that planes have very dry air, while submarines are very humid. Your friend says this is because of the different air pressures. Why is he wrong? (Assume the temperatures inside the plane and submarine are equal). ii) Given that submarines are completely sealed from their surroundings, while planes draw air from the surrounding atmosphere, make a beger explana&on for the difference in humidity on planes and submarines. iii) Will the boiling point of water be higher than, lower than or exactly 100 C in: a plane: a submarine: Why?

19 Answers A) In winter, when the temperature outside is significantly colder than the temperature inside, it is common to see water condensing on the inside of a window. In terms of vapour pressure, explain why this occurs. The cold window creates a thin layer of cold air on the inside. The maximum vapour pressure of water in this thin layer is lower than the vapour pressure of water in the warm room so the excess water vapour condenses (on the window). (B) i) The air pressure inside a plane is slightly lower than 100 kpa (1 atm) while the air pressure in a submarine is slightly higher. It is known that planes have very dry air, while submarines are very humid. Your friend says this is because of the different air pressures. Why is he wrong? (Assume the temperatures inside the plane and submarine are equal). Humidity is related to vapour pressure but vapour pressure depends only on temperature, not total pressure. ii) Given that submarines are completely sealed from their surroundings, while planes draw air from the surrounding atmosphere, make a beger explana&on for the difference in humidity on planes and submarines. The air surrounding a plane is very cold (sub- zero) so the vapour pressure of water in this air will be extremely low, and so the air in the plane will be very dry. Submarines recirculate their air so the vapour pressure of water can reach its maximum value. iii) Will the boiling point of water be higher than, lower than or exactly 100 C in: a plane: lower, total pressure is low a submarine: higher, total pressure is high

20 Reading and Problems Reading: Chapter 5: 5-7 Chapter 12: 12-1, 12-2, 12-9, (12-15) Chapter 13: Problems: Chapter 5: 46 Chapter 12: 50, 72 (note, 760 torr = 1 atm) Chapter 13: 23, 37 (two parts)