Developing FuelsDefintions Exothermic- Describes a process which emits energy. Endothermic- Describes a process that absorbs energy. Standard State- The standard state gives the state (solid, liquid or gas) and colour of a substance at room temperature (298K) and at 1 atmosphere. Enthalpy change of combustion- The process when one mole of a compound is completely burnt in excess oxygen Enthalpy change of reaction- The energy change at constant pressure and a stated temperature for a process in which a specified amount of reactants are converted into products. Enthalpy change of formation- The process when one mole of a compound is formed from its elements in their standard states. Hesss Law- The enthalpy change of a reaction is the same, whether it occurs in one step or a series of steps, providing that the initial and final conditions are the same for each route. Entropy- The measure of how disordered a system is. Catalyst- A substance which changes the rate of a chemical reaction without undergoing any change itself.

Energy changes and chemical reactions Some chemical reactions give out heat, these are called exothermic reactions and have a negative enthalpy change (H= -e).Other chemical reactions are endothermic, meaning that they take in heat (H= +e).When a reaction gives out heat, the chemical reactants are losing energy. This energy is used to heat up the surroundings. The products end up with less energy than what the reactants had, but the surroundings end up with more energy. In an endothermic reaction, the reactants take in energy from the surroundings, resulting in the products having more energy than the reactants. We call the heat changes associated with chemical reactionsenthalpy changes (H).The equation to work out the enthalpy change is H Products H Reactants The enthalpy change is defined as the heat exchange with the surroundings at constant pressure, but generally it does not matter about the pressure.

Enthalpy changes are measured inKilojoules per mol (KJmol-1). CH4(g)+ 2O2(g)CO2(g)+ H2O(l)H= -890 KJ mol-1

This means that for every one mole of methane that has reacted, 890 KJ are released as heat to the surroundings.

When calcium carbonate decomposes, it takes in heat. CaCO3(s)+ CaO(s)+ CO2(g) H= +572 KJ mol-1

572 KJ of energy is needed to decompose 1 mole of calcium carbonate.Standard Conditions The enthalpy change is affected by the temperature and pressure/concentration of solutions. When referring to enthalpy changes, the standard used is 1 atmosphere of pressure (1.01x105Pascal's), a temperature of 25oC (298K), and a concentration of 1 mol dm-3.

IfH refers to the standard conditions, it is written asH.Standard enthalpy of combustion This refers to the enthalpy change that occurs when 1 mole of fuel is burned fully. In theory the fuel needs to burn in standard conditions, but in practice this is near impossible. C8H18(l)+ 12O2(g)8CO2(g)+ 9H2O(l)H= -5470 KJ mol-1 The enthalpy change for octane is much higher than that of Methane, because burning octane involves more breaking and making of bonds.

Enthalpies of combustion always have a minus enthalpy change.Standard enthalpy of formation This refers to the enthalpy change when 1 mole of a compound is formed from its reactants. H2(g)+ O2(g)+ H2O( H= -286 KJ mol-1 Notice that the equation refers to 1 mole of H2O (l)and so only O2 (g)is needed.

So for every mole of water that is made, 286 KJ of heat are dissipated to the surroundings.

It is often impossible to measure enthalpy changes of formation directly, as the reactions don't occur under normal conditions.

The enthalpy change of formations are measure indirectly using energy cycles.

Measuring the enthalpy change of combustion of different fuelsWhen measuring the energy produced from a compound, water is used to measure the amount of heat; this is because of its specific absorption of energy. The experiment above is very inefficient. This is because not all of the energy is absorbed by the water; some is absorbed by the air, while some is absorbed by the container holding the water. Also, the methanol may not have entirely reacted.

Hess's Energy CycleEnthalpy changes using combustion data Many enthalpy changes of formation reactions do not occur under standard conditions, and so they cannot be directly measured; however they can be measured indirectly using energy cycles. Elements react to form a compound. When the compound is combusted it forms the same substances as when the elements are combusted. Using this relationship we can work out the standard enthalpy of formation indirectly.

Hc1=Hf+ Hc2 Thetotal enthalpy of combustion for the reactantsis equal to theformation energy of the productsplus thecombustion energy of the products. This makes sense asEnergy can neither be created nor destroyed,so as long as the starting point and the finishing point are the same, then the enthalpy change will always be the same no matter how you get from start to finish. So if a process can be written in several steps, the enthalpy change of the whole process is equal to the sum of the steps.To find out the Enthalpy change using Heats of combustion: Start with an equation to represent the energy change. Using the combustion data complete a Hess law cycle. Balance the cycle and add symbols representing H1H2andH3. Remember to get the direction of change correct (arrows go form reactants and products down to the combustion products). Use Hess's law to find the relationships between the enthalpy changes and rearrange to find the value wanted. So, if we can measure Hc2and Hc1, we can work out Hf.Hf=Hc2-Hc1 It is minusHc1because the reaction is going in the opposite direction to what we want. Hc2is the sum of enthalpy changes of combustion of 1 mole of carbon and 2 moles of hydrogen andHc1is the enthalpy change of combustion for methane, andHfis the enthalpy change of formation for methane, which is what we are trying to find out.Enthalpy changes using formation data Sometimes it is necessary to use enthalpy change of formation to work out the enthalpy change of combustions.

Hf1=Hf2+Hc TheEnthalpy change from the elements in their standard states to the productsis equal to theformation energy of the productsplus theEnthalpy change from the elements in their standard states to the reactants.To find out the Enthalpy change using Heats of combustion:1.Start with an equation to represent the energy change.2.Using the formation data complete a Hess law cycle.3.Balance the cycle and add symbols representing H1H2andH3.4.Remember to get the direction of change correct (arrows go form elements in standard states to the reactants and products).5.Use Hess's law to find the relationships between the enthalpy changes and rearrange to find the value wanted.

Bond Energies All chemical reactions involve the breaking and making of bonds. Bonds break in the reactants and then reform to make the products. The energy changes in the reactions come from the difference in energies between when the bonds are broken, and when they are made. Chemical bonds are electrostatic forces of attractions between atoms or ions. To break the bond you must give the elements energy until the atoms or ions are far enough apart to no longer be attracted to each other.

The diagram above shows two hydrogen atoms. In the first picture they are bonded together by a covalent bond. When the atoms are given energy, they pull apart from each other, eventually this pulling force overcomes the electrostatic attraction, and the bond breaks. The energy needed to break a bond is called thebond enthalpyorbond energy. The bond enthalpy of a H-H bond is 436 KJ mol-1, so it requires 436 KJ to break every H-H bond, and 436 KJ is given out for every H-H bond that is formed. Bond energies also tell us how strong the bond is. The higher the bond energy, the stronger the bond is. Double bonds have a much higher bond enthalpy than single bonds. In general, the higher the bond enthalpy, the shorter the bond, as the attraction is stronger.Measuring bond lengths Bond energies are very hard to measure, because there are usually many bonds in a compound. This is why energy cycles are used.Breaking and making bonds in a chemical reaction The breaking and making bonds in a reaction are responsible for the energy change. For example,

CH4 (g)+ 2O2(g)CO2(g)+ 2H2O(g) This reaction involves making and breaking bonds. The CH4(g)contains 4 C-H bonds, and the 2 moles of oxygen contain 1 O=O bond each. These bonds are broken and this requires energy. The CO2contains 2 C=O bonds, and the 2 water molecules contain 2 O-H bonds. Energy is produced when these bonds are formed. The difference between these two energies is the enthalpy change. The energy required to break the bonds and start the reaction is called theActivation energy. Some reactions require little energy which is available from the room temperature. Other reactions require more energy, which can come from them being heated. It isnt necessary for all bonds to break before the reaction starts. Chemists represent energy changes using an energy profile.

The bond-breaking and bond-making can also be represented in an energy cycle. Going through H1 is the same as going through H2 and H3. Bond energies do have one disadvantage, and that is that the bond enthalpy is only an average. Enthalpy changes of combustion and formation are much more accurate.Example:Work out the enthalpy change of combustion of Methane, using bond energies.CH4 (g)+ 2O2 (g)CO2 (g)+ H2O (g)

This value is different from the -890 KJ mol-1that is should be. This is because the bond energies are only averages, and the elements are all in their gaseous states, which is not the case in standard enthalpy changes.

Organic molecules All organic molecules contain carbon; this is due to its chemical properties that allow it to produce the individuality of living things. A Carbon atom has four electrons in its outer shell, and becomes stable when it loses or gains four electrons. This would be too many to lose or gain, as the charge when losing and gaining four electrons would be either 4+or 4-, which is too highly charged. So carbon forms covalent bonds. For example, in methane a carbon atom bonds with four hydrogen atoms, sharing their electrons. Carbon can form a strong covalent bond with itself (catenation) and so can form long chains. This leads to a limitless variety of carbon compounds.Hydrocarbons Hydrocarbons are compounds containing Hydrogen and Carbon only. There three different categories of Hydrocarbon: alkanes, alkenes and aromatic hydrocarbons. Aromatic hydrocarbons are derived from benzene. Aliphatic hydrocarbons have an open chain structure.The Alkanes Alkanes are saturated hydrocarbons; they only contain single bonds. They are called saturated because they contain the maximum amount of hydrogen atoms possible. The general formula for an alkane isCnH2n+2where n is a positive integer.

Above is a table with the first ten alkanes. Note that at the end of each name is -ane this is the same for all alkanes. At the beginning of the word is the stem meth-, eth-.. This relates to how many carbons there are in the alkane. A series of compounds that are all related, like alkanes, are called a homologous series. All the series have the same general formula, but each member differs from the next by a CH2unit. All the compounds in the series have similar chemical properties, but different physical properties such as boiling point and density. However the physical properties change gradually in the series as the number of carbons increase.Alkane structural Isomers As well has just having a straight line chained carbons, it is often possible for branches of chains to branch off the primary chain. There are often many different structural formulas for a given molecular formula.For Example:C4H10has two possible structural formulae:

These two compounds are isomers, because they have the same chemical formula, but different structural formulae. Notice that the branched alkane has a different name; it is considered to have been formed from a propane group with an added methyl group. It is therefore called methyl propane. To name branched alkanes:

1)Find the longest chain of carbons. This doesn't have to be a straight line - it can go around corners, as long as it follows the carbon chain. This is the constituent chain and goes on the end of the name. In the above example the longest possible chain is 3 (either a horizontal line or the one at the top, bottom middle then either bottom left or right). This number then names the isomer: e.g. three carbons means that the name of this ends with propane.2)Using the longest chain, identify any branching out from it. In the above isomer there is just one branch - this branches off the second carbon from either end (if the numbers from either end were different, the lowest would be taken). The branch consists of just one carbon, or a methyl group.3)The table below contains the names of the groups with more carbons in them:

As well open chained alkanes, it is also possible for alkane molecules to form rings. These rings are called cyclo-alkanes, and have the same general formula as an alkene CnH2n. They have two less hydrogens, because there are no CH3groups at the end of the chains.

Every corner on the skeletal formula represents a CH2group. Skeletal diagrams are very useful when drawing cyclic hydrocarbons. Skeletal diagrams can also be used to draw chained and branched isomers.

The above represents pentane, every kink in the line represents a CH2 group, and the ends represent a CH3group.

The above represents 4 methyl heptane.Shapes of Alkanes Representing three dimensional shapes in 2D on paper can give a misleading image of what the molecule looks like. The pairs of electrons in covalent bonds repeal each other and so arrange themselves around the carbon atom as far apart as possible. The C-H bonds in methane are directed so that they form a tetrahedron shape, with bond angles of109o

The structure of ethane:

The structure of butane:

Hydrocarbon chains are not really straight, but are "Zig-Zagged".All the bond angles are 109oAlkane characteristics What an alkane looks like at room temperature depends upon its size.

Alkanes contain non-polar molecules and mix well with each other; however they do not mix well with non-polar substances such as water. Alkanes are unreactive towards many laboratory reagents; they are unaffected by acids, alkalis, metals and oxidising agents. When they do react, it is usually in the gas phase, and energy is needed to get the reaction started.Melting and Boiling points of Alkanes As the molecular number increases, the boiling and melting point of the alkane increases.

C1to C4are gases, C5to C17are liquids, C18and higher are solids. The increase in the boiling point is due to the stronger intermolecular forces. In hydrocarbons the only intermolecular forces are instantaneous dipole, induced dipole interactions. In a molecule, electrons are mobile, and at one point they may all find themselves at one end of the molecule. This will form a temporary dipole, affecting neighbouring molecules, pulling the molecules together. At the next instant the electrons could end up at the other side reversing the polarity of the molecule.

This permanent moving around of the electrons causes rapidly fluctuating dipoles which hold together the hydrocarbons. The higher the molecular weight of the compound, the more electrons there are in the molecules, meaning the stronger these Van der Wahl forces. This is why the melting points and boiling points of the larger molecules are higher. Branched alkanes have a lower Van der Wahl force than chains, due to their shape, which doesnt allow the molecules get close enough to each other.Oxidation of Alkanes Alkanes do not react readily with air, but if they are heated they can combust to form carbon dioxide and water. The reaction has a high activation energy; the energy must be supplied in order for the reaction to begin. Alkanes must be vaporised before they can combust, thus more volatile hydrocarbons ignite easier. Once ignited, the reaction is very exothermic, which is why alkanes are often used for fuels. If the air supply is limited, the combustion may be incomplete, producing carbon dioxide and soot, along with partially combusted hydrocarbons.Molecular and Empirical Formula of Alkanes A molecular formula shows how many atoms of each type are present in a given compound. The empirical formula shows the smallest ratio of the different types of atom present. TheMolecular Formula = M x Empirical Formula(where M= a positive integer). The empirical formula of a compound can be worked out from its percentage composition. For a hydrocarbon this can be found out by burning a known mass in oxygen and working out the amounts of carbon dioxide and water formed. This is called combustion analysis.

Octane numbers The octane number of a fuel is based on how fuels auto ignite, under compression. It is based on the scale where isooctane is 100 (minimal ignition under pressure) and heptane is 0 (easily ignites under pressure). For example, a petrol with an octane number of 92 has the same knock as a mixture of 92% isooctane and 8% heptane. Octane rating decreases with an increase in the carbon chain length. Octane ratings increase with carbon chain branching. Octane ratings increase in aromatics with same number of carbons. It is important to know octane numbers for petrol because the auto ignition of fuels causes a knocking effect in petrol engines. This is where it ignites twice; once due to the high pressure and again when the spark ignites the petrol. This causes the car engine to be less efficient and it can also damage the engine. However, diesel engines rely on this knocking effect, as they have no spark plugs and rely on the effect of compression to make their fuel/air mixtures ignite.How octane rating of a fuel can be increased There are two ways to increase the octane number of a fuel. One is to put special additives into the fuel which discourage auto ignition, and the other is to blend high-octane fuels in with the ordinary petrol.Anti-knocking additives Anti-knocking additives are substances which reduce the tendency of a fuel to auto-ignite, and so increase the octane number. Since the 1920s, small amounts of lead compounds have been used as economical and effective anti-knock additives. Concern over the environmental effects of leaded petrol has led to a gradual phasing out of leaded petrol. Petrol companies have focused their attention on refining and blending to get high octane ratings.The right kind of alkane The best kind of alkane for fuels is not the most plentiful in crude oil. The refinery doctors the fuel for our needs. The shorter the alkane chain, the higher the octane number. However, the volatility of the hydrocarbon increases with a decrease in the chain length. The short alkane chains have to be mixed in petrol to lower their volatility and higher the octane number of the petrol. The other factor that affects the octane number is the degree of branching in the alkane chain. The more branched the alkane is, the higher the octane number. Crude oil contains both straight-chain and branched alkanes, but it does not contain enough branched alkanes to give it a high octane number. Chemists have solved this problem; increasing the octane numbers by isomerism, reforming and cracking.Isomerism Isomerization involves heating up straight chained alkanes in the presence of suitable catalysts, so that the chains break. When the chains join together again, they are more likely to be branched. Oil refineries do this on a large scale with C6H14and C5H12. The isomerization reaction is reversible and higher temperatures favour the production of straight chained isomers. However the temperature cannot be too low or the reaction would take too long, so a compromise is made between yield and rate of reaction. The temperature used is 250oC. The catalyst involved in the isomerization is platinum; the products are then passed over a form of zeolite, which acts as a molecular sieve, separating the straight chains from the branched chains.Reforming Reforming is where alkanes are converted to cyclo-alkanes and cyclo alkanes to aromatic hydrocarbons. Substances such as hexane (which are common in crude oil) are converted to cyclo-hexane. Hydrogen is also produced, which is valuable.Cracking Cracking is one of the most important reactions in the petroleum industry. Cracking involves breaking up large alkanes which have too many carbons for petrol into shorter chains that can be used in petrol. These shorter chains are often branched, which gives them a higher octane number. Cracking also helps to solve the supply and demand problem. Some fractions are abundant in the crude oil, but are not needed. However, other fuels are in great demand but are not common in the crude oil. Most of the cracking is done by heating up the fuel in the presence of a catalyst. This process is referred to as cat cracking. The fuel that is heated can be kerosene, diesel oil, or any other heavier fuels from the residue. Zeolites are once again used in this process, as they act as excellent catalysts. Zeolite Y is used to crack the fuel, as it particularly effective at producing fuels with a high octane rating. As well as the shorter chained alkanes, alkenes are also formed. These are more often branched and are important in the petrochemicals industry.

One problem with cracking is that coke (carbon) tends to form on the catalyst surface, and so the catalyst eventually becomes inactive. To overcome this, the catalyst is circulated with the products and regenerated after each cycle.Catalyst cracking plant The cracking takes place in a 60m long vertical tube, 2m in diameter called a riser reactor. The vaporised hydrocarbons are fed into the bottom and forced upwards by steam. It takes the mixture about 2 seconds to move from the bottom to the top of the tube. The hydrocarbons arent in contact with the catalyst for very long. The mixture then passes into a separator where jets of steam separate it from the solid catalyst. The catalyst goes into a regenerator, where the coke is burnt off it. The catalyst is then ready for a repeat of the cycle.

Isomerism Two molecules which have the same chemical formula but differ in the way that their atoms are arranged are called isomers. Isomers are distinct compounds with different physical and chemical properties.

Structural isomers have the same molecular formula, but have a different order in which the atoms are bonded.Chain Isomerism These isomers occur because of the different possibilities of branching alkanes, for example there are two isomers of butane:

Both these molecules have the same molecular formula, C4H10. Their different structures lead to different properties; Butane has a boiling point of 273k, and methyl propane has a boiling point of 261k. The octane number of a branched alkane is higher than that of a straight chain. As the number of carbons increases in the hydrocarbon, the number of isomers also increases.Position Isomerism This is where an atom or group of atoms are replaced in the carbon chain or ring. These are called functional groups. The isomerism occurs when the functional group is situated in different positions in the molecule.

1-bromopropane, and 2-bromopropane each have the molecular formula C3H7Br; however the bromine atom is situated in different parts of the molecule. Another example occurs with alcohols:

Both have the same molecular formula C4H9OH, but the OH group is situated in different parts of the molecule.Functional group isomerism It is possible for compounds with the same molecular formula to have different functional groups:

C2H6O can either be an alcohol, or an ether depending upon its structural layout.

The formula C3H6O can either be a ketone (propanol) or an acetone, depending upon the layout of the atoms within the molecule.

Alcohols and Ethers All alcohols contain an OH group, for example ethanol. Alcohols have similar chemical properties. All ethers contain an O group, for example methoxymethane. Ethers have similar chemical properties. Groups such as the OH and O which are substituted in a hydrocarbon chain are called functional groups. They dominate the chemistry of a molecule and give it characteristic properties. The hydrocarbon chain is the unreactive part of the molecule and is called the side chain and given the symbol R.

Alcohols Alcohols are hydrocarbon chains with an OH group replacing an H atom. They are named from the parent alkane by dropping the -e from the end of the alkane and replacing it with an -ol. For alcohols containing more than two carbon atoms, isomeric compounds are possible. To distinguish between them it is necessary to label the position of the OH group. As with alkanes the number is the shortest that it can be, so instead of butan-4-ol, it would be butan-1-ol. Polyhydric alcohols have more than one OH group and are named diol for 2 and triol for 3. Compounds with an alcohol attached directly to a benzene ring are called phenols.

Physical properties of alcohols Alcohols are like water, apart from one of the hydrogen atoms has been replaced by an alkyl group.

Water molecules and alcohol molecules are polar (has a negative and positive end) because of their O-H bond. The strong attractive forces between the molecules are known as hydrogen bonds. Hydrogen bonds are weaker than covalent bonds, but stronger than other attractive forces between covalent molecules (for example, van der waals forces). The hydrogen atoms within the O-H group are slightly positive, because the electrons are pulled away from them by the highly electronegative oxygen atoms. The oxygen atoms are slightly negative, because the electrons are closer to them. The positive hydrogen atoms and negative oxygen atoms in neighbouring molecules are attracted to each other forming a hydrogen bond approximately 5% as strong as a covalent bond.

It is because of the strong hydrogen bond in alcohols that makes them have a higher boiling point than their corresponding alkanes which have a similar Mr. The solubility of alcohols decreases as you increase the size of the molecule. When an alcohol is added to water, the hydrogen bonds in the water and alcohol have to be broken in order for them to mix. Breaking bonds requires energy; however (in smaller molecules) when the hydrogen bonds are reformed between the alcohol and water molecules energy is released, and this compensates for the initial energy needed to break the bonds. In larger molecules, the hydrocarbon tail of the molecule needs to fit in between the water molecules, and so it breaks more hydrogen bonds between the molecules. The tail cannot form hydrogen bonds, so the hydrogen bonds are replaced by van der waals forces. These forces are much weaker, and so not as much energy is released when they form; not all of the energy needed to break the hydrogen bonds has been compensated for. So larger molecules do not dissolve as easily in water in comparison to smaller molecules.Ethers Ethers are derived from alkanes by replacing a H atom for a alkoxyl group (-OR). For example:

Ethoxyethane is formed from butane, but two of the hydrogen atoms have been replaced by an oxyen atom. To name the ether the longest hydrocarbon chain is placed at the end and an oxy is placed before it (oxyalkane). Ethers are derived from water, in that they have the central oxygen atom, with chains of hydrocarbons coming from each side. There are no Hydrogen bonds holding together the structure of ethers; only relatively weak forces hold the slightly polar molecules together. The boiling points of ethers are much the same as their counterpart alkanes. Lower ethers are extremely volatile and dangerously flammable. Ethers are only slightly soluble in water, but mix well with other non-polar molecules such as alkanes.

Entropy When petrol is spilt into an enclosed space such as a garage, the petrol vaporises and diffuses through the air to occupy all the available space. This is why petrol is a serious fire risk; it mixes with the air to form a highly flammable mixture. Chance and probability are the reason for diffusion.

In the diagram above, there is a container with a partition down the middle. The molecules in the left hand side of the container are constantly moving; travelling in a straight line until they hit another molecule or the side of the container. When the partition is removed, each of the molecules moving around can either end up in the left hand side of the container or the right. It is pure chance which side the molecules can end up in after a given amount of time. There are four molecules altogether and each one has two places they can be, so the total number of ways the molecules could arrange themselves is 24= 16. Each of these ways is equally likely. Only one of these arrangements has all of the molecules in the left hand side of the container where they started. So the chance that the molecules will stay in one side of the container is 1/16. The molecules diffuse because there is more chance that they will spread out than stay in one place. In a real-life situation there will be billions upon billions of molecules instead of just five. So the possibilities of the arrangement of the gas are vast, meaning that there is an extremely small chance that the molecules will all stay in one place. The events that happen are the ones that are most likely to happen (the gas is more likely to spread throughout the container); the more ways an event can occur, the more likely it is to happen. The mixing of liquids is also another example of this rule. Liquids mix because there are more ways of being mixed than staying unmixed. In theory the liquids can stay unmixed, but the chance of this happening is extremely low. However, not all liquids mix; for example, petrol and water do not mix. This is because there is something to prevent the natural mixing process from happening. There are small forces between all of the molecules, but if the forces are stronger between one liquid than another, then the molecules will stay together, therefore not mixing. The general rule about mixing isthat substances always tend to mix unless there is something stopping them(like attractive forces holding molecules together, preventing them from easily separating).Measuring Entropy It is very important for chemists, biologists and physicists to be able to work out the number of ways an event can happen; so that they can work out how likely an event is to occur. Entropy is used to measure the number of ways an event can happen. The higher the entropy, the more number of ways there are for an event to happen. This means that generally the more disordered a system, the higher the entropy. Gases have higher entropies than liquids, which have higher entropy than solids.

This is because the molecules in gases are more randomly spread out than the water molecules which are more randomly spread out than the solid molecules. Substances with more complex molecules have higher entropy than substances with simple molecules.Volumes of Gases The number of molecules in one mole of gas is always 6 x 1023(Avagadro's constant). The molecules of a gas are extremely spread out, and so the size of them is negligible compared to the total volume the gas occupies. So one mole of gas always occupies the same volume, and at standard conditions (1 atmosphere, 298K) that volume is 24dm3. We can use this idea of molar volumes to work out the volumes of gases involved in a chemical reactionPollution

Cars produce lots of emissions that are harmful to the environment. These emissions are causing great concern around the world. Cars emit carbon dioxide, carbon monoxide, water, un-burnt hydrocarbons, sulphur dioxide, nitrogen and nitrogen oxides. Hydrocarbonemissions result when the fuels in engines dont burn completely. They are also released during refuelling, and during hot days when the petrol evaporates. Hydrocarbons react in the presence of nitrogen oxides and sunlight to form ground level ozone (smog). Ozone irritates the eyes, damages the lungs and aggravates respiratory problems. Nitrogen Oxidesare formed when nitrogen and oxygen atoms in air react under high temperatures and pressures present in the engine. Nitrogen oxides, like hydrocarbons, lead to the formation of ozone. They also lead to acid rain. Carbon monoxideis formed from the incomplete combustion of hydrocarbons, and occurs when carbons in the fuel are partially oxidised, rather than being fully oxidised. Carbon monoxide is a poisonous gas, as it bonds with haemoglobin in the blood. Carbon dioxideis produced when hydrocarbons are burned. Carbon dioxide is not poisonous, but it is a greenhouse gas and leads to excessive global warming. Sulphur Dioxideis formed from impurities in petrol. Sulphur dioxide leads to acid rain.Catalysts A catalyst is a substance which alters the rate of a reaction without undergoing any permanent damage. Catalysts are not used up during a reaction (so they can be reused), and they are not damaged chemically during a reaction. However, catalysts can become physically damaged, for example the surface of the catalyst can crumble. Only small amounts of catalysts are needed in a reaction, as they are being re-used. Catalysts only affect the rate of the reaction, not the amount of product formed. Catalysts usually speed up chemical reactions; however some catalysts slow down reactions and these are called inhibitors, or negative catalysts. Catalysts are usually specific for one type of reaction, as are biological catalysts (enzymes). Catalysts can be added to cars (catalytic converters) in order to reduce the amount of pollution being emitted.

In these reactions the pollutants are being converted to carbon dioxide, water and nitrogen which are naturally present in the air. These pollutions do react on their own, but catalysts make them go faster. Catalysts are present in catalytic converters. A lean burn engine uses an oxidation catalyst which removes CO and Hydrocarbons, converting them to carbon dioxide and water. This catalyst system does not remove nitrogen oxides. Catalytic converters can be fitted to ordinary cars; instead of an oxidation catalyst, a three-way catalyst system is needed, which catalyses oxides, CO, hydrocarbons, and nitrogen oxides. This catalyst only works if the air-petrol mixture is controlled so that it is exactly stoichiometric. The reduction catalystis made from platinum and rhobium and it reduces NOxemissions.

2NO(g)N2(g)+ O2(g)or2NO2N2(g)+ 2O2(g) The oxidation catalystis made from platinum and palladium and it is used to lower emissions of un-burnt hydrocarbons and carbon monoxide.

2CO(g)+ O2(g)2CO2(g).Types of Catalysts If the reactants and catalysts are in the same physical state, for example they are both in an aqueous solution), then the reaction is said to involve homogenous catalysis. In many cases, the catalyst and reactants are in a different physical state. This kind of reaction is said to have involved heterogenous catalysis.How Catalysts Work All chemical reactions require bond breaking, and bond making. Breaking the bonds requires energy known as the activation energy. The energy supplied must be higher than the activation energy, in order for the reaction to occur. If the activation energy is high, very few molecules will have enough energy to overcome it, and so the reaction will take place at a slow rate. Catalysts speed up chemical reactions by providing an alternative pathway for breaking and remaking bonds. In the reaction above, the activation energy is made lower, and so more bonds can be broken and reformed, hence a faster reaction.How a Heterogeneous catalyst works When a solid catalyst is used to speed up the reaction between liquids or gases, the reaction occurs on the surface of the catalyst. This is why it is important for the catalyst to have a high surface area, so that more of the reactants can react on the surface. Solid catalysts are usually found in a finely divided form or as a fine wire mesh. In catalytic converters the catalyst is supported on a porous material to increase its surface area.

Catalyst poisoning Catalyst poisoning occurs when other gases are absorbed more readily onto the catalyst than the reactants. This prevents the reactants from doing so. Catalyst poisons in cars include lead and sulphur.