Chemistry Unit 3 (C7) Revision Slides Hydrocarbons in crude oil
Many compounds in crude oil only contain the elements carbon and hydrogen. They are called hydrocarbons. Most hydrocarbons in crude oil are compounds called alkanes. Alkanes contain a single chain of carbon atoms with hydrogen atoms bonded along the side. What are alkanes? Alkanes are a family of hydrocarbon compounds with the general formula CnH2n+2. The simplest alkane is methane. It has the formula CH4. The second simplest alkane is ethane. It has the formula C2H6. Teacher notes The alkanes are a homologues series and it should be pointed out to students that the general formula allows the molecular formula of any alkane to be determined. The naming of alkanes could also be introduced, making it clear that all alkanes end in -ane. The start of the name denotes the number of carbon atoms in each molecule. The third simplest alkane is propane. It has the formula C3H8. How can crude oil be made useful?
Crude oil itself has no uses it must first be processed or refined. This is done in an oil refinery. The first step is to separate compounds in the oil into groups called fractions. Each fraction contains a mix of compounds with a similar number of carbon atoms. Photo credit: BP plc The Singapore Refinery, formerly owned by BP. Molecule size and boiling point
Molecules in crude oil can contain anything from just 1 carbon atom to well over 50. The more carbon atoms in a hydrocarbon molecule, the larger the molecule. How does this affect its boiling point? Generally, the larger a hydrocarbon, the higher its boiling point. This is because the intermolecular forces between large molecules are stronger than the intermolecular forces between small molecules. Teacher notes The boiling point of hydrocarbons increases with the size of the molecule due to increasing surface area. A larger surface area means a larger van der Waals intermolecular force between adjacent molecules. More energy is needed to break the forces between large molecules, and so the boiling point is higher. Fractional distillation of crude oil
Crude oil is separated into fractions by fractional distillation. 1.Oil is heated to about 450 C and pumped into the bottom of a tall tower called a fractionating column, where it vaporizes. 2.The column is very hot at the bottom but much cooler at the top. As the vaporized oil rises, it cools and condenses. 3.Heavy fractions (containing large molecules) have a high boiling point and condense near the bottom of the column. 4.Lighter fractions (containing small molecules) have a lower boiling point and condense further up the column. Supply and demand The amount of each type of fraction obtained by fractional distillation does not usually match the amount of each fraction that is needed. Crude oil often contains more heavier fractions than lighter fractions. Lighter fractions are more useful and therefore more desirable. The large hydrocarbon molecules in the heavier fractions can be broken down into smaller, more useful, molecules to meet demand for raw materials for fuels and plastics. Catalytic cracking Large hydrocarbon molecules can be broken down into smaller molecules using a catalyst. This is called catalytic cracking, and is an example of a thermal decomposition reaction. The hydrocarbon molecules are heated until they turn into vapour, and then mixed with a catalyst. The molecules break apart, forming smaller alkanes and alkenes. Teacher notes It may be worth pointing out to students that the molecules crack rather than burn when heated, because oxygen is kept out of the reaction vessel. Alkenes are reactive molecules that are used to make plastics and other chemicals. What are alkenes? Alkenes are a family of hydrocarbon compounds with the general formula CnH2n. Alkenes are very similar to alkanes, but they have one important difference: they contain at least one double covalent bond between carbon atoms. The simplest alkene is ethene. It has the formula C2H4. Teacher notes The alkenes are a homologues series and it should be pointed out to students that the general formula allows the molecular formula of any alkene to be determined. The naming of alkenes could also be introduced, making it clear that all alkenes end in -ene. The start of the name denotes the number of carbon atoms in each molecule. The second simplest alkene is propene. It has the formula C3H6. Cracking decane Decane from the naphtha fraction can be cracked to form pentane (for use in petrol), propene and ethene. decane (C10H22) pentane (C5H12) propene (C3H6) ethene (C2H4) + Saturated vs. unsaturated
Alkanes are examples of saturated compounds. A saturated compound only contains single covalent bonds between carbon atoms. Alkenes are examples of unsaturated compounds. An unsaturated compound contains at least one double covalent bond between carbon atoms. A test to distinguish between saturated and unsaturated compounds is to add red bromine water. In the presence of unsaturated compounds, the red colour disappears. Boiling point of fractions
Each fraction of crude oil contains a mixture of different compounds. This means that the boiling point of the fraction is not a fixed temperature but a range. LPG gasoline naphtha kerosene lubricating oil diesel fuel oil residue Fraction Boiling point (C) < 0 20-200 20-180 >330 n/a Volatility and flammability
Fractions that have a low boiling point evaporate easily. The easier a fraction evaporates, the more volatile it is. When fractions burn, they react with oxygen in the air. The more volatile a fraction is, the easier it mixes with air. This means the fraction ignites and burns easily. Photo credit: Eric Terry Fractions that ignite and burn easily are flammable. Generally, the smaller the molecules in a fraction, the more volatile and flammable the fraction. What is viscosity? Some fractions of crude oil are thin and runny. Other fractions are thick and sticky. The runniness of a liquid is called viscosity. For example, the residue from fractional distillation has a very high viscosity (it is viscous) and cannot be easily poured. Gasoline has a low viscosity and pours easily. What is the relationship between the length of a hydrocarbon chain and the viscosity of a fraction? The longer the hydrocarbon chains in a fraction, the more viscous the fraction will be. Molecule size and viscosity
Why are fractions with large hydrocarbon molecules more viscous than fractions with small hydrocarbon molecules? The longer chains of large hydrocarbon molecules are easily entangled. Smaller molecules have shorter chains and are less likely to become entangled. Alternative fuels Teacher notes
This true-or-false quiz could be used as a starter exercise to work on alternative fuels. Students could be given coloured traffic light cards (red = false, green = true) to vote on the statements shown. To stretch students, they could be asked to explain their voting. The need for alternative fuels
Most vehicles in the world use petrol or diesel as a fuel. These are produced from crude oil, a fossil fuel. Although fossil fuels are convenient sources of energy, they are very polluting, and will one day run out. As a result, some people have already begun using alternative fuels to power their vehicles, such as biofuels and hydrogen. Photo credit: Witold Barski Why is it important to develop and use these fuels before oil supplies run out? What are biofuels? Biofuels are renewable fuels produced from plant material, such as agricultural crops. Two types of biofuel used in vehicles are bioethanol and biodiesel. They can be safely combined with normal petrol or diesel and used in conventional engines to reduce levels of harmful emissions without causing engine damage. Photo credit: Ana Schaeffer What is bioethanol? Bioethanol is an alcohol produced by the natural fermentation of the carbohydrates (such as starch) in sugar beet/cane or wheat crops. Photo credit: Ivana De Battisti Flexi-Fuel vehicles, fitted with modified fuel injection systems, can run on E85 fuel (85% bioethanol, 15% petrol), which cuts carbon dioxide emissions by 70% compared to normal petrol-engine cars. What is biodiesel? Biodiesel is produced by chemically reacting vegetable oils or animal fats with alcohol and a catalyst. The process can be completed in 12 hours. Biodiesel can be mixed with conventional diesel, which significantly reduces emissions, especially toxic hydrocarbons, particulates and carbon monoxide. Photo credit: Andre Veron There are few garages in the UK that sell biodiesel. Home-made fuels, usually from waste vegetable oils, are heavily taxed. Advantages of biofuels
What are some of the advantages of using biofuels? Biofuels are carbon neutral: the carbon released during combustion comes from the carbon dioxide the plants took in when they were growing. Storage, transport and distribution costs are low as biofuels can be handled in the same way as conventional fuels. Photo credit: Charles Bensinger/Renewable Energy Partners of New Mexico/NREL Triple biofuels dispenser at Baca Street Biofuels Stations. By-products of production, such as pressed seedcake, can be burnt in power stations instead of fossil fuels or used a animal feed. Disadvantages of biofuels
What are some of the disadvantages of using biofuels? Although biofuels themselves produce relatively little when combusted, their production needs energy from fossil fuels. There are few UK producers of biofuels, and only small quantities of fuel are made. Biofuels therefore need to be imported, mainly from Brazil and South-East Asia. The high demand for land to plant biofuel crops can lead to deforestation and habitat loss, for example in Malaysia. Exothermic and endothermic reactions
What are exothermic and endothermic reactions? exothermic reactions release energy they get hot ex = out (as in exit) thermic = relating to heat endothermic reactions absorb energy they get cold en = in (as in entrance) Most chemical reactions are exothermic. Exothermic reactions Exothermic reactions release thermal energy (heat) into their surroundings. Exothermic reactions can occur spontaneously and some are explosive. What are some examples? combustion respiration neutralization of acids with alkalis reactions of metals with acids the Thermit Process. Photo credit: Jupiterimages Corporation Teacher notes The Thermit Process is a displacement reaction between aluminium and iron oxide, and is used in welding iron and steel. See the Chemical Reactions presentation for more information on the Thermit Process. See the GCSE Science (Chemistry) Combustion and Alternative Fuels presentation for more information on combustion. Reversible reactions and energy
Reversible reactions are exothermic in one direction and endothermic in the other direction. For example: endothermic anhydrous copper sulfate hydrated copper sulfate + water CuSO4.5H2O CuSO4 5H2O + exothermic The amount of energy transferred in each direction is exactly the same. Magnesium and hydrochloric acid Exothermic reaction: energy transfer
What happens to energy in the reaction between magnesium and hydrochloric acid? No external heat source is used so the heat released during the reaction must come from the reactants. During the reaction, chemical energy in the reactants is converted to thermal energy (heat). This causes the temperature of the reaction mixture to rise. This thermal energy is eventually lost to the surroundings and the temperature of the reaction mixture returns to normal. Exothermic reaction: energy levels Exothermic reactions: summary
Teacher notes This completing sentences activity could be used as a plenary or revision exercise on exothermic reactions. Students could be asked to write down the missing words in their books and the activity could be concluded by the completion on the IWB. Endothermic reactions
Endothermic reactions absorb thermal energy, and so cause a decrease in temperature. What are some examples? thermal decomposition, e.g. calcium carbonate in a blast furnace photosynthesis some types of electrolysis sherbet! Photo credit: Janet Goulden Teacher notes Endothermic reactions must absorb energy in order to proceed, and so cannot occur spontaneously. Melting, boiling, and evaporation are examples of endothermic changes of state, but not chemical reactions. Sherbet is made from sugar, bicarbonate of soda and powdered citric acid. When the acid and bicarbonate of soda dissolve in saliva, they produce fizzing sodium citrate, water and carbon dioxide, and a cooling sensation on the tongue. Ammonium nitrate and water Endothermic reaction: energy transfer
What happens to energy in the reaction between ammonium nitrate and water? During the reaction, thermal energy from the reaction mixture is converted to chemical energy in the products. This causes the temperature of the reaction mixture to fall. Thermal energy from the surroundings is transferred to the reaction mixture, and the temperature eventually returns to normal. Endothermic reaction: energy levels Endothermic reactions: summary
Teacher notes This completing sentences activity could be used as a plenary or revision exercise on endothermic reactions. Students could be asked to write down the missing words in their books and the activity could be concluded by the completion on the IWB. Exothermic or endothermic?
Teacher notes Appropriately coloured voting cards could be used with this classification activity to increase class participation. Energy transfer: true or false?
Teacher notes This true-or-false activity could be used as a plenary or revision exercise on energy transfer, or at the start of the lesson to gauge students existing knowledge of the subject matter. Coloured traffic light cards (red = false, yellow = dont know, green = true) could be used to make this a whole-class exercise. Making and breaking chemical bonds
Most chemicals will break up (decompose) if they are heated strongly enough. This means that energy is needed to break chemical bonds an endothermic process. energy absorbed Because bond-breaking is endothermic, bond-making must therefore be exothermic. This means that energy is released when chemical bonds are made. energy released Bonds and exothermic reactions Bonds and endothermic reactions What is activation energy?
All reactions need a certain amount of energy to get started. This is called the activation energy (Ea). Activation energy is needed to start breaking the bonds of the reactants. In most chemical reactions, some existing bonds need to be broken (an endothermic process) before new bonds can be made (an exothermic process). Ea Do different reactions need different Ea?
In some reactions, the bonds are easily broken and a low activation energy is needed; for example, the reaction between sodium hydroxide and water starts at room temperature. In other reactions, the bonds are strong and not easily broken. The reaction needs lots of activation energy. Photo credit: Dennis Taufenbach An example is the combustion of charcoal (carbon) it needs lots of heating before it will start to burn. Ea: exothermic reactions Ea: endothermic reactions Bond energies The amount of energy needed to break or make a bond is called the bond energy. Different chemical bonds have different bond energies. For example: H H Cl Cl H Cl Bond energy (kJ) 432 240 Teacher notes Bond energies taken from The Elements, third Edition 1998, John Emsley. Clarendon Press, Oxford. 428 The energy changes in a reaction can be calculated from the bond energies of the reactants and the products. Calculating bond energies
What are the energy changes in the reaction between hydrogen and chlorine? hydrogen hydrogen chloride + chlorine H2 2HCl + Cl2 energy for bond-breaking energy from bond-making = H H+Cl Cl = H Cl + H Cl = 432 kJ kJ = 428 kJ kJ Teacher notes Is the total energy change for the reaction endothermic or exothermic? = 672 kJ = 856 kJ total energy change = energy out energy in = 856 kJ 672 kJ = 184 kJ Energy level diagram for H2 + Cl2 True or false? Teacher notes
This true-or-false activity could be used as a plenary or revision exercise on bonds and activation energy, or at the start of the lesson to gauge students existing knowledge of the subject matter. Coloured traffic light cards (red = false, yellow = dont know, green = true) could be used to make this a whole-class exercise. Glossary activation energy The amount of energy needed to start a reaction. bond-breaking A process that requires energy and so is endothermic. bond-making A process that releases energy and so is exothermic. bond energy The energy needed to break a bond, or released when a bond is made. endothermic A type of reaction that absorbs thermal energy. exothermic A type of reaction that releases thermal energy. Anagrams Exothermic or endothermic
Teacher notes Appropriately coloured voting cards could be used with this classification activity to increase class participation. Multiple-choice quiz Teacher notes
This multiple-choice quiz could be used as a plenary activity to assess students understanding of energy transfer. The questions can be skipped through without answering by clicking next. Students could be asked to complete the questions in their books and the activity could be concluded by the completion on the IWB. Reactions, particles and collisions
Reactions take place when particles collide with a certain amount of energy. The minimum amount of energy needed for the particles to react is called the activation energy, and is different for each reaction. The rate of a reaction depends on two things: the frequency of collisions between particles the energy with which particles collide. Teacher notes See the Energy Transfer presentation for more information on activation energy. If particles collide with less energy than the activation energy, they will not react. The particles will just bounce off each other. Changing the rate of reactions
Anything that increases the number of successful collisions between reactant particles will speed up a reaction. What factors affect the rate of reactions? increased temperature increased concentration of dissolved reactants, and increased pressure of gaseous reactants increased surface area of solid reactants use of a catalyst. Slower and slower! Reactions do not proceed at a steady rate. They start off at a certain speed, then get slower and slower until they stop. As the reaction progresses, the concentration of reactants decreases. This reduces the frequency of collisions between particles and so the reaction slows down. 0% 25% 50% 75% 100% percentage completion of reaction reactants product Graphing rates of reaction
Teacher notes This animated graph summarizes the qualitative information provided by the gradient of a graph that plots amount of product in a reaction against time. Reactantproduct mix Teacher notes
This animated graph follows-on from the graph on the previous slide, and illustrates how the change in the rate of a reaction can be explained in terms of changing amounts of reactants and product. Effect of concentration on rate of reaction
The higher the concentration of a dissolved reactant, the faster the rate of a reaction. Why does increased concentration increase the rate of reaction? At a higher concentration, there are more particles in the same amount of space. This means that the particles are more likely to collide and therefore more likely to react. lower concentration higher concentration Concentration and particle collisions
Teacher notes This simulation illustrates how increasing the concentration increases the number of collisions between particles. Effect of pressure on rate of reaction
Why does increasing the pressure of gaseous reactants increase the rate of reaction? As the pressure increases, the space in which the gas particles are moving becomes smaller. The gas particles become closer together, increasing the frequency of collisions. This means that the particles are more likely to react. lower pressure higher pressure What are catalysts? Catalysts are substances that change the rate of a reaction without being used up in the reaction. Catalysts never produce more product they just produce the same amount more quickly. reaction (time) energy (kJ) Ea without catalyst Different catalysts work in different ways, but most lower the reactions activation energy (Ea). Ea with catalyst Teacher notes See the Energy Transfer presentation for more information on activation energy. Everyday catalysts Many catalysts are transition metals or their compounds. For example: Nickel is a catalyst in the production of margarine (hydrogenation of vegetable oils). Iron is a catalyst in the production of ammonia from nitrogen and hydrogen (the Haber process). Platinum is a catalyst in the catalytic converters of car exhausts. It catalyzes the conversion of carbon monoxide and nitrogen oxide into the less polluting carbon dioxide and nitrogen. Photo credit: 2007 Jupiterimages Corporation Teacher notes See the Reversible Reactions presentation for more information on the Haber process. Catalysts in industry Why are catalysts so important for industry?
Products can be made more quickly, saving time and money. Catalysts reduce the need for high temperatures, saving fuel and reducing pollution. Teacher notes See the Enzymes Biology presentation for more information on enzymes as catalysts and their use in industrial processes. Catalysts are also essential for living cells. Biological catalysts are special types of protein called enzymes. Glossary activation energy The amount of energy needed to start a reaction. catalyst A substance that increases the rate of a chemical reaction without being used up. concentration The number of molecules of a substance in a given volume. enzyme A biological catalyst. rate of reaction The change in the concentration over a certain period of time. Rates of reaction: summary
Teacher notes This completing sentences activity provides the opportunity for some informal assessment of students understanding of rates of reaction. Multiple-choice quiz Teacher notes
This multiple-choice quiz could be used as a plenary activity to assess students understanding of rates of reaction. The questions can be skipped through without answering by clicking next. Students could be asked to complete the questions in their books and the activity could be concluded by the completion on the IWB. Irreversible reactions
Most chemical reactions are considered irreversible the products that are made cannot readily be changed back into their reactants. For example, when wood burns it is impossible to turn it back into unburnt wood again! Similarly, when magnesium reacts with hydrochloric acid to form magnesium chloride and hydrogen, it is not easy to reverse the reaction and obtain the magnesium. Photo credit: 2007 Jupiterimages Corporation What are reversible reactions?
Reversible reactions occur when the backwards reaction (products reactants) takes place relatively easily under certain conditions. The products turn back into the reactants. + A B C D (reactants) (products) For example, during a reversible reaction reactants A and B react to make products C and D. Teacher notes In equations for reversible reactions, reactants and products are joined by a two-way arrow. However, products C and D can also undergo the reverse reaction, and react together to form reactants A and B. Reversible biochemical reactions
Many biochemical reactions (those that take place inside organisms) are reversible. For example, in the lungs, oxygen binds to haemoglobin (Hb) in red blood cells to create oxyhaemoglobin. When the red blood cells are transported to tissues, the oxyhaemoglobin dissociates back to haemoglobin and oxygen. Hb + 4O2 Hb.4O2 There are also some very important industrial reactions, like the Haber process, that are reversible. Heating copper sulfate
Teacher notes This animation can be used to introduce the heating of hydrated copper (II) sulfate, and subsequent rehydration of anhydrous copper (II) sulfate as an example of a reversible reaction. It could be shown as precursor to running the experiment in the lab, or as a revision exercise. Heating ammonium chloride
An ammonium salt can be made by reacting ammonia with an acid. Some of the salt will decompose back into the reactants when heated. ammonium chloride ammonia + hydrogen chloride NH3 (g) NH4Cl (s) HCl (g) NH4Cl reforms in the cooler part of the test tube Teacher notes See the Chemical Reactions presentation for more information on thermal decomposition. NH4Cl decomposes back into NH3 and HCl gases when heated Reversible or irreversible?
Teacher notes Appropriately coloured voting cards could be used with this classification activity to increase class participation. What is dynamic equilibrium?
In some reversible reactions, the forward and backward reactions largely occur in the same conditions and at the same rate. These reactions are said to be in dynamic equilibrium there is no overall change in the amount of products and reactants, even though the reactions are ongoing. reactant A + product reactant B Dynamic equilibrium can only take place in a closed system, otherwise the products would escape. What happens in dynamic equilibrium?
What is special about the forward and backward reactions at dynamic equilibrium? Setting dynamic equilibrium
The position of dynamic equilibrium is not always at a half-way point, i.e. when there are equal amounts of products and reactants. It may be at a position where there are mainly reactants with a little product, or vice versa. The position of equilibrium is influenced by two main factors: temperature concentration (or pressure for reactions involving gases) Adding a catalyst speeds up the time it takes to reach equilibrium, but does not change the position of equilibrium. True or false? Teacher notes
This true-or-false activity could be used as a plenary or revision exercise on dynamic equilibrium, or at the start of the lesson to gauge students existing knowledge of the subject matter. Coloured traffic light cards (red = false, yellow = dont know, green = true) could be used to make this a whole-class exercise. Opposing change Whenever a change is made to a reversible reaction in dynamic equilibrium, the equilibrium will shift to try and oppose the change. Temperature Concentration Pressure Condition Effect Increasing the temperature shifts the equilibrium in the direction that takes in heat. Increasing the concentration of a substance shifts the equilibrium in the direction that produces less of that substance. Teacher notes This is Le Chateliers principle. Increasing the pressure shifts the equilibrium in the direction that produces less gas. Exothermic and endothermic reactions
All reactions are exothermic (give out heat) in one direction and endothermic (take in heat) in the other. If the temperature is increased: equilibrium shifts to decrease the temperature equilibrium shifts in the endothermic direction If the temperature is decreased: Teacher notes See the Energy Transfer presentation for more information on exothermic and endothermic reactions. equilibrium shifts to increase the temperature equilibrium shifts in the exothermic direction Opposing changes in temperature
Nitrogen dioxide is in constant equilibrium with dinitrogen tetroxide. The forward reaction is exothermic and the backwards reaction is endothermic. N2O4 (g) 2NO2 (g) nitrogen dioxide dinitrogen tetroxide What will happen if the temperature is increased? The equilibrium will shift to decrease the temperature, i.e. to the left (endothermic). More NO2 will be produced. If the temperature is decreased, more N2O4 will be produced. Concentration and equilibrium
Changing the concentration of a substance affects the equilibrium of reversible reactions involving solutions. increasing the concentration of substance A equilibrium shifts to decrease the amount of substance A = decreasing the concentration of substance A equilibrium shifts to increase the amount of substance A = Opposing changes in concentration (1)
Bismuth chloride reacts with water to produce a white precipitate of bismuth oxychloride and hydrochloric acid. bismuth oxychloride bismuth chloride water hydrochloric acid + BiOCl (s) BiCl3 (aq) H2O (l) 2HCl (aq) What will happen if more H2O is added? The equilibrium will shift to decrease the amount of water, i.e. to the right. More BiOCl and HCl will be produced. If H2O is removed, more BiCl3 and H2O will be produced. Opposing changes in concentration (2)
Chlorine gas reacts with iodine chloride to produce iodine trichloride. iodine trichloride chlorine iodine chloride + ICl3 (s) Cl2 (g) ICl (l) pale green brown yellow What effect will adding more Cl2 have on the colour of the mixture? It will become more yellow. What effect will removing Cl2 have on the colour of the mixture? It will become more brown. Pressure and equilibrium
Changing the pressure has an effect on the equilibrium of reversible reactions involving gases. If the pressure is increased: equilibrium shifts to decrease the pressure equilibrium shifts in the direction of fewest molecules If the pressure is decreased: equilibrium shifts to increase the pressure equilibrium shifts in the direction of most molecules Opposing changes in pressure
Nitrogen dioxide is in constant equilibrium with dinitrogen tetroxide. Two molecules of nitrogen dioxide react to form one molecule of dinitrogen tetroxide. N2O4 (g) 2NO2 (g) dinitrogen tetroxide nitrogen dioxide What will happen if the pressure is increased? The equilibrium will shift to reduce the number of molecules, i.e. to the right (only 1 molecule). More N2O4 will be produced. If the pressure is decreased, more NO2 will be produced. Dynamic equilibrium and change
Teacher notes This completing sentences activity could be used as a plenary or revision exercise on dynamic equilibrium. Students could be asked to write down the missing words in their books and the activity could be concluded by the completion on the IWB.