SUPPLEMENTARY TOPIC 3 ENERGY AND CHEMICAL REACTIONS

Transcription

1 SUPPLEMENTARY TOPIC 3 ENERGY AND CHEMICAL REACTIONS Rearranging atoms. In a chemical reaction, bonds between atoms in one or more molecules (reactants) break and new bonds are formed with other atoms to produce different molecules (products). These reactions can only occur if the molecules involved collide with sufficient energy to allow the bonds to break, a process which always requires energy input. When the new bonds form as a result of the collision, energy is released. Overall, there may be a nett release of energy (EXOTHERMIC reaction) or there may be a nett requirement for energy to be supplied (ENDOTHERMIC reaction). The heat supplied by burning gas is an example of an exothermic reaction, while the production of the metal iron from its ore, iron oxide, is endothermic, the heat energy required being supplied in a blast furnace. During chemical reactions, although bonds are broken and new bonds are formed, the same atoms remain in the same quantities after the reaction as there were before. It is a fundamental law of chemistry that atoms are not created or destroyed during chemical change. For this reason, whenever a balanced chemical equation is written for a reaction, all atoms which appear on the reactants side must also appear on the products side of that equation. The following equation shows the formation of water from its elements, and includes structural formulas to emphasise the bonds broken and formed. H H + O=O + H H H O H + H O H This would of course usually be written as 2H 2 + O 2 2H 2 O. Energy changes. All molecules have energy stored in their chemical bonds. Some of this energy can be released (mostly as heat) when they react to form molecules which have less energy than the reactants, the reaction being classed as EXOTHERMIC. Alternatively, in an ENDOTHERMIC reaction, additional energy is supplied to the reactants so that the products finish up with more energy stored in their chemical bonds than the reactants had originally. The total energy stored in a collection of molecules is called the ENTHALPY of that substance (H), and for the overall reaction, the difference (enthalpy of products enthalpy of reactants) is the energy released or taken up. This enthalpy difference is given the symbol ÄH. The enthalpy change for a chemical reaction can be measured experimentally by observing the temperature change (increase or decrease) when the reaction is carried out in a thermally insulated ST3-1

2 ST3-2 container basically similar to a thermos flask. CALORIMETER. Such a container is called a As an example, the burning of carbon in oxygen releases 394 kj for each mole of carbon (12.01 g) used. This is represented by the THERMODYNAMIC EQUATION which shows the enthalpy change as well as the reactants and products and their physical states. C(s) + O 2 (g) CO 2 (g) The sign indicates the energy is released. For an endothermic reaction, a + sign is used for ÄH. ÄH = 394 kj/mol Thus for the electrolysis of water, an endothermic process, the corresponding thermodynamic equation is H 2 O(l) H 2 (g) + ½O 2 (g) ÄH = +285 kj/mol This reaction can be reversed by burning hydrogen in oxygen to produce water as a vapour which then condenses to form liquid water. The energy released by this exothermic reaction is numerically exactly the same as the energy supplied (per mole of water) in the electrolysis reaction, but with the sign reversed. Thus the process could be represented by the following thermodynamic equation H 2 (g) + ½O 2 (g) H 2 O(l) ÄH = 285 kj/mol Consider another example - the burning of methane, the main component of natural gas, to form gaseous water and carbon dioxide. This exothermic reaction releases 889 kj from each mole of methane used so the thermodynamic equation for the reaction is CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(g) ÄH = 889 kj/mol Energy units. There are various units used to express an amount of energy. In science, the normal unit used is the joule, abbreviated as J. Note that energy and work units are identical as these two are interchangeable - energy is expended doing work or work can be harnessed to produce energy. The use of kj per mole in the above thermodynamic equations can be somewhat confusing until one realises that the per mole part merely indicates that all quantities are expressed in moles. The actual numbers of moles of each constituent is given by the stoichiometric coefficients as written in the equation. It is customary to write the per mole as mol 1 " using the exponential notation. Thus while ÄH for the reaction equation H 2 (g) + ½O 2 (g) H 2 O(l) ÄH = 285 kj mol 1 shows that 285 kj is released for the formation of 1 mole of H 2 O,

3 ÄH for the reaction equation ST3-3 2H 2 (g) + O 2 (g) 2H 2 O(l) ÄH = 570 kj mol 1 shows that 2 ( 285) = 570 kj is released for the formation of 2 moles of H 2 O. Consequently a ÄH value for a reaction must be accompanied by the equation to which it refers. Check your understanding of this section. (a) Write a thermodynamic equation for the combustion of methane (natural gas). A typical household uses 100 megajoules each quarter on average. Calculate what mass of methane this would correspond to having been burnt. (1 megajoule = 10 6 joules) (b) Given H 2 (g) + Cl 2 (g) 2HCl(g) ÄH = 184 kj mol 1, calculate ÄH for the reaction 1 / 2 H 2 (g) + 1 / 2 Cl 2 (g) HCl(g). Activation energy: why you need a match to light the gas? Consider the reaction of hydrogen with oxygen again. A mixture of these two gases in a balloon remains indefinitely without reacting. However, application of a small amount of energy such as from a spark or a match causes the reaction to proceed instantaneously to completion, releasing the 285 kj of energy for every mole (2.0 g) of H 2 reacting. [This is the fuel mixture that was used in the Space Shuttle.] The reaction could not start without the heat supplied by the match because there is an "energy barrier" between the reactants and products. Recall that energy is always needed to break bonds, and that before any energy can be obtained through the formation of new bonds, at least some of the old bonds must be broken. This energy barrier is called the ACTIVATION ENERGY of the reaction. Apparently, in the original mixture, not enough of the hydrogen and oxygen molecules have sufficient energy when they collide for reaction to occur no matter how long one waits. By supplying the additional energy from the match to relatively few molecules, they are then able to react and in the process release energy to the remaining molecules, thereby providing the activation energy for all the remaining molecules to react, as the reaction is exothermic. This can be illustrated on an energy diagram as shown.

4 ST3-4 Check your understanding of this section. The process of charging a car battery involves some electrolysis of the sulfuric acid / water mixture in the battery. Why should one not use a match to inspect the electrolyte level in the battery? Catalysts. Catalysts are substances which, when present, reduce the activation energy for a given reaction by providing a different, lower-energy pathway, thereby causing the reaction to proceed faster. Catalysts are not consumed in the reaction, and do not give a greater final yield of products - they only make the reaction faster, thereby increasing the yield obtained in a given time. The following diagram represents the enthalpy change for a reaction with and without a catalyst present. There are many different types of catalysts. Some are simply metals which have a suitable surface on which to promote a particular reaction - eg platinum gauze can catalyse the reaction between hydrogen and oxygen. All modern motor cars have a canister containing metal catalysts in their exhaust systems to catalyse the reaction of unburnt hydrocarbons to carbon dioxide and water. The catalyst used would become "poisoned" if any lead came in contact with it, and this was the original reason why unleaded fuel was introduced, although later concern about lead levels in blood may cause one to believe that this was the reason. As replacement canisters of catalyst for cars cost about $500, it was most important not to ever allow leaded fuel to enter the petrol tank of a car requiring unleaded fuel. The elimination of leaded fuel has removed this risk.

5 ST3-5 Many industrial processes would not be feasible without the use of catalysts which allow greater yields in a given amount of time and/or allow a lower temperature to be used, thereby saving on fuel costs. The Haber synthesis, a method whereby nitrogen and hydrogen are combined to form ammonia, is responsible for about half of the nitrogen fixed on earth. This synthesis would not be feasible without the catalyst used in the industrial process. The search is currently under way to find catalysts that can lead to the commercial production of hydrogen economically enough to become a fuel for motor vehicles. Energy content of some common fuels. The following illustration shows the energy supplied by complete combustion of some common fuels. The units used are kilojoules per gram of the fuel. The chemical energy released from the fuel can be harnessed for other purposes - e.g. to do work such as in an internal combustion engine or to generate electricity which in turn can be used to do work via an electric motor. Note that the process of conversion of the original energy from the fuel into useful work is seldom even 50 % efficient. Energy and living systems. The same laws governing energy and work apply equally well to living systems (in vivo) as they do to chemical changes in test tubes (in vitro). Apart from nuclear energy, earth is ultimately dependent on energy from the sun in the form of electromagnetic radiation (EMR) in the infra-red, visible and ultra-violet frequencies. The energy carried by EMR is converted to chemical energy stored in the bonds of CARBOHYDRATES by plants through the process of PHOTOSYNTHESIS, in particular via formation of the monosaccharide called GLUCOSE as shown in the following equation. 6CO 2 (g) + 6H 2 O(l) C 6 H 12 O 6 (s) + 6O 2 (g) ÄH = kj mol 1

6 ST3-6 Glucose and other simple carbohydrates are combined to form polymers called POLYSACCHARIDES, such as CELLULOSE and STARCH. n C 6 H 12 O 6 (C 6 H 10 O 5 ) n where n is > 1000 Cellulose provides structural strength for plant cells and starch provides a reserve store of glucose for energy requirements. The energy stored in the chemical bonds of oil, coal and natural gas represents reserves of solar energy from millions of years past, laid down at a time when the climate favoured prolific plant growth. Objectives of this Topic. After studying this Topic, you should have achieved the following goals: 1. Recognise that energy is stored in chemical bonds. 2. Know that when chemical reactions occur, energy is required to break bonds and is released when new bonds form, and that there will be an overall energy change equal to the difference between the energy stored in the bonds of the products and reactants. 3. Know that both endothermic and exothermic reactions require some energy input to initiate them - called the activation energy for that reaction. 4. Understand that the energy change for a reaction proceeding one way is identical, but of opposite sign, to the energy change for the same reaction in reverse. 5. Be able to write a thermodynamic equation and interpret the ÄH data associated with it. 6. Understand the role of catalysts in lowering the activation energy barrier by providing an alternative pathway for the reaction. 7. Know that energy can be obtained from fuels and foods substances which undergo exothermic reactions and that the energy released can in part be made to do work or support living systems.

7 XVI - 7 SUMMARY. Chemical reactions involve the breaking of existing bonds in the reactants and the forming of new bonds in the products. These changes are accompanied by energy changes - exothermic if there is a nett release of energy and endothermic if there is a nett gain of energy by the components of the reaction. The total energy stored in a collection of molecules is called its enthalpy and is given the symbol H. When a reaction occurs, the total energy change is represented as ÄH. If the reaction is exothermic, the enthalpy change is given a negative sign while if the reaction is endothermic, absorbing energy, the enthalpy change is given a positive sign. The energy change for a reaction going in the forward direction has the same magnitude as that for the reaction going in the reverse direction. An equation which shows the reactants and their physical states as well as the accompanying enthalpy change is called a thermodynamic equation. Reacting molecules must collide with enough energy to break bonds (always an endothermic process) before any energy can be obtained from the formation of new bonds (always exothermic). Consequently every reaction always has an activation energy requirement which must be supplied before the reaction can proceed. This activation energy barrier can be lowered by substances called catalysts which provide an alternative reaction pathway. They are not consumed in the reaction but merely allow it to proceed much more rapidly. Energy derivable from fuel and foods can be used to do work or to allow living systems to function. Recommended follow up chemcal modules: Section: General Chemistry Module: Chemical Energy and Calorimetry Topics covered: Chemical energy; calorimetry; enthalpy; Hess's law; bond enthalpies. TUTORIAL QUESTIONS - SUPPLEMENTARY TOPIC What is meant by the terms "exothermic reaction" and "endothermic reaction"? Give two examples of each.

8 ST What is the reason why chemical reactions are usually accompanied by gain or loss of heat? 3. Why is it that a reaction which is exothermic such as the combustion of hydrogen gas still requires heat to be supplied before it starts? 4. Utilisation of glucose as a food releases 2807 kj of energy per mole of glucose. Where did the energy to make glucose originate? What happens to the energy released when glucose is utilised? 5. Explain the role of a catalyst. What are the characteristics of catalysts? 6. The Haber synthesis of ammonia produces half of all the nitrogen that is fixed on earth. This reaction combines nitrogen gas and hydrogen gas at high pressure and temperature using a catalyst to achieve large yields economically. Given that for the reaction N 2 (g) + 3H 2 (g) 2NH 3 (g) ÄH = 91.8 kj mol 1 calculate the amount of energy released in the production of 100 g of ammonia.

9 ST3-9 ANSWERS TO TUTORIAL SUPPLEMENTARY TOPIC Exothermic reactions are those that liberate heat while endothermic reactions absorb heat as a result of the reaction occurring. Burning fuels such as methane (natural gas) or petrol are exothermic. Endothermic reactions include the smelting of metal ores such as iron oxide to form the metal. Another example is the formation of carbohydrates by plants using solar energy to provide the required energy. 2. There is usually a difference between the energy stored in the bonds of the reactants and the products. This energy difference is made apparent by the gain or loss of heat during the reaction. 3. To initiate reactions requires some energy to be supplied because the first step involves the breaking of bonds in the reactants, a process that always needs energy input. Once even a small number of bonds have been broken, the energy released from the formation of new bonds may sustain the reaction so continued heating may not be required. For example, a mixture of hydrogen and oxygen gases will not combine unless a spark or similar small amount of heat is supplied and then the mixture explodes instantaneously. The energy required to make a reaction proceed is called its activation energy. 4. The chemical energy stored in the bonds of glucose originated as energy from the sun. Glucose undergoes a series of reactions in cells of the body, ultimately being transformed into water and carbon dioxide and releasing energy in the process. The energy released is used to drive chemical reactions required for processes involved in the metabolism within living cells. 5. Catalysts are substances that enable the activation energy of a reaction to be lowered by using a different pathway between the reactants and the products. Catalysts are not consumed in the reaction and do not increase the yield but speed it up, allowing more product to be obtained in a shorter time and often with the need for less heat to be supplied because the activation energy is smaller. 6. The thermodynamic equation for the reaction, N 2 (g) + 3H 2 (g) 2NH 3 (g) ÄH = 91.8 kj mol 1 shows that the production of 2 moles of ammonia releases 91.8 kj of energy. Molar mass of NH 3 = g mol 1 2 moles of NH 3 has a mass = = g and releases 91.8 kj of energy. production of 100 g of NH 3 releases / kj = 269 kj of energy.