5.1 Energy Changes in Chemical and Nuclear Reactions

Transcription

1 Figure 1 Our use of fossil fuels is usustaiable. New sources of gree, sustaiable eergy are eeded to meet the world s growig demad. 5.1 Eergy Chages i Chemical ad Nuclear Reactios Our ability to haress eergy has cotributed greatly to the developmet of huma civilizatio. For most of huma history, the oly way to trasport people ad thigs over lad was by walkig or through aimal labour. The, durig the Idustrial Revolutio of the 1800s, ivetors foud ways to haress thermal eergy to do work. Eergy released by burig coal was used to heat water, releasig steam fast eough to move mechaical parts i machies. People ad goods could the be moved log distaces by steam-powered locomotives ad boats. The ext advace took place i the late 1800s i the form of the iteral combustio egie, which produces eergy by burig gasolie. Today, there are more tha half a billio vehicles that are powered by the iteral combustio egie (Figure 1). As this umber grows, the demad for iexpesive ad reliable sources of fuel also icreases. Most of these vehicles use fossil fuels, which are o-reewable ad egatively effect the eviromet. How will we meet our eergy eeds for trasportatio ad other activities i more sustaiable ways? To aswer this challege, we eed to uderstad the eergy chages that take place i chemical ad uclear reactios. The Nature of Eergy thermochemistry the study of the eergy chages that accompay physical or chemical chages i matter eergy the ability to do work; SI uits joules (J) work the amout of eergy trasferred by a force over a distace; SI uits joules (J) potetial eergy the eergy of a body or due to its positio or compositio kietic eergy the eergy of a object due to its motio The study of eergy chages durig physical or chemical chages i matter is called thermochemistry. The study of thermochemistry uses some familiar terms i very precise ways. Scietists defie eergy as the ability to do work. Work is the eergy trasferred to a object by a force that causes the object to move. For example, the eergy to lift satellites ito orbit comes from simple chemical reactios that release huge quatities of thermal eergy. The hot expadig gases released durig the rocket lauch do work o the rocket by pushig it ito the sky. No work is doe, however, if the force of the gases is iadequate to get the rocket off the groud. Both eergy ad work are measured i joules (J). All forms of eergy ca be classified as either kietic eergy or potetial eergy. Potetial eergy is eergy due to the positio or compositio of a object. Kietic eergy is the eergy of motio. For example, the water behid a dam has a great deal of potetial eergy because of its elevatio. This eergy ca be used to do work o the blades of turbies i a hydroelectric geeratig statio. As the water falls, the potetial eergy is trasformed ito kietic eergy. The fallig water causes the turbie blades to spi. The kietic eergy of the spiig turbies is trasferred to a geerator ad trasformed ito electrical eergy, aother form of potetial eergy. The eergy associated with chemical bods is also potetial eergy. Chemical bods are the result of attractive forces betwee the ucleus of oe atom or io ad the egative charges of electros i the other atom or io. The amout of eergy released or absorbed i a chemical reactio equals the differece betwee the potetial eergy of the bods i the reactats ad the potetial eergy of the bods i the products. For example, eergy is released durig the combustio of gasolie because the products have less potetial eergy tha the reactats. I a iteral combustio egie, the products of this combustio reactio are hot gases. The molecules i the hot gases that are produced by the egie move much faster tha the molecules i the icomig cool air. Therefore, the combustio gases have more kietic eergy. This eergy is used to do work o the mechaical parts iside the egie that evetually make the car move. 286 Chapter 5 Thermochemistry NEL

2 Thermal Eergy, Heat, ad Temperature The total quatity of potetial eergy ad kietic eergy of a substace is called thermal eergy. I geeral, the quatity of thermal eergy of a substace depeds o how fast its etities atoms, ios, molecules, or polyatomic ios are movig. Whe a substace absorbs thermal eergy, its etities move at a greater speed, ad the substace warms up. Whe a substace releases thermal eergy to its, its etities move more slowly, ad the substace cools. I sciece, heat is a verb that refers to the trasfer of thermal eergy from a warm object to a cool object. So, whe you heat water i a kettle o a stove, you trasfer thermal eergy from the burer to the kettle to the water. Similarly, whe a object cools, it trasfers thermal eergy to its. Temperature is a measure of the average kietic eergy of the etities i a substace. As a substace is warmed, some of its particles move faster. The average kietic eergy of the substace s etities icreases ad so does the temperature of the substace. For example, a cup of hot water has a higher temperature tha a iceberg. The average water molecule i the hot water has more kietic eergy tha the average water molecule i the iceberg. However, a iceberg cotais billios more water molecules tha a cup of hot water. Therefore, the total quatity of thermal eergy of a iceberg is greater tha that of a cup of hot water. Law of Coservatio of Eergy Oe importat characteristic of eergy is that it is always coserved. We caot create ew eergy or ca we destroy eergy by usig it. Eergy ca oly be coverted from oe form ito aother. Thik about the combustio of propae i a barbecue (Figure 2). Chemical eergy stored i propae is coverted ito thermal eergy ad light eergy. However, the total amout of eergy before ad after the combustio reactio occurs remais costat. These ideas are summarized i the law of coservatio of eergy: Law of Coservatio of Eergy Eergy caot be created or destroyed. To help aalyze eergy coversios ivolvig chemical reactios, chemists fid it useful to divide the uiverse ito two parts: the chemical ad its. A chemical is composed of the reactats ad products beig studied. The are all the matter that is ot part of the. I the propae gas barbecue, the chemical is composed of the reactats (propae gas, C 3 H 8 (g), ad oxyge gas, O 2 (g)), ad the products (carbo dioxide gas, CO 2 (g), ad water vapour, H 2 O(g)). The iclude the barbecue ad the food o the grill. Chemical s ca be classified as beig ope or closed. A ope ca exchage both eergy ad matter with its. A operatig propae barbecue is a example of a ope. Propae gas flows ito the burer ad thermal eergy ad the products of combustio flow out. A chemical reactio that produces a gas i a solutio i a usealed beaker is also a ope, sice eergy ad matter ca flow ito or out of the to the. The iclude the beaker ad the air aroud the beaker. A closed ca exchage eergy, but ot matter, with its. A glow stick (Figure 3) is a example of a closed. Light eergy released whe chemicals iside the stick are mixed flows out ito the. The matter (that is, the chemicals) remai sealed iside. A third kid of exists, called a isolated. I a isolated, either matter or eergy ca move ito or out of the. However, it is impossible to set up a true isolated o Earth. thermal eergy the total quatity of kietic ad potetial eergy i a substace heat the trasfer of thermal eergy from a warm object to a cooler object temperature a measure of the average kietic eergy of etities i a substace Figure 2 The potetial eergy of the propae fuel i the barbecue is coverted to thermal eergy. chemical a group of reactats ad products beig studied all the matter that is ot part of the ope a i which both matter ad eergy are free to eter ad leave the closed a i which eergy ca eter ad leave the, but matter caot Figure 3 Each of these glow sticks forms a closed. Matter caot escape; oly light eergy. isolated a ideal i which either matter or eergy ca move i or out NEL 5.1 Eergy Chages i Chemical ad Nuclear Reactios 287

3 exothermic releasig eergy to the Edothermic ad Exothermic Reactios Durig a chemical reactio, the chemical bods i the reactat(s) are broke, ad ew bods are formed to produce the product(s). The potetial eergy of the etities i a stable structure, such as the atoms i a molecule or the ios i a ioic compoud, is lower tha the potetial eergy of the idividual isolated atoms. Therefore, bod breakig requires eergy, while bod formatio releases eergy. From the law of coservatio of eergy, you kow that the total eergy i a chemical ad its must be the same before ad after a chemical reactio occurs. If more eergy is released from the formatio of ew bods i the products tha is required to break bods i the reactats, the the chemical reactio will release some eergy to the. A chemical reactio that releases eergy to the is exothermic (the prefix exo- meas out of ). The products of a exothermic reactio have lower potetial eergy (ad stroger bods o average) tha the reactats. For example, the combustio of methae is a exothermic reactio. The balaced chemical equatio for this reactio is CH 4 1g2 1 2 O 2 1g2 S CO 2 1g2 1 2 H 2 O1g2 1 eergy Notice that a eergy term is icluded o the product side of the equatio to show that eergy is released. This eergy is the differece betwee the eergy absorbed by breakig bods i the reactats ad the eergy released by formig bods i the products. The quatity of this eergy is the differece i potetial eergy betwee the reactats ad products ad has the symbol DE p. Sice the combustio of methae is exothermic, eergy is released to the, maily as thermal eergy ad light eergy. Figure 4 illustrates the chages that occur durig this reactio i a ope. The quatity of matter i the (represeted by the ta box) remais the same, ad the released eergy (the large arrow) flows to the (the yellow area). Whe a exothermic reactio releases thermal eergy, the temperature of the icreases. Eergy Chages durig a Exothermic Reactio i a Ope System 1 mol CH 4 (g) 2 mol O 2 (g) ( reactats) E p eergy released to the 1 mol CO 2 (g) 2 mol H 2 O(g) (products) Figure 4 The combustio of methae releases a quatity of eergy with the magitude E p, which flows to the, maily as thermal eergy ad light eergy. This is a exothermic reactio. edothermic absorbig eergy from the A reactio that absorbs thermal eergy from its is edothermic (the prefix edo- meas iside ). Durig a edothermic reactio, the chemical absorbs eergy from the ad icreases its potetial eergy. This meas that the products have higher potetial eergy (ad weaker bods o average) tha the reactats. The sythesis of itric oxide from its elemets is a example of a edothermic reactio, represeted by the equatio N 2 1g2 1 O 2 1g2 1 eergy S 2 NO1g2 288 Chapter 5 Thermochemistry NEL

4 Notice that, for a edothermic reactio, the eergy term is icluded o the reactats side of the equatio. Figure 5 illustrates the chages that occur durig this reactio i a ope. The quatity of matter i the agai remais the same, but ow eergy flows from the to the. Eergy Chages durig a Edothermic Reactio i a Ope System 1 mol N 2 (g) 1 mol O 2 (g) ( reactats) E p 2 mol NO(g) (product) eergy absorbed from the Figure 5 The sythesis of itric oxide from its elemets absorbs a quatity of thermal eergy with the magitude E p from the. This is a edothermic reactio. Uit TASK BOOKMARK How ca you apply what you have leared about edothermic ad exothermic reactios i your Uit Task (o page 402)? Nuclear Eergy Ulike chemical reactios, all uclear reactios are exothermic. Per uit of mass, uclear reactios release much more eergy tha exothermic chemical reactios. Two uclear reactios ivolvig large quatities of eergy are fissio ad fusio. A fusio reactio occurs whe uclei of small atomic mass combie to form larger, heavier uclei. Fusio reactios are resposible for the release of vast amouts of eergy i stars, icludig the Su (Figure 6). The Su cosists of 73 % hydroge, 26 % helium, ad 1 % other elemets. Hydroge uclei uder immesely high pressure i the Su udergo fusio ad form helium uclei. The fusio process releases eergy, some of which reaches Earth. The fusio of hydroge atoms produces kj of eergy for each mole of helium produced. Let us review uclear otatio. A elemet with the chemical symbol X is represeted as A ZX, where A is the mass umber (umber of protos plus umber of eutros) ad Z is the atomic umber (umber of protos). A eutro is a subatomic etity, ad the symbol 1 0 is used to show that a eutro has a mass umber of 1 ad a atomic umber of 0. Usig uclear otatio, the equatio for the fusio reactio of hydroge atoms is 2 1H 1 3 1H S 4 2He Scietists have worked for decades to fid a way to develop fusio reactors to geerate eergy, but the itese pressure ad heat eeded for fusio to occur have made this difficult to achieve to date. CAREER LINK Durig fissio, large uclei with high atomic mass are split ito smaller, lighter uclei by collisio with a eutro. The uclei of all elemets above atomic umber 83 are ustable ad ca udergo fissio. Fissio does ot ormally occur i ature. Fissio reactios produce vast quatities of eergy millios of times more tha is released i chemical reactios. fusio the process of combiig two or more uclei of low atomic mass to form a heavier, more stable ucleus Figure 6 The most commo fusio reactios that occur i the Su ivolve hydroge atoms. fissio the process of usig a eutro to split a ucleus of high atomic mass ito two uclei with smaller masses NEL 5.1 Eergy Chages i Chemical ad Nuclear Reactios 289

5 Nuclear power plats use the fissio of uraium-235 to produce electricity. Whe a eutro collides with a U ucleus, the uraium ucleus splits ito smaller uclei ad releases eergy ad additioal eutros (Figure 7). These eutros collide with more uraium uclei, causig these uclei to split ad release more eergy ad eve more eutros Kr eergy U U (ustable ucleus) Ba Figure 7 The uraium-235 ucleus udergoes fissio whe a eutro strikes it. The fissio produces two lighter uclei, free eutros, ad a large amout of eergy. The equatio for the fissio of uraium is U S 92 36Kr Ba Uraium is used i uclear power plats because it udergoes a fissio chai reactio. About 15 % of all electrical eergy i Caada is produced by the fissio of uraium i uclear power plats. Nuclear power plats ca geerate much more electricity from a small amout of fuel tha ca power plats that use fossil fuels. For example, uraium fissio ca produce about 26 millio times more eergy tha the combustio of a equal mass of methae. WEB LINK Eergy chages occur durig ay physical, chemical, or uclear chage. Table 1 gives a example of each of these types of eergy chages, ad the magitude of the potetial eergy chage i the. Notice that the potetial eergy chage varies cosiderably. For example, to show the graph of the uclear chage o the same scale as the other chages would require a textbook about 1000 km log! Table 1 Examples of Magitudes of Potetial Eergy Chages durig a Phase Chage, Chemical Chage, ad Nuclear Chage Phase chage Chemical chage Nuclear chage H 2 O(g) S H 2 O(l) Potetial Eergy Chage durig a Phase Chage H 2 O(g) (reactat) E p = 44.0 kj/mol H 2 O(l) (product) H 2 (g) O 2(g) S H 2 O(l) Potetial Potetial Eergy Eergy Chage Chage Durig durig a Chemical Chage a Chemical Chage 1 H 2 (g) 2 O 2 (g) (reactats) E p = kj/mol H 2 O(l) (product) 2 1H 1 3 1H S 4 2He Potetial Potetial Eergy Eergy Chage Chage Durig durig a Nuclear Chage a Nuclear Chage 1 2 H 3 1 H (reactats) E p = kj/mol 4 2He 1 0 (products) 290 Chapter 5 Thermochemistry NEL

6 5.1 Review Summary Eergy is the ability to do work. Potetial eergy is the eergy of positio or compositio. Kietic eergy is the eergy of motio. Thermal eergy is the sum of the potetial ad kietic eergy i a substace. Heat is the trasfer of thermal eergy from a warmer object to a cooler object. Temperature is a measure of the average kietic eergy of the particles i a substace. The law of coservatio of eergy states that eergy caot be created or destroyed but ca be coverted from oe form to aother. A ope ca exchage both eergy ad matter with its. A closed ca exchage eergy, but ot matter, with its. A exothermic reactio releases eergy to its. A edothermic reactio absorbs eergy from its. Fusio is a uclear reactio i which two or more uclei of low atomic mass combie to form a heavier ucleus. Fissio is a uclear reactio i which a eutro collides with a ucleus of high atomic mass ad splits it ito uclei with smaller masses. Questios 1. For each of the followig, describe which is greater the potetial eergy or the kietic eergy. K/U (a) a stick of dyamite prior to explodig (b) a space shuttle before lauch (c) the soot risig above a campfire (d) the chemicals iside a ew battery 2. For each of the followig, idetify whether the reactio process is exothermic or edothermic. K/U (a) water evaporatig ito steam (b) a cadle burig (c) the combustio of gasolie (d) the meltig of ice (e) the splittig of a itroge molecule, N 2, ito idividual atoms (f) dissolvig barium hydroxide, Ba(OH) 2, i water ad observig a temperature decrease i the 3. Cosider the followig statemet: The law of coservatio of eergy states that eergy caot be created or destroyed. Therefore, whe thermal eergy is trasferred from a to the, the quatity of thermal eergy lost by the must be equal to the amout of thermal eergy gaied by the. Does this statemet accurately describe what happes to the thermal eergy? Explai why or why ot. K/U A 4. Distiguish betwee heat, thermal eergy, ad temperature usig examples from your experiece. K/U A 5. Compare ad cotrast uclear fissio ad fusio. K/U T/I 6. The law of coservatio of eergy ca be stated as the total eergy of the uiverse is costat. Explai how this is a restatemet of the defiitio give i the text. K/U T/I 7. A fire is started i a fireplace by strikig a match ad lightig crumpled paper uder some logs. Explai all the eergy trasfers i this sceario usig the terms exothermic, edothermic,,, potetial eergy, ad kietic eergy. K/U T/I A 8. Explai why the water i a swimmig pool at 24 C has more thermal eergy tha a cup of boilig water at 100 C. K/U T/I A 9. Liquid water at 0 C turs to ice. Is this process edothermic or exothermic? Explai what is occurrig usig the terms,, thermal eergy, potetial eergy, ad kietic eergy. K/U T/I A NEL 5.1 Eergy Chages i Chemical ad Nuclear Reactios 291