CHEMICAL REACTIONS INVOLVE ENERGY The study energy and its transrmatins is knwn as thermdynamics. The discussin thermdynamics invlve the cncepts energy, wrk, and heat. Types Energy Ptential energy is stred energy that matter pssesses as result its cmpsitin r psitin relative t ther bjects. E P =mgh Kinetic energy is energy matter has as a result its mtin. K E = ½mv 2 Chemical energy is due t ptential energy stred in arrangements atms the substance. Likewise, the energy an bject pssesses because its temperature (thermal energy) is assciated with the kinetic energy the mlecules in the bject. COMMON MISCONCEPTIONS Dn t cnuse pwer and energy r heat and temperature. Heat is the energy that is transerred rm a htter bject t a clder ne. Energy is the capacity t d wrk r t transer heat. Pwer is the time rate at which wrk is dne r energy transerred. Temperature is a measure average kinetic energy a substance. ENERGY UNITS The SI unit r energy is the jule. With mass expressed in kilgrams and velcity expressed in meter per secnds, kinetic energy has the units jules ( 1J = 1 kg m 2 /s 2 ). A jule is nt a large amunt energy, thus kiljules (kj) is ten used. Traditinally, energy changes accmpanying chemical reactins have been expressed in calries. A calrie (cal) was riginally deined as the amunt energy required t raise the temperature 1 g water by 1 C. It is nw deined in terms the jule: 1 cal = 4.184 J (exactly) A related energy unit used in nutritin is the nutritinal Calrie. 1 Cal = 1 kcal 1
SYSTEMS AND SURROUNDINGS When we study energy changes, we cus ur attentin n a limited and well-deined part the universe. The prtin we single ut r study is called the system; everything else is called the surrundings. I we bserve a chemical reactin in the lab, the chemicals usually cnstitute the system The cntainer and everything beynd it are cnsidered the surrundings. The systems we can mst readily study are called clsed systems (matter can t be exchanged with the surrunding). THE FIRST LAW OF THERMODYNAMICS Science has bserved that energy can be neither created nr destryed: energy is cnserved. Any energy lst by the system must be gained by the surrundings, and vice versa. This imprtant undamental bservatin is knwn as the irst law thermdynamics, which is als called the law cnservatin energy. INTERNAL ENERGY The ttal energy a system is the sum all the kinetic and ptential energies its cmpnent parts. Parts being O 2 and H 2 mlecules. We deine the change in internal energy as the dierence between the internal energy the system at the cmpletin a prcess and that at the beginning: E = E inal - E initial The quantity E has three parts: a number, a unit, and a sign. A psitive E results when E inal > E initial, thus the system has gained energy rm its surrunding. A negative E results when E inal < E initial, here the system has lst energy t its surrundings E = E inal - E initial H 2(g), O 2(g) In a chemical reactin the initial state the system reers t the reactants and the inal state reers t the prducts. H 2 + O 2 H 2 O + heat Internal energy E < 0 E > 0 H 2 O (l) When hydrgen and xygen rm water, the system lses energy t the surrundings as heat; the energy cntent the prducts is less than that the reactants, thus E is negative. A psitive E results when E inal > E initial, thus the system has gained energy rm its surrunding. A negative E results when E inal < E initial, here the system has lst energy t its surrundings. 2
RELATING E TO HEAT AND WORK Any system can exchange energy with its surrundings in tw general ways: as heat r wrk. When a system underges any chemical r physical change, the accmpanying change in its internal energy, E, is given by the heat added t r liberated rm the system,q, plus the wrk dne n r by the system,w. E = q + w This is the irst law thermdynamics in algebraic rm. When Heat is transerred rm the surrunding t the system, q has a psitive value. E Heat q > 0 Wrk w > 0 Surrundings Likewise, when wrk is dne n the system by the surrunding, w has a psitive value. When Heat is transerred rm the system t the surrundings, q has a negative value. E Heat q < 0 Wrk w < 0 Surrundings The H 2 and O 2 gases in the cylinder are ignited. As the reactin ccurs, the system lses 1150 J heat t the surrundings. The pistn mves up and des 480 J wrk n the surrundings. What is the change in internal energy the system? Likewise, when wrk is dne n the surrundings by the system, w has a negative value. ENDOTHERMIC AND EXOTHERMIC PROCESSES When a prcess ccurs in which the system absrbs heat, we say the the prcess endthermic (end =int). A prcess that results in the evlutin heat is called exthermic (ex = ut ). STATE FUNCTIONS One has n way knwing the precise value the internal the internal energy a system, hwever ne des knw that it has a ixed value r a given set cnditins. The cnditins that inluence this energy include the temperature and pressure. And energy is an extensive prperty. Sate unctins d nt depend n the pathway, nly n the initial and inal states. 3
E =q + w ENTHALPHY A quantity deined by the relatinship H = E + PV. The enthalpy change, H r a reactin that ccurs at cnstant pressure, is the heat evlved r absrbed in the reactin: H=q p In shrt, enthalphy is a cnventin used t make the measuring heats reactin simple by remving wrk. ENTHALPIES OF REACTION Surrundings Surrundings The enthalpy change r a chemical reactin is given by the enthalpy the prducts minus that the reactants: H = H(prducts) - H(reactants) Heat Heat 2 H 2(g) + O 2(g) 2 H 2 O (g) H = -483.6 kj H 2(g), O 2(g) H >0 Endthermic H < 0 Exthermic H < 0 Exthermic Enthalpy H 2 O (g) GUIDELINES FOR USING THERMOCHEMICAL EQUATIONS 1. Enthalpy is an extensive prperty. CH 4(g) + 2O 2(g) CO 2(g) + 2H 2 O (g) H = -802 kj/ml 2. The enthalpy change r a reactin is equal in magnitude but ppsite in sign t H r the reverse reactin. CO 2(g) + 2H 2 O (g) CH 4(g) + 2O 2(g) H = 802 kj 3. The enthalpy change r a reactin depends n the state the reactants and prducts. 2H 2 O (g) 2H 2 O (l) H = -88kJ The value H can be determined experimentally by measuring the heat lw accmpanying a reactin. When heat lws int r ut a substance, the temperature the substance changes. Experimentally, we can determine the heat lw assciated with a chemical reactin by measuring the temperature change it prduces. The measurement heat lw is calrimetry; an apparatus that measures heat lw is a calrimeter. 4
HEAT CAPACITY AND SPECIFIC HEAT The temperature change experienced by an bject when it absrbs a certain amunt energy is determined by its heat capacity. Fr pure substances the heat capacity is usually given r a speciied amunt the substance. The heat capacity 1 mle substance is called its mlar heat capacity. The heat capacity 1 g a substance is called speciic heat capacity, r just its speciic heat. Quantity heat transerred Speciic heat = (grams substance) x (temperature change) q Speciic heat = m x T I ne +4.184 J heat is added t 1 gram water its temperature will rise by 1 0 K Hw much heat is required t heat 300.0g water rm 22 C t 100 C? 1 g H 2 O @ 20 0 C + 4.184 J heat = 1 g H 2 O @ 21 0 C q Speciic heat = m x T q = (spec. heat water)(m x T) Slve r q (4.184 J/g x K)(300g)(78 K)= 97.91kJ 1 g Fe @ 20 0 C + 4.184 J heat = 1 g Fe @ 29.3 0 C CONSTANT-PRESSURE CALORIMETRY Many slutin reactins ccur at cnstant pressure s that H=q p q rxn = -q sln q sln = (speciic heat slutin) x (grams slutin) x T When 50.0 ml 0.100M AgNO 3 and 50.0mL 0.100M HCl are mixed in a calrimeter, the temperature the mixture increase rm 22.3 t 23.11 C Calculate H r this reactin: AgNO 3 + HCl AgCl(s) + HNO 3 Assume that the cmbined slutin has a mass 100g and speciic heat 4.184J/g C H = q rxn = -q sln q sln = (speciic heat slutin) x (grams slutin) x T (4.184J/ g C) (100g)( 0.81) = 338.9 J/0.005 mles H = -67.8 kj 5
CONSTANT VOLUME CALORIMETRY CONSTANT VOLUME CALORIMETRY Calculate the heat capacity a calrimeter (C calrimeter ). Cmbustin exactly 1g benznic acid prduces 26.38 kj heat. When 1g benzic acid is cmbusted in ur calrimeter it causes a temperature increases 5.022 C. The heat capacity the calrimeter is: 26.38 kj 5.022 C = 5.022kJ/ C I 1g hydrazine, which has a heat reactin -618kJ/ml, is cmbusted in a calrimeter, that has a heat capacity 5.022kJ/ C, what will be the expected rise in temperature? -618 kj - C (1 g N 2 H 4 ) 1 ml N 2 H 4 = 3.85 C ml 5.022kJ 32.0g HESS S LAW I a reactin is carried ut in a series steps, H r the reactin will be equal t the sum the enthalpy changes r the individual steps. H r any prcess can be calculated using H values rm ther reactins. + Using the llwing enthalpies cmbustin: 1) C(s) + O 2(g) CO 2(g) H = -393.5 kj 2) CO (g) + ½ O 2(g) CO 2(g) H = -283.0 kj calculate the enthalpy cmbustin r the cmbustin C t CO. C (s) + O 2(g) CO 2(g) H = -393.5 kj CO 2(g) CO (g) + ½ O 2(g) H = 283.0 kj C (s) + ½ O 2(g) CO (g) H = -110.5 kj ENTHALPIES OF FORMATION The heat reactin r many chemical reactins can be calculated rm a ew tabulated H values. Extensive tables exist enthalpies vaprizatin r cnverting liquids t gas, enthalpies usin r cnverting liquid t slids) and enthalpies cmbustin. An imprtant prcess used r tabulating thermchemical data is the rmatin a cmpund rm its cnstituent elements. This is called the enthalpy rmatin H. In rder t cmpare the enthalpies dierent reactins, it is cnvenient t deine a set cnditins, called a standard state, at which mst enthalpies are tabulated. The standard state a substance is its pure rm at atmspheric pressure and 298K (25 C). 6
The standard enthalpy a reactin is deined as the enthalpy changed when all reactants and prducts are in their standard state. We dente a standard enthalpy as H, where the superscript indicates standard-state cnditins The standard enthalpy rmatin a cmpund, H is the change in enthalpy r the reactin that rms 1 mle the cmpund rm its elements, with all substances in their standard states. 2C (graphite) + 3H 2(g) + ½O 2 (g) C 2 H 5 OH (l) H = -277.7 kj Using Enthalpies rmatin t Calculate H rxn I an element exists in mre than ne rm under standard cnditins, the mst stable rm that element is used r the rmatin reactin. H = n H (prducts) m H (reactants) rxn = ( prducts) H (reactants) rxn n H m H Use standard enthalpies rmatin t calculate H rxn r the reactin: 2SO 2 (g) + O 2 (g) 2SO 3 (g) H, kj/ml -296.83 0-395.72 Frm Appendix C p987 H rxn = [2 ml SO 3 H (SO 3 )]-[2 ml SO 2 H (SO 2 )+ 1ml O 2 H (O 2 )] [2(-395.72)]-[2(-296.83) + 1(0)] = -197.78kJ 7