KEY POINTS: NOTE: OCR A

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1 KEY: A.J.F.S DEFINITION: KEY POINTS: NOTE: OCR A Chemistry Mdule 3- Energy 2.3 (1) Enthalpy What is chemical energy? - Chemical energy is a special frm f ptential energy that lies within chemical bnds - Chemical bnds bind the atms in cmpunds tgether - When chemical react Bnd break and new bnds frm - Such a prcess changes the chemical energy f the atms Enthalpy - Enthalpy, H, is the heat cntent stred in a chemical system - It s practically impssible t measure directly the enthalpy f the reactants and prducts - Hwever, able t measure the energy released/absrbed t the surrundings during a chemical change - Often the energy is in the frm f heat, but als can be light and electrical. - Cnservatin f energy Energy is never created r destryed The amunt f energy released frm a chemical system is exactly equal t the amunt released int the surrundings S in the case heat: Heat lss in a chemical system is equal t heat gain in surrunding Heat gained in a chemical system is equal t heat lss in surrundings - Enthalpy change Smetimes prducts have mre energy than the reactants Otherwise reactants have mre chemical energy than the prducts An enthalpy change is, H, : Heat exchange with the surrundings during a chemical reactin at cnstant pressure The difference between the enthalpy f the prducts and enthalpy f reactants H = H prducts - H reactants Exthermic Reactins - In exthermic reactins the enthalpy f the prducts is smaller than the reactants - As there is heat lst, hence = EXO- - H is a negative sign as heat is lst by the chemical system Endthermic Reactins - In endthermic reactins the enthalpy f prducts is greater than the enthalpy f the reactants - There is heat gained frm the surrundings, hence = ENDO- - H has a psitive as heat is gained within the chemical system 2.3 (2) Exthermic and Endthermic Reactins Exthermic Reactins - Oxidatin f fuels: Cmmn example: Methane Carbn Dixide + Water H = -890kJ ml -1 Prducts have less enthalpy than reactants. Hence excess energy is released CH 4 (g) + 2O 2 (g) 2H 2 O (l) + CO 2 (g) H = -890kJ ml -1 Enthalpy change is included in the equatin Units are kj ml -1 Therefre 1 mle f CH 4 reacts with 2 mles f O 2 t frm 2 mle H 2 O and 1 mle f CO 2, with a releasing f 890kJ ml -1 f heat energy - Respiratin: Mst exthermic reactin in a life frm Reactin = C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O H = -2801kJ ml -1 Withut respiratin, life wuldn t exist

2 Endthermic Reactins - Thermal decmpsitin f limestne Limestne cntains CaCO 3 CaCO 3 decmpses t prduce CaO CaCO 3 (s) CaO (s) + CO 2 (g) H = +178kJ ml -1 Calcium xide is cmmnly knwn as lime Lime is used in cement and treating acidic sils used by farmers - Phtsynthesis Mst imprtant exthermic reactin fr life During phtsynthesis C 6 H 12 O 6 is prduced frm H 2 O + CO 2 Light frm the sun allws phtsynthesis t take place 6CO 2 (g) + 6H 2 O (l) C 6 H 12 O 6 (aq) + 6O 2 (g) H = +2801kJ ml -1 Withut phtsynthesis, life wuldn t exist at all 2.3 (3) Enthalpy Prfile Diagrams Simple Enthalpy Prfile Diagrams - Reactins and their enthalpy changes can be shwn in a enthalpy prfile diagram - An enthalpy prfile diagram, is a diagram which shws the reactants enthalpy cmpared with the prducts enthalpy - Exthermic reactins H prducts < H reactants H Is negative value - Endthermic reactins H prducts > H reactants H Is psitive value Activatin Energy - Chemical reactins d have a chemical barrier which prevents many reactins ccurring spntaneusly - This is knwn as the activatin energy E a - Activatin energy is the minimum energy required t start a reactin by the breaking f bnds - Exthermic reactins: Even thugh the prducts have lwer energy than the reactants, sme energy is actually required t be inputted t initiated the reactin Activatin energy is supplied by a spark, r by heating the chemicals]natural gas requires a spark t start the reactin, because afterwards the reactin is led by itself (heat is expelled which is used t cntinue the reactin Once an exthermic reactin begins the activatin energy is regenerated and the reactin becmes self-sustaining If activatin energy didn t exist, neither wuld fuel (as it wuld just spntaneusly cmbust), see diagram abve n where E a lies.

3 2.3 (4) Standard Enthalpy Changes Standards - Enthalpy changes are depended n the cnditins they are in - Chemists therefre use standard cnditins in enthalpy changes - Clse t nrmal cnditins - Standard cnditins: 100kPa (1 Atmspheric pressure) Temperature f -298K (25 C) Cncentratin f 1 mle dm -3 Standard enthalpy change is shwn by the symbl H - Standard states: Fr a standard enthalpy change, any substance must be in a standard state i.e. Mg(s) is magnesium s standard state Standard Enthalpy Changes - Standard enthalpy change f reactin H r - We require an equatin t find ut mles f reactants and prducts - The enthalpy change, giving the enthalpy change f mlar quantities - Standard enthalpy change f cmbustin H c - Standard enthalpy change f frmatin H f 2.3 (5) Determinatin f Enthalpy Changes Determinatin f Enthalpy Changes - Heat lss frm chemical system = Heat gain t surrundings - Heat gain t chemical system = Heat lss frm surrundings - T determine the heat exchange we need t knw: M The mass f the surrundings invlved in the heat exchange C The specific heat capacity f the surrundings t Temperature change f the surrundings - Q = MC t Units are Jules Direct Determinatin f Enthalpy Changes - Fr many reactins yu can use direct experimental results t calculate enthalpy change - Many reactins ccur when they are just mixed, therefre yu can use a calrimeter - Therefre the calrimeter acts as an insulatr, therefre reactins ccur within the calrimeter and temperatures that changed can be measured 2.3 (6) Enthalpy Change f Cmbustin Cmbustin - Is a reactin that uses xygen t frm xides - Enthalpy change f cmbustin is the change f enthalpy that accmpanies the cmplete cmbustin f 1 mle f substances in excess xygen Experimental Determinatin f H c - Generally are f the easiest experiments t carry ut - What yu have t d: Burn a knwn mass f a substance in air T heat a knwn mass f water And measure the temperature change in the water - Yu measure the vlume f water in a beaker, remember 1cm 3 = 1g - Burner cntaining fuel is weighed - Initial temperature f water is nted and then the burner is lit and water is heated until the temperature has risen by a reasnable amunt taking int accunt the temperature change, - Then re-weigh the mss f the burner

4 - Cmparisn f experimental value with standard enthalpy change There is a difference between the value f H c and H This is because, heat maybe lst t surrundings, r there can be incmplete cmbustin T vercme such issues we need t cut dwn n heat lss and ensure cmplete cmbustin des ccur This is dne by using a much mre better apparatus such as a bmb calrimeter O 2 is freely available = Cmplete Cmbustin and the water is well insulated, reducing the heat lst t surrundings 2.3 (7) Bnd enthalpies Bnd Enthalpy - Chemical bnds are the strehuses fr chemical energy - Get infrmatin abut the strength f a chemical bnd frm its bnd enthalpy - Bnd enthalpies tell yu hw much energy is needed t break different bnds - Bnd breaking is endthermic - Bnd making is exthermic - Average bnd enthalpies (Given t yu in the exam) Bnd Average Bnd Enthalpy/kJ ml -1 C-H +413 O=O +497 O-H +463 C=C +612 H-H +436 Breaking and making bnds - Chemical reactin Bnds brken and new bnds are made - Energy is required at first t break the bnd = Endthermic - Energy is released t make bnds f prducts = Exthermic - Ex r End Exthermic reactins Bnds that are frmed are strnger than the bnds brken Endthermic reactin Bnds that are frmed are weaker than the bnds brken H = Sum f bnd enthalpies brken Sum f bnd enthalpies f bnd made 2.3 (8) Enthalpy changes frm H c Measuring enthalpy changes indirectly - It isn t pssible t measure enthalpy change f a reactin directly this maybe because f: A high activatin energy Slw rate f reactin Mre than 1 reactin taking place - Hence chemists use Hess law - Hess law states that if a reactin can take place by mre than 1 rute and the initial and final cnditins are the same, then ttal enthalpy change is the same fr each rute 2.3 (9) Enthalpy changes frm H f Using enthalpy changes f frmatin - Same as enthalpy f cmbustin (as abve) - Here, reactants and prducts are frmed frm the elements - Use the fabulus bucket methd by Peter. Reactants Prducts Cmbustin prducts/ Elements (frmatin) - What yu simply d is using the values given, frm a bucket underneath the reactin, shuld the values be cmbustin, draw arrws frm each cmpund twards the bucket, ther way rund fr frmatin. Then simply fllw thrugh using the arrws, if yu re ging against the arrw, then make sure yu turn the value frm psitive t negative (if it is psitive) and negative t psitive (if it is negative)

5 2.3 (10) Rates f Reactins Cllisin Thery Intrductin - Cmbustin reactins and explsins have a very high rate f reactin - Explsins ccur within a fractin f a secnd - Rusting f Irn is a very slw prcess and can range frm days t years - Rate f reactin f Diamnd int Graphite can take millins f years t frm - Rate f reactin: Defined as the change in cncentratin f a reactant r a prduct in a given time Rate = Change in cncentratin/time Units = mldm -3 s -1 At start f reactin each reactant is at its greatest cncentratin, hence rate f reactin is at its greatest As the reactin prceeds, the cncentratin gradually decreases, therefre rate f reactin decreases As ne f the reactants is used up Rate f reactin is 0 reactin has stpped - Factrs that alter the rate f a chemical reactin: Can ften be affected by, temperature, pressure, cncentratin, surface area and if a catalyst is present T explain these factrs yu use the cllisin thery S: When 2 mlecules cllide, a reactin may take place if certain cnditins are met The mlecules must have sufficient energy t vercme the activatin energy f the reactin - The effect f cncentratin n reactin rates: Increasing in cncentratin means mre mlecules in the same vlume Mlecules are mre clser tgether = Increase likely hd f successful rate f cllisins Therefre mre cllisins will ccur in a certain length f time - The effect f pressure n reactin rate: As pressure f gasses in increased, mre mlecules are pushed tgether Therefre same number f mlecules ccupy a smaller vlume Increasing pressure in gases is just like increasing the cncentratin 2.3 (11) Catalysts What is a catalyst? - Swedish scientist Jns Jakb Berzelius discvered the term catalyst - A catalyst increases the rate f a chemical reactin by lwering the activatin energy and isn t used up in the prcess - It desn t underg permanent change in the reactin - A catalyst lwers the activatin energy required fr a reactin t carry ut Thus prviding an alternative rute fr reactin t fllw Reducing Energy Cnsumptin and Helping the Envirnment - Many industrial prcesses rely n catalysts Reducing csts - 80% f industrial prcesses use a catalyst - There are numerus catalysts currently in use, half f the peridic table either is a catalyst r made up in a catalyst - In many industrial prcesses a catalyst will greatly speed up a chemical reactin, this is dne by lwering the activatin energy - Therefre less energy is required t be inputted fr them t react

6 - Much f the energy is taken by burning crude il r electrical supply - If a prcess then requires less energy, then energy csts are reduced - Als this helps the envirnment as less fssil fuels is burnt = Less emissins f CO 2 int the envirnment - Sme catalysts imprve the percentage yield Ecnmic Imprtance - Used extensively in the chemical industry - Required fr prductin f ply(ethene), nyln and many ther husehld items we knw tday - Catalyst develpment leads t greater prfitability = Prducts can be quicker, easier and requiring less energy Reduced running csts - Haber prcess fr the prductin f ammnia is f great ecnmic imprtance Ammnia is used as a basis f a fertiliser - Ammnia is manufactured via N 2 (g) + 3H 2 (g) 2NH 3 (g) - The triple bnd in N 2 has t be brken, hence requiring a large input f energy - The Ziegler Natta Catalyst was an imprtant develpment f nn-branched ply(ethene) - Catalytic cnverters play an imprtant rle in imprving ur air quality by reducing txic emissins fr vehicles Enzymes as Bilgical Catalysts - Bicatalysis is any prcess where the catalyst is an enzyme - It s a grwing industry, that s been knwn fr ver years - Babylnians and Egyptians used enzymes in yeast t ferment fruit , Luis Pasteur carried ut experiments led t hw enzyme-catalysed reactins can lead t the frmatin f a single pure prduct knwn as specificity - Enzymes are large prtein mlecules - They perate at mild cnditins, lw temperatures, atmspheric pressure and an ptimum ph Enzymes in Industry - Often used in synthesis f rganic chemicals - Industrial use f enzymes has many benefits Lwer temperature and pressure = Saving energy and csts Often allw a reactin t take place which frms pure prducts N side reactins Cnventinal catalysts are ften pisnus, therefre causing dispsal prblems Enzyme d bidegrade - Enzymes are used in a variety f imprtant industrial prcesses - Manufacturing cmpanies use enzymes fr fd and drink prductin - Dairy prducts, alchlic drinks and fruit juices are prduced cause f enzymes - Other materials such as detergents, cleaning agers are als relied upn enzymes - In prductin f Ibuprfen, enzyme actin is used t separate the drug frm mixture 2.3 (13) The Bltzmann distributin The Bltzmann Distributin - Frm a gas r a liquid, the mlecules mve inside the cntainer - They therefre cllide with each ther - As mlecules are mving they have kinetic energy - In a sample f gas: Sme mlecules mve fast and have high energy Sme mlecules mve slwly and have lw energy Majrity f mlecules have average energy - The Bltzmann distributin shws the distributin f mlecular energies in a gas at cnstant temperature - Imprtant features f the Bltzmann distributin Area under the curve is equal t the ttal number f mlecules in the sample DOESN T CHANGE UNDER DIFFERENT CONDITIONS

7 N mlecules within the system with 0 energy N maximum energy fr a mlecule Only the mlecules with energy greater than E a are able t react - The Effect f temperature f temperature n reactin rate At higher temperatures, the kinetic energy increases f the mlecule The Bltzmann distributin flattens and shifts t the right (Number f mlecules desn t change at all) As temperature increases, s des the rate f reactin This is because f mre cllisins due t mlecules having mre kinetic energy Higher prprtin f mlecules have an energy that is greater than the activatin energy = Mre cllisins successful t cause a reactin - The effect f a catalyst n reactin rate Activatin energy f the reactin is reduced Mre mlecules in the system vercme the activatin energy (the lwer ne) Therefre mre successful cllisins in certain length f time = Rate f reactin increases Reversible Reactins - Reactive metal reacts with an excess acid slutin = Salt and H 2 prduced - Reactin stps after all the metal has reacted - Reactin is said t have gne int cmpletin, hence the sign - Hwever sme reactins are reversible - Therefre the sign fr a reversible reactin is (Due t restrictin f a cmputer, basically arrws in bth directins) Dynamic Equilibrium - When a system is in a state f equilibrium, then there is n bservable change Can t bserve anything happening - Hwever the system is said t be dynamic = As fast as prducts are frmed = As fast as reactants are frmed - Dynamic equilibrium nly exists in a clsed system, where frward rate f reactin is equal t backward rate f reactin - Equilibrium nly ccurs if the system remains islated = N materials added r remved - Factrs affecting the psitin f equilibrium Reversible reactins remains in equilibrium n if it is islated int a clsed system = Nthing being added r remved = Nthing being added r remved Psitin f equilibrium can be altered by 3 ways: Cncentratins f reactants and prducts Pressure in reactins invlving gas Temperature - The effect f cncentratin n equilibrium Fr a system in equilibrium, change in cncentratin f reactants r prducts causes a shift in equilibrium Increasing the cncentratin f a reactant Causes the psitin f equilibrium t mve in the directin f less reactant cncentratin The system decreases the cncentratin f the reactant by remving it System is ppsing it Therefre psitin f equilibrium mves t the right hand side = Mre prducts frmed Increasing the cncentratin f a prduct causes the psitin f equilibrium t mve in the directin that decreases the cncentratin f the reactants = Mre reactants - The effect f pressure n equilibrium Changing pressure f a system will nly change the psitin f equilibrium if there are gases present Side with greater mles = Higher pressure Therefre increasing pressure causes psitin f equilibrium t mve t the side with fewer gas mlecules = Decrease in pressure

8 Decreasing ttal pressure = System causes psitin f equilibrium t mve t side with greater gas mlecules Increase the pressure 2.3 (15) Equilibrium and Industry The Effect f Temperature n Equilibrium - Effect f changing the temperature is dependent n the enthalpy sign - Frward reactin = Exthermic = Negative enthalpy sign - Reverse reactin = Endthermic = Psitive enthalpy sign - Increasing the temperature f the system causes the psitin f equilibrium t mve in the directin that decreases the temperature Enthalpy = Endthermic reactin - Decreasing the temperature causes the system t mve in such a way t increase the temperature Enthalpy = Exthermic The Effect f a Catalyst n Equilibrium - Catalyst desn t alter the psitin f equilibrium r the cmpsitin f an equilibrium system - Catalyst speeds up the rate f the frward reactin and reverse reactin equally - Increases the rate f which equilibrium is established Equilibrium and Industry - Many imprtant chemical prcesses exist as equilibrium systems - Such examples are ammnia via the Haber Prcess and sulphur trixide in the cntact prcess N 2 (g) + 3H 2 (g) 2NH 3 (g) -92kJ ml -1 - Hydrgen and Nitrgen must readily be available - Nitrgen is btained frm fractinal distillatin, als hydrgen is prepared by reacting methane and water - What cnditins favur prductin f Ammnia Ammnia is prduced frm a frward reactin Therefre high pressure is used Hwever there are drawbacks Because even thugh lw temperature shuld prduce a high equilibrium yield reactin wuld be very slw A high pressure increases the gases cncentratin Therefre shuld get high percentage yield Hwever its quite cstly fr the equipment t prduce a high pressure - The mdern ammnia plant Prduces a sufficient percentage yield Therefre a cmprmise is made between rate f reactin and yield Temperature between C Pressure abut 200atm is usually used There cnditins prduce 15% yield Virtually all the H 2 and N 2 will be cnverted t ammnia as everything is repeatedly recycled