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Chemical Reactions Chemical Reaction - Observation Reaction (1) CH4 + 2O2 CO2 + 2H2O Reaction (2) CH4 + CO2 2CO + 2H2 When carrying out these reactions we found that at 400K (123°C), the reaction (1) will proceed and reaction (2) will not at 1000K(723°C), reactions (1) & (2) both proceed; but rxn (1) can go complete (until CH4 or CO2 consumed completely) rxn (2) won’t complete (with a feed CH4=CO2=1 & CO=H2=0, max. conv.=63% at 1000K) The reaction (1) will give out heat, but the reaction (2) will require heat. Why? 1 Chemical Reactions Chemical Reaction Thermodynamics Each molecule contains certain types and quantity of chemical energy There is always energy change In a chemical reaction because of breaking / reformation of chemical bonds out-giving or in-taking heat There are different energies associated with a substances & a reaction (A systematic study of various forms of energy & their changes is called Thermodynamics) We will learn some of these energies The meanings How to get values / do simple calculate How to use them as a tool to study chemical reactions 2 Chemical Reactions Chemical Reaction Thermodynamics The heat of formation, H, (also called Enthalpy of Formation or Enthalpy) H is an energy associated with heat H is specific for each substance and is dependent of temperature & pressure e.g. at 1000K: H°CH4=-89, H°O2=0, H°CO2=-394, H°H2O=-241, H°CO=-111, H°H2=0 (kJ/mol) (H values for various substances can be found in physical chemistry/Chem Eng handbooks) In a reaction we are interested in the enthalpy change, DH, which is calculated using DHT0 ( vi Hi0,T )prod ( vi Hi0,T )reac refers to standard pressure (1 atm.) Temperature For Rxn(1) CH4 + 2O2 CO2 + 2H2O DH°1000=-801 kJ/mol Rxn (2) CH4 + CO2 2CO + 2H2 DH°1000=+260k J/mol The meaning When DH<0, a reaction releases heat reaction is exothermic, as in rxn (1) When DH>0, a reaction requires heat reaction is endothermic, as in rxn (2) 3 Chemical Reactions Chemical Reaction Thermodynamics The Gibbs Free Energy, G, (also called Free Energy) G is a thermodynamic function related to a reaction. It is a function of H, T & S (entropy) G is specific for each substance & is a function of H, T & S (entropy) e.g. at 1000K: G°CH4=-+30, G°O2=0, G°CO2=-395, G°H2O=-192, G°CO=-200, G°H2=0 (kJ/mol) (G values for various substances can be found in physical chemistry/Chem Eng handbooks) The Gibbs Free energy change, DG, in a reaction can be calculated using DGT0 ( viGi0,T )prod ( viGi0,T )reac For Rxn(1) CH4 + 2O2 CO2 + 2H2O DG°400 DG°1000=-801 kJ/mol Rxn (2) CH4 + CO2 2CO + 2H2 DG°400=+145, DG°1000=-24 kJ/mol Use of DG - Rxn(1) DG <0 at 400 & 1000K-spontaneous, Rxn(2) DG <0 at 400K, will not proceed for a reaction at constant T, P, DGT 0 reaction can proceed (but we don’t know how fast it will be!) DGT 0 reaction at equilibrium (no further change possible-‘dead’ state) DG 0 reaction will NOT proceed (or can proceed backward!) T 4 Chemical Reactions Chemical Reaction Thermodynamics Example of DH°T calculation• CH4(g) + 2O2(g) CO2(g) + 2H2O(g) Coeff. H°400( H°1000 1 -77 -89 2 0 0 Equation to use 1 -393 -394 2 -242 -248 CH4 + CO2 2CO + 2H2 1 -77 -89 1 -393 -394 2 -110 -111 2 0 0 kJ/mol kJ/mol DHT0 ( vi Hi0,T )prod ( vi Hi0,T )reac DH°400 =[1x(-393)+2x(-242)]-[1x(-77)+2x(0)]= -800 kJ/mol DH°1000 =[1x(-394)+2x(-248)]-[1x(-89)+2x(0)]= -801 kJ/mol DH°400 =[2x(-110)+2x(0)]-[1x(-77)+1x(-393)]=+250 kJ/mol Reaction (1) Reaction (2) DH°1000=[2x(-111)+2x(0)]-[1x(-89)+1x(-393)]= +260 kJ/mol Note: The heat of formation of single element gases (O2, H2, N2 etc) is defined as zero. 5 Chemical Reactions Chemical Reaction Thermodynamics Example of DH°T calculation• CH4(g) + 2O2(g) CO2(g) + 2H2O(g) Coeff. 1 2 1 2 CH4 + CO2 2CO + 2H2 1 1 2 2 G°400( -42 0 -394 -224 -42 -394 -146 0 kJ/mol G°1000 +19 0 -396 -193 +19 -396 -200 0 kJ/mol Equation to use DGT0 ( viGi0,T )prod ( viGi0,T )reac DG°400 =[1x(-394)+2x(-224)]-[1x(-42)+2x(0)]= -800 kJ/mol DG°1000 =[1x(-396)+2x(-193)]-[1x(+19)+2x(0)]= -801 kJ/mol DG°400 =[2x(-146)+2x(0)]-[1x(-42)+1x(-394)]=+144 kJ/mol Reaction (1) Reaction (2) DG°1000=[2x(-200)+2x(0)]-[1x(19)+1x(-396)]= -23 kJ/mol Note: The Gibbs Free Energy of single element gas (O2, H2, N2 etc) is defined as zero. 6 Chemical Reactions Chemical Reaction Thermodynamics The values of DG°T and DH °T Equations DHT0 ( vi Hi0,T )prod ( vi Hi0,T )reac DGT0 ( viGi0,T )prod ( viGi0,T )reac In both cases G° and H° values for the reactants and products have to be those at the reaction temperature, indicated by the subscript. For common substances, G° and H° values are given as a function of T in handbooks - okay For some less common substances, you may only find values at 298K, G°298 and H°298 How do you convert values of G°298 and H°298 to those of G°T and H°T ? Here is the equations you can use to calculate the values of G°T and H°T from G°298 and H°298 H i0,T H i0,298 DC p ,i dT DH phasechange,i T 298 Gi0,T H i0,T - T Si0,T T 0 where Si0,T S 298 DC p ,i 298 dT Q phase T Tj in which, S°T is the entropy and Cp is the heat capacity at constant pressure 7 Chemical Reactions Chemical Reaction Thermodynamics Summary Will a reaction proceed in the direction specified? Check DG°T value of the reaction. The DG°T value of a reaction can be calculated by DGT0 ( viGi0,T )prod ( viGi0,T )reac The G° values of reactants / products can be found in literature. Remember for a reaction at constant T, P, DGT 0 reaction can proceed (but we don’t know how fast it will be!) DGT 0 reaction at equilibrium (no further change possible-‘dead’ state) DG 0 reaction will NOT proceed (or can proceed backward!) T Is a reaction exothermic or endothermic? Check DH°T value of the reaction. The DH°T value of a reaction can be calculated by DHT0 ( vi Hi0,T )prod ( vi Hi0,T )reac The H° values of reactants / products can be found in literature 8