1 Chemical Reaction - Observation Reaction (1) CH 4 + 2O 2  CO 2 + 2H 2 O Reaction (2) CH 4 + CO 2  2CO + 2H 2 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 CH 4 or CO 2 consumed completely)  rxn (2) won’t complete (with a feed CH 4 =CO 2 =1 & CO=H 2 =0, max. conv.=63% at 1000K)  The reaction (1) will give out heat, but the reaction (2) will require heat. Why? Chemical Reactions

2 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 Chemical Reactions

3 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,  H, which is calculated using For Rxn(1) CH 4 + 2O 2  CO 2 + 2H 2 O  H° 1000 =-801 kJ/mol Rxn (2) CH 4 + CO 2  2CO + 2H 2  H° 1000 =+260k J/mol  The meaning  When  H<0, a reaction releases heat  reaction is exothermic, as in rxn (1)  When  H>0, a reaction requires heat  reaction is endothermic, as in rxn (2) Chemical Reactions refers to standard pressure (1 atm.) Temperature

4 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,  G, in a reaction can be calculated using ForRxn(1) CH 4 + 2O 2  CO 2 + 2H 2 O  G° 400  G° 1000 =-801 kJ/mol Rxn (2) CH 4 + CO 2  2CO + 2H 2  G° 400 =+145,  G° 1000 =-24 kJ/mol  Use of  G - Rxn(1)  G <0 at 400 & 1000K-spontaneous, Rxn(2)  G <0 at 400K, will not proceed Chemical Reactions reaction can proceed (but we don’t know how fast it will be!) reaction at equilibrium (no further change possible-‘dead’ state) reaction will NOT proceed (or can proceed backward!) for a reaction at constant T, P,

5 Chemical Reaction Thermodynamics Example of  H° T calculation CH 4 (g) + 2O 2 (g)  CO 2 (g) +2H 2 O(g) CH 4 + CO 2  2CO + 2H 2 Coeff H° 400( kJ/mol H° kJ/mol Equation to use  H° 400 =[1x(-393)+2x(-242)]-[1x(-77)+2x(0)]= -800 kJ/mol  H° 1000 =[1x(-394)+2x(-248)]-[1x(-89)+2x(0)]= -801 kJ/mol  Reaction (1)  H° 400 =[2x(-110)+2x(0)]-[1x(-77)+1x(-393)]=+250 kJ/mol Reaction (2)  H° 1000 =[2x(-111)+2x(0)]-[1x(-89)+1x(-393)]= +260 kJ/mol Note: The heat of formation of single element gases (O 2, H 2, N 2 etc) is defined as zero. Chemical Reactions

6 Chemical Reaction Thermodynamics Example of  H° T calculation CH 4 (g) + 2O 2 (g)  CO 2 (g) +2H 2 O(g) CH 4 + CO 2  2CO + 2H 2 Coeff G° 400( kJ/mol G° kJ/mol Equation to use  G° 400 =[1x(-394)+2x(-224)]-[1x(-42)+2x(0)]= -800 kJ/mol  G° 1000 =[1x(-396)+2x(-193)]-[1x(+19)+2x(0)]= -801 kJ/mol Reaction (1)  G° 400 =[2x(-146)+2x(0)]-[1x(-42)+1x(-394)]=+144 kJ/mol Reaction (2)  G° 1000 =[2x(-200)+2x(0)]-[1x(19)+1x(-396)]= -23 kJ/mol Note: The Gibbs Free Energy of single element gas (O 2, H 2, N 2 etc) is defined as zero. Chemical Reactions

7 Chemical Reaction Thermodynamics  The values of  G° T and  H ° T Equations 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 in which, S° T is the entropy and C p is the heat capacity at constant pressure Chemical Reactions

8 Chemical Reaction Thermodynamics  Summary  Will a reaction proceed in the direction specified?  Check  G° T value of the reaction. The  G° T value of a reaction can be calculated by The G° values of reactants / products can be found in literature. Remember  Is a reaction exothermic or endothermic?  Check  H° T value of the reaction. The  H° T value of a reaction can be calculated by The H° values of reactants / products can be found in literature Chemical Reactions reaction can proceed (but we don’t know how fast it will be!) reaction at equilibrium (no further change possible-‘dead’ state) reaction will NOT proceed (or can proceed backward!) for a reaction at constant T, P,