Thermochemistry

THERMOCHEMISTRYEnergy in Chemical Reactions

COMPARISONS OF ENERGY RELEASE Name a chemical reaction that releases

heat or light energy. Which would have more energy in those cases, the reactants or products?

Think of reaction that absorbs heat and think about the reactants and products energy.

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THERMODYNAMICS “Thermo” means “heat” “Dynamics” means “power: The study of energy and its transformations Energy- the capacity to do work or to

transfer heat Work-the energy used to cause an object

with mass to move against a force Heat- energy used to cause the

temperature an object to increase

ENERGY (P 8-9) Kinetic –energy of motion Ek = ½ mv2 m = mass v = velocity Potential – energy of position Ep = mgh m=mass g=gravity(9.8m/s2)

h=height Electrostatic energy- opposite charges attract

each other, and like charges repel each other Eel =κ Q1 Q2 κ = proportionality constant 8.99x109J-m/C2

D Q=electrical charges magnitude of electronD = distance

ELECTROSTATIC Example:

ENERGY FLOW System- the part of the universe of

interest Surroundings-everything relevant to

system Example: system might be reactants in a beaker while the surroundings would be the room temperature, pressure and humidity

INTERNAL ENERGY Energy within the system is found by: ∆E =Efinal – Einitial = Eproducts - Ereactants ∆E = ∆U : internal energy If Eproducts > Ereactants E= “+” If Eproducts < Ereactants E= “-”

HEAT – THERMAL ENERGY/WORK Heat – q; Energy transferred between a

system and it’s surroundings due to ______difference

Work- w; Energy transferred when an object is moved by a force∆E = q + w

Energy moving into a system is “ “Energy moving out of a system is “ “

+-

ENTHALPY Sum of internal energy plus the product

of the pressure and volume ∆H = ∆E + P∆V (remember liquids and solids undergo

negligible volume change)(note that if gases are present-volume ∆

small!) Therefore- often ∆H = ∆E or close to

it

CALORIMETRY Heat change absorbed by water can be

measured as: Specific Heat Capacity (c)- Which had higher c : Al vs. water ?

q = c x mass x ∆T q = quantity of heat

c = specific heat capacity

∆T = change in temperature

STOICHIOMETRY IN THERMOCHEM Heats of reaction correspond in

magnitude to the amount reacted in moles and their states of matter (solid/liquid/gas)

For example: 2H2O(l) 2H2(g) + O2(g) ∆Hrxn =

572kJ Reverse: 2H2(g) +O2(g) 2H2O(l) =-

572kJ Or… H2(g) + ½ O2(g) H2O(l)

∆Hrxn=-286kJ

Do all our reactions with exactly one mole? Create a potential problem with one of the above 3 equations giving the mass which would then be converted into moles. How much energy would be absorbed or released?

HESS’S LAW OF SUMMATION The overall enthalpy is the sum of

enthalpy steps leading to the final product.

The enthalpy of the combustion of C to CO2 is -393.5 kJ/mol C and the enthalpy for the combustion of CO2 is -283.0 kJ/mol CO:

(1) C(s) + O2(g) CO2(g) ∆H1 = -393.5 kJ

(2) CO(g) + ½ O2 CO2(g) ∆H2 = -283.0 kJ

This data can be found in Appendix B Calculate the enthalpy for the combustion of C to

CO: (3) C(s) + ½ O2(g) CO(g) ∆H3 =

-110.5 kJ

STANDARD STATE ASSUMPTIONS: Gases- are at 1 atm pressure Aqueous solutions-are 1 M concentration Pure substances-1 atm pressure; 298 K— What state are they in now?

Standard Heat of formation ∆Hf Take apart the equation pg 254…what is

the enthalpy?

SUM TOTAL! ∆Hrxn = ∑ m∆Hf - ∑ n∆Hi

Try to find the total enthalpy of the reaction:

4NH3(g) + 5O2(g) 4NO(g) + 6H2O(l)

LOOK AGAIN… Entropy: S ∆Srxn = ∑ m Sproducts - ∑ n Sreactants

How are they similar?Calculate ∆S for the synthesis of ammonia from N2(g) and H2(g) at 298 K

N2(g) + 3 H2(g) 2 NH3(g)

∆S = -198.3 J/K

PREDICTING ENTROPY SIGNS Temperature increases causes entropy

increase –directly proportional Physical state/phase change-as a

compd absorbs heat (q>0) S˚increases Dissolving a solid/liquid-increases

disorder and therefore S˚ Dissolving a gas dispersion increases

S˚ Increase in atomic size/complexity

increases

SPONTANEOUS EXOTHERMIC REACTIONS In Exothermic- (∆H < 0) the heat released

by the system increases the total entropy (S) of the system + surroundings: ∆Suniverse > 0;

∆Ssystem + ∆Ssurroundings > 0 In Exothermic- (∆H < 0) the heat released

by the system increases the entropy of the surroundings such that it overwhelms the decrease in the system then ∆Suniverse>0

SPONTANEOUS ENDOTHERMIC REACTIONS For an endothermic reaction the

∆Suniverse is positive if the ∆Ssystem large enough (∆S >>0) to overwhelm the ∆Ssurroundings (∆Ssurroundings<0)

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SPONTANEOUS VALUE? Brainstorm an equation that would

combine the concepts of entropy & enthalpy to determine if a reaction is spontaneous-Enter Gibbs Free Energy!

∆G = measure of the spontaneity of a process and of the useful energy available from it.

G = H – TSH = Enthalpy T= Temperature S = EntropyTHE SIGN OF G TELLS US WHETHER A REACTION IS SPONTANEOUS!

SIGNS OF S & G ∆Suniverse>0 Spontaneous ∆Suniverse<0 Non-spontaneous ∆Suniverse =0 Equilibrium ∆G < 0 Spontaneous ∆G > 0 Non-spontaneous ∆G = 0 Equilbrium

CALCULATING GIBBS FREE ENERGY ∆G˚ = ∆H˚sys - T∆S˚sys

Also: ∆G˚ = ∑m∆G˚f(products) - ∑n∆G˚f(reactants)

With your shoulder partner, read Sample Problem 20.4. After attempting it and following through the solution. Try Follow-up Problem 20.4

ENTROPY/GIBBS FUN! http://www.youtube.com/watch?v=OsldTzBaIEw

http://www.youtube.com/watch?v=V48r4IArzLc&feature=related