Warm-up (7 mins)1.Take out a sheet of paper and answer the

following question on it: “What do you think you know about Hydrogen Fuel Technology?

2. Once you have answered the above question, unfold the sheet of paper that was handed to you as you walked through the door and follow the instructions on that paper.

Image: http://failuremag.com/index.php/feature/article/the_hindenburg_disaster/

How Green are Hydrogen and Corn-Based Fuels?

(Review) What is this?

CH4(g) + 2 O2(g) CO2(g) + 2 H2O(l)

Image: http://office.microsoft.com/en-us/images/results.aspx?qu=natural+gas&ex=1#ai:MC900014755|

Thermochemical Equation

• A Thermochemical Equation is a chemical equation with the enthalpy change that occurs during the reaction also written.

• Thus, the chemical equation from the previous slide written as a thermochemical equation is:CH4(g) + 2 O2(g) CO2(g) + 2 H2O(l) ΔH = -890 kJ

ORCH4(g) + 2 O2(g) CO2(g) + 2 H2O(l) + 890 kJ

&

What do you notice about…• 5 C6H12O6(s) + 30 O2(g) 30 CO2(g) + 30 H2O(g) ΔH =

- 12,719.9 kJ

• When given a specific reaction, as above, the total change in enthalpy is called the reaction enthalpy.

• While reaction enthalpies can be written for any reaction, reaction enthalpies are usually reported in thermochemical equations that are written using lowest common denominators.

What is the difference between…

CH4(g) + 2 O2(g) CO2(g) + 2 H2O(l) ΔH = -890 kJ

and CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g) ΔH = -802 kJ

• Reaction enthalpies depend on the phases of the reactants and products.

• Reaction enthalpies also depend on the temperature and pressure of the system.

&

Standard State

• The standard state of a reactant or product is when it is in its pure form at 1 bar.

• For solutions, the standard state is also when the solutes concentration is 1 M.

&

Standard Reaction Enthalpy

• When a reaction enthalpy is reported, if the reactants and products are all in their standard states, it is called a standard reaction enthalpy

• The symbol is written as ΔHo where “o” means standard state

• Lastly, the standard reaction enthalpies in your book are reported at 298.15 K.

&

Look in the notes

Hess’s Law

• Definition: The enthalpy change for a reaction is the sum of the enthalpy changes for the individual reactions that lead to this final reaction

• This summation occurs regardless of pathway (even if the pathway is theoretical).

• (This is just an extension of the fact that enthalpy is a state property. A state property is only dependent on a beginning and an end, not the pathway taken to get from the beginning to the end.)

Thus…

• Standard reaction enthalpy equations can be multipled by a factor:

2 x [SO2(g) S(s) + O2(g) ΔHo = 296.8 kJ]

is2 SO2(g) 2 S(s)

+ 2 O2(g) ΔHo = 593.6 kJ

And…

• Standard reaction enthalpy equations can be flipped:

SO2(g) S(s) + O2(g) ΔH = 296.8 kJ

flipped isS(s)

+ O2(g) SO2(g) ΔH = -296.8 kJ

And…

• The number of reaction steps needed to get to the final reaction can be an infinite amount.

Source

• Peter Atkins and Loretta Jones “Chemistry Molecules, Matter, and Change 3rd Ed.” W. H. Freeman and Company:New York, 1997, pgs. 194-200, 212-217