4.7 [ENDORSED]

[Jenna 1G]

4.7 (a) Calculate the work that must be done against the atmosphere for the expansion of the gaseous products in the combustion of 1.00 mol c6 H6(l) at   25°C and 1.00 bar. (b) Using data in Appendix 2A, calculate the standard enthalpy of the reaction. (c) Calculate the change in internal energy, ΔU°, of the system.

how would I solve this equation?

[Chem_Mod]

Part a

First write the balanced chemical equation to determine the net change in moles of gas.

When you have the net change in moles of gas, Delta n, then use the ideal gas equation PV = nRT.

Since the external pressure and the temperature are constant one obtains:

P Delta V = Delta n RT

Since w = -P Delta V

Then w = - Delta n RT

This is very similar to the example I did in class and showed the same above equations.
Rewatch my lecture on this as I went through it step-by-step.

Part b

Look up the standard enthalpies of formation and use them to calculate Delta H(rxn).
See the detailed example I did in class.

Delta H(rxn) = standard enthalpies of formation products - standard enthalpies of formation of reactants

Part c

I also covered in detail in class.

Delta U = q_p + w = Delta H + w (use the numbers from part a and b)

You can see the detailed numbers for each step in the Student Solutions Manual on Achieve.

[kylanjin]

You'd first want to balance the combustion reaction. For part a, use the ideal gas law to solve for change in V, which you can then use to solve for the work using w=PdeltaV. For enthalpy of formation, look up the values in the appendix and subtract the sum of the products from the reactants to find q. Since internal energy is equal to q+w, add the two values to find change in U.