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Heats of Formation And Hess’s Law

THERMOCHEMISTRY. Heats of Formation And Hess’s Law. HEATS OF FORMATION. Another method of calculating the enthalpy of a reaction is by using heats of formation. There are tables of Δ H form that we can gather information from Pure elements are always 0

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Heats of Formation And Hess’s Law

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  1. THERMOCHEMISTRY Heats of Formation And Hess’s Law

  2. HEATS OF FORMATION • Another method of calculating the enthalpy of a reaction is by using heats of formation. • There are tables of ΔHform that we can gather information from • Pure elements are always 0 • ΔHform is dependent on the number of moles • We also need to use the equation presented earlier: DHrxn = ∑Hproducts - ∑Hreactants

  3. HEATS OF FORMATION Calculate ΔH for the following reaction: 8 Al(s) + 3 Fe3O4(s)  4 Al2O3(s) + 9 Fe(s)

  4. CLASSROOM PRACTICE Use heats of formations calculations to determine the combustion of which hydro-carbon will produce the most energy per mole… (CH4= -74.81 kJ/mol; C2H6= -84.68 kJ/mol; C3H8= -104.5; C4H10= -126.5 kJ/mol) CH4 + 2O2 CO2 + 2H2O 2C2H6 + 7O2 4CO2 + 6H2O C3H8 + 5O2 3CO2 + 4H2O 2C4H10 + 13O2 8CO2 + 10H2O

  5. HESS’ LAW • The ΔH of a process is a state function, meaning that regardless of the path to reach your goal, the energy to get there is constant. • For instance if you want to vap-orize a solid, you have 2 pathways • You can melt it into a liquid and then vaporize it into a gas. • Or sublime the solid into a gas. • both paths get the same results & both paths requires the same amnt of heat energy, this is Hess’s Law.

  6. The idea that we can calculate Hsub by combining the Hfus with the Hvap is an illustration of Hess’ Law.

  7. HESS’S LAW • During any Hess’s Law calc, there are 2 things that we are allowed to do to the given rxns as manipulations. • Reverse the rxn in order to make the products reactants, as long as we change the sign of the enthalpy • Increase or decrease the amounts of reactants or products by multiplying by a factor, as long as we multiply the enthalpy by the same factor • The key is to keep our eye on the prize, the goal rxn

  8. HESS’S LAW • For example, use Hess’s Law to calc the ΔHf for the following rxneqn: • 2N2(g) + 5O2(g)  2N2O5(g) ΔH°f = ? • Given the following rxneqns: 2NO(g)+O2(g)2NO2(g) ΔH°rxn= -114kJ/mol 4NO2(g)+O2(g)2N2O5(g) ΔH°rxn= -110kJ/mol N2(g)+O2(g)2NO(g) ΔH°rxn= +181kJ/mol

  9. HESS’S LAW • Example 2: • Given the following information: C2H6C2H4 + H2 137kJ/mol 2H2O2H2+O2 484kJ/mol 2H2O+2CO2C2H4+3O2 1323kJ/mol Find the value of H° for the rxn: 2C2H6 + 7O2 4CO2 + 6H2O

  10. CLASSROOM PRACTICE Before pipelines were built to deliver natu-ral gas, towns & cities contained plants that produced a fuel known as town gas by passing steam over red-hot charcoal. C(s) + H2O(g)  CO(g) + H2(g) Calculate ΔH° for this rxn from the following information: C(s) + ½O2(g) CO(g)H = -110.53 kJ CO(g) + ½O2(g) CO2(g)H = -282.98 kJ C(s) + O2(g)CO2 (g)H = -393.51 kJ H2(s) + ½O2(g)H2O(g)H = -241.82 kJ

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