2ALSO CALLED … Law of constant heat summation First law of thermodynamics
3HESS’S LAW States that: The total enthalpy of a reaction is independent of the reaction pathway.
4PARAPHRASE OF HESS’S LAW The heat evolved or absorbed in a chemical process is the same whether the process takes place in one or in several steps.Mathematically,ΔH overall = sum of ΔH’s of individual reactions.
6IMPORTANCE OF HESS’ S LAW Allows us to calculate the enthalpy changes of reactions that cannot be measured directly in the laboratory.
7EXAMPLEIn the production of CO2, there are 2 reaction paths that can be taken before final product is obtained. When the enthalpy change of these 2 reactions are added, the enthalpy change of the overall reaction is obtained.
8EXAMPLE C + ½ O2 CO H1 = Y kJ CO + ½ O2 CO2 H2 = Z kJ C + O2 CO2 H3 = XkJUsing Hess’s law,H3 = H1 +H2H3 = Y kJ + Z kJH3 = X kJ
9MORE QUESTIONS S(s) + 1½O2(g) SO3(g) Two reactions occurring in the manufacture of sulfuric acid are shown below:S(s) +O2(g) SO2(g) ΔH1Ө = –297 kJSO2(g) + ½O2(g) SO3(g) Δ H2Ө = –92 kJDeduce the Δ HӨ value of this reaction:S(s) + 1½O2(g) SO3(g)(November 2005)
10RULES FOR SOLVING QUESTIONS ON HESS’S LAW. When it is necessary to reverse a chemical equation, change the sign of ΔH for that reaction.When multiply equation coefficients, multiply values of ΔH by the same factor.Make sure to rearrange the given equations so that reactants and products are on the appropriate sides of the arrows.
11QUESTION 1 Calculate the enthalpy change, ΔH4 for the reaction C + 2H2 + ½O2 CH3OH ΔH4Using Hess’s Law, and the following information.CH3OH + 1½O2 CO2 + 2H2O ΔH1 = -676 kJ mol-1C + O2 CO ΔH2 = -394 kJ mol-1H2 + ½O2 H2O ΔH3 = -242 kJ mol-1(May 2006)
12QUESTION 2 Using the equations below Cu(s) + ½ O2(g) → CuO(s)∆Hο = –156 kJmol-12Cu(s) + ½ O2(g) → Cu2O(s)∆Hο = –170 kJmol-1What is the value of ∆Hο (in kJmol-1) for the following reaction?2CuO(s) → Cu2O(s) + ½ O2(g)A. 142B. 15C. –15D. –142(May 2004)
13QUESTION 3 Consider the following equations. Mg(s) + ½O2(g) → MgO(s) ∆Hο = –602 kJ H2(g) + ½ O2(g) → H2O(g) ∆Hο = –242 kJWhat is the ∆H° value (in kJ) for the following reaction?MgO(s) + H2(g) → Mg(s) + H2O(g)A. –844B. –360C. +360D. +844(November 2004)
15C + ½ O2 CO H = – 110.5 kJ CO + ½ O2 CO2 H = – 283.0 kJ ReactantsH = – kJCO+ ½ O2IntermediateEnthalpyH =– kJH =– kJNote that the states should be included in the diagram.ProductsCO2
16Steps in drawing enthalpy diagrams Balance the equation(s).Sketch a rough draft based on H values.Draw the overall chemical reaction as an enthalpy diagram with the reactants on one line, and the products on the other line.Draw a reaction representing the intermediate step by placing the relevant reactants on a line.Check arrows.Look at equations to help complete balancing (all levels must have the same number of all atoms).Add axes and H values.NB: Endothermic reactions should have their arrows pointing upward as the enthalpy change is increasing whereas exothermic reactions should have arrows pointing downwards as the enthalpy change is decreasing.
18TO TRY Calculate the enthalpy change for the reaction below Using Hess’s law and the following information:With the above information, draw an enthalpy diagram for the reaction using the knowledge acquired from the above examples.NO(g) + ½ O2(g) NO2(g)NO(g) ½ N2(g) + ½ O2(g) H= – kJ½ N2(g) + O2(g) NO2(g) H= kJ