What does Hess's law say about the enthalpy of a reaction?

Question

Naomi Aronson · Accepted Answer

According to the law, a reaction's total enthalpy change occurs regardless of how many steps are involved in the process. Stated differently, if a chemical change occurs through multiple pathways, the total enthalpy change remains constant, irrespective of the pathway used (as long as the starting and ending conditions remain the same). When a reaction cannot be measured directly, Hess' law allows the enthalpy change (ΔH) to be calculated. This is done by applying previously determined values for the enthalpies of formation to basic algebraic operations based on the chemical equation of reactions. A net equation, also called an overall equation, is created when chemical equations are added together. The enthalpy change for the net equation is obtained if the enthalpy change for each equation is known. AN EXAMPLE Determine the heat of combustion, #ΔH_"c"#, of CS₂, given the following equations. Resolution Put down the equation you are aiming for, the target equation. CO2(g) + 2SO2(g) → CS₂(l) + 2O₂(g) Commence with equation 3, which includes the target's first compound (CS₂). To move the CS₂ to the left, we need to flip equation 3 and its ΔH, which yields equation A below. A. CS₂(l) → C(s) + 2S(s); -#ΔH_"f"# = -87.9 kJ We now remove C(s) and S(s) one at a time. Since C(s) is present in Equation 1, we express it as Equation B below. B. C(s) + O₂(g) → CO₂(g); #ΔH_"c"# = -393.5 kJ We use Equation 2 to eliminate the S(s), but we have to double it to get 2S(s). We also double its #ΔH#. We then get equation C below. C. 2S(s) + 2O₂(g) → 2SO₂(g); #ΔH_"c"# = -593.6 kJ The target equation is then obtained by adding equations A, B, and C, and canceling terms that appear on opposite sides of the reaction arrows. A. CS₂(l) → C(s) + 2S(s); -#ΔH_"f"# = -87.9 kJ
B. C(s) + O₂(g) → CO₂(g); #ΔH_"f"# = -393.5 kJ
C. 2S(s) + 2O₂(g) → 2SO₂(g); #ΔH_"f"# = -593.6 kJ CS₂(l) + 3O₂(g) → CO₂(g) + 2SO₂(g);  #ΔH_"c"# = -1075.0 kJ

Cooper Adams · Answer

undefined Hess's law states that the total enthalpy change for a reaction is the same whether the reaction takes place in one step or multiple steps.