Hess's Law W3

• Hess's Law of Constant Heat Summation

  States that regardless of the multiple stages or steps of a reaction, the total enthalpy change for the reaction is the sum of all changes
• Hess's Law


  • It is a manifestation that enthalpy is a state function
• Hess's Law is named after Russian Chemist and Doctor Germain Hess
• Hess helped formulate the early principles of thermochemistry
• Hess's most famous paper, which was published in 1840, included his law on thermochemistry
• Hess's law


  It allows us to calculate the overall change in enthalpy by simply summing up the changes for each step of the way, until product is formed
• All steps have to proceed at the same temperature and the equations for the individual steps must balance out
• The principle underlying Hess's law does not just apply to Enthalpy and can be used to calculate other state functions like changes in Gibbs' Energy and Entropy
• Hess's Law


  The heat of any reaction ΔH∘fΔ೦೦° for a specific reaction is equal to the sum of the heats of reaction for any set of reactions which in sum are equivalent to the overall reaction
• Applicability of this law requires that all reactions considered proceed under similar conditions, specifically at constant pressure
• Calorimetry reveals that this reaction requires the input of 90.1 kJ of heat for every mole of C(s) consumed
• When heat is absorbed during a reaction, we consider the quantity of heat to be a positive number
• When heat is evolved, the reaction is exothermic and the quantity of heat is negative by convention
• The heat input in the hydrogen gas generation reaction

  Is exactly equal to the difference between the heat evolved in the combustion of carbon and the heat evolved in the combustion of hydrogen
• Taking the combustion of carbon and "subtracting" the combustion of hydrogen (or more accurately, adding the reverse of the combustion of hydrogen) yields the hydrogen gas generation reaction
• This result, known as Hess's Law, is general and can be applied to many chemical reactions
• Hess's Law (pictorial view)

  The net heat evolved or absorbed during a reaction is independent of the path connecting the reactant to product, provided all reactions occur under constant pressure conditions
• The net heat transferred in a complete circuit through the reaction steps is exactly zero, which is a statement of the conservation of energy
• Enthalpy (H)


  An energy function whose value for the reactants is independent of how the reactant state was prepared, and whose value in the product state is independent of how the products are prepared

  
• Enthalpy (H) is a state function, since its value depends only on the state of the materials under consideration, that is, the temperature, pressure and composition of these materials
• Standard enthalpy of formation (ΔH°f)


  The heat involved in the reaction of forming a material from its constituent elements in their standard states
• Extensive tables of ΔH°f values have been compiled that allows us to calculate with complete confidence the heat of reaction for any reaction of interest