## At A Glance

### Hess's Law of Constant heat Summation

Check us out at http://www.tutorvista.com//videos born Russian chemist and doctor. The law is based on the principle of conservation of energy and the path independence of energy changes. Hess's law can be used to predict energy changes that are not easily measured. Explanation: The law states that the energy change for any chemical or physical process is independent of the pathway or number of steps required to complete the process provided that the final and initial reaction conditions are the same. In other words, an energy change is path independent, only the initial and final states being of importance. This path independence is true for all state functions.Hess's law allows the enthalpy change for a reaction to be calculated even when it cannot be measured directly. This is accomplished by performing arithmetic operations on chemical equations and known values. Chemical equations may be multiplied or divided by a whole number. When an equation is multiplied by a constant, its ΔH must be multiplied by the same number as well. If an equation is reversed, ΔH for the reaction must also be reversed .Addition of chemical equations can lead to a net equation. If enthalpy change is included for each equation and added, the result will be the enthalpy change for the net equation. If the net enthalpy change is negative the reaction will be exothermic and is more likely to be spontaneous; positive ΔH values correspond to endothermic reactions. Note that entropy also plays an important role in determining spontaneity, so some reactions with a positive enthalpy change are nevertheless spontaneous.Hess's Law says that enthalpy changes are additive. Thus the ΔH for a single reaction can be calculated from the difference between the heat of formation of the products and the heat of formation of the reactants.
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