ch 7 thermochemistry Hess's law of constant heat summation

statement

if a chemical change takes place by several different routes, the overall energy change is the same, regardless of the route by which the chemical change occurs, provided that the initial and final conditions are the same.

del H = del H1 = del H2

OR

The sum of enthalpy changes in a closed cycle is zero.

sigma del H(cycle) = 0

explanation

Let A be converted into D directly in a single step in one route and in the steps in second route.

When A is directly converted into D directly in a single step then heat change is del H. When A is converted into D in three steps then heat changes during three steps are del H1, del H2, del H3 respectively.

acc to Hess’s law: del H = del H1 + del H2 + del H3

del H + (-delH1)+ (-del H2) + (-del H3) = 0.

mathematically,

sigma del H(cycle) = 0

applications of Hess’s law

hess’s law is used for determination of heat of formation of compounds such as CCl4 becuase

its heat of formation cannot be determined by calorimetric method

ccl4 cannot be prepared directly by combining graphite and chlorine.

heat of formation of al2o3 and b2o3 cannot be measured directly. it is difficult to burn these substances completely in oxygen as protective layer of their oxides is formed on the surface.

heat of formation of CO cannot be measured directly due to the formation of CO2 with it.

enthalpy of formation of na2co3

-89.08 kj/mol

enthalpy of formation of CO

-110.7 kj/mol