State Hess law. Give its methamatical form?
Answers
Explanation:
Hydrogen gas, which is of potential interest nationally as a clean fuel, can be generated by the reaction of carbon (coal) and water:
C(s)+2H2O(g)→CO2(g)+2H2(g)(2)
Calorimetry reveals that this reaction requires the input of 90.1 kJ of heat for every mole of C(s) consumed. By convention, when heat is absorbed during a reaction, we consider the quantity of heat to be a positive number: in chemical terms, q>0 for an endothermic reaction. When heat is evolved, the reaction is exothermic and q<0 by convention.
It is interesting to ask where this input energy goes when the reaction occurs. One way to answer this question is to consider the fact that the reaction converts one fuel, C(s) , into another, H2(g) . To compare the energy available in each fuel, we can measure the heat evolved in the combustion of each fuel with one mole of oxygen gas. We observe that
C(s)+O2(g)→CO2(g)(3)
produces 393.5kJ for one mole of carbon burned; hence q=−393.5kJ . The reaction
2H2(g)+O2(g)→2H2O(g)(4)
produces 483.6 kJ for two moles of hydrogen gas burned, so q=-483.6 kJ. It is evident that more energy is available from combustion of the hydrogen fuel than from combustion of the carbon fuel, so it is not surprising that conversion of the carbon fuel to hydrogen fuel requires the input of energy. Of considerable importance is the observation that the heat input in equation [2], 90.1 kJ, is exactly equal to the difference between the heat evolved, -393.5 kJ, in the combustion of carbon and the heat evolved, -483.6 kJ, in the combustion of hydrogen. This is not a coincidence: if we take the combustion of carbon and add to it the reverse of the combustion of hydrogen, we get
C(s)+O2(g)→CO2(g)(1)
2H2O(g)→2H2(g)+O2(g)(2)
C(s)+O2(g)+2H2O(g)→CO2(g)+2H2(g)+O2(g)(5)
Canceling the O2(g) from both sides, since it is net neither a reactant nor product, equation [5] is equivalent to equation [2]. Thus, taking the combustion of carbon and "subtracting" the combustion of hydrogen (or more accurately, adding the reverse of the combustion of hydrogen) yields equation [2]. And, the heat of the combustion of carbon minus the heat of the combustion of hydrogen equals the heat of equation [2]. By studying many chemical reactions in this way, we discover that this result, known as Hess's Law, is general.