thermochemistry(08-23-08)

you know what thermochemistry means? thermochemistry is a branch of chemistry that deals with the relationship between chemical action and heat or the quantitative study and measurement of heat and enthalpy changes. thermochemistry, generally, is concerned with the heat exchange accompanying transformations, such as mixing, phase transitions, chemical reactions, etc., which includes calculations of such quantities as the heat capacity, heat of combustion, heat of formation, etc.
thermochemical equation shows the relationship energy changes and mass relationship.
example:
H2(g) + 1/2 O2(g) --> H2O(l); ∆H = -285.8 kJ (exothermic-heat is released)
HgO(s) --> Hg(l) + 1/2 O2(g); ∆H = +90.7 kJ (endothermic-heat is absorbed)

there are guidelines that are to be remembered in writing this equations:

• always specify the physical states of all reactants and products

example:

liquid H2O to solid H2O the ∆H is +6.01 kJ/mol

solid H2O to liquid H2O the ∆H is -6.01 kJ/mol

• when both sides of thermochemical equation is multiplied by a factor ,n, then ∆H must also change by the same factor

example:

2H2O -> 2H2O ∆H=2(6.01 kJ/mol)=12.02 kJ/mol

• when we reverse the equation, we change the role of products and reactants. as a consequence, the sign changes but the magnitude of ∆H for the equation remains the same

example:

liquid H2O to solid H2O the ∆H is +6.01 kJ/mol
solid H2O to liquid H2O the ∆H is -6.01 kJ/mol

solving for thermochemical equations:

problem:

Calculate the heat evolve when 74.6 g of SO2 is converted to SO3.

SO2(g) + 1/2 O2(g) -> SO3(g) ∆H = -99.1 kJ/mol

get the number of moles of SO2:

74.6g SO2x 1 mol of SO2 / 64.07g SO2 = 1.16 mol SO2

*64.07 g of SO2 is the molar mass of SO2 which is to be computed first if it is not given. to compute the molar mass, get the mass of S and 2O. after you get their mass, combine them by adding the mass of S and 2O.

multiply the answer to the ∆H which is -99.1 kJ/mol:

= (1.16 mol SO2)(-99.1 kJ/mol) ->cancel the mol

heat evolve = -115 kJ