Writing Thermochemical Equations Review

*Rules*

-Thermochemical equations must be balanced -The states of all reactants & products are noted in parentheses -The change in enthalpy, or enthalpy of reaction (△H) , is written at the right -(△H can be written in units of kJ or kJ/mol) -Endothermic reactions give +△H -Exothermic reactions give -△H

Thermochemical Equation

a chemical equation that also shows the enthalpy of a chemical reaction

Calculating Net Enthalpy

you can calculate the net enthalpy if you know the initial enthalpy & the enthalpy from the change of state. You can also calculate the initial enthalpy if you know the net enthalpy

*Example* What is the enthalpy of reaction for the initial combustion of liquid methane given the △H for vaporization is =88kJ & the net △H is -890kJ (you have to write the equation from scratch) Unbalanced Equation: CH4+O2 △CO2+H2O Balanced Equation: CH4+2O2 △2CO2+2H2O

net enthalpy = enthalpy of the initial combustion+enthalpy of the vaporization: △Hinitial+ △Hvaporization= △Hnet △Hinital= (△Hnet- △Hvaporization)

Thermochemical Equations

show-reactants & products; can be-endothermic (+△H)-or-exothermic (-△H)

The Change in Enthalpy (△H)

the change in thermal energy between the reactants & products

*Example* 4Fe+3O2 △2Fe2O3 △H=-3.926kJ If oxygen is in excess, how much heat (q) will the hand warmer produced when all the iron is oxidized (molar mass Fe=55.8 g/mol

Step 1: Write down what was given Mass=100g, Molar Mass Fe= 55.8g/mol, △H= -3.926 Step 2: Convert the △H in kJ to kJ/mol △H(kJ/mol)= △H in kJ/mole Fe; △H (kJ/mol) = 3926kJ/4 mol Fe= 981.5kJ/mol Step 3: Calculate the number of moles of Iron oxidized Mole Fe=m/molar mass Fe; Mol Fe= 100g/55.8g/mol= 1.79 mol

Net Enthalpy

The net enthalpy is equal to the sum of the enthalpies from the initial combustion & the change of state