An enthalpy change is the heat energy transfered between system and surrondings in units of J/mol of reactant at constant pressure.
The system refers to the process under investigation/the responding variable while the surrondings is everything outside the system in a thermodynamic process.
As for enthalpy changes, the calculations are similar to calculating stoichometry.
#2SO_2(g) +O_2 -> 2SO_3(g) +198kJ
Calculate the enthalpy change that occurs when 58.0 g of sulfur dioxide is reacted with excess oxygen.
Since every 2 moles of SO_2 gives out 198kJ of energy, we first convert 58.0(g) of sulfur into moles, giving us 0.90625 moles.
Then, in order to convert it back into grams, we take the number of moles and multiply it by 198kJ before multiplying by 2, giving us the final answer of -89.7185, which I would then round off to 2 decimal place -89.72kJ.