Textbook 4D.15 Strategy (Hess’s Law)

[705968765]

Hi,

As I was doing 4D.15 in the textbook I noticed a useful strategy that the answer key used. The first step in this problem is to write out the combustion reactions for each of the compounds given and to then balance those equations. What I noticed was that when the solution manual balanced the equations, it intentionally avoided using whole number integers. Here’s why (I think): If you can find a way to balance the equations in a way that matches the stoichiometric coefficients of the compounds you want in your final equation to the final equation given, it will make it much easier to visualize how to use Hess’s law to combine the equations together.

For example, if you know that H2 will be in your final equation and you have the equation

H2+O2—>H20, rather than balancing it by doing 2H2+O2—>2H2O, you can use fractions and write H2+1/2O—>H2O. In this manner, you keep H2 in the stoicheometric coefficient desired for the final equation.

This may overcomplicate things for some people and make it simpler for others. I hope this is helpful for some.

[Chem_Mod]

This method of doing H₂ + 1/2 O₂ --> H₂O is used over 2H₂ + O₂ --> 2H₂O specifically because you are then finding the standard enthalpy of 1 H₂O formation. Make sure to just check what is given in a standard enthalpy table. If H₂O is given, definitely do it with the 1/2 coefficient.