hess's law formulae
Answers
Explanation:
Chemical Reactions
Most chemical reactions are actually a series of reactions. Take for example the breakdown of sugar into energy:
Glucose reaction
This actually occurs over dozens of steps. Glucose is first changed into glucose-6-phospahte and eventually into pyruvate, and then into carbon dioxide. Some of the electrons are put onto ADP to make ATP. When these electrons are removed for energy, oxygen is used. Through this entire process, we end up losing water. But it is a lot easier and more efficient to simplify it into one reaction. Everything that ends up being both a product and reactant gets canceled out until we end up with sugar and oxygen as reactants, and carbon dioxide and water as products.
Let's look at a simple example using A's and B's. Let's say that:
A+B = AB
and
AB + C = ABC
Then we can say that A+B+AB+C=AB + ABC. But since AB is both a product and reactant, we can cancel those out and simplify this further into A+B+C=ABC.
Hess's Law
This doesn't just make it easier to write chemical formulas. It also simplifies the process of determining the total enthalpy change. Hess's law states that no matter the multiple steps or intermediates in a reaction, the total enthalpy change is equal to the sum of each individual reaction. It is also known as the conservation of energy law. So, this means that we can determine the total enthalpy change of A+B+C=ABC by determining this actual reaction's enthalpy change. Or, we can determine the enthalpy change for A+B=AB and AB+C=ABC and then add these two together.
Another way to look at this is with a graph. Let's say that it takes a lot of energy for A and B to form AB. But then, once it combines with C into ABC it releases a lot of energy.
So, we can determine the enthalpy change by simply looking at delta H1, or we can add delta H2 and delta H3. This may not make sense at first. Why we would add delta H2 and delta H3 instead of subtracting delta H2 from delta H3? But we need to remember that delta H1 and delta H3 are negative (because they go down on the enthalpy chart) while delta H2 is positive (because it goes up on the enthalpy chart), so the adding and subtracting is already worked in by making the numbers either positive or negative.