Hess's Law of Constant Heat Summation
Definition and meaning of Hess's Law of Constant Heat Summation in chemistry.
Hess's law of constant heat summation states that the total enthalpy change of a chemical reaction is constant. This value is the same whether the reaction happens in one step or in several steps. The law only requires that the starting materials and final products are identical.
In more detail
This law works because enthalpy is a state function. A state function is a property whose value depends only on the current state of a system. The path taken to reach that state does not affect the value.
Therefore, we can add, subtract, or multiply thermochemical equations just like algebraic equations. This process lets us calculate the heat change of a reaction that is hard to measure. A common student misconception is that the steps must actually occur in a laboratory.
In reality, the intermediate steps can be entirely hypothetical. You only need to make sure the steps add up to the net reaction. When you reverse a chemical reaction, you must change the sign of its enthalpy change.
If you multiply a reaction by a number, you must multiply the enthalpy by that same number. This law is the main tool for finding standard enthalpies of reaction. Chemists use tabulated values, such as heats of formation, to calculate changes for complex reactions. This method saves time and keeps scientists safe from dangerous experiments.
Key facts
| Field | Physical Chemistry |
|---|---|
| Proposed by | Germain Hess, 1840 |
| Underlying principle | Enthalpy (H) is a state function |
| Main use | Calculating ΔH for reactions not directly measurable |
| Alternative name | Hess's Law |
Consider making carbon dioxide gas from solid carbon and oxygen gas. The direct reaction releases 393.5 kilojoules of heat per mole. You can also calculate this value by adding two separate steps. First, burn carbon to make carbon monoxide, which releases 110.5 kilojoules. Second, burn carbon monoxide to make carbon dioxide, releasing 283.0 kilojoules. Adding these two values gives the same total of 393.5 kilojoules.
Frequently asked questions
Why does Hess's law work?
It works because enthalpy is a state function. The total energy change depends only on the starting materials and the final products. The specific pathway or number of steps does not change this value.
How is Hess's law applied in practice?
Chemists combine known chemical equations to build a target reaction. They add or reverse the equations so that unwanted compounds cancel out. They then combine the individual enthalpy values to find the total change.
What happens to the enthalpy value if I reverse a chemical reaction?
If you reverse a reaction, you must change the sign of the enthalpy change value. A process that releases heat in the forward direction will absorb heat in the reverse direction.