Hess's Law Calculator
The "Equation Manipulator". Reverse reactions, multiply coefficients, and sum the steps to find Total Enthalpy (ΔH).
Energy Staircase
Enter reaction steps to visualize the path of enthalpy change.
The Law of the Path
Hess's Law is the ultimate shortcut in chemistry. It proves that you don't need to measure every single reaction in the lab. If you can build a path from A to B using known steps, you can calculate the energy change mathematically.
The Core Concept
The Mountain Analogy
Imagine climbing a mountain. Whether you take the steep direct trail (Reaction 1) or the winding switchbacks (Reaction 2 + 3 + 4), the change in altitude is exactly the same.
State Functions
Enthalpy (H) is a State Function. It only cares about the Start State (Reactants) and the End State (Products). The journey between them is irrelevant to the energy math.
The Two Golden Rules
To solve Hess's Law problems, you are allowed to manipulate the equations in two specific ways:
If you flip the equation, flip the sign.
A $\rightarrow$ B (+100kJ) becomes B $\rightarrow$ A (-100kJ)
If you multiply coefficients, multiply energy.
2A $\rightarrow$ 2B requires $2 \times$ Energy (+200kJ)
Why use Intermediates?
Direct measurement is often impossible. For example, trying to burn Carbon to only Carbon Monoxide ($CO$) is hard because some of it will keep burning to become $CO_2$.
Instead, we measure:
- Step 1: C + $O_2$ $\rightarrow$ $CO_2$ (Easy to measure)
- Step 2: CO + $\frac12O_2$ $\rightarrow$ $CO_2$ (Easy to measure)
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Frequently Asked Questions
What is Hess's Law?
Hess's Law of Constant Heat Summation states that the total enthalpy change for a chemical reaction is the same regardless of the path taken. Whether you go directly from A to B, or take a detour through C and D, the net energy change is identical.
Why is Enthalpy a State Function?
A State Function is a property whose value does not depend on the path taken to reach that specific value. Altitude is a state function: Standing on top of Mt. Everest is 8,849m high whether you climbed it or flew there by helicopter.
What happens if I reverse an equation?
If you flip the direction of a reaction (turning reactants into products), the sign of $\Delta H$ flips. If A $\rightarrow$ B is $+50$ kJ (Endothermic), then B $\rightarrow$ A is $-50$ kJ (Exothermic).
What if I multiply the coefficients?
Enthalpy is an Extensive Property, meaning it depends on the amount of matter. If you double the amount of stuff reacting (multiply coefficients by 2), you must also double the energy change (multiply $\Delta H$ by 2).
How do I solve Hess's Law problems?
- Look at your Target Equation.
- Manipulate the given steps (Reverse or Multiply) so that when you add them up, the intermediate chemicals cancel out and you are left with the Target Equation.
- Add up the adjusted $\Delta H$ values.
What is an Intermediate?
Intermediates are chemicals that are produced in one step and consumed in a later step. They do not appear in the final overall equation because they cancel out.
Can I use this for Bond Energies?
Sort of. Bond energy calculations are a specific application of Hess's Law where you break all reactant bonds (Endothermic) and form all product bonds (Exothermic). Formula: $\Delta H = \Sigma \text{Bonds Broken} - \Sigma \text{Bonds Formed}$.
What is Standard Enthalpy of Formation?
It is the change in enthalpy when one mole of a substance is formed from its pure elements under standard conditions. Often used as the 'steps' in Hess's Law problems.
Is this related to the Born-Haber Cycle?
Yes! The Born-Haber Cycle is essentially Hess's Law applied to ionic lattices. It breaks down the formation of an ionic solid into steps like ionization energy, electron affinity, and lattice energy.
Why is my result negative?
A negative $\Delta H$ means the reaction is Exothermic (releases heat). A positive $\Delta H$ means it is Endothermic (absorbs heat).
Does pressure affect Hess's Law?
Hess's Law assumes constant pressure (usually 1 atm). If pressure changes significantly, other thermodynamic factors like work ($PV$) come into play.
Who was Germain Hess?
Germain Hess was a Swiss-born Russian chemist who formulated this law in 1840, establishing the early principles of thermochemistry.