Enthalpy Change
Heat absorbed or released at constant pressure.
Formula
Variables
Example
Negative = exothermic
How it works
This formula calculates the enthalpy change, ΔH, by comparing the enthalpy of the products with the enthalpy of the reactants. Because enthalpy is a measure of the heat content of a system at constant pressure, the difference tells you whether heat was taken in or given out during the reaction. If the products end up at a higher enthalpy than the reactants, energy must have been absorbed; if they are lower, energy was released.
When to use it
Use this when analysing a chemical reaction carried out at constant pressure, such as in open beakers or typical lab reactions. It is also useful when interpreting thermochemical equations, calorimetry results, or predicting whether a reaction is endothermic or exothermic.
Worked example, step by step
Step 1: Write the formula: ΔH = H_products - H_reactants Step 2: Substitute the values: H_products = 180 kJ mol^-1 and H_reactants = 250 kJ mol^-1 Step 3: Calculate: ΔH = 180 - 250 Step 4: ΔH = -70 kJ mol^-1 Step 5: Interpret the sign: the reaction released 70 kJ mol^-1 of heat, so it is exothermic.
Common mistakes to avoid
- Reversing the subtraction and doing reactants minus products
- Forgetting that a negative ΔH means energy is released
- Mixing up enthalpy values with temperature or heat capacity
Did You Know?
Exothermic reactions release heat with negative enthalpy, which is why hand warmers feel warm.
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