Internal Energy and Enthalpy
Simple Explanation
A substance's internal energy is the total energy stored in its particles' motion and bonds. Enthalpy (H) is a closely related quantity — essentially the heat content of a system at constant pressure, which is how most reactions (open to the atmosphere) actually happen.
Why Do We Need It?
Chemists use enthalpy rather than raw internal energy because most reactions are carried out in open containers at roughly constant atmospheric pressure, and enthalpy change is exactly the quantity that can be measured directly as heat released or absorbed under those conditions.
Formula
Heat Energy (Calorimetry)
q = mcΔT
The heat energy absorbed or released by a substance, calculated from a measured temperature change — the standard way an enthalpy change is measured experimentally.
- q
- — heat energy transferred, in joules (J)
- m
- — mass of the substance (usually the water in a calorimeter), in grams (g)
- c
- — specific heat capacity of the substance, in J/(g·°C) — for water, c = 4.18 J/(g·°C)
- ΔT
- — change in temperature, in °C
When to use it: Whenever a reaction's heat is measured by tracking the temperature change it causes in a known mass of (usually water-based) surroundings.
Worked Example
Calculate heat released using calorimetry
A reaction heats 100 g of water from 25.0°C to 45.0°C in a calorimeter. Find the heat energy released (c for water = 4.18 J/(g·°C)).
Why Does This Work?
Measuring the temperature rise of a known mass of water (or another substance with a known specific heat capacity) that absorbs the heat from a reaction is a direct, practical way to measure how much heat energy the reaction released, without needing to observe the bonds directly.
Real-Life Example
Food energy labels
The Calories listed on a food label come from burning a sample of the food in a calorimeter.
A bomb calorimeter measures the temperature rise of a known mass of water surrounding the burning food sample, and q = mcΔT converts that temperature rise directly into the food's energy content.
Practice
A reaction raises the temperature of 50 g of water by 12.0°C. How much heat (in J) was released? (c = 4.18 J/(g·°C))
MediumCommon mistake
Forgetting that q = mcΔT calculates the heat absorbed BY the water (or calorimeter), which is the negative of the heat released by an exothermic reaction — the reaction itself loses exactly the heat energy the surroundings gain.
Quick Review
- Enthalpy (H) is essentially heat content at constant pressure.
- q = mcΔT lets you calculate heat from a measured temperature change.
- Calorimetry is the standard experimental method for measuring reaction heat.