Energy, Heat & Enthalpy
- Distinguish endothermic from exothermic changes and read their energy diagrams
- Separate the ideas of heat, temperature, and enthalpy
- Interpret the sign of ΔH and scale it with the amount of reactant
System, surroundings, and the flow of energy
Thermodynamics splits the universe into two parts: the system (the reaction you care about) and the surroundings (everything else — the solvent, the beaker, the air). Energy is conserved, so any energy the system loses the surroundings gain, and vice versa. All of chemistry's heat bookkeeping starts from this single rule.
Exothermic vs. endothermic
An exothermic reaction releases energy to the surroundings — the products store less energy than the reactants, so the surroundings warm up. An endothermic reaction absorbs energy from the surroundings — the products store more energy than the reactants, so the surroundings cool down. On an energy diagram, exothermic reactions end below where they started; endothermic reactions end above.
Heat, temperature, and enthalpy are three different things
Temperature measures the average kinetic energy of particles — how fast they jiggle. Heat (q) is energy transferred because of a temperature difference; it always flows from hot to cold. Enthalpy (H) is the heat content of a system at constant pressure, and what we track is the change ΔH = H(products) − H(reactants). A big pot of warm water holds more heat than a spark, even though the spark is far hotter.
For N₂(g) + 3 H₂(g) → 2 NH₃(g), ΔH = −92 kJ. How much heat is released when 4.0 mol of NH₃ is formed?
- 1.The equation as written makes 2 mol NH₃ and releases 92 kJ (ΔH is negative, so heat is released).
- 2.Set up the ratio: 92 kJ released per 2 mol NH₃ = 46 kJ per mol NH₃.
- 3.Scale to 4.0 mol: 4.0 mol × 46 kJ·mol⁻¹ = 184 kJ.
ΔH is an extensive quantity — it scales with amount. If you double the moles of reaction, you double the heat. Always tie a ΔH value to the balanced equation it belongs to, and watch the sign: releasing heat is negative.
In an exothermic reaction, the products have ___ enthalpy than the reactants, and ΔH is ___.
Which statement correctly distinguishes heat from temperature?
On an energy diagram for an endothermic reaction, the products sit ___ the reactants, and ΔH is ___.
A quick reflex for the exam: "exo = exit," heat exits the system, ΔH < 0, surroundings warm. "Endo = into," heat goes into the system, ΔH > 0, surroundings cool. The temperature change you feel in the beaker is the opposite sign of the system's ΔH.
Answer the 3 checkpoints as you read.
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