• Hess’s Law: if you add chemical equations to get an overall equation, then you can also add the heat changes (ΔH) to get the overall heat change
  1. If two identical substances are on opposite sides of the arrow, they will cancel (reduce)
  2. If two identical substances are on the same side of the arrow, add the coefficients together
  3. Keep substances on the same side of the arrow in the final equation


Calculations via Hess’s Law

  1. Reverse any reactions as needed to get substances on correct side → If a reaction is reversed, (the sign of) ΔH is also reversed
  1. If needed, multiply reactions by an integer to give the correct number of reactants and products → ΔH is multiplied by that same integer


  • Tips: Start process with compounds that only appear once; once used one elementary step, cross through it bcuz not going to use it again

Laws of Thermodynamics

  • Thermodynamics: study of energy transfers and transformations

First Law: “Principle of Conservation of Energy”

  • Energy can be transferred and transformed into a different form but it cannot be created or destroyed

         ○ Forms include kinetic energy and potential energy

         ○ So organisms must get all their resources from the environment

  • The energy of the universe is constant

Internal energy

  • Internal Energy (E) (energy of an object): the sum of kinetic and potential energies of a system ○ Can be changed by a flow of work, heat or both

            ○ ΔE = + internal energy of the system increased

            ○ ΔE = – internal energy of the system decreased

            ○ Q = + heat flowed into the system

            ○ Q = – heat flowed out of the system

            ○ W = + the surroundings do work on the system (gas compression)

            ○ W = – the system does work on the surroundings (gas expands)

  • To convert between L·atm and Joules, use 1 L·atm = 101.33 J