Energy Changes in Chemical Reactions

Energy Changes in Chemical ReactionsThermochemistry1Thermodynamics and ThermochemistryThermodynamics = the study of heat and its transformations.Thermochemistry = the part of thermodynamics that deals with changes in heat that take place during chemical reactions as in most chemical reactions, energy is absorbed or released.

2

What is Internal Energy?Internal Energy (E)- the combined kinetic and potential energy of all particles in a system.The total internal energy of a system is not usually known. Energy changes (DE) can be measured. 3Examples of Potential and Kinetic that Contribute to Internal EnergyPotential Energy- Intermolecular forces of attraction or repulsionIntramolecular forces of attraction or repulsionKinetic EnergyRotational nature of moleculesVibrational nature of molecules

4interactions between particles of matter down to the subatomic level (p, n, and e-) but we are generally concerned about interactions at the molecular level.

SystemWhat is the system? The object or substance undergoing a change of physical state or reaction. What we are studying. Surroundings are everything around the system that can exchange energy with the system.5Types of SystemsOpen system can exchange mass and energy (usually in the form of heat) with the surroundings.Closed system can exchange energy but not mass.Isolated system no transfer of energy or mass.

6

State FunctionsState functions are path independent. They depend only on present state and are independent of history of the system. The change in a state function does not depend on how the process is carried out.7

State Functions8

Total Internal Energy (E) in a SystemEtotal = mgh + mv2 OR Potential Energy + Kinetic EnergyInternal energy of the system = sum of kinetic and potential energies making up a substance down to the subatomic level. We often dont know the total energy of a system.Chemists arent so interested in total energy in a system- theyre more interested in the energy changes that happen during chemical reactions.

9First Law of ThermodynamicsThe total energy in the universe is constant (Law of Conservation of Energy).You cant get something for nothingThere are essentially two ways to change the energy of a system- heat(q) and work(w).DE = q + w10Energy Change DEEnergy change in a system can be determined:DE = Energy can be transferred as heat + workDE = Ef Ei = q + wWhere work = force X distance OR pressure X volume changew = p DVNote: Expansion volume = -pDV. Work is being done by the system on the surroundings.

11Work in Chemical ReactionsOften times in chemical reactions, there isnt a volume change or a force applied over a distance.Therefore, for the chemist, heat changes are the primary concern when considering the energy changes that exist in a chemical reaction.DE = q(for many, not all, chemical reactions)

12Focus on Heat Heat = thermal energy (motion of atoms)Something is hot because it has thermal energy and its atoms are moving rapidly.The more energy, the faster they move.The total thermal energy of a substance is the sum of the individual thermal energies of the atoms and molecules making up a substance.13Chemists are Interested in Heat Transfer (q) during chemical reactionsHeat is a form of energy transfer.Heat transfers from hot object to cooler object.q>0 Heat is transferred from surroundings to system. ENDOTHERMICq