Chem 30 - Unit 6 - Thermodynamics

Energy

The capacity to do work, create heat, and/or generate electricity

1st Law of Thermodynamics

Energy is always conserved, cannot be created or destroyed

2nd Law of Thermodynamics

Energy is always lost as heat through conversions, from highest to lowest

What loses or gains energy in the combustion of Methane?

System loses, surroundings gain

What loses or gains energy in electrolysis?

System gains from the surroundings

Five sources of energy

Chemical, nuclear, solar, geothermal, hydroelectric

Enthalpy

Amount of energy released or absorbed by the system during a reaction.

Endothermic vs exothermic enthalpy change

Endothermic is positive enthalpy. Exothermic is negative.

Molar enthalpy

The amount of energy released by a substance per mol of substance

Thermochemical equations

Chemical equations where energy is included in the equation

Three ways of expressing enthalpy

Part of a reaction, on the side, or in an enthalpy diagrams

What do catalysts do to enthalpy diagrams?

They decrease activation energy needed, but do not change the enthalpy change

What is the equation for energy change in a system?

Q = mct, where t is temperature change, c is specific heat capacity, and m is mass of object.

Specific heat capacity

Amount of energy needed to increase 1 gram of substance by 1 degree celsius, J/gc

Calorimeter

Used to measure energy changes in an ALMOST isolated system. Usually, some form of cup with a low specific heat capacity on top of another with water. The water absorbs the reaction's heat.

Hess's law

H = sum of n*rH(P)- sum of n*rH(R)

Intermediates

Chemical entities that form with varying stability at the end of a step in a reaction mechanism. More stable than activated complex, but less than reactants and products.

What is the enthalpy of a thermally stable compound?

A very small (high negative) number