Notes on Heat

Nomenclature “Heat” can mean several different things Sometimes it used to mean “energy” (sloppy) Sometimes it is “hotness” (i.e temperature) Sometimes it refers to stored heat ( Potential Energy)

Heat is a part of energy First Law of Thermodynamics  Energy =  q(heat) +  w(work) The other part (work) we chemists try to ignore. (Physics pays a lot of attention to that part.) The only time we can NOT ignore it is when there is a change in the number of gas molecules. The gas changes in volume or exerts a pressure on the container and that involves work.

Heat can describe “Hotness” Example: Exothermic reaction: C 3 H 8 + 5O 2  3CO H 2 O + Heat (Propane burning, things get hot) The “Heat” here makes “hotness”, which is temperature Definition of Temperature = Average Kinetic Energy Heat here is a type of Kinetic Energy Notice that hot object can heat up cold objects “Heat”, in this sense, can transfer from one object to another

Heat can refer to Potential Energy Heat can be stored in chemical bonds as “Enthalpy” (a type of potential energy) Potential Energy is a negative scale Weak bond have a small negative value Strong bonds have a large negative value Potential Energy is stored, it can NOT be transferred to other potential energies. But it can be lost or gained to kinetic energy. That is what chemical reactions do.

Heat can be talked about in different ways…Careful! C 3 H 8 + 5O 2  3CO H 2 O + Heat Here “heat” is Kinetic Energy It is saying that heat (“hotness”) is produced. C 3 H 8 + 5O 2  3CO H 2 O  Heat = kJ/mol Here “heat” is Potential Energy It is saying that the chemicals have lost energy ( and so the value is negative) and released it to the environment.

Two ways of Describing Heat Transfer   H  Also known as “Enthalpy”  Measures the amount of heat per one mole of material  Units are kJ(kiloJoules)/mol  q  Refers to the heat transferred in a particular reaction  The reaction will often describe the number of grams or moles involved  “+” means heat gained, “-” means heat lost

Heat Transfer Only Kinetic Energy can be transferred directly from one object to another. Hot objects transfer (and so lose) KE to cold objects (which gain KE) The two objects will stop transferring energy when they have the same temperature Potential Energy (chemical energy) can be lost or gained When lost or gained, it causes a gain or lose of kinetic energy to the surrounding environment. KE C + O 2 CO 2 PE KE (hot!!!) -KE + KE

Heat Transfer Formulas Kinetic Energy: a change in temperature q = mc  T q = heat, m = mass, c = constant for that material,  T = change in temperature (T final – T initial )( o Celcius or Kelvin)

Heat Transfer Formulas Potential Energy: a chemical change q =  H*mol q = heat, mol = moles of chemical reacting  H = change potential energy (-  H is losing PE, so releasing KE and exothermic) (+  H is gaining PE, so releasing KE and endothermic)

Putting it together… A chemical change….(burning propane) Can cause a physical change….(heating water) Propane The mass (and moles) of propane decreases The temperature of the water increases Water Mass (g) Temp ( o C)

Putting it together… Propane The mass (and moles) of propane decreases The temperature of the water increases Water Mass (g) Temp ( o C) Burning PropaneHeating Water Potential EnergyKinetic Energy  H* mol = q = m c  T