Thermodynamics Formulas and Sign Conventions, Study notes of Chemistry

Download Thermodynamics Formulas and Sign Conventions and more Study notes Chemistry in PDF only on Docsity! Chapters Five and Nineteen: Thermodynamics “I heard you tellin' Dave to shoot me in the brain with a laser, out behind the dumpster.” Sign Convention Signs matter. In the case of work and heat, a positive sign (+) means it is ENTERING while an negative sign (-) means it is LEAVING. Units Function Abbreviation Unit State Function? Work w Joules No Heat q Joules No Internal Energy E Joules Yes Enthalpy H Joules Yes Entropy S Joules/Kelvin Yes Gibbs Free Energy G Joules Yes Formulas Most of the work in these two chapters is relatively simple math; the hard part is knowing which formula to use. Formula Important Things… When to use… ?E = q + w The 1st law of Thermodynamics Relating internal energy, heat, and work. Always valid. w = -P? V Expansion work; a gas expanding against a pressure P (in Pascals) does work w in Joules. The ? V is the change in volume of the gas; needs to be in m3. Use when a gas it expanding and you need to know something about work. Should be very obvious, as the problem will have to give you both an external pressure and a volume. ?H = qp ONLY AT CONSTANT PRESSURE! … but lots of things are at constant pressure. Problems at constant pressure always explicitly say so. Tm q atSpecificHe ∆ = * Temperature has to be in Kelvin. Mass usually needs to be in grams. The word “calorimeter” really gives things away. ? H0rxn = ? n H 0 products - ?nH 0 reactants DO NOT FORGET MOLAR COEFFICEINTS! This ? H0 isn’t necessarily the same thing as ?H. “standard molar enthalpy of formation” T q S =∆ Temperature has to be in Kelvin. Watch the sign on q; if the system is releasing heat and you want to know the ?S of the surroundings, q needs to be positive. Use when you want to know an entropy change associated with a given heat transfer at a given temperature. T H S trstrs =∆ A sort of special case form of the formula above. Watch the sign on H. Phase transitions; melting, freezing, sublimation… ? S0rxn = ? n S 0 products - ?nS 0 reactants DO NOT FORGET MOLAR COEFFICEINTS! This ? S0 is not necessarily the same thing as ?S. “standard molar entropy”