University:
Bradley University
Course:
CHM 100 | Fundamentals of General Chemistry
Academic year:

2023
Views:

280

Pages:

2
Author:

cravelingstnw

Week 7 Discussion Notes First Law of Thermodynamics: The total energy of the universe (system and surroundings) is constant; energy can neither be created or destroyed. Introduction: Energy Units: Energy, heat and work: E = q + w q = heat transferred & w = work done = P×ΔV Heat flow: q = m⋅C⋅ΔT q = heat transferred C = specific heat capacity (J/g⋅K or J/g⋅oC) ΔT == Tfin − Tin (oC or K) Change in Internal Energy: ΔE = qreaction mol of reactant Any flow of heat between the system and surroundings manifests itself in the temperature change of the surroundings (water). If heat leaves the system, it enters the surroundings and the temperature of the surroundings increases. The reverse of this is also seen when heat leaves the surroundings into the system. Since there is no heat lost to the outside world between the system and surroundings, one can write: (1) qsystem + qsurroundings (water) = 0 Which means, (2) qsystem = − qsurroundings Chemical equations (thermochemical) are just like a regular chemical equation, with an energy term. Example: The combustion of methane has a reaction enthalpy (ΔHrxn) of −802 kJ. The negative sign indicates that the reaction is exothermic such that the thermochemical equation can be written: CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) + 802 kJ This implies that the energy term can be treated as a product resulting in stoichiometric conversion factors like: 802kJ 2mols H2O or etc... 1mol CH4 802kJ The enthalpy of a reaction scales with the reaction: Reverse a reaction, reverse a the enthalpy sign: Double a reaction, double the enthalpy sign: CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) ΔrH = − 802 kJ CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) CO2(g) + 2H2O(g) → CH4(g) + 2O2(g) ΔrH = + 802 kJ 2CH4(g) + 4O2(g) → 2CO2(g) + 4H2O(g) ΔrH = − 1604 kJ Enthalpies of reaction can be measured by coffee cup calorimeter experiments: qrxn + qsolution = 0 where m = the total mass of the solution & qrxn = − qsolution = − m × C × ΔT ΔrH = − 802 kJ Bomb or Constant Volume Calorimetry: Heats of combustion (ΔEocombustion ) are measured using a device called a Bomb Calorimeter. The bomb is placed in a bath of water where the temperature change due to the reaction is measured. qrxn + qbomb + qwater = 0 so qrxn = −qbomb − qwater = − Cbomb×ΔT −mwater×Cwater×ΔT ΔErxn = qrxn mole of reactant