Chp. 6 Thermodynamics/Thermochemistry Remember

Thermodynamics/Thermochemistry

• is the study of energy

Energy

• is the capacity to do work or produce heat

Law of Conservation of Energy

Energy cannot be created or destroyed, just transferred

1st Law of Thermodynamics

The energy in the universe is constant

State Functions

• depend only on the current state of the system. P, V, T, energy

Heat

• heat flows from hot to cold: high KE [kinetic energy], high velocity particles collide with cold/slower particles, making them speed up, increasing their KE

• When T is up, KE is up

Exothermic

• Exo - exist, thermic - heat

• Heat energy flows out of a system into its surroundings. (hot pack)

• EX. burning paper & burning a candle

Endothermic

• Endo - in, thermic - heat

• Heat energy flows into a system from its surroundings. (cold pack)

• EX. ice cube melting & Cooking an egg

the First Law of Thermodynamics

energy cannot be created or destroyed, only transferred between a system and its surroundings through heat and work

• ΔE = q + w

• Δ = delta = ‘change of’

• ΔE = change in internal energy of a system (joules)

• q = heat

• w = work

• +q = heat flows into the system (endothermic)

• -q = heat flows out of the system (exothermic)

• -w = the system does work (loses energy)

• +w = the surroundings do work on the system

Pressure-Volume Work

the work done by a gas when it expands or contracts against an external pressure

• w = -PΔV

  P = pressure

• ΔV = V_f - V_i (change in volume)

  V_{f =} final volume, V_i = inital volume

• 1 L x atm = 101.3 J !!

Calorimetry

is the science of measuring heat

• ΔE = q + w

  w = -PΔV

  q = smΔT

  where s = specific heat capacity,

  m = mass, and ΔT = T_f - T_i (T_f: final & T_i: inital)

Specific Heat Capacity (s)

is the energy required to raise the temperature one degree Celsius for one gram of substance

• s for H₂O = 4.184 J/(g x ^oC)

  = 1 calorie,

  s for Fe = 0.45 J/(g x ^oC)

Molar Heat Capacity

is the same except for 1 mol of substance

J/(mol x ^oC)

• so the “q” equation would have mols for “m,” instead of mass

Enthalpy (H)

is the heat energy

• q = H at constant P

Hess’s Law

The enthalpy change (ΔH_r^o) for a reaction is the sum of the enthalpy changes for a series of reactions, that add up to the overall reaction

• Steps

  For each reaction:

  1) Check to see, if the compounds are on the correct sides of the reaction. **If not, reverse the entire reaction, & change the sign of ΔH

  2) Check to see, if all the unwanted compounds will cancel completely. **If not, multiply an entire reaction by a # so that they do cancel completely & multiply ΔH by that same #