CHEM 2B Ch9

hayashi

q > 0

system absorbs heat

When energy is added to the system as heat, the effect is to increase the internal energy of the system (ΔE > 0), and q must be a positive number.

q < 0

system evolves heat

When the system loses energy as heat to the surroundings, the effect is to decrease its internal energy (ΔE < 0), and q must be a negative number

w > 0

surroundings does work on system

When the surroundings does work on the system, the effect is to increase the internal energy of the system, and w must be a positive number

w < 0

system does work on surroundings

When the system does work on the surroundings, the effect is to decrease its internal energy and w must be a negative number

(U)

Internal Energy

(U) = Thermal energy + Chemical Energy

Thermal Energy

~Kinetic energy

Chemical Energy

~Potential Energy

q

heat, internal energy transferred between system and surroundings

J

SI Unit for heat, 1 cal = 4.184 J

q_sys

heat of the system, positive when it gains heat, negative when it loses heat

q_surr

heat of the surroundings, positive when it gains heat, negative when it loses heat

Relationship between q_sys and q_surr

q_sys+q_surr = 0

q_sys = -q_surr

C

heat capacity

C_sp and C(molar) in relation to q

Bond breaking

Chemical energy absorbed, heat

Bond forming

Chemical energy released, heat released

Exothermic

releases heat, -H, q_rxn = negative

Endothermic

absorbs heat, +H, q_rxn = positive

Bomb Cal equations

q_rxn = -q_cal

Pressure-volume work

w_sys = (-F)(h)

= -(F/A)(hA)

only works if P is constant

Sign convention of work

Intensive state function

Does not depend on the quantity of the substance

Ex) pressure, temp

Extensive state function

Depends on the quantity of the substance

Ex) volume, U

Path dependent function

functions that depend on how the system changes from the initial state to the final state

Ex) Heat, work

Law of thermodynamics in relation to U

If we know q and w we can get deltaU

The energy of a isolated system is constant

q=0 w=0 → dealtaU=0

Enthalpy

H = U +PV

at constant P, the change in enthalpy is the heat gained or lost

Enthalpy of fusion

The enthalpy change of 1 mol of a substance during fusion

Enthalpy of vaporization

enthalpy of change of 1 mol of a substance during the vaporization

Fusion

melting

Heating curve