Aula #07 · A Comparison of DH and DU DH 2Na (s) + 2H 2 O (l) 2NaOH (aq) + H2 (g) DH = -367.5 kJ/mol DU DH = DU + pDV DU = DH-pDV DU ≡DH-Dn(g) RT = -367.5 - 2.5 = -370.0 kJ/mol

Aula #07

Thermodynamics/Thermochemistry

Work ….

p as function of V …p ..constant…isobaric expansion

W1….W2….

- W2 > - W1dW = -pdV

e.g. p=RT/V

Thermodynamics &Thermochemistry

H ≡ U + pV (Defined as…)DU = q + w If dV=0… constant volume

dU = dq + dwDU = q

dH ≡ dU + d(pV)

DH = q

If dp=0… constant pressure dH ≡ dU + d(pV)

Work …. dW = -pdV

If dV=0… constant volume

If dp=0… constant pressureDH = q

DU = qWork ….

dW = -pdV

Thermodynamics/Thermochemistry dU = dq + dw

W2….

A Comparison of DH and DU

2Na (s) + 2H2O (l) 2NaOH (aq) + H2 (g) DH = -367.5 kJ/molDH

DU

DH = DU + pDV

P = 1 bar, T = 25 ºC

Dn(g) = 1 mol

DH = q

Thermodynamics &Thermochemistry

A Comparison of DH and DU

2Na (s) + 2H2O (l) 2NaOH (aq) + H2 (g) DH = -367.5 kJ/molDH

DU DH = DU + pDV

DU = DH - pDV

DU ≡ DH - Dn(g) RT = -367.5 - 2.5 = -370.0 kJ/mol

(pDV = Dn(g)RT)

P = 1 bar, T = 25 ºC

Dn(g) = 1 mol

Thermodynamics &Thermochemistry

dW = -pdV

Thermochemistry

Constant-Pressure CalorimetryNo heat enters or leaves!

qsys = qwater + qcal + qrxn

qrxn = - (qwater + qcal)

qwater = m x c x Dt qcal = Ccal xDt

Reaction occurs at Fixed P DH = qrxn

qsys = 0

Thermochemistry

Constant-Volume Calorimetry

qsys = qwater + qbomb + qrxn

qsys = 0

qrxn = - (qwater + qbomb)

qwater = m x c x Dt

qbomb = Cbomb xDt

Reaction at Constant V DU = qrxn

Atomization

Thermodynamics &Thermochemistry

e.g. The Solution Process for NaCl

DHsoln = Step 1 + Step 2 = 788 – 784 = 4 kJ/mol

Thermodynamics &Thermochemistry

Acetone

e.g. The Solution Process for NaCl

NaCl

Thermodynamics &Thermochemistry

water

Thermodynamics &Thermochemistry

Acetone

e.g. The Solution Process for NaCl

NaCl

Thermodynamics &Thermochemistry

water