Calorimetry
5.60 Spring 2004
Lecture #8
page 1
Calorimetry The objective is to measure
DH rx (T1 )
•
Reactants (T1)
isothermal
=
Products (T1)
constant p
Constant pressure
(for solutions)
adiabatic constant p
reaction calorimeter
I) II)
DH I
React. (T1) + Cal. (T1)
DH II
Prod. (T2) + Cal. (T2)
adiabatic
=
Prod. (T2) + Cal. (T2)
constant p
=
Prod. (T1) + Cal. (T1)
constant p
______________________________________________________
DH rx (T1 )
React. (T1) + Cal. (T1)
=
Prod. (T1) + Cal. (T1)
constant p
DH rx (T1 ) = DH I + DH II (I) Purpose is to measure (T2 - T1) Adiabatic, const. p
fi
qp = 0
fi DH I = 0
(II) Purpose is to measure heat qp needed to take prod. + cal. from T2 back to T1.
5.60 Spring 2004
Lecture #8
page 2
T2
qp = Ú C p (Pr od.+ Cal.)dT = DH II T1
\
T2
DH rx (T1 ) = -ÚT C p (Pr od.+ Cal.)dT 1
•
Constant volume
(when gases involved)
adiabatic constant V
reaction calorimeter
I) II)
DU I
React. (T1) + Cal. (T1)
DU II
Prod. (T2) + Cal. (T2)
adiabatic
=
Prod. (T2) + Cal. (T2)
constant V
=
Prod. (T1) + Cal. (T1)
constant V
______________________________________________________
DU rx (T1 )
React. (T1) + Cal. (T1)
=
Prod. (T1) + Cal. (T1)
constant V
DU rx (T1 ) = DU I + DU II (I) Purpose is to measure (T2 - T1) Adiabatic, const. V
fi
qV = 0
fi DU I = 0
(II) Purpose is to measure heat qV needed to take prod. + cal. from T2 back to T1.
†
5.60 Spring 2004
Lecture #8
page 3
T2
qV = Ú CV (Pr od.+ Cal.)dT = DU II T1
\
T2
DU rx (T1 ) = -ÚT CV (Pr od.+ Cal.)dT 1
Now use H =U + pV or DH = DU + D ( pV ) Assume only significant contribution to D( pV ) is from gases. Ideal gas fi D( pV ) = R D (nT) Isothermal T = T1 fi D( pV ) = R T1Dngas \
DHrx (T1 ) = DU rx (T1 ) + RT1 Dngas T2
DHrx (T1 ) = - Ú CV (Pr od.+ Cal.)dT + RT1 Dngas T1
e.g.
4 HCl(g) + O2(g) = 2 H2O(l) + 2 Cl2(g)
T1 = 298.15 K DU rx (T1 ) = -195.0 kJ
Dngas = -3 moles
DH rx (T1 ) = -195.0 kJ + (-3 mol)( 298.15 K)(8.314x10-3 kJ/K-mol) = -202.43 kJ
5.60 Spring 2004
Lecture #8
page 4
Bond energies: An approximate method for estimating DH °f Really bond enthalpies, but usually D(pV)
Lecture #8
page 1
Calorimetry The objective is to measure
DH rx (T1 )
•
Reactants (T1)
isothermal
=
Products (T1)
constant p
Constant pressure
(for solutions)
adiabatic constant p
reaction calorimeter
I) II)
DH I
React. (T1) + Cal. (T1)
DH II
Prod. (T2) + Cal. (T2)
adiabatic
=
Prod. (T2) + Cal. (T2)
constant p
=
Prod. (T1) + Cal. (T1)
constant p
______________________________________________________
DH rx (T1 )
React. (T1) + Cal. (T1)
=
Prod. (T1) + Cal. (T1)
constant p
DH rx (T1 ) = DH I + DH II (I) Purpose is to measure (T2 - T1) Adiabatic, const. p
fi
qp = 0
fi DH I = 0
(II) Purpose is to measure heat qp needed to take prod. + cal. from T2 back to T1.
5.60 Spring 2004
Lecture #8
page 2
T2
qp = Ú C p (Pr od.+ Cal.)dT = DH II T1
\
T2
DH rx (T1 ) = -ÚT C p (Pr od.+ Cal.)dT 1
•
Constant volume
(when gases involved)
adiabatic constant V
reaction calorimeter
I) II)
DU I
React. (T1) + Cal. (T1)
DU II
Prod. (T2) + Cal. (T2)
adiabatic
=
Prod. (T2) + Cal. (T2)
constant V
=
Prod. (T1) + Cal. (T1)
constant V
______________________________________________________
DU rx (T1 )
React. (T1) + Cal. (T1)
=
Prod. (T1) + Cal. (T1)
constant V
DU rx (T1 ) = DU I + DU II (I) Purpose is to measure (T2 - T1) Adiabatic, const. V
fi
qV = 0
fi DU I = 0
(II) Purpose is to measure heat qV needed to take prod. + cal. from T2 back to T1.
†
5.60 Spring 2004
Lecture #8
page 3
T2
qV = Ú CV (Pr od.+ Cal.)dT = DU II T1
\
T2
DU rx (T1 ) = -ÚT CV (Pr od.+ Cal.)dT 1
Now use H =U + pV or DH = DU + D ( pV ) Assume only significant contribution to D( pV ) is from gases. Ideal gas fi D( pV ) = R D (nT) Isothermal T = T1 fi D( pV ) = R T1Dngas \
DHrx (T1 ) = DU rx (T1 ) + RT1 Dngas T2
DHrx (T1 ) = - Ú CV (Pr od.+ Cal.)dT + RT1 Dngas T1
e.g.
4 HCl(g) + O2(g) = 2 H2O(l) + 2 Cl2(g)
T1 = 298.15 K DU rx (T1 ) = -195.0 kJ
Dngas = -3 moles
DH rx (T1 ) = -195.0 kJ + (-3 mol)( 298.15 K)(8.314x10-3 kJ/K-mol) = -202.43 kJ
5.60 Spring 2004
Lecture #8
page 4
Bond energies: An approximate method for estimating DH °f Really bond enthalpies, but usually D(pV)
