Essential Thermochemistry Definitions

 

1. Standard enthalpy of formation, DHfo: the standard reaction enthalpy for the formation of one mole of a substance from its elements in their most stable form at 1 atm and a specified temperature, usually 25oC. The equation associated with the standard enthalpy of formation and any thermochemical equation must specify the state (s, l, g, aq) of each compound in the equation. The "standard" in the definition is indicated by the superscript "o" in the symbol where "standard" simply means 1 atm pressure.

What is the common form (formula and state) of each element at 25oC and 1 atmosphere?
123456789101112131415161718
H2(g)He(g)
Li(s)Be(s)B(s)C(s)N2(g)O2(g)F2(g)Ne(g)
Na(s)Mg(s)Al(s)Si(s)P4(s)S(s)Cl2(g)Ar(g)
K(s)Ca(s)Sc(s)Ti(s)V(s)Cr(s)Mn(s)Fe(s)Co(s)Ni(s)Cu(s)Zn(s)Ga(s)Ge(s)As(s)Se(s)Br2(l)Kr(g)
Rb(s)Sr(s)Y(s)Zr(s)Nb(s)Mo(s)Tc(s)Ru(s)Rh(s)Pd(s) Ag(s) Cd(s)In(s)Sn(s)Sb(s)Te(s)I2(s)Xe(g)
Cs(s)Ba(s)La(s)*Hf(s)Ta(s)W(s)Re(s)Os(s)Ir(s)Pt(s)Au(s)Hg(l)Tl(s)Pb(s)Bi(s)Po(s)At2(s)Rn(g)
Fr(s)Ra(s)Ac(s)*

Example: write the equation that represents the standard enthalpy of formation of calcium carbonate.
Answer: Ca(s) + C(graphite) + 3/2O2(g) -----> CaCO3(s)
What are the units of standard enthalpy of formation?

 

2. Standard enthalpy of combustion, DHco: the standard reaction enthalpy for the combustion of one mole of a substance at 1 atm. 

Example: write the equation that represents the standard enthalpy of combustion of methane.
Answer: CH4(g) + 2O2(g) -----> CO2(g) + 2H2O(l).

 

3. Bond enthalpy: the change in enthalpy when a bond is broken in the gaseous state to form the corresponding gaseous atoms. Bond enthalpies are always positive. Why?

Example: write the equation that represents the bond enthalpy of H-Br.
Answer: H-Br(g) -----> H(g) + Br(g)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Sample Problems

1.
Calculate DH values for each of the following reactions from the data below.

  DHfo, kJ.mol-1  Bond Enthalpy, kJ.mol-1
HCl(g) -92.31  
Cl-(aq) -167.16  
H+(aq) 0  
H-Cl   431

(a) H(g) + Cl(g) -----> HCl(g)
(b) H2(g) + Cl2(g) -----> 2HCl(g)
(c) H+(aq) + Cl-(aq) -----> HCl(g)

 

2.
Calculate the standard enthalpy of formation of HCl(g) from the data below.

  DHfo, kJ.mol-1  Bond Enthalpy, kJ.mol-1
H(g) 217.9  
Cl(g) 121.4  
H-Cl   431

 

 

 

 

 

 

 

 

 

 

 

 

The First Law

DU = q + w

Sign

Contribution to DU
work done on system

+w

+
work done by system

-w

-
heat gained by system

+q

+
heat lost by system

-q

-

Example:
A CD player and its battery together do 500 kJ of work. The battery releases 250 kJ of energy as heat and the CD player releases 50 kJ as heat due to friction from spinning. What is the change in internal energy of the system? The battery and CD player are considered the system.
Answer

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Interesting Examples of Thermochemistry

1. "Ice that burns": methane hydrate