When we calculate how much energy is needed to break a chemical bond, it makes a

Question

When we calculate how much energy is needed to break a chemical bond, it makes a difference if we are talking about a single molecule in a vacuum, or lots of molecules contained in a fluid. The latter, more realistic and less abstract situation requires us to consider not just the energy to break the bond, but the energy required to "deal with" the environment. If, for example, as is common in chemistry (or biology), the reaction takes place at a constant pressure and temperature, then some of the energy put in to break up the molecules might flow off as thermal energy or as work to create a space for the additional particles created. This is why we might use enthalpy as our energy needed to "break the bond" rather than just the energy needed to break the bond itself. The energy needed to simply pull the bond apart is called the dissociation energy. For this problem, we'll work out the difference in the energy needed to break a bond and the enthalpy needed to break a bond. One of the difficulties in thinking about these issues is the use of different units. In old-fashioned chemistry, energy is measured in calories. More modern chemistry uses SI units for energy -- Joules. And quantum chemistry, where the focus is on individual atoms or molecules, often uses electron Volts (eV). Here are the conversions among them:

1 calorie = 4.185 Joules

1 eV = 1.602 x 10-19 Joules

A. Some confusion is introduced by the fact that physicists might focus on an individual molecule whereas a chemist might focus on a mole of them (1 mole = 6.023 x 1023 molecules or atoms). If a particular molecule had a bond dissociation energy of 1 eV, how much energy would be needed (in kJ = kiloJoules) to break all the bonds in one mole of molecules -- counting no energy for interactions with the environment?_________ kJ/mole

B. For a single H2 molecule, H2 2H, the dissociation energy is 4.52 eV. How much energy would have to be put in (in kJ) to dissociate a mole of hydrogen molecules? _______kJ/mole

C. Suppose we are putting in energy to dissociate a bubble consisting of 1 mole of hydrogen molecules at STP (p = 1 atmosphere = 105 N/m2, and T = 300 K). As we put in energy to dissociate the hydrogens, some of the energy we put in will go into expanding the bubble, some will heat up the gas and some energy will flow out to maintain T = 300 K. Calculate the factor pV needed to find the enthalpy change by using the ideal gas law, pV = nRT, where n is the number of moles of gas. __________ kJ

D. What is the total enthalpy change (in kJ/mol) for the dissociation of a mole of hydrogen gas at STP? _________kJ/mol

Explanation / Answer

A) energy needed to break bonds of 1-mole molecule = 6.023 x 1023 eV

= 6.023 x 1023 x 1.602 x 10-19 J/mole

= 9.648846 x 104 x 10-3 KJ/mole = 96.48846 KJ/mole

B) as the energy required in this case is 4.52 times the energy required in case A)

Energy required = 4.52 x 96.48846 = 436.12784 KJ/mole

C) as P and T are constant

pV = (n)RT

now one 1 mole of H2 is dissociating, volume is getting doubled, 1 mole --> 2 mole

n = 2-1 =1

pV = 1 x RT = 1 x 8.314 x 300 J = 2494.2 J = 2.4942 KJ

D) H = U + (pV)

now T = constant,   U = 0

p is constant

H = 0 + p(V) = 2.4942 KJ/mole

Related Questions

Navigate

• Browse (All)
• Browse W
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.