Why is there a difference in the heats of formation of CF4 and CCl4? I know that the bond length of CCl4(C--Cl) is longer than that of CF4(C--F), does this affect the heat of formation. Heat of formation are -225.709kcal/mol(CF4) and -28.120kcal/mol(CCl4).
Wouldn't you expect the heat of formation of a stronger bond to be different than that of a weaker bond? I would.
Thanks. I also have a similar question about cis and trans isomers.why is the heat of formation lower for a trans isomer? the only thing i could think of is that the dipole moment is 0.But does this have anything to do with the heats of formation?
That sounds like the answer is in the ball park. Also, consider that in the cis isomer, there is a repulsion of the two atoms on the same side that is not there when they are trans to each other. It takes extra energy to overcome the repulsion present in the cis configuration.
The difference in the heats of formation of CF4 (carbon tetrafluoride) and CCl4 (carbon tetrachloride) can be attributed to various factors, including differences in bond energies and molecular properties.
One important factor that influences the heat of formation is the strength of the bonds formed within the molecule. In this case, the difference in the bond length between C--Cl and C--F bonds is relevant. The C--Cl bond is longer than the C--F bond because chlorine atoms are larger and have more electrons than fluorine atoms. Due to its larger size and greater electron-electron repulsion, the chlorine atom has longer bond distances compared to fluorine.
The longer C--Cl bond in CCl4 can be expected to have weaker bonding compared to the shorter C--F bond in CF4. Weaker bonds are associated with lower bond energies, meaning that it takes less energy to break a C--Cl bond compared to a C--F bond. Consequently, CCl4 has lower bond energies than CF4.
The heat of formation of a compound is the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states. Since breaking a bond requires energy input (endothermic), while forming a bond releases energy (exothermic), the differences in bond energies affect the overall heat of formation.
In the case of CF4, four strong C--F bonds are formed during its formation. Breaking these strong bonds during the formation of CF4 requires significant energy input, resulting in a relatively high heat of formation. On the other hand, the formation of CCl4 involves weaker C--Cl bonds, and less energy is required to break these bonds. As a result, CCl4 has a lower heat of formation compared to CF4.
Therefore, the difference in the heats of formation between CF4 and CCl4 can be explained by the difference in bond energies, which is influenced by the bond length and the sizes of the atoms involved.