covalent bond strength most simply, the strength of a bond is measured by determining how much...
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Covalent Bond Strength
• Most simply, the strength of a bond is measured by determining how much energy is required to break the bond.
• This is the bond enthalpy.• The bond enthalpy for a Cl—Cl bond,D(Cl—Cl), is measured to be 242 kJ/mol.
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Average Bond Enthalpies
• This table lists the average bond enthalpies for many different types of bonds.
• Average bond enthalpies are positive, because bond breaking is an endothermic process.
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Average Bond Enthalpies
NOTE: These are average bond enthalpies, not absolute bond enthalpies; the C—H bonds in methane, CH4, will be a bit different than theC—H bond in chloroform, CHCl3.
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Enthalpies of Reaction
• Yet another way to estimate H for a reaction is to compare the bond enthalpies of bonds broken to the bond enthalpies of the new bonds formed.
• In other words, Hrxn = (bond enthalpies of bonds broken)
(bond enthalpies of bonds formed)
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Enthalpies of Reaction
CH4(g) + Cl2(g)
CH3Cl(g) + HCl(g)
In this example, one
C—H bond and one
Cl—Cl bond are broken; one C—Cl and one H—Cl bond are formed.
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Enthalpies of Reaction
So,
Hrxn = [D(C—H) + D(Cl—Cl) [D(C—Cl) + D(H—Cl)
= [(413 kJ) + (242 kJ)] [(328 kJ) + (431 kJ)]
= (655 kJ) (759 kJ)
= 104 kJ
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Bond Enthalpy and Bond Length
• We can also measure an average bond length for different bond types.
• As the number of bonds between two atoms increases, the bond length decreases.
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Work
When a process occurs in an open container, commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings).
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WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston.
w = −PV
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Enthalpy• If a process takes place at constant pressure
(as the majority of processes we study do) and the only work done is this pressure-volume work, we can account for heat flow during the process by measuring the enthalpy of the system.
• Enthalpy is the internal energy plus the product of pressure and volume:
H = E + PV
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Enthalpy
• When the system changes at constant pressure, the change in enthalpy, H, is
H = (E + PV)
• This can be written
H = E + PV
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Enthalpy• Since E = q + w and w = −PV, we can
substitute these into the enthalpy expression:
H = E + PVH = (q+w) − w
H = q
• So, at constant pressure the change in enthalpy is the heat gained or lost.