The net potential energy EN between two adjacent ions, is sometimes represented by the expression

EN = -C/r + Dexp (-r/p)

in which r is the interionic separation and C, D, and rho are constants whose values depend on the specific material.

Derive an expression for the bonding energy E0 in terms of the equilibrium interionic separation r0 and the constants D and rho using the following procedure:
1. Differentiate EN with respect to r, and then set the resulting expression equal to zero.
2. Solve for C in terms of D, rho and r0.
3. Determine the expression for E0 by substitution for C in the equation above.
What is the equation that represents the correct expression for E0?

Respuesta :

The expression for the bonding energy E₀ in terms of the equilibrium interionic separation r₀ and the given constants is;

E₀ = rD[(exp(-r₀/ρ) + exp(-r/ρ)] - EN/r₀

Bonding Energy and Net Potential Energy

We are given the expression;

EN = -C/r + Dexp(-r/ρ)

where;

EN is net potential energy

r is the interionic separation

C, D, and rho(ρ) are constants whose values depend on the specific material.

The formula for the bonding energy is usually;

E₀ = -C/r₀ + D exp(-r₀/ρ)

Step 1; We are to differentiate EN with respect to r. Thus, we have;

dEN/dr = C/r² - D exp(-r/p)

Step 2; We are to solve for C in terms of D, rho(ρ) and r₀. Thus;

E₀ + (C/r₀) = -D*exp(-r₀/ρ)

⇒ C/r₀ = -Dexp(-r₀/ρ) - E₀

Thus, multiplying both sides by r₀ gives;

C = -r₀(Dexp(-r₀/ρ) + E₀)

Step 3; We are to determine the expression for E₀ by substitution for C in the equation given. This gives us;

EN = -r₀(Dexp(-r₀/ρ) + E₀)/r + Dexp(-r/ρ)

EN = r₀*D*exp(-r₀/ρ) - (r₀E₀/r) + D*exp(-r/ρ)

EN + (r₀E₀/r) = r₀*D*exp(-r₀/ρ) + D*exp(-r/ρ)

r₀E₀/r = D[(exp(-r₀/ρ) + exp(-r/ρ)] - EN

Thus;  E₀ = rD[(exp(-r₀/ρ) + exp(-r/ρ)] - EN/r₀

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