Chapter 27, Problem 26P

To determine

Energy needed for the chemical reaction.

Answer to Problem 26P

Solution:

Energy needed for the chemical reaction is $1.97\text{eV}$ or $45.39\text{kcal/mol}$.

Explanation of Solution

Given:

Threshold wavelength, $\lambda =630\text{nm}$

Formula used:

The energy of the photon is calculated as

$W=\frac{hc}{e\lambda }$

Here,

$e$ is the charge of electron

$h=6.62\times {10}^{-34}\text{J-s}$ is the Planck’s constant

$c=3\times {10}^8\text{m/s}$ is the speed of light

$\lambda$ is the wavelength

Calculation:

The maximum energy of the photon is calculated as

$W=\frac{hc}{e\lambda }$

Plugging the values in the above equation

$\begin{array}{c}W=\frac{6.62\times {10}^{-34}\times 3\times {10}^8}{1.6\times {10}^{-19}\times 630\times {10}^{-9}}\\ =1.97\text{eV}\end{array}$

The energy in $\text{kcal/mol}$ is calculated as

$\begin{array}{c}W=\frac{1.97\times 1.6\times {10}^{-19}\times 6.02\times {10}^{23}}{4.18\times {10}^3}\\ =45.39\text{kcal/mol}\end{array}$