Photochemistry is when light causes a chemical reaction to occur. A simple example is when light breaks a Cl₂ molecule apart into 2 Cl atoms. If the energy required to break apart a mole of chlorine molecules is 243 kJ, a photon of what frequency will break apart a single Cl₂ molecule?

Question

Quizplus · Accepted Answer

The energy of a photon is given by $E = h\nu$, where $E$ is the energy, $h$ is Planck's constant ($6.626 \times 10^{-34} \, \text{Js}$), and $\nu$ is the frequency of the photon. To find the frequency required to break apart a single Cl$_2$ molecule, we first convert the energy from kJ to J (1 kJ = $10^3$ J), so $243 \, \text{kJ} = 243 \times 10^3 \, \text{J}$. Since this is the energy for a mole of molecules, and one mole contains Avogadro's number ($6.022 \times 10^{23}$) of molecules, the energy to break apart a single molecule is $\frac{243 \times 10^3 \, \text{J}}{6.022 \times 10^{23}}$. Solving for $\nu$ gives a frequency in the order of $10^{14} \, \text{s}^{-1}$, which matches the order of magnitude for choice A.

Photochemistry Is When Light Causes a Chemical Reaction to Occur