Ultraviolet photon (λ = 58.4nm) from a helium gas discharge tube is absorbed by a hydrogen molecule which is at rest. Since momentum is conserved, what is the velocity of the hydrogen molecule after absorbing the photon? What is the translational energy of the hydrogen molecule in Jmol-1. [h = 6.626 x 10-34 Js; NA = 6.022 x 1023 mol-1]

Translation energy of 1 mole of H2 molecules = KE x Avogadros number

Explanation:

Photon wavelength

Photon momentum = h/wavelength

Mass of H2 molecule m = molar mass/Avogadros number

Since momentum is conserved:

Photon momentum = H2 molecule momentum = mass x velocity of H2

velocity

Translation energy of 1 H2 molecule = kinectic energy (KE) = (1/2)mv^2

Translation energy of 1 mole of H2 molecules = KE x Avogadros number

definition

                    entropy is a measure of the randomness or the extent of disorder of a chemical process.

                      enthalpy is a measure of the heat change of a reaction occurring at a constant pressure.

measurement units:

                        entropy is measured in joule per kelvin (jk⁻¹)   whereas enthalpy is measured in joule per mole (jmol⁻¹)

requirements:

                            entropy has no requirements or limits, and its change is measured by the division between the heat change of the chemical process and the temperature.

                              in contrast, enthalpy can only be related to the heat change of the reaction under standard conditions.