Methane and chlorine in UV light sim
Shine a UV lamp on methane and chlorine in a graduated tube over water: with no UV nothing happens; with it the chlorine fades, oily drops form, the water rises, and CH₃Cl, then CH₂Cl₂, CHCl₃ and CCl₄ build up.
Organic chemistry: alkanes and substitution · covers the O-Level, IGCSE topic
What the sim shows
Alkanes such as methane are unreactive, except that they burn and that they react with chlorine in ultraviolet (UV) light. With no UV light, methane and chlorine do not react at all. UV light gives the energy to start the reaction (it is a photochemical reaction). Then one hydrogen atom at a time is replaced (substituted) by a chlorine atom, and hydrogen chloride forms every time: CH₄ + Cl₂ → CH₃Cl + HCl, then CH₃Cl + Cl₂ → CH₂Cl₂ + HCl, CH₂Cl₂ + Cl₂ → CHCl₃ + HCl, CHCl₃ + Cl₂ → CCl₄ + HCl. So a mixture of products forms; with more chlorine, more of the hydrogen is replaced. In the tube, the pale yellow-green chlorine fades, oily drops form on the glass (dichloromethane, trichloromethane and tetrachloromethane are liquids at room temperature), and the water rises because hydrogen chloride dissolves in it (the solution turns acidic). The gas volume read on the tube is methane + chloromethane (a gas) + chlorine left. The model counts all the dichloromethane as liquid; in a real tube some of it stays as vapour, so the real fall is a little smaller. Afterwards the solution can be tested with damp blue litmus, once the chlorine colour has gone (chlorine bleaches litmus). Rates are illustrative: each molecule reacts in proportion to the hydrogen atoms it has left, and the light falls off with distance squared (twice as far gives a quarter of the light). The lamp's distance is read on the rail's scale; the drawing is not to scale. Equal volumes of gas hold equal numbers of molecules, so the amounts are given in cm³.
- Graduated tube: the yellow-green chlorine fades; the water rises as HCl dissolves
- UV lamp on a rail: drag it nearer for more light; click its switch
- Gas jars of chlorine and methane, used to fill the tube
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