Illustrated Guide to Home Chemistry Experiments

310 DIY Science: Illustrated Guide to Home Chemistry Experiments

EvdRE y Ay pHoToCHEmISTRy

Photochemical reactions are used frequently in laboratory syntheses, but they are also commonplace in everyday life.

Here are just a few examples:

• When you spend the day at the beach, your skin tans as a result of a photochemical reaction. Skin cells called melanocytes
  are stimulated by the ultraviolet light present in sunlight to produce a brown pigment called melanin, which diffuses in the
  skin and protects it by absorbing additional ultraviolet light.


• Smog is produced by a photochemical reaction of the nitrogen oxide (NO) present in automobile exhaust. The raw nitrogen
  oxide reacts with atmospheric oxygen and unburned hydrocarbons to produce various toxic gases, including nitrogen
  dioxide (NO 2 ), ozone (O 3 ), and peroxyacetyl nitrate (CH 3 COONO 2 ), all of which are components of photochemical smog.


• When you shoot a roll of 35 mm film, you are using a photochemical reaction to record the images. In fact, human
  vision depends on the photochemical isomerization of retinaldehyde (also commonly called “retinal”), so you are using
  photochemistry right now to read this page.


• And then there’s the most important photochemical reaction of all, photosynthesis, in which the chlorophyll present in
  green plants uses the energy from sunlight to combine water and carbon dioxide from the atmosphere to produce the
  carbohydrates that are the foundation of our diets, and without which plant and animal life could not exist.

LABORATORY 1 7.1:

pHoToCHEmICAL REACTIoN of IodINE ANd oxALATE

Ammonium oxalate reacts in solution with

elemental iodine to form ammonium iodide and

carbon dioxide, but the reaction rate is very low

at room temperature. In this laboratory session,

we investigate the effects on the reaction

rate of elemental iodine with oxalate ions by

exposing these reactants to various types and

intensities of light for differing periods of time.

RIREEqU d EqUIpmENT ANd SUppLIES

£ goggles, gloves, and protective clothing

£ balance and weighing paper (optional)

£ test tubes (12)

£ stopper (to fit test tubes)

£ test tube rack

£ 150 mL beaker (1)

£ graduated cylinder, 100 mL

£ graduated cylinder (10 mL) or 5 mL measuring pipette

£ dropper or Beral pipette (1)

£ aluminum foil

£ clear household ammonia (25 mL)

£ oxalic acid solution (dissolve 2.5 g or 3/4 tsp. in 25

mL of water)

£ tincture of iodine (small bottle)

£ light sources (incandescent light, fluorescent light,

sunlight, open shade)

The balanced equation shows that one molecule of aqueous
ammonium oxalate reacts with one molecule of iodine to form
two molecules of ammonium iodide and two molecules of
carbon dioxide.

(NH 4 ) 2 C 2 o 4 (aq) + I 2 (aq) → 2 NH 4 I(aq) + 2 Co 2 (g)

Or, looking at the individual atom and ion species,

2 NH 4 +(aq) + C 2 o 4 2–(aq) + 2 I^0 (aq)
→ 2 NH 4 +(aq) + 2 I–(aq) + 2 Co 2 (g)

Oxalate ions are oxidized to carbon dioxide, and iodine is
reduced to iodide ions. Iodine (oxidation state 0) is strongly
colored in aqueous solution—an intense orange in the
concentration we’re using—and iodide ions (oxidation state –1)
are colorless. By observing the color change, if any, we can judge