Manipulating Chemical Equilibrium
Chemical reactions tend to go to equilibrium. It is possible to shift the equilibrium by changing the conditions under which the reaction occurs. Factors that can affect chemical equilibrium include the concentration of reactants and products, temperature, and pressure. In this lab, you will observe a chemical reaction and use your observations to predict how one factor will shift the equilibrium of the reaction. Then, you will perform an experiment to test your prediction.
Problem How can you change the equilibrium of a chemical reaction?
Materials
iodine-starch solution
150-mL beaker
4 dropper pipets
spot plate
ascorbic acid (vitamin C) solution
chlorine bleach (sodium hypochlorite, NaOCl) solution
Skills Formulating Hypotheses, Designing Experiments, Observing
Procedure
Part A: Observing a Reversible Reaction
Pour 50 mL of iodine-starch solution into the 150-mL beaker. The dark color of this solution is due to the presence of iodine molecules (I2) within the grains of starch. CAUTION Handle iodine solutions with care. Iodine is toxic.
Use a dropper pipet to transfer 3 drops of iodine-starch solution from the beaker to one well on the spot plate.
Use another clean dropper pipet to add one drop of ascorbic acid solution to the iodine-starch solution on the spot plate. Continue to add ascorbic acid solution to the mixture on the spot plate, one drop at a time, until the mixture becomes clear. When an iodine molecule reacts with ascorbic acid, the iodine molecule is reduced and breaks down into two colorless iodide ions (2I–).