Question

Starch+Iodine are reacted to produce a Starch-Iodine complex. 10mL were then placed in 2 test tubes. test tube one was heated to 80 degrees Celsius, and the result was a color change to yellow. testtube two was cooled in an ice water bath, and no change was observed. based on this, "Discuss how and why equilibrium was affected by each stress. be specific, inlcude the exothermicity/endothermicity of the rxn as it applies.

Answer 1

The basic reaction when you have the starch-iodine complex is as follows:

Starch + Iodine   Starch-Iodine Complex (the double arrow indicates that the reaction can go both ways)

If you ever had done this experiment you will know that the complex at natural state has a blue-black color indicating that it has amylose in the mixture. This picture shows how the two molecules are related in the complex:

Basically the amylose "hugs" the iodine. When we increase the temperature our reaction reverse like this:

Starch-Iodine Complex Starch + Iodine

Why? Because as you add heat to the complex you are increasing its internal energy, basically it´s an endothermic reaction, when the starch and the iodine gets separated the mixture turns yellow. So the true reaction goes like this

Starch-Iodine Complex + Heat (Increase in temperature) Starch + Iodine (if the heat is in the reactants is an endothermic rection)

And when you cool down your reaction you are not giving any heat to the mixture so, the complex does not suffer any change and the mixture stays without any visible change of colors.

EQUILIBRIUM: As it states in the Van 't Hoff equation, the equilibrium (constant) can be changed with a change in the temperature. So as you add temperature to your mixture your equilibrium states that the reaction has to follow as the last reaction we give to you, if your temperature goes down it has to go in the opposite way.