Answer to Question #172279 in General Chemistry for dsad

Question

Experiment: Le Chatelier’s Principle

• Do your observations support your prediction based on Le Chatelier’s Principle? Explain clearly using your color observations as evidence.
• Explain the direction equilibrium shifted for each activity in this lab. Justify why the equilibrium shifted in that direction.
• Based on the color change you observe, is the equilibrium for the solution in the ice water shifting left or right?
• Based on the color change you observe, is the equilibrium for the solution in the hot water shifting left or right?
• Do your observations indicate that the reaction is exothermic or endothermic? That is, based on your observations, is heat a product or a reactant? How do you know?
• What is the net ionic equation of this reaction with heat included in the equation?
• Are equilibrium shifts reversible? Justify using evidence from the lab.

Answer 1

Let's take the reaction

In an aqueous solution, the chromate ion, CrO42-(aq), can be converted to the dichromate ion, Cr2O72-(aq), and vice versa, the dichromate ion, Cr2O72-(aq)ion can be converted to the chromate ion, CrO42-(aq) ion.

2Na₂CrO₄ + H₂SO₄ = Na₂Cr₂O₇ + Na₂SO₄ + H₂O

Exothermic reaction

chromate-dichromate (K2CrO4) equilibrium:

changes color from yellow to orange

2CrO4^2-+ 2H+↔H2O + Cr2O7^2-

dichromate-chromate (K2Cr2O7) equilibrium:

changes color from orange to yellow

Cr2O7^2-+ 2OH-↔H2O + 2CrO4^2-

 Cr2O72-ions present with a small amount of CrO42-ions.Le Chatelier’s Principle states that if a stress is applied to a reaction at equilibrium, the reaction will shift to offset the stress applied. The addition of hydrogen ions or hydroxide ions constitutes a stress, as does the removal of either ion, and so it can be expected that the above equilibria will shift in response to concentration changes in these ions. For example, in eqn. 1, if [H+] is increased by adding HCl, there are more ions available for reaction and the reaction will favour the formation of products. Conversely, if [H+] is decreased (by adding OH-) there are fewer ions available for the forward reaction and the reaction shifts reverse to favor reactant formation.