CHROMATE – DICHROMATE EQUILIBRIUM• Addition of NaOH and HCl

• Addition of Barium Nitrate

pieferrer/Chem3.SY2013-2014

THE CHROMATE - DICHROMATE EQUILIBRIUM

• When solid potassium chromate, K2CrO4 is dissolved in water it forms a yellow solution. When solid potassium dichromate, K2Cr2O7 is dissolved in water the resulting solution is orange.

• The colors come from the negative ions: CrO4 2-(aq) and Cr2O7 2-(aq).

• However, in solution these ions are actually in equilibrium as indicated by the equation:

2 CrO4 2-(aq) + 2 H+ (aq) ⇌ Cr2O7 2-(aq) + H2O (l)

• In a solution we call a chromate solution, there is also a little bit of dichromate, but the predominant color will be yellow.

2 CrO42-(aq) + 2 H+ (aq)   Cr2O7

2-(aq) + H2O (l)

• In a solution we call a dichromate solution, there is also a little bit of chromate, but the predominant color will be orange.

2 CrO42-(aq) + 2 H+ (aq)     Cr2O7

2-(aq) + H2O (l)

ADDITION OF HCL• Hydrochloric acid is a source of H+ ions. If there was no chemical reaction

then adding HCl should increase the amount of H+(aq).

If we added H+ to the chromate solution and there was no chemical reaction:

2 CrO4 2-(aq) + 2 H+ (aq) ⇌ Cr2O7 2-(aq) + H2O (l) would become ?

If we added H+ to the dichromate solution and there was no chemical reaction:

2 CrO4 2-(aq) + 2 H+ (aq) ⇌ Cr2O7 2-(aq) + H2O (l) would become?

• Hydrochloric acid is a source of H+ ions. If there was no chemical reaction then adding HCl should increase the amount of H+(aq).

If we added H+ to the chromate solution and there was no chemical reaction:

2 CrO4 2-(aq) + 2 H+ (aq) ⇌ Cr2O7 2-(aq) + H2O (l)

If we added H+ to the dichromate solution and there was no chemical reaction:

2 CrO4 2-(aq) + 2 H+ (aq) ⇌ Cr2O7 2-(aq) + H2O (l)

Chromate + HCl

• From Le Chatelier’s principle, we know the reaction will try to remove some of the H+  we have added.  Some of the added H+ reacts with the CrO4

2-, removing some of each, and making more Cr2O72-. 

2 CrO4 2-(aq) + 2 H+ (aq) ⇌ Cr2O7 2-(aq)

+ H2O (l)

Dichromate + HCl

2 CrO4 2-(aq) + 2 H+ (aq) ⇌ Cr2O7 2-(aq)

+ H2O (l)

ADDITION OF NAOH• One of the simplest ways to think about the effect of adding sodium

hydroxide is to realize that it removes H+ ions because of this reaction: H+ (aq) + OH-(aq) H2O (l)

•  Sodium hydroxide removes H+ ions.  If there was no chemical reaction then adding NaOH should decrease the amount of H+(aq).

• If we removed H+ from the chromate solution and there was no chemical reaction:

It would become:

2 CrO4 2-(aq) + 2 H+ (aq) ⇌ Cr2O7 2-(aq) + H2O (l)

2 CrO4 2-(aq) + 2 H+ (aq) ⇌ Cr2O7 2-(aq) + H2O (l)

• If we removed  H+ from the dichromate solution and there was no chemical reaction:

It would become:

2 CrO4 2-(aq) + 2 H+ (aq) ⇌ Cr2O7 2-(aq) + H2O (l)

2 CrO4 2-(aq) + 2 H+ (aq) ⇌ Cr2O7 2-(aq) + H2O (l)

• From Le Chatelier's principle, we know the reaction will try to restore some of the H+  we have removed.  The required H+ comes from turning some of the Cr2O7

2- back into the CrO42-. 

Chromate solution: Dichromate solution:

ADDITION OF BARIUM NITRATE

• The NO3- part of the barium nitrate has no effect in this reaction -- it is just

a spectator ion.  However, the Ba2+ can react with the CrO42- to form a

solid precipitate of BaCrO4.  The solubility of BaCr2O7 is much higher, and it does not form a precipitate.

• The primary role of the Ba2+ ions in this reaction is to show the amount of CrO4

2- present.  The more CrO42- there is, the greater the quantity of

precipitate that will be formed.

When Ba2+ ions are added to a chromate solution, a lot of BaCrO4 precipitate forms:

When Ba2+ ions are added to a dichromate solution, a small amount of BaCrO4 precipitate forms:

• Most of the Ba2+ ions remain  unreacted

Addition of acid to the dichromate solution

• Because the reaction of the H+ ions will cause the small amount of CrO4

2- ions present to become even smaller, the small amount of precipitate will dissolve.  There are no longer enough CrO4

2- ions for the Ba2+ to react with.   

Addition of sodium hydroxide to the dichromate solution

• Because the reaction of the OH- ions will remove some of the H+, this will cause a large amount of CrO4

2- ions to form in the solution.  Much more precipitate will form since there are now enough CrO4

2- ions for theBa2+ to react with.  

ADDITION OF ACID AND BASE

Predict the effect of the addition of acid and base to the chromate – dichromate equilibrium.

SUMMARY

PART I. COLOR

Chromate + HCl light orange

Dichromate + HCl orange

Chromate + NaOH yellow

Dichromate + NaOH Yellow/dark yellow

Chromate + HCl + NaOH yellow

Dichromate +HCl +NaOH yellow orange

Chromate + NaOH + HCl Light orange

Dichromate +NaOH + HCl orange

PART II COLOR

Chromate + NaOH + Barium nitrate (A) Yellow with precipitate

Dichromate + HCl + Barium nitrate (B) Orange to dark orang w/o ppt

A + HCl Orange to dark w/o ppt

B + NaOH Yellow with ppt

Dichromate + Barium nitrate Yellow with ppt

Chromate + Barium nitrate Orange with few ppt

• Le Chatelier's Principle states that if a system is at equilibrium and something is changed so that it is no longer at equilibrium, the system will respond in an effort to counteract that change.

• If more reactant is added, the equilibrium will shift forward in order to consume some of the extra reactant (since there are more ions available for reaction), resulting in more product.

• If some of the product is removed from the system, the equilibrium will shift forward to produce more of that product. The equilibrium can be shifted reverse by either adding product to or removing reactant from the system.

• Yellow chromate ion and orange dichromate ion are in equilibrium with each other in aqueous solution.

• The more acidic the solution, the more the equilibrium is shifted to favour the dichromate ion. As hydrochloric acid is added to the potassium chromate solution, the yellow colour turns to orange. When sodium hydroxide is added to the potassium chromate solution, the orange colour turns back to yellow. The sodium hydroxide reacts with hydrogen ions, removing them from the solution. When one reactant is removed from an equilibrium system, the equilibrium shifts reverse, in this case forming the yellow chromate ion again.

• Barium reacts with chromate ions (but not dichromate ions) to form an insoluble salt. Solid barium chromate can be converted to soluble barium dichromate by adding a source of H+ ions to the solution. This reaction can be reversed again by adding OH- ions, which decreases the H+

concentration, thus favouring barium chromate.

• First Quarter Coverage

The Concept of EquilibriumThe Equilibrium Constant

Calculating Equilibrium ConstantsMagnitude of the Equilibrium Constant

Equilibrium ExpressionsHeterogeneous EquilibriaManipulating Equilibrium Expressions

Using Equilibrium Expressions to Solve ProblemsPredicting the Direction of a ReactionCalculating Equilibrium Concentrations

Factors that Affect Chemical EquilibriumAddition or Removal of a SubstanceChanges in Volume and PressureChanges in Temperature