Question

Another interesting equilibrium system is that between the polyatomic ions chromate, CrO₄^2-, and dichromate, Cr₂O₇^2-. In an acidic environment (high H₃O⁺), the chromate will react to form dichromate ion. Barium will react with chromate, but NOT dichromate, to form an insoluble precipitate.

2 CrO₄^2- (aq, yellow) + 2 H₃O⁺ (aq) <---> Cr₂O₇^2- (aq, orange) + 2 H₂O (l)

Describe and explain how the chemical equilibrium will shift and what changes you would observe under the following scenarios:

a) Addition of chromate

b) Addition of dichromate

c) Addition of HCL

d) Addition of NaOH

e) Addition of BaCL₂ (aq)

f) Increase in pressure

g) Increase in volume

h) Addition of catalyst

i) Addition of NaNO₃ (aq)

j) Increase in temperature

Answer 1

Answer:

at equilibrium system the reaction between chromate CrO42- (aq) ions shows yellow i.e more yellow indicates high chromate is present and the reaction between dichromate CrO72-  (aq) ions shows orange colour i.e more orange indicates more dichromate is present . It is so happened (colour change) due to presence of negative ions. In solutions these ions are in equilibrium as shown below 2CrO42- (aq) + 2H+ (aq) CrO72- (aq) + H2O (l) Here there is no change in chemical reaction, but ionic concentration (H+ ions ) is different

Let us observe how the chemical equilibrium shifting occurs and what changes occurs by following stages

a) addition of chromate

here the chromate (yellow) and dichromate (orange) are at equilibrium in aq. state,

the more acidic solution, more the euilibrium shift to dichromate ion, so colour will become orange

b) addition of dichromate

here the dichromate (orange) and chromate (yellow) are in equilibrium in aq. state,

the more acidic solution, more the euilibrium shift towards dichromate ion, so colour will remains orange

c) Addition of HCL:

here HCL is having H⁺ ions. so the addition of HCL is offers increasing the H⁺ (aq) in the reaction, H⁺ (aq) ions converts chromate ion to dichromate ion, so the chromate colour will get change to paler colour, because there is producing of water in the reaction. i.e yellow shift to orange.

CrO₄^2- (aq) + 2H⁺Cl(aq) CrO₃^2- (aq) + H₂O (l) + 2Cl

i.e more acidic the solution equilibrium shift towards left dichromate

d) Addition of NaOH

here NaOH take away H⁺ ions (due to acid-base neutralization), by that way reducing H⁺ (aq) in the reaction or OH^- inceases, more H⁺ (aq) ions converts dichromate ion to chromate ion,so the dichromate colour willnget shift from orange to yellow.

(CrO₄^2-)₂ + 2NaOH (OH)₂ + 2NaCrO₄^2-

i.e the system try to reduce shift but solution equilibrium effect right by taking chromate ion out of equilibrium.

e) Addition of BaCL₂ (aq.)

as barium reacts with chromate but not with dichromate

by increasing barium chloride concentration the chromate precipitates more i.e yellow colour get more yellowish, dichromate precipitate inceases by forming insoluble salt, & doesn't change orange colour.

so shift the equilibrium to right, by taking chromate ion out of equilibrium.

f) Increase in pressure:

the equilibrium will get shift towards right, if pressure increases i.e due to less gas in the reaction cause colourless

g) Increase in volume:

the equilibrium will get shift towards right, if volume increases, i.e concentration of H₂O increases the chromate ions concentration also increases in the solution, so the colour get change to yellow.

h) Addition of catalyst:

catalysts are involved in speedup the reactions either reverse or forward, i.e just increase in equilibrium attainment.

So catalyst doesn't shift an equilibrium.

i) Addition of NaNO₃:

as sodium reacts with chromate but not with dichromate

by increasing NaNO₃ concentration the chromate precipitates more i.e yellow colour get more yellowish, dichromate precipitate inceases by forming insoluble salt, & doesn't change orange colour.

j) Increase in temperature:

the equilibrium will get shift towards left, if temperature increases, i.e chromate ion converts to dichromate ion,

so increase in temperature will shift equilibrium in the energy absorbed direction i.e called endothermic