1. Home
  2. Chemistry
  3. Equilibria of other reaction
  4. Precipitation and dissolution

15.1 Precipitation and dissolution  (Page 5/17)

<< Chapter < Page
  Chemistry     Page 5 / 17
Chapter >> Page >

Predicting precipitation

The equation that describes the equilibrium between solid calcium carbonate and its solvated ions is:

CaCO 3 ( s ) ⇌ Ca 2+ ( a q ) + CO 3 2− ( a q )

We can establish this equilibrium either by adding solid calcium carbonate to water or by mixing a solution that contains calcium ions with a solution that contains carbonate ions. If we add calcium carbonate to water, the solid will dissolve until the concentrations are such that the value of the reaction quotient ( Q = [ Ca 2 + ] [ CO 3 2− ] ) is equal to the solubility product ( K sp = 8.7 × 10 –9 ). If we mix a solution of calcium nitrate, which contains Ca 2+ ions, with a solution of sodium carbonate, which contains CO 3 2− ions, the slightly soluble ionic solid CaCO 3 will precipitate, provided that the concentrations of Ca 2+ and CO 3 2− ions are such that Q is greater than K sp for the mixture. The reaction shifts to the left and the concentrations of the ions are reduced by formation of the solid until the value of Q equals K sp . A saturated solution in equilibrium with the undissolved solid will result. If the concentrations are such that Q is less than K sp , then the solution is not saturated and no precipitate will form.

We can compare numerical values of Q with K sp to predict whether precipitation will occur, as [link] shows. (Note: Since all forms of equilibrium constants are temperature dependent, we will assume a room temperature environment going forward in this chapter unless a different temperature value is explicitly specified.)

Precipitation of mg(oh) 2

The first step in the preparation of magnesium metal is the precipitation of Mg(OH) 2 from sea water by the addition of lime, Ca(OH) 2 , a readily available inexpensive source of OH – ion:

Mg(OH) 2 ( s ) ⇌ Mg 2+ ( a q ) + 2OH − ( a q ) K sp = 8.9 × 10 − 12

The concentration of Mg 2+ ( aq ) in sea water is 0.0537 M . Will Mg(OH) 2 precipitate when enough Ca(OH) 2 is added to give a [OH – ] of 0.0010 M ?

Solution

This problem asks whether the reaction:

Mg(OH) 2 ( s ) ⇌ Mg 2+ ( a q ) + 2OH − ( a q )

shifts to the left and forms solid Mg(OH) 2 when [Mg 2+ ] = 0.0537 M and [OH – ] = 0.0010 M . The reaction shifts to the left if Q is greater than K sp . Calculation of the reaction quotient under these conditions is shown here:

Q = [ Mg 2+ ] [ OH − ] 2 = (0.0537)( 0.0010) 2 = 5.4 × 10 − 8

Because Q is greater than K sp ( Q = 5.4 × 10 –8 is larger than K sp = 8.9 × 10 –12 ), we can expect the reaction to shift to the left and form solid magnesium hydroxide. Mg(OH) 2 ( s ) forms until the concentrations of magnesium ion and hydroxide ion are reduced sufficiently so that the value of Q is equal to K sp .

Check your learning

Use the solubility product in Appendix J to determine whether CaHPO 4 will precipitate from a solution with [Ca 2+ ] = 0.0001 M and [ HPO 4 2− ] = 0.001 M .

Answer:

No precipitation of CaHPO 4 ; Q = 1 × 10 –7 , which is less than K sp

Got questions? Get instant answers now!

Precipitation of agcl upon mixing solutions

Does silver chloride precipitate when equal volumes of a 2.0 × 10 –4 - M solution of AgNO 3 and a 2.0 × 10 –4 - M solution of NaCl are mixed?

(Note: The solution also contains Na + and NO 3 − ions, but when referring to solubility rules, one can see that sodium nitrate is very soluble and cannot form a precipitate.)

Solution

The equation for the equilibrium between solid silver chloride, silver ion, and chloride ion is:

AgCl ( s ) ⇌ Ag + ( a q ) + Cl − ( a q )

The solubility product is 1.6 × 10 –10 (see Appendix J ).

AgCl will precipitate if the reaction quotient calculated from the concentrations in the mixture of AgNO 3 and NaCl is greater than K sp . The volume doubles when we mix equal volumes of AgNO 3 and NaCl solutions, so each concentration is reduced to half its initial value. Consequently, immediately upon mixing, [Ag + ] and [Cl – ] are both equal to:

1 2 ( 2.0 × 10 − 4 ) M = 1.0 × 10 − 4 M

The reaction quotient, Q , is momentarily greater than K sp for AgCl, so a supersaturated solution is formed:

Q = [ Ag + ] [ Cl − ] = (1.0 × 10 − 4 ) ( 1.0 × 10 − 4 ) = 1.0 × 10 − 8 > K sp

Since supersaturated solutions are unstable, AgCl will precipitate from the mixture until the solution returns to equilibrium, with Q equal to K sp .

Check your learning

Will KClO 4 precipitate when 20 mL of a 0.050- M solution of K + is added to 80 mL of a 0.50- M solution of ClO 4 − ? (Remember to calculate the new concentration of each ion after mixing the solutions before plugging into the reaction quotient expression.)

Answer:

No, Q = 4.0 × 10 –3 , which is less than K sp = 1.05 × 10 –2

Got questions? Get instant answers now!

Questions & Answers

What is inflation
Bright Reply
a general and ongoing rise in the level of prices in an economy
AI-Robot
What are the factors that affect demand for a commodity
Florence Reply
differentiate between demand and supply giving examples
Lambiv Reply
differentiated between demand and supply using examples
Lambiv
what is labour ?
Lambiv
how will I do?
Venny Reply
how is the graph works?I don't fully understand
Rezat Reply
information
Eliyee
devaluation
Eliyee
t
WARKISA
hi guys good evening to all
Lambiv
multiple choice question
Aster Reply
appreciation
Eliyee
explain perfect market
Lindiwe Reply
In economics, a perfect market refers to a theoretical construct where all participants have perfect information, goods are homogenous, there are no barriers to entry or exit, and prices are determined solely by supply and demand. It's an idealized model used for analysis,
Ezea
What is ceteris paribus?
Shukri Reply
other things being equal
AI-Robot
When MP₁ becomes negative, TP start to decline. Extuples Suppose that the short-run production function of certain cut-flower firm is given by: Q=4KL-0.6K2 - 0.112 • Where is quantity of cut flower produced, I is labour input and K is fixed capital input (K-5). Determine the average product of lab
Kelo
Extuples Suppose that the short-run production function of certain cut-flower firm is given by: Q=4KL-0.6K2 - 0.112 • Where is quantity of cut flower produced, I is labour input and K is fixed capital input (K-5). Determine the average product of labour (APL) and marginal product of labour (MPL)
Kelo
yes,thank you
Shukri
Can I ask you other question?
Shukri
what is monopoly mean?
Habtamu Reply
What is different between quantity demand and demand?
Shukri Reply
Quantity demanded refers to the specific amount of a good or service that consumers are willing and able to purchase at a give price and within a specific time period. Demand, on the other hand, is a broader concept that encompasses the entire relationship between price and quantity demanded
Ezea
ok
Shukri
how do you save a country economic situation when it's falling apart
Lilia Reply
what is the difference between economic growth and development
Fiker Reply
Economic growth as an increase in the production and consumption of goods and services within an economy.but Economic development as a broader concept that encompasses not only economic growth but also social & human well being.
Shukri
production function means
Jabir
What do you think is more important to focus on when considering inequality ?
Abdisa Reply
any question about economics?
Awais Reply
sir...I just want to ask one question... Define the term contract curve? if you are free please help me to find this answer 🙏
Asui
it is a curve that we get after connecting the pareto optimal combinations of two consumers after their mutually beneficial trade offs
Awais
thank you so much 👍 sir
Asui
In economics, the contract curve refers to the set of points in an Edgeworth box diagram where both parties involved in a trade cannot be made better off without making one of them worse off. It represents the Pareto efficient allocations of goods between two individuals or entities, where neither p
Cornelius
In economics, the contract curve refers to the set of points in an Edgeworth box diagram where both parties involved in a trade cannot be made better off without making one of them worse off. It represents the Pareto efficient allocations of goods between two individuals or entities,
Cornelius
Suppose a consumer consuming two commodities X and Y has The following utility function u=X0.4 Y0.6. If the price of the X and Y are 2 and 3 respectively and income Constraint is birr 50. A,Calculate quantities of x and y which maximize utility. B,Calculate value of Lagrange multiplier. C,Calculate quantities of X and Y consumed with a given price. D,alculate optimum level of output .
Feyisa Reply
Answer
Feyisa
c
Jabir
Got questions? Join the online conversation and get instant answers!
Jobilize.com Reply
<< Chapter < Page Page > Chapter >>
Practice Key Terms 4

Read also:

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now




Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Chemistry' conversation and receive update notifications?

Ask