Precipitation and Equilibra Reactions

8/12/2019 Precipitation and Equilibra Reactions

1/5

Precipitation reactions

A precipitationreaction is when a solid is produced from a reaction of two soluble compounds, resulting in the

production of an insoluble solid which is suspended in the solution. Precipitation occurs because not all ionic

compounds are soluble and that results in an insoluble compound being precipitated, if the separate ions are added

together in solution. It is assumed that if the precipitation reaction goes to completion that either one or both of the

ions involved will be completely precipitated.

Determining solubility

Writing ionic equations

Often chemical equations involve ions, rather than neutral molecules, which require ionicequationsto be properly

represented. Precipitation reactions are best represented using ionic equations.

Some disadvantages of using a neutral species equation to represent precipitation reactions include:

The neutral species equation implies that before the reaction take place the ions are in some way stuck together,

i.e. bonded together. However, what is actually happening is the ions exist separately in water.

The equation implies that two of the ions (the spectator ions) have changed in some way during the reaction.

However, these ions are called spectator ions because they do not take part in the chemical reaction. They still

exist as ions dissolved in the solution.

The best way to represent precipitation reactions is to:

1. Ions are written separately when they are in solution

2. Spectator ions which, although they may be present, do not take part in the reaction are not included

(these are called spectator ions)

3. Numbers of atoms and charge must be balanced.

With a precipitation reaction it is important to always use state symbols, as this indicates which compound is the

reci itate, and which com ound still exists dissolved in the solution as ions.

Soluble compounds:

All nitrates

All acetates

All sulfates (except those of calcium, barium, lead, mercury and silver).

All chlorides, bromides and iodides (except those of lead, mercury and silver)

All ammonium compounds

All sodium, potassium (and the rest of Group 1) compounds.

Insoluble compounds:

All phosphates, sulfites, carbonates (except those of Group 1 elements and ammonium)

All oxides and hydroxides (except those of Group 1 elements, calcium, magnesium, barium and ammonium)


2/5

Precipitation and ion interactions

Dissolution with insoluble compounds

A saturated solution is a solution where no more solute will dissolve into the solvent at existing conditions. If more

solute tries to be dissolved in results in the presence of undissolved solid. Saturation, the limit of dissolving the

solute,

is reached earlier with an insoluble compound than a soluble compound.

When an insoluble ionic compound is added to a solvent such as water, a small amount of the crystal lattice will break

into free ions. When the solution reaches saturation, no more of the compound will dissolve and some of the ionic

compound remains undissolved in the solution.

Ion movement in precipitation

Insoluble compounds will precipitate from solutions. Precipitation will occur when the quantity of insoluble ions

exceeds the solubility of the compound formed from these ions.

Precipitation is essentially the opposite process to dissolution (dissolving). In a saturated solution, the ions in the solid

are continually leaving the crystal to go into the solution, while at the same time, and at the same rate, different ions in

the solution precipitate out, resulting in neither reaction going to completion.

Equilibrium

In saturated solutions, a situation called equilibrium occurs when precipitation and dissolution occurs at the same time

and at the same rate resulting in there being no apparent activity. During this situation, the total concentration of ions

and the amount of precipitate will remain constant.

In order to set up such a situation, the containing vessel must be sealed to prevent evaporation of the solvent and the

temperature needs to be kept constant.

Concentration

Concentrationis defined as the amount of solute in a specific amount of solvent. This is expressed in many ways,

because:

Concentration is used by people other than chemists, who may be unfamiliar with the concept of molarity.

In commerce and industry, the measure of concentration emphasises the mass or volume of the solute, rather than

the moles.

In areas such as environmental studies or drug testing, the concentration of the solute is very small, so molarity is

impractical, and thus concentration is used instead. The concentration used here is usually parts per million (ppm)

Some common ways of expressing concentration are:

Mass of solute per volume of solvent or solution.

Volume of solute per volume of solvent or solution.

Mass per volume as a percentage %(w/v)

Volume per volume as a percentage %(v/v)

Mass per mass as a percentage %(w/w)

Equilibriumis a dynamic situation in a closed system, where there is continual interchange between reactants and

products on an atomic level, with no noticeable change in observation or physical properties.

Equilibrium Equations:

Double arrows () are used to show the dynamic equation of the process.


3/5

Parts per million (ppm), mass in milligrams per kilogram of solution


4/5

Molarity

The concentration of a solution in moles per litre indicates the number of moles of solute dissolved in a litre of

solution. This form of concentration is the most commonly used in chemistry, as it allows from the measurement of a

definite volume of solution of known concentration, the number of moles of solvent present in the solution.

The concentration of a solution in moles per litre is referred to as molarconcentrationor molarity. It is sometimes

given the symbol M. (A 0.50 mol L1

solution is the same as a 0.50 M solution)

Preparing and diluting solutions

Preparing solutions of known concentration

A weighed mass of substance is dissolved in water and the solution made up to a definite volume in a volumetric

flask.

The beaker is washed out to ensure that all the solute is transferred to the volumetric flask. If this is not done

properly, then the concentration will be different. The volumetric flask needs to be filled to the marked level. If this

is not done the concentration will be different and also harder to calculate.

The expected solution can be calculated using the formula for calculating concentration from moles.

Diluting solutions

First, a solution of known concentration is prepared.

A sample (determined by the needed dilution) is then taken and transferred to another volumetric flask, which also

needs to be filled up to the marked level

Calculations with solutions

c =n

V

Calculating molarity:

where C is the concentration in mol L1

(moles per litre), n is number of moles and V is volume in litres

Calculating dilution:

C1V1 = C2V2

where C1is the original concentration and C2is the final concentration in mol L1

(moles per litre), V1is the original

volume and V is the final volume in litres.


5/5

Documents

Precipitation and Equilibra Reactions