Section 8.1 Water and Chemical Reactions The reactions that are most important to us happen in...

Section 8.1Water and Chemical ReactionsThe reactions that are most important to

us happen in water. Ex.: Metabolic reactions happen in a watery

environment (inside or outside cells) Ex.: Oxygen dissolves in blood (mostly

water) and binds to hemoglobin on red blood cells so it can be distributed to body cells; oxygen is used to release energy from food in a series of reactions.

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What are the driving forces that cause chemical reactions to happen?

Several “tendencies” drive reactants to form products:

Formation of a solid Formation of water Transfer of electrons Formation of a gas

Formation of a Solid:Precipitation Reaction A solid forms new substance

formed chemical reaction has happened

A common form of precipitation is the formation of a solid ionic compound from a mixture of two aqueous compounds

The solubility of the new compound determines whether a precipitate forms

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Formation of solid AgCl (precipitation)Formation of solid AgCl (precipitation)from a mixture of KCl and AgNOfrom a mixture of KCl and AgNO33

Solid/powder = precipitate


1. Ions from an ionic compound dissociate (separate) when mixed in water.2. A new combination of ions (cation and anion) may form.3. If the new compound is insoluble in water, it will precipitate

as a solid.

Possible Ion Combinations (not in notes):Possible Ion Combinations (not in notes):


Cl- NO3-

K+ KCl KNO3

Ag+ AgCl AgNO3

Reactants:Reactants:KCl and KCl and AgNO3

Possible new compounds:Possible new compounds:AgCl and KNO3

Which of these is insoluble in water?


Using Using solubility solubility

rules:rules:

Solubilities Solubilities of common of common compoundscompounds

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Predicting the Products of a Precipitation Reaction

1. Write proper, neutral formulas -- consider the products that could form by exchanging the ions of the added salts;

2. Balance the equation using coefficients;

3. Add physical states -- use the solubility rules to decide whether a solid forms and predict its identity.

Equations Used to Describe a Precipitation Reaction

Ex.: Precipitation of Silver Chloride

Molecular Equation – shows overall reaction with molecular formulas:

AgNO3(aq) + KCl(aq) AgCl(s) + KNO3(aq)

Complete Ionic Equation – shows strong electrolytes (aq) as ions:

Ag+(aq) + NO3

- (aq) + K+

(aq) + Cl- (aq) AgCl(s) + K+ (aq) + NO3

-(aq)

Net Ionic Equation – shows only the components that form the precipitate; spectator ions are not shown:

Ag+(aq) + Cl- (aq) AgCl(s)

Section 8.1 Water and Chemical Reactions The reactions that are most important to us happen in...

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