Aqueous Reactions and Solution Stoichiometry

Chapter 4 BLB 12th

4.1 General Properties of Aqueous Solutions

Solution – homogeneous mixture (ch. 13) Solvent – dissolving medium; aq-water Solute – dissolved substance

Electrolytic Properties Electrolyte – a substance whose aqueous

solution contains ions; conducts electricity Nonelectrolyte – substance that does not

form ions in solution

Aqueous Solutions, cont.Ionic Compounds in Water

Ionic solids dissociate (or ionize) into ions as they dissolve.

hydration – process of dissolving an ionic substance in water

solvation – dissolving in any solvent; dissolution

Why? Water is polar.

Polarity of molecules

Electrons are shared unequally.

Results in partial charges (δ), and a…

Dipole moment

Hydration of NaCl(s)

How many ions does an ionic compound produce when it dissociates?

KCl,

MgCl2, or

K2SO4?

Aqueous Solutions, cont.Molecular Compounds in Water

nonelectrolytes – contain only molecules (no ions); do not dissociate; do not conduct electricity; may dissolve in water

Most molecular compounds are nonelectrolytes.

Some may have strong interaction with water (alcohols).

Some may dissociate (acids).

Methanol (CH3OH) in water

Aqueous Solutions, cont.Strong and weak electrolytes – depend on

the extent of dissociation Strong – completely dissociate into ions

- all water-soluble ionic compounds, strong acids & bases

Weak – remain mostly as neutral molecules and produce very few ions; establish chemical equilibrium- weak acids (like acetic acid) and weak bases (like amines); water

Note: strong doesn’t mean soluble and vv.

4.2 Precipitation Reactions

Marked by the formation of an insoluble product (precipitate)

Solubility – amount of solute that can be dissolved in a given amount of solvent at a certain temperature; g/100g or g/L or mol/L

Insoluble – solubility < 0.01 mol/L Solubility Rules – Table 4.1, p. 121 Metathesis (or exchange) reactions

Note: All common compounds of Group I metals and NH4+ are soluble in water.

Metathesis (or exchange) reactionsMolecular:BaCl2(aq) + Na2SO4(aq) → 2 NaCl(aq) + BaSO4(s)

Complete ionic:

Net ionic:

Metathesis (or exchange) reactionsMolecular:NaI(aq) + Pb(NO3)2(aq) →

Complete ionic:

Net ionic:

Metathesis (or exchange) reactionsMolecular:NaOH(aq) + Co(NO3)2(aq) →

Complete ionic:

Net ionic:

4.3 Acids, Bases, and Neutralization Reactions

Involve H+

Acid – H+ donorBase – H+ acceptor

Neutralization:acid + base → salt + waterHCl + NaOH → NaCl + H2O

4.3 Acids, Bases, and Neutralization ReactionsStrong and Weak

Strong – completely dissociate Weak – only partially ionize Neutralization:

acid + base → salt + waterHCl + NaOH → NaCl + H2O

4.4 Oxidation-Reduction (Redox) Reactions Involve transfer of e¯ Oxidation – loss of e¯

Reduction – gain of e¯ Oxidation number – a “charge” assigned

to an atom to keep track of electrons transferred during redox

Displacement reaction – ion in solution is replaced through oxidation of an element.

4.5 Concentrations of Solutions Molarity (M) – mole solute/L solution

M • V = mol Dilution – adding solvent to decrease

concentrationM1V1 = M2V2

mol1 = mol2; only volume changes

Calculate the concentration (in M) if 2.50 g (NH4)2SO4 is dissolved in enough water to form 250 mL of solution.

How many grams of K2Cr2O7 are needed to make 50.0 mL of 0.850 M solution?

Ion Concentration: 0.850 M K2Cr2O7

Concentration (M) of Cr2O72-?

Concentration (M) of K+?

What volume (in mL) of 6.0 M HNO3 is needed to make 250 mL of 1.0 M HNO3?

4.6 Solution Stoichiometry and Chemical Analysis

Use M and volume to obtain moles Titration – process used to determine the

concentration of a solution (p. 145 ff) Standard solution – one of precisely known

concentration Analyte – solution of unknown concentration

Stoichiometry Overview, p. 144