Molecular View of Reactions

in Aqueous Solutions

CHAPTER 5

Chemistry: The Molecular Nature of Matter, 6th editionBy Jesperson, Brady, & Hyslop

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CHAPTER 5: Molecular View of Reactions

Learning Objectives Define a solution, solute, solvent Solubility Rules Concentration

Dilution Solution Stoichiometry Writing chemical equations for reactions in

solutions Definition/basics of solution chemistry of Acids

& Bases Titrations

Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

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Solutions Definition

• For reaction to occur

– Reactants needs to come into physical contact

• Happens best in gas or liquid phase

– Movement occurs

Solution• Homogeneous mixture

– Two or more components mix freely

– Molecules or ions completely intermingled

– Contains at least two substances

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Solutions Examples of Solutions

Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

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Solutions Solutes and Solvents

Solvent• Medium that dissolves solutes

– Component present in largest amount – Can be gas, liquid, or solid– Aqueous solution—water is solvent

Solute• Substance dissolved in solvent

– Solution is named by solute– Can be gas—CO2 in soda– Liquid—ethylene glycol in antifreeze– Solid—sugar in syrup

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Solutions Electrolytes

In Water…• Strong electrolytes produce ions and conduct an

electric current. • Weak electrolytes produce a few ions. • Nonelectrolytes do not produce ions.

Brady & Senese 5th Ed

Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

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Solutions Acids & Bases

• Arrhenius Theory– Acids produce hydronium when dissolved in water– Bases produce hydroxide when dissolved in water

• Bronsted-Lowry Theory– Acids are proton (H+) donors. – Bases are proton (H+) acceptors.

Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

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Solutions Acids & Bases

• Neutralization: metathesis reaction in which acid + metal hydroxide or metal oxide forms water and salt – NaOH(aq) + HCl(aq) →H2O(l) + NaCl(aq)

• Acid-base reaction: reaction of weak base and acid transferring a H+ ion, driven by the formation of a weaker acid.– HCl(aq) + NH3(aq) →NH4Cl(aq)

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Solutions Writing Solution Chemical Equations

• hydrated ions, with the symbol (aq), are written separately

• Na2SO4(s) → 2Na+(aq) + SO4

2-(aq)

• you might encounter the equation as: • Na2SO4(s) → 2Na+ + SO4

2-

– Accepted because only 2 states allow for dissociated ions (plasma and aqueous). Aqueous is far more common

– It is vague and not preferred

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Solutions Writing Solution Chemical Equations

• Molecular equation: – Balanced, shows states, all substances electrically neutral– AgNO3(aq) + KCl(aq) →AgCl(s) + KNO3(aq)

• Ionic equation:– Balanced, shows states, shows strong electrolytes as

dissociated ions, net charges balance– Ag+

(aq) + NO3-(aq) + K+

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

-(aq)

• Net ionic equation:– Balanced, shows states, eliminates spectator ions from

the ionic equation, net charges balance– Ag+

(aq) + Cl-(aq) →AgCl(s)

Brady & Senese 5th Ed

Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

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Solutions Criteria for Balancing Ionic & Net Ionic Equations

Material Balance• There must be the same number of atoms of

each kind on both sides of the arrow

Electrical Balance• The net electrical charge on the left must equal

the net electrical charge on the right

• Charge does not have to be zero

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Solutions Solubility

• saturated –no more solute can be dissolved at the current temperature in the given amount of solvent

• solubility - the amount of solute that can dissolve in the specified amount of solvent at a given temperature (usually g solute/ 100 g solvent or moles solute/L solution)

• unsaturated - contains less solute than the solubility allows

• supersaturated- contains more solute than solubility predicts

Brady & Senese 5th EdJesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

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Solutions Solubility & Temperature

Brady & Senese 5th Ed

Solubility:•Depends on temperature.

•Of most solids increases as temperature increases.

•Of gases decreases as temperature increases.

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Solutions Solubility & Pressure

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•Henry’s Law states •The solubility of a gas in a liquid is directly related to the pressure of that gas above the liquid.

•At higher pressures, more gas molecules dissolve in the liquid.

Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

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Solutions Concentration

• In solutions, solutes are dispersed in a larger volume• Molarity expresses the relationship between the moles

of solute and the volume of the solution• Molarity (M)=moles solute/L solution

– Hence, a 6.0M solution of HCl contains 6.0 mole HCl in a liter of solution

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Solutions Dilution

Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

• Can take solution of higher concentration and dilute it to a lower concentration.

• Amount of MOLES does NOT change Remains the same

Small Volume

Concentrated Solution

LargeVolume Dilute

Solution

Add solvent

• moles of solute do not change, hence CstockVstock= CnewVnew• C=concentration• V=volume

Vdil Mdil = Vconc MconcBrady & Senese 5th Ed

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Solutions Solubility Rules

A general idea as to whether a fair amount of solid will dissolve is achieved using solubility rules

1. All compounds of the alkali metals (Group IA)

2. All salts containing NH4+, NO3

−, ClO4−, ClO3

−, and C2H3O2−

3. All chlorides, bromides, and iodides (salts containing Cl−, Br−, or I−) except when combined with Ag+, Pb2+, and Hg2

2+

4. All sulfates (salts containing SO42−) except those of Pb2+,

Ca2+, Sr2+, Hg22+, and Ba2+

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Solutions Solubility Rules

A general idea as to whether a fair amount of solid will dissolve is achieved using solubility rules

5. All metal hydroxides (ionic compounds containing OH−) and all metal oxides (ionic compounds containing O2−) are insoluble except those of Group IA and Group IIA

• When metal oxides dissolve, they react with water to form hydroxides. The oxide ion, O2−, does not exist in water. For example, Na2O(s) +H2O(l)

→ 2NaOH(aq)

6. All salts that contain PO43−, CO3

2−, SO32−, and S2−

are insoluble, except those of Group IA and NH4+.

Brady & Senese 5th Ed

Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

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Solutions Solution Stoichiometry

• Often work with solutions when conducting reactions• How do we determine amounts needed to completely react

one compound?• Like any other stoichiometry problem• Now use volume and molarity to obtain moles of each

substance.• Sometimes we need to know concentrations of ions• Important for net ionic reaction stoichiometry• Molar concentration of particular ion equals molar

concentration of salt multiplied by number of ions of that kind in one formula unit of salt.

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Solutions Titrations

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• Is the controlled addition of one reactant (titrant) to a known quantity of another reactant of unknown concentration (analyte) until the reaction is complete

• Often, an indicator is used to signal the reaction completion

• Endpoint: the volume of titrant required to complete the reaction

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Solutions Titrations

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