8/3/2019 Chemistry 101 Chapter 5 Notes

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Chemistry 101 Chapter 5 Notes: Introduction to Reactions inAqueous Solutions

y Aqueous Solution: Ionic or molecular compounds dissolved in watero Ionic compounds dissociate into ionso Molecular compounds polarize

y Electrolytes: Substances that conduct electricity when dissolved in watery Ions: Charged particles that carry electrons from one electrode to the other

through water, completing the circuit

y Anode: Positively charged electrode; attracts negative chargesy Cathode: Negatively charged electrode; attracts positive chargesy Strength ofElectrolytes:

o Strong Electrolytes Dissolve in water D

issociate completely into ions Ex. Soluble ionic salts, strong acids, strong bases

o Weak Electrolytes Dissolve in water Do not dissociate completely into ions Ex.Weak acids, weak bases

o Non Electrolytes Dont dissolve in or produce ions in water

o Ex. Insoluble ionic salts, molecular compounds, solidsy When writing equations to describe aqueous solutions, reactions involving

strong electrolytes should be written using a single arrow in the forwarddirection, while weak electrolyte reactions should be written using a doublearrow pointing in opposite directions.

y Precipitation Reactions: Chemical reactions that result in the formation of aninsoluble precipitate (solid). These are usually formed when combiningsolutions of two soluble salts.

y Ionic Equations: Chemical equations that describe how ionic compoundsdissolve in water and the reactions that occur when aqueous ions are mixed.

y MolecularEquation: Chemical equation in which all substances arerepresented by the formulas for the compounds even though individual ionsexist in the solution.

y Total Ionic Equation: Chemical equation in which ionic compounds are writtenas ions in solution.

y Spectator Ions: Ions that are physically present in a reaction, yet play no partin it. Can be cancelled in net ionic equations.

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y Net Ionic Equation Chemical equation showing only ions that are changedin the reaction. Spectator ions are not shown.

y Solubility: The amount of a substance that will dissolve in another substance.o Usually the second substance is water.o

Some ionic compounds do not dissolve at all in water (INSOLUB

LE

).It takes energy to break apart the compound and if not enough energy

is gained by the formation of the aqueous ions, the substance wontdissolve.

o Solubility Rules can be used to predict the extent to which an ioniccompound is soluble.

1. Salts of group 1 cations (with some exceptions for Li) and theammonium cation are soluble.

2. Nitrates, acetates, and perchlorates are soluble.3. Salts of silver, lead, and mercury(I) are insoluble.4. Chlorides, bromides, and iodides are insoluble.5. Carbonates, phosphates, sulfides, oxides, and hydroxides are

insoluble. (Sulfides of group 2 cations and hydroxides of calcium,strontium, and barium are slightly.

6. Sulfates are soluble except for those of calcium, strontium, andbarium.

y Acid: A substance that produces H+ ions when dissolved in water (Arrheniusdefinition)

o Sour tasteo

Reactive with metals and carbonateso Strong acids completely ionize in water

HCl HBr HI H2SO4 HNO3 HClO4

o Weak acids do not completely ionize in watero Actually, H+ ions do not exist in water in hydrated form the way that

other ions we have discussed do. Instead, the hydrogen ions form acovalent bond with a water molecule, and a hydronium ion is formed(H3O+).

y Base: A substance that produces OH- ions when dissolved in watero Bitter tasteo Slippery feel

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o Strong bases are ionic salts containing the OH- ion that completelyionize in water

Group 1 & 2 hydroxide compoundso Weak bases produce some OH- ions in solution

All other basesy Acid Base Reactions

o Neutralization Acid and Base yield salt and water

y Oxidation Reduction (Redox) Reaction: A chemical reaction in whichelectrons are transferred from one element to another. Redox reactions alsoinclude reactions in which an elements oxidation state changes.

o Oxidation: Loss of electrons; the oxidation state increaseso Reduction: Gain of electrons; the oxidation state decreaseso Redox reactions always occur in pairso Half Reaction: Redox reaction divided into 2 separate reactions,

oxidation and reduction

y Balancing Redox Reactions1. Write half reactions.2. For each half reaction:

a. Balance the atoms of all the elements except H and O.b. Balance O using H2O.c. Balance H using H+.d. Balance charge using electrons.

3.

If necessary, equalize the number of electrons in the half reactions bymultiplying 1 of the equations through by an appropriate integer.4. Add the half reactions, cancelling species common to both sides of the

overall equation.5. Check that atom numbers and charges balance.

a. Stop here for redox reaction in aqueous acidic solutions.6. For redox reactions in basic solutions add a number of OH- ions equal

to the number of H+ ions to both sides of the overall equation.7. On the side of the overall equation containing both H+ ions and OH-

ions, combine to form water. Cancel water appearing on both sides of

the equation if necessary.8. Check that atom numbers and charges balance.

y Oxidizing Agent (Oxidant): Element (or compound containing element) whoseoxidation state decreases in a redox reaction

o Causes another species in the reaction to be reducedo Elements with high oxidations states are good oxidizing agents

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y Reducing Agent (Reductant): Element (or compound containing element)whose oxidation state increases in a redox reaction

o Causes another species in the reaction to be oxidizedo Elements with low oxidations states are good reducing agents

y Disproportionation Reactions: Redo

xreactions in which the same element isoxidized and reduced at the same time.

o Ex. Hydrogen peroxide yields water and oxygen gas; thiosulfate yieldssolid sulfur, sodium dioxide, and water

y Assigning Oxidation States1. An atom or molecule in its elemental state has an oxidation state of 0.2. The sum of oxidation states in:

a. a neutral compound is 0.b. a polyatomic ion is equal to the charge on the polyatomic ion.

3. In a compound, group 1 metals have an oxidation state of +1 andgroup 2 metals have an oxidation state of +2

4. In a compound, fluorine has an oxidation state of -1.5. In a compound, hydrogen usually has an oxidation state of +1.6. In a compound, oxygen usually has an oxidation state of -2.7. In binary compounds with metals, group 17 elements have an

oxidation state of -1; group 16 of -2; and group 15 of -3.

y Titrations: A reaction carried out by the carefully controlled addition of onesolution to another.

o Allows for the determination of an unknown concentrationo

Involves a titrant of known molarity in a buret, and a solution ofunknown molarity in a volumetric flask.o Equivalence Point: Point in titration when both reactants have reacted

completelyo Indicator: A substance added to the volumetric flask during a titration

that changes color at the equivalence point of the reaction