chemistry exam 3

Exceptions to the oct rule

1. odd- electron species: molecules or ions with an odd # of electrons

2. Incomplete octets: molecules or ions that are stable with fewer than 8 electrons per atom

3. expanded octet: molecules or ions that are stable with more than 8 electrons per atom.

Free radicals or radicals

molecules or ions with an odd number of electrons in their lewis structures. example nitrogen monoxide

Incomplete octet

certain elements form compounds with incomplete octets

Expanded octets 

• Elements in the third row of the periodic table and beyond are often observed to form compounds with expanded octets.

• Often have 12 electrons 

• preferred altenative to having high formal charges 

Lewis theory

model for predicting the structures of covalent complexes

VSEPR theory

allows us to predict molecular shape

Molecular shape

• Based on the number of electron groups around an atom 

• Electron groups: lone pairs, single, double, or triple bonds 

• based on the idea that electron groups repel one another 

VSEPR electron group

• Each lone pair of electrons constitutes one electron group on a central atom. 

• Each bond consitutes one electron group on a central atom regardless of whether it is a single double or triple bond. 

• maximum of six bonding electron groups 

VSEPR shapes linear geometry

• Two electron groups

• occupy positions on opposite sides of the central atom 

• bond angle is 180

VSEPR shape trigonal planar

• Three electron groups 

• shape of a triangle around the central atom 

• bond angle is 120 

VSEPR shapes tetrahedral geometry

• Four electron groups 

• shape of a tetrahedron 

• bond angle 109.5 

triangle and dashes lines

• triangle means going forward 

• dash lines means going backwards 

VSEPR shapes trigonal bipyramidal geometry

• Five electron groups 

• position above and below the central atom called axial positions 

• positions in the same base plane as the central atom called equatorial positions 

• bond angle is 120 

• bond angle between axial and equatorial is 90 

Electron geometry

the geometrical arrangement of electrons groups

Molecular geometry

the geometrical arrangement of atoms

VSEPR shapes octahedral geometry

• six electron groups

• all position are equivalent

• bond angle is 90 

Steric number

total number of electric groups

VSEPR theory predicting molecular geometry

1. draw lewis structure for the molecule

2. determine the total number of electrons groups

3. determine the number of bonding groups 

4. determine the number of lone pairs 

Polarity 

• unequal sharing of covalent bonds results in a polar bond. for molecules with only two atoms a polar bond results in a polar molecule 

• for molecules with more than one bond a polar bind doesn’t mean polar molecule 

how to determine molecule polarity?

we add together the dipole moments of each bond

Mass percent composition

• How much of an element is in a compound 

• mass % of element X= Mass of element X/ total mass of the compound x 100

Mass percent from chemical formulas

• For a pure substance with a known chemical formula the mass % composition of each element in the compound can be calculated.

• Mass % of element X= Mass of element X in 1 mol of compound/ Mass of 1 mol of the compound x 100

Decomposition

• Breaking the sample into its constituent elements.

• by determining the moles of each constituent element we can find the simplest mole ratio of the elements. (can determine the empirical formula)

Empirical formula

the simplest whole number ration of the moles of each element in a compound

Molecular formula 

the actual number of moles of each element in a compound 

determining empirical formula from mass percent

1. assume a 100g sample —> convert given percents to grams

2. convert given masses to mole of each element 

3. divide moles of each element by smallest number of moles

4. if necessary multiply entire formula to get whole numbers 

how to get molecular formula

1. the empirical formula- calculate 

2. the molecular mass- given 

procedure for determining molecular formula from empirical formula and molar mass

1. calculate empirical mass ( mass of empirical formula)

2. find the ration of molecular mass: empirical mass ( molar mass/ em) 

3. multiply empirical formula by the determined ratio

Molarity

• solution concentration expressed units. 

• amount of solute (in moles) divided by the volume of solution (in liters) 

• Molarity(M)= amount of solute (in mol)/ volume of solution (in L) 

solution preparations 

• Stock solutions: prepare concentrated solution and then dilute it as needed for experiments. 

• the process is called dilution 

process of calculating dilution

• M1V1=M2V2

• M1: stock solution concentration (M)

• V1: volume of stock solution (L)

• M2: concentration of desired solution (M)

• V2: volume of desired solution (L) 

chemical reaction 

• Describes the process by which one or more substances are converted into one or more different substances. 

• Represented by a chemical equation. 

Law of conservation of mass

atoms are not created or destroyed in a chemical reaction they are simply re-arranged to from different compounds.

strategies for balancing chemical equations

1. when balancing chemical equations can only change the coefficients 

2. do not change the chemical formula of any compounds by changing their subscripts 

3. if there is an element on its own add its coefficient last

4. in a precipitation /double replacement reactions treat platonic ions as a group not individual elements 

5. always check your work 

solubility

• The extent to which a substance may be dissolved in a solvent.

• compounds with large solubilites as double 

• compounds with low solubilities as insoluble 

Understanding solubility

solids dissolve when the attractive forces holding a solid molecule together are overcome by the attraction between solute and solvent

Aqueous solutions

• When water is added ionic compounds dissociate into their positive and negative ions to from free ions 

• cations are attracted to the electron- rich oxygen of H20

• anions are attracted to the electron-poor hydrogen of H20

• added water to covalent compounds can dissolve too. 

mobile charges

results of dissociation of ionic compounds. covalent compounds form no mobile charges as they do not dissociate these are called non electrolytes. 

electrolytes

dissociated ionic compounds solution that conducts electricity

non electrolytes

covalent compounds do not dissociate forming no mobile charges

complete ionic equation

shows all free ions in solution

net ionic equation

shows only the species that change during the reaction 

spectator ions

all ions not included in the net ionic equation

Acids

a compound that dissociates hydrogen ion H+ in water

Bases

a compound that dissociates hydroxide ion OH- in water

neutralization reaction

when acids and bases react together they produce water and salt.

oxidation reduction or redox reactions 

occur when electrons are transferred from one reactant to the other. oxidation and reduction must occur together.

oxidized

compound that loses electrons

reduced

compound that gains electrons.

OIL RIG

• O:oxidation 

• I: is 

• L: loss 

• R: reduction

• I: is 

• G: gain 

Coefficient in a chemical equation

specify the relative amounts in moles of each of the substances involved in the reaction. 

Stoichiometry

mass to mass calculations

stoichiometric realtionships

1. the reactants are combined in stoichiometrically equivalent amounts. 

2. one of the reactants runs out first 

Limiting reactant

reactant that runs out or is fully consumed

Excess reactant

reactant that does not run out

Theoretical yield or calculated yield

is the amount of product that can be made in a chemical reaction based on the amount of limiting reactant

steps for finding the limiting reactant and theoretical yield 

1. calculate how much product can be formed from each reactant 

2. whichever reactant produces less product is the limiting reactant 

3. the theoretical yield of a reaction is the amount product calculated from the limiting reactant 

actual yield

the amount of product that can be made in a chemical reaction based on the amount of limiting reactant.

Percent yield 

% yield = actual yield/ theoretical yield x 100