chem quiz 3

percent yield equation

(actual yield/theoretical yield) x 100

rules for assigning oxidation numbers

- an atom in its elemental state has an oxidation number of 0 - a monatomic ion has an oxidation number equal to its charge - in a molecular compound, an atom usually has the same oxidation number it would have if it were a monatomic ion (hydrogen almost always +1, oxygen almost always -2, nitrogen often but not always -3, halogens almost always -1) - for compounds with more than one nonmetal element, the more electronegative element has a negative oxidation number and the less electronegative element has a positive oxidation number - the sum of the oxidation numbers in a neutral compound is 0

endothermic reactions:

- if the input of energy to break bonds is more than the amount of energy released when forming bonds, the excess is absorbed - the heat absorbed is a reaction reactant - the heat of reaction is positive because heat is "gained" during the reaction - positive ΔH

two factors determine the spontaneity of a chemical or physical change:

- the release or absorption of heat - the increase or decrease in entropy

how can you tell when a redox reaction is taking place?

- when ions are involved, determine whether there is a change in charges - for reactions involving metals and nonmetals, predict a gain or loss of electrons - molecular substances can be analyzed in terms of loss and gain of oxygen.

exothermic reactions

- when strength of the bonds formed in the products is greater than the strength of the bonds broken in the reactants, energy is released - the heat released is a reaction product - the heat of reaction is negative because heat is lost during the reaction - negative ΔH

general rules on solubility

1. a compound is probably soluble if it contains one of the following cations: group 1A cation: Li+, Na+, K+, Rb+, Cs+ ammonium cation: NH4+ 2. a compound is probably soluble if it contains one of the following anions: halide: Cl-, Br-, I-(except Ag+, Hg22+, and Pb2+ ) compounds nitrate (NO3-), perchlorate (ClO4-), acetate (CH3CO2-), sulfate (SO42-) (except Ba2+, Hg22+, and Pb2+sulfates)

generalizations about redox behavior

1. in reactions involving metals and nonmetals, metals tend to lose electrons while nonmetals tend to gain electrons. the number of electrons lost or gained can often be predicted based on the position of the element in the periodic table 2. in reactions involving nonmetals, the "more metallic" element (farther down and/or to the left in the periodic table) tends to lose electrons, and the "less metallic" element (up and/or to the right) tends to gain electrons

avogadro's number (NA)

6.022 × 10^23

one mole of any substance contains _______ formula units

6.022 × 10^23

T or F: gases have higher entropy than liquids, and liquids have higher entropy than solids

T

endothermic

a chemical change that absorbs heat (bond breaking) is endothermic

free-energy change (ΔG)

a measure of the change in free energy as a chemical reaction or physical change occurs.

the standard enthalpy of formation

a measure of the energy released or consumed when one mole of a substance is created under standard conditions from its pure elements

heat

a measure of the kinetic energy of the particles that make up the molecule

coefficient

a number placed in front of a formula to balance a chemical equation

oxidation number

a number that indicates whether an atom is neutral, electron-rich, or electron-poor

spontaneous process

a process or reaction that, once started, proceeds on its own without any external influence

product

a substance that is formed in a chemical reaction and is written on the right side of the reaction arrow in a chemical equation

reactant

a substance that undergoes change in a chemical reaction and is written on the left side of the reaction arrow in a chemical equation

solubility

amount of a compound that will dissolve in a given amount of solvent at a given temperature

bond dissociation energy

amount of energy that must be supplied to break a bond and separate the atoms in an isolated gaseous molecule

theoretical yield

amount of product formed assuming complete reaction of the limiting reagent

ionic equation:

an equation in which ions are explicitly shown

net ionic equation

an equation that does not include spectator ions, includes only the ions that undergo change

chemical equation

an expression in which symbols and formulas are used to represent a chemical reaction

a net ionic equation must be balanced both for ______ and for _______

atoms; charge

molecular weight (MW)

average mass of a substance's molecules

atomic weight

average mass of an element's atoms

tabular bond energies

average values

combustion

burning of a fuel by rapid oxidation with oxygen in air

mass-to-mass conversions _______ be carried out directly

cannot

mole-to-mass and mass-to-mole conversions

carried out using molar mass as a conversion factor

mole-to-mole conversions

carried out using mole ratios as conversion factors

more moles to less moles entropy _____

decreases

stable

describes a substance that has little remaining potential energy and, consequently, little tendency to undergo further change - whether a reaction occurs, and how much energy or heat is associated with the reaction, depends on the difference in the amount of potential energy contained in the reactants and products

corrosion

deterioration of a metal by oxidation (such as the rusting of iron in moist air)

heat of reaction/enthalpy change (ΔH)

difference between the heat absorbed in breaking bonds and the heat released in forming bonds

(aq)

dissolved in aqueous solution

the reverse of an exothermic reaction is

endothermic

law of conservation of energy

energy can be neither created nor destroyed in any physical or chemical change

kinetic energy

energy of motion

entropy change (ΔS)

entropy change (ΔS) is a measure of the increase in disorder (ΔS = +) or decrease in disorder (ΔS = −) as a chemical reaction or physical change occurs.

the reverse of an endothermic reaction

exothermic

reduction

gain of one or more electrons

when attractive forces between ions or atoms result in the formation of ionic or covalent bonds, the potential energy is often converted into _____

heat

In reactions some bonds break (energy ___) some bonds form (energy ___)

in; out

spectator ions

ions that undergo no change during the reaction, they appear on both sides of the reaction but play no role

the more stable a molecule, the _____ reactive it is

less

if the products have _____ potential energy than the reactants, the products are more ______ than the reactants.

less; stable

if a substance has a low solubility, then it is ______ to precipitate from an aqueous solution.

likely

oxidation

loss of one or more electrons

actual bond energies

may vary depending on the chemical environment in which the bond is found.

entropy (S)

measure of the amount of molecular disorder in a system cal/(mol•K) or J/(mol•K).

in chemical reactions, the unit to specify the relationship between reactants and products is the ____

mole

Coefficients can be put in the form of _____ _____, which _____________________________________________________

mole ratios; act as conversion factors when setting up factor-label calculations

spontaneous endothermic processes have an increase in _______________

molecular disorder/randomness

coefficients in a chemical equation tell how many ________, and thus how many ______, of each reactant are needed and how many of each product are formed

molecules; moles

if heat is released (exothermic), then the sign is _____ to indicate energy is _____ by the substance (bond-forming)

negative; lost

if a substance has a high solubility in water, then _____ precipitate will form

no

the reverse of a spontaneous process is always

nonspontaneous

when enthalpy/entropy are unfavorable, a process is

nonspontaneous

oxidation and reduction always _______ __________

occur together - when one substance loses an electron (is oxidized), another substance must gain that electron (be reduced)

by comparing the _________ __________ of an atom before and after a reaction, we can tell whether the atom has gained or lost shares in electrons

oxidation number

historically: oxidation/reduction

oxidation referred to the combination of an element with oxygen reduction referred to the removal of oxygen from an oxide to yield the element

an atom is _______ when it loses a share in electrons

oxidized

if heat is absorbed (endothermic), then the sign is _________ to indicate energy is _________ by the substance (bond-breaking)

positive; gained

the attractive forces between ions or atoms are a form of _____ ______

potential energy

respiration

process of breathing and using oxygen for the many biological redox reactions that provide the energy that living organisms need

acid-base neutralization reactions

processes in which an acid reacts with a base to yield water plus an ionic compound called a salt - a neutralization reaction removes H+and OH-ions from solution and yields neutral H2O - any ionic compound produced in an acid-base reaction is called a salt - the most common kind of neutralization reaction occurs between an acid (HA) and a metal hydroxide (MOH) to yield water and a salt.

precipitation reactions

processes in which an insoluble solid called a precipitate forms when reactants are combined in an aqueous solution

oxidation-reduction reactions or redox reactions

reactions in which electrons are transferred from one atom to another - as a result of the electron transfer in a redox reaction, the number of electrons assigned to individual atoms in the various reactants changes - reactions involving covalent compounds are considered redox reactions because electrons are rearranged as bonds are broken and new bonds are formed

an atom is _______ when it gains a share in electrons

reduced

alkali metals and alkaline earth metals are the most powerful __________ _______

reducing agents - they will react with pure water - they have low ionization energy - as ionization energy increases, reducing power decreases

metallurgy

science of extracting and purifying metals from their ores, makes use of numerous redox processes

the direction of energy flow in a chemical change is indicated by the ____

sign

whether a precipitation reaction will occur on mixing aqueous solutions of two ionic compounds depends on the _________ of the potential products.

solubilities

a release of heat favors _____________

spontaneity

an increase in molecular disorder favors ______________

spontaneity

when enthalpy and entropy are both favorable (ΔH negative, ΔS positive), a process is _____________

spontaneous

exergonic event

spontaneous reaction or process that releases free energy and has a negative ΔG

the greater the bond dissociation energy, the more ______ the chemical bond

stable

steps for balancing equation

step 1: write an unbalanced equation, using the correct formulas for all reactants and products (for example, hydrogen and oxygen must be written as H2 and O2 rather than as H and O because both elements exist as diatomic molecules) (subscripts in chemical formulas cannot be changed) step 2: add appropriate coefficients to balance the numbers of atoms of each element (begin with elements that appear in only one compound or formula on each side of the equation) (leave elements that exist in elemental forms, such as oxygen and hydrogen, until last) if a polyatomic ion appears on both sides of an equation (as in the cases of sulfate and hydroxide), it is treated as a single unit step 3: check the equation to make sure the numbers and kinds of atoms on both sides of the equation are the same step 4: make sure the coefficients are reduced to their lowest whole-number values

steps for determining mass relationships among reactants and products

step 1: write the balanced chemical equation step 2: choose molar masses and mole ratios to convert the known information into the needed information step 3: set up the factor-label expression step 4: calculate the answer and check the answer against the ballpark estimate you made before you began your calculations

potential energy

stored energy

the ______ of a covalent bond is measured by the amount of energy that must be supplied to break the bond

strength

nonspontaneous process

takes place only in the presence of a continuous external influence

mole

the amount of a substance whose mass in grams is numerically equal to its molecular or formula weight

actual yield

the amount of product actually formed in a reaction

molar mass

the mass in grams of 1 mole of a substance, numerically equal to molecular weight

balanced chemical equation

the numbers and kinds of atoms must be the same on both sides of the reaction arrow

percent yield

the percent of the theoretical yield actually obtained from a chemical reaction

limiting reagent

the reactant that runs out first - only rarely are all reactants converted to products - when running a chemical reaction, we don't always have the exact amounts of reagents to allow all of them to react completely

exothermic

the reverse of bond breaking is bond formation, a process that releases heat and is described as exothermic

oxidizing agent

the substance that gains an electron and causes the oxidation - gains one or more electrons - may lose oxygen atoms reactant that causes an oxidation by taking electrons from another reactant

reducing agent

the substance that gives up an electron and causes the reduction - loses one or more electrons - may gain oxygen atoms reactants that causes reduction of another reactant by giving up electron

a substance's molecular weight (or formula weight for an ionic compound)

the sum of the atomic weights for all the atoms in the molecule or formula unit.

standard enthalpies of formation*, Hfo, equation

Δ Hf(reaction)= SΔ Hf(products)- SΔ Hf(reactants)

free-energy change (ΔG) equation

ΔG=ΔH-TΔS ΔH=heat of reaction T=temperature in Kelvins ΔS=entropy change

ΔH equation

Σ(Bond dissociation energies)reactants- Σ(Bond dissociation energies)products