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
