CHEM 131 - Midterm 3

What is the Law of Conservation of Mass used for?

used for balancing equations → need an equal number of atoms of each element on both sides of the arrow

what is the reaction coefficient?

coefficient we put in front of atoms to specify the number of moles of each of the substances in a chemical reaction

what determines if a reaction is a combustion reaction or not?

O2 as a reactant + typically has H2O and O2 as products

when balancing equations, which elements do last?

free elements balance last

what are solutions?

homogenous mixtures (where the components are uniformally distributed throughout)

what is a solvent?

the majority component (dissolves in)

what is a solute?

the minority component (dissolves)

what is an aqueous solution?

a solution in which water is the solvent

what type of molecules dissolve in water?

polar molecules and ionic molecules

how do ionic compounds dissolve in water?

1) water molecules surround the solid

2) water pulls ions away from the solid (negative oxygen in H2O attracts to positive ion, positive hydrogen in H2O attracts to negative ions)

3) Ions become fully surrounded → ions flow freely and are able to conduct electricity

what are electrolytes?

materials that dissociate (ionic compound breaking into constituent ions) in water to form a solution that will conduct electricity (ex: NaCl)

what are electrolytes that partially dissociate called?

weak electrolytes

what are electrolytes that completely dissociate called?

strong electrolytes

what are nonelectrolytes?

materials that only dissolve in water and form a solution that will not conduct electricity (ex: sugar)

why does AgNo3 readily dissolve in water, but AgCl doesn’t?

the ionic bond between Ag and Cl is very strong - lots of energy required to break them apart (lattice energy)

in AgNO3, ions are easily separated

when do aqueous reactions occur?

can occur when 2+ solutes dissociate in an aqueous solution and the resulting ions react with each other (chemical reaction may/may not occur)

• form solid products - precipitation

• form liquid water - neutralization (acid-base)

• form gaseous products

• oxidation-reduction reactions

what is the symbol for a reversible reaction?

two arrows on top of each other, pointing different directions

what is the symbol for an irreversible reaction?

one arrow pointing in one direction

what are acid-base/neutralization reactions?

an acid reacts with a base, and the two neutralize each other, yielding water and a salt.

what is an acid?

substance that produces H+ in aqueous solutions (ex: HCl → H+ and Cl-)

what do H+ ions from acids bond with in solutions?

bonds with water to form hydronium ions (H3O+)

what are polyptoptic acids?

Acids that can donate more than one proton or hydrogen ion (H+) per molecule in a solution. Examples include sulfuric acid (H2SO4) and phosphoric acid (H3PO4).

• the first ionizable proton is strong, while subsequent protons are weak

what is a base?

substance that produces OH- ions in aqueous solutions (ex: NaOH → Na+ + OH-)

What are the strong acids?

HI, H2SO4, HClO4, HClO3, HNO3, HBR, HCL

what are the strong bases?

LiOH, NaOH, KOH, Ca(OH)2, Sr(OH)2, Ba(OH)2

what is always the net ionic equation for a strong acid/base reaction?

H+ (aq) + OH- (aq) → H2O (l)

what are oxidation-reduction/redox reactions?

reactions in which electrons are transferred from one reactant to the other

• many involve the reaction of a substance with oxygen

what is oxidation?

the loss of electrons (LEO)

what is reduction?

the gain of electrons (GER)

what are oxidation states?

a number chemists assign to each element, allowing them to determine the electron flor in the reaction, like electron bookkeeping. This number indicates the degree of oxidation or reduction of an atom.

what are the rules for assigning oxidations states?

1) free elements have an oxidation state equal to 0

2) monatomic ions have an oxidation state equal to their charge (ex: Ca2+ → 2+, C;- → -1)

3) the sum of oxidation states of all of the atoms in a compound is equal to 0

what is the oxidation state of group 1 metals?

+1

what is the oxidation states of group 2 metals?

+2

what is the oxidation state of oxygen?

-2

what is the oxidation state of hydrogen?

+1

what is the oxidation state of flourine?

-1

what is the oxidation state of group 7A

-1 (halogens)

what is the oxidation state for group 7b?

-2 (Mn, Tc, Re)

what is the oxidation state for group 5a?

+3, +5, -3 (N, P, As, Sb)

what is a reducing agent?

the element that is oxidized (causes reduction)

what is an oxidizing agent?

the element that is reduced (causes oxidation)

what do prefix multipliers do?

change the value of the unit by a power of 10

kilo

10³

centi

10^-2

milli (m)

10^-3

micro

10^-6

nano

10^-9

pico

10^-12

what is volume

a measure of space, units of length cubed or liters

what is density

the ratio of a substances mass to volume (less dense substance floats)

what is mass number

the total number of protons and neutrons in an atomic nucleus (represented by A)

1st type of nuclide symbol

AZ (a on top, z on bottom) next to X

• a - mass number

• z - atomic number

• x - chemical symbol

  • charge included when necessary

2nd type of nuclide symbol

x - A

• x - chemical symbol or name

• A - mass number

what is atomic mass number?

a unit used to express the masses of atoms and subatomic particles,

amu: defined as approximately 1/12th the mass of a carbon-12 atom.

how do you calculate average atomic mass

sum (fraction of isotope n) * mass of isotope n for all isotopes present in the sample.

what does the mole help us do

is a way to represent large numbers of atoms (also molecules and other things) in a more reasonable way

what is the molar mass

the mass of a mole of atoms - numerical value equal to the atomic mass

when do i use avogadro’s number

when you need to convert between moles and number of particles, such as atoms or molecules.

• 6.02214 × 10^23

how do we find molarity, and what is it equal to

molarity is equal to concentration

M (molarity) = moles of solute (mol)/L of solution (L)

what is the equation for mass % composition?

mass percent of X = (mass of element X in 1 mol of compound / mass of 1 mole of the compound) * 100

what is the empirical formula

gives the relative number of atoms of each element in a compound in its simplest form, not necessarily the actual numbers of atoms.

what is a molecular formula?

gives the actual number of atoms of each element in the molecule of a compound

what is the empirical formula for H2O4?

H2O

what is the multiplier equation to get from the empirical formula to the molecular formula?

n = molar mass (g/mol) / empirical formula molar mass (g/mol)

what is stoichiometry?

the study of the numerical relationship between chemical quantities in a chemical reaction

what is the limiting reactant

the reactant that makes the least amount of product (other reactant is the one in excess)

• completely consumed in a chemical reaction and limits the amount of product

what is the theoretical yield

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

what is the actual yield

amount of product actually produced

what is the percent yield

(AY/TY) * 100

what is thermochemistry?

heat transfer through a physical or chemical process

what is thermodynamics

the study of energy and heat transfer, focusing on the laws governing energy conservation

what is the law of conservation of energy

the total amount of energy in the universe is constant - can’t produce energy without using energy

what does conservation of energy require?

the sum of the energy changes in the system + the surroundings must be 0

what are state functions?

a mathematical function whose result depends only on the initial and final conditions, not on the process used

what are some examples of state functions

temperature, volume, and enthalpy (Xfinal - Xinitial = delta H)

what is heat?

the exchange of thermal energy between a system and its surroundings

when does heat exchange occur

when the system and surroundings have a temperature difference

what is temp

the measure of the thermal energy within a sample of matter

what are temperature increases directly proportional to

the amount of heat absorbed

what is q, in the equation q=mc(delta t)

the amount of heat transferred / amount of heat gained or lost by a sample

what is m, in the equation q=mc(delta t)

mass

what is c, in the equation q=mc(delta t)

the specific heat capacity of the substance (amount of heat associated with changing temperature of 1g of substance by 1 degree C)

what is delta t, in the equation q=mc(delta t)

the change in temperature of the substance, calculated as the final temperature minus the initial temperature.

what is q reaction equal to ?

-q solution, if q reaction was negative then q solution would be positive

what does a larger specific heat capacity lead to

the larger the specific heat capacity, the smaller the temperature rise will be for a given amount of heat

what is thermal equilibrium

two items of different temps reach the same final temp through energy exchange

what is qsystem equal to

-q surroundings (signs can switch)

what can we conclude when qfinal>qinitial

its endothermic - absorbed energy (heat absorbed/added to the system) - +q

give an example of an endothermic reaction

ice (s) → water (l) → steam (g)

• +q

what can we conclude when qfinal<qinitial

its exothermic - releasing energy (decrease of heat in the system

• -q

give an example of an exothermic reaction

water vapor (g) → water (l) → ice (s)

• -q

what is heat of fusion?

amount of heat associated with changing one mole of substance from solid to liquid (+q) or liquid to solid (-q)

what is heat of vaporization

amount of heat associated with changing one mole of substance from liquid to gas (+q) or gas to liquid (-q)

what is the equation for heat associated with a phase change

q = n ∆H

what are heating/cooling curves

graphs that visualize the temperature changes of a substance (phase changes)

• x axis: q added

  • y axis: temp

what is enthalpy?

heat absorbed/released by a reaction at constant pressure (H)

what does deltaH = qp mean?

the change in enthalpy of a system is equal to the heat (q) added or released by the system at constant pressure (p)

• when pressure doesn’t change, the delta H tells you directly how much heat the system absorbed or released

what is the change of enthalpy in a reaction (delta H reaction) equal to

Hfinal - Hinitial

Hproducts - Hreactants

what can you conclude when Hproducts > Hreactants

delta H is positive

• endothermic - absorbing energy - temp decreases

what can you conclude when Hproducts < Hreactants

delta H is negative

• exothermic → releasing energy → temp. increases