MCAT General Chemistry Term Review
MCAT General Chemistry Term Review
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Date Created: 03/25/14
Acids and Bases The Bronsted and Lowry s definition of an acid is any species that donates a proton when it dissociates in a solution The Bronsted and Lowry s definition of a base is any species that accepts a proton in solution Lewis defined a base as an electron pair donor and an acid as an electron pair acceptor Every Bronsted Lowry acid has a matching Bronsted Lowry base Every Bronsted Lowry base has a matching Bronsted Lowry acid and the matching pairs are called conjugates pH and pOH Rule number 18 pH equals negative logarithm of the hydrogen ion concentration pOH equals negative logarithm of the hydroxide ion concentration To find the pH of a solution we take the exponent of the hydrogen concentration and change its sign then subtract to the log of the coefficient from the exponent Since the log of the coefficient is always less than one the pH is always between the negative log of the exponent and 1 less than that value Water Solubility Constant Rule number 19 Kw is the solubility constant for water and it is equal to the hydrogen concentration times the hydroxide ion concentration Kw is equal to lxl0 l4 If we now take the log of each concentration we get pH plus pOH equals 14 Acids in solution have low pH values and high pOH values Bases in solution have high pH values and low pOH values Because pH and pOH add up to 14 the pH scale varies from 0 to 14 Neutral solutions have a pH of 7 Anything higher than that is on the basic side and anything lower than that is on the acidic side Elements There are 103 elements and they are all set out on the periodic table The smallest existing unit of any element is one atom of that element All atoms have a nucleus which contains protons and neutrons Each proton has a positive charge of 1 Neutrons have no charge Outside its nucleus the atom contains electrons and each electron has a charge of 1 In every atom the normal number of electrons outside the nucleus is equal to the number of protons inside the nucleus so the atom is electrically neutral Knowing that an atom has a positive charge in its nucleus and negative charge outside its nucleus you can see that the atom is held together by the attractive force of opposite charges For any atom the number of protons in the nucleus is called the atomic number The atomic number is shown by a subscript next to the symbol of the element and it tells you what element the atom is Isotopes For any atom the number of protons or electrons is not necessarily equal to the number of neutrons If two atoms of the same element differ in their numbers of neutrons they are called isotopes Isotopes have the same atomic number and the same net charge but they differ in their mass numbers The mass of an electron is negligible therefore an atom s mass derives entirely from its protons and neutrons The mass number for any atom is equal to the sum of its protons and neutrons The mass number is represented by the superscript of an element Now remember that the mass number is not the same as atomic weight In nature elements typically occur as combinations of their isotopes so the atomic weight of an element represents the average weight per atom within the combination of isotopes of that element that usually occurs in nature The unit of measurement for atomic mass is atomic mass units AMU Ions If an atom loses an electron it acquires a charge of 1 and is called a positively charged ion If an atom gains an extra electron it acquires a charge of 1 and is called a negatively charge ion Let us summarized what we have just said Proton s charge equals 1 Neutron has no charge and electron s charge equals 1 Atomic number equals number of protons Mass number equals number of protons and neutrons Atomic weight AMU equals approximate average weight of an atom calculated from natural isotopes Atomic weight does not equal mass number Electrons exist in orbitals Electrons are located outside the nucleus in electron orbitals Each orbital can have two electrons Orbitals are located in subshells and the subshells are located in shells To specify a particular electron of an atom we indicate that electrons four quantum numbers Shells are the first quantum number which is symbolized by the letter N When an electron is located in the first shell the electron is said to have a first quantum number of one Subshells are the second quantum number which is symbolized by the letter L An electron s second quantum number specifies its subshell There are four subshells called s p d and f The first subshell is called the s subshell The second shell has two subshells the s subshell and the p subshell The third shell has three subshells the s subshell the p subshell and the d subshell The fourth fifth sixth and seventh shells have four subshells the s subshell the p subshell the d subshell and the f subshell Orbitals are the third quantum number which is symbolized by the letter M To find the number of orbitals in a particular subshell we take the quantum number that designates the subshell multiply it by two and add one Subshell s has quantum number zero subshell p has quantum number one subshell d has quantum number two and subshell f has quantum number three So to find the number of orbitals that are in the f subshell we multiply its quantum number three by two which will give us six and then we add one which makes it seven Knowing that each orbital can hold two electrons we know that the f subshell can hold 14 electrons Electrons have spin which is the fourth quantum number symbolized by Ms An electron spin is either plus 12 for a positive spin or minus 12 for a negative spin Electron configuration 2 The electron configuration of an element tells us the order in which electrons fill the subshells from lower energy orbitals to higher ones The following are the electron configurations of the first three elements Hydrogen lsl helium ls2 lithium ls2 2sl So lithium for example has 1 electron in the 2s sub shell and 2 electrons in the ls subshell Galvanic and Electrolytic Cells Here is a quick comparison of galvanic and electrolytic cells In a galvanic cell the type of redox reaction is spontaneous In an electrolytic cell the type of redox reaction is non spontaneous In a galvanic cell electron ow is created In an electrolytic cell electron ow is required In a galvanic cell the site of oxidation is the anode In an electrolytic cell the site of oxidation is the anode In a galvanic cell the site of reduction is the cathode In an electrolytic cell the site of reduction is the cathode In a galvanic cell the positive electrode is the cathode In an electrolytic cell the positive electrode is the anode In galvanic cell the negative electrode is the anode In an electrolytic cell the negative electrode is the cathode In a galvanic cell the ow of electrons is from anode to cathode In an electrolytic cell the ow of electrons is from anode to cathode Entropy Entropy is the measure of disorder in a system When left alone systems tend to go from conditions of greater order low entropy to conditions of greater disorder higher entropy Entropy means disorder Entropy has the symbol S and the change in entropy after a reaction is AS You may have to figure out whether a reaction has positive or negative AS If you do think about whether the products are more or less random If the products are more random the AS is positive If the products are less random then the AS is negative Remember that liquids are more random than solids and gases are more random than liquids Enthalpy The enthalpy of a substance is the amount of energy that comes from its bonds The symbol for enthalpy is H and different substances have different amounts of enthalpy The amount of enthalpy in the substance changes when that substance reacts since bonds are broken and formed during the reaction The change in enthalpy from a reaction AH is often called the heat of reaction It is equal to the difference between the enthalpy of the products of the reaction and the enthalpy of the reactants Rule number 16 Change in enthalpy equals enthalpy of the product minus the enthalpy of the reactant or AH HP HR If the products of the reaction have less enthalpy than the reactants then the reaction is called exothermic which means that heat has been released by the reaction and the AH is negative If the products of the reaction have more enthalpy than the reactants then the reaction is called endothermic which means that heat has been absorbed by the reaction and the AH is positive Generally all things tend to move toward two distinct states high entropy and low enthalpy We can use Gibbs free energy to find out how spontaneous a reaction is The symbol for Gibbs free energy is G Rule number 17 AG AH TAS T is the temperature in Kelvin If the AG obtained from this reaction is negative then the reaction is spontaneous If the AG obtained from this reaction is positive then the reaction is nonspontaneous If the AG is zero then the reaction is at equilibrium Covalent Bonds When we say covalent bond we are talking about a bond in which two atoms share a pair of electrons The shared electrons revolve around the nuclei of both atoms in the bond In order to form the pair each atom contributes one unpaired electron If an atom has one unpaired electron it can form one covalent bond If it has two unpaired electrons it can form two covalent bonds If it has three unpaired electrons it can form three covalent bonds Think about a molecule of water The oxygen atom has two unpaired electrons Each of the hydrogen atoms has one unpaired electron so the oxygen atom can make two covalent bonds and each of the hydrogen atoms can make one covalent bond So remember that the number of covalent bonds an atom can form is equal to the number of unpaired electrons in its outer shell Polar Bonds If two atoms form a covalent bond and one of the atoms is more electronegative than the other the bond will be a polar bond since it is not balanced based on charge Hydrogen Bonds A hydrogen bond is an attraction between a hydrogen atom attached to one molecule and a highly electronegative atom in another molecule Remember that hydrogen bonding typically occurs with oxygen nitrogen and uorine Kinetic Molecular Theory The kinetic molecular theory states that for ideal gases 1 The kinetic energy of a gas is directly proportional to its absolute temperature so the greater the temperature the greater the kinetic energy 2 Compared to the volume of the space that the particles move in the volume of gas particles is small 3 No forces of attraction occur between the particles so when a collision takes place there is no loss of kinetic energy 4 The time between collisions is significantly larger than the time during collisions Units of Pressure 760 torr 760 millimeters of mercury one atmosphere 1013 x 105 pascals We measure pressure using a barometer For gases the standard temperature and pressure STP means 0 degrees Celsius C and one atmosphere Remember that at STP a mole of any gas will take up 224 liters of volume For ideal gases pressure and temperature are directly proportional Rule Number 8 Initial pressure initial temperature final pressure final temperature P1T1 P2T2 If the temperature remains constant then pressure is inversely related to volume Rule Number 9 Boyle s Law Initial pressure gtlt initial volume final pressure gtlt final volume P1V1 P2V2 Now if the pressure is kept constant the volume is directly related to the temperature Rule Number 10 Charle s Law Initial volume initial temperature final volume final temperature V1T1 V2T2 If we combine Boyle s law with Charles s law we get rule 11 Rule Number 11 Combined Gas Law Initial pressure gtlt initial volume initial temperature final pressure gtlt final volume final temperature P1V1T1 P2V2T2 Rule Number 12 Ideal Gas Law Pressure gtlt volume number of moles gtlt ideal gas law constant gtlt temperature PVNRT The ideal gas law constant is 00821 liters atmosphere per moles Kelvin You ll be given this number so you don t need to memorize it You may be asked to compare the speeds of two gas molecules in a closed container You can do this by using Graham s law Rule Number 13 V1 V2 m2 ml V is the Velocity and m is the mass of each molecule Oxidation Numbers Oxidation numbers are numbers for each atom that describe the number of electrons it loses or gains when it reacts For a molecule the oxidation numbers from all the atoms must add up to zero to make a neutral molecule Try to remember the following oxidation numbers group 1A oxidation number l group 2A oxidation number 2 group 3A oxidation number 3 uorine oxidation number 1 oxygen oxidation number 2 noble gases oxidation number 0 Now if you need to figure out the oxidation number of an atom in a molecule you can use these Values to figure out the rest of them and find the Value so they all add up to zero Oxidation reduction reaction In an oxidation reduction redox reaction the oxidation numbers change because in the newly formed compounds the atoms have to donate or accept fewer or more electrons to the bonds Reduction is when the oxidation number decreases and the atom gains electrons Oxidation is when the oxidation number increases and the atom loses electrons Oxidizing and reducing agents 2 An oxidizing agent or oxidant causes another atom to be oxidized and gets reduced in the process A reducing agent or reductant causes another atom to be reduced and gets oxidized in the process Phase Cha nge In a phase change diagram the straight horizontal lines show how it takes heat for a substance to change its phase and the slanted lines show how the temperature increases when heat is added When a solid becomes a liquid it is called melting The reverse is called freezing or fusion The constant for melting or freezing is called the heat of fusion When a liquid becomes a gas its called evaporation A liquid evaporates when its vapor pressure is equal to the surrounding atmospheric pressure The reverse is condensation The constant for evaporating or condensing is called the heat of vaporization or the heat of condensation When a solid directly becomes a gas it is called sublimation The reverse is deposition Specific heat Specific heat is the number of calories it takes to get a substance to raise its temperature by a degree Rule number 14 Specific heat Heat equals mass times specific heat times change in temperature or Q mcAt Solutions Think about dissolving sugar in a glass of Water because the Water dissolves the sugar it s called the solvent And because the sugar is dissolved in the water it is called the solute Sugar water is an aqueous solution which means that the solution is water A relatively weak solution is called dilute and a relatively strong solution is called concentrated Measuring Concentrations The most commonly used unit for concentration is called molarityM It gives the number of moles of solute dissolved per liter of solution So molarity M Moles of Solute Liters of Solution Molality is another unit for measuring concentration and its the number of moles of solute per kilogram of solvent So molality m Moles of Solute Mass Kg of Solvent Solubility and Saturation If you take a glass of water and keep adding tablespoon after tablespoon of sugar you will reach a point at which the water refuses to dissolve any additional sugar At that point we say that the solution is saturated Boiling Point Elevation Freezing Point Depression and Vapor Pressure When a solid solute dissolves in a liquid solvent it raises the solvent s boiling point and lowers its freezing point Rule number 15 The change in boiling point or freezing point is always equal to sum constant K times the molality of the solution times the number of dissolved particles produced by each molecule of solute or AT KMX If we are dealing with boiling point the constant is called the boiling point elevation constant If we are dealing with freezing point the constant is called the freezing point depression constant Boiling point elevation constants and freezing point depression constants vary with the solvent Each solvent has its own Vapor Pressure Vapor pressure is the pressure from the gas that is evaporating from the exposed surface of a liquid If we were to add some solute to a liquid its Vapor pressure would decrease according to the amount of solute we add A liquid that has a high Vapor pressure is said to be Volatile Let us summarize what we have just said When you add non Volatile substances to a solution one the Vapor pressure is lowered Two the boiling point is raised And three the freezing point is lowered Hund39s Rule and Radiation Hund s rule says that within any one s subshell electrons occupy orbital singly unless and until all of the orbitals in that subshell have one electron in them Energy can travel One of the modes by which energy travels is the wave of electromagnetic radiation Its speed is 3 X 108 meters per second in a vacuum This number 3 X 108 meters per second is called the speed of electromagnetic radiation and is symbolize with the letter C You may have seen this as the speed of light That is because visible light is a type of electromagnetic radiation Let us think of an electromagnetic wave as a sinusoidal wave oscillating about an aXis It is useful to describe such wave with the following measurements 1 A wavelength tells us the length of one full cycle of the wave It can be measured in nanometers per cycle 2 A frequency F tells the number of cycles that passes by a given point during one second The inverse of frequency is called the period of the wave or T 3 An amplitude which is the height of the wave it can also be thought of as the maximum displacement from the aXis Waves of electromagnetic radiation may differ in their wavelengths frequencies and amplitudes but all waves of electromagnetic radiation have a velocity of 3 X 108 meters per second symbolized with the letter C Rule number 1 Wavelength times frequency equals the speed of light or AF C Using this formula you can figure out the wavelength if you know the frequency and you can figure out the frequency if you know the wavelength All matter has wave characteristics This is called the de Broglie hypothesis Rule number 2 The de Broglie hypothesis Wavelength equals Planck s constant over mass times velocity or A Hmv Planck s constant is equal to 663 X 1034 joule seconds The De Broglie hypothesis is mostly used in electrons That is because all electrons have the same mass but they can differ in velocity Some electromagnetic waves produce light Visible light is mostly a miXture of different waves of electromagnetic radiation within the wavelength range of 390 to 700 nanometers If all colors are present in the miXture the result is white light Lights other than white contain only some wavelengths and therefore only some colors When light is passed through a prism its component wavelengths are separated and a spectrum of different colors appears These different colors correspond to the wavelengths that were present in the original light If we pass white light through a prism we get something called a continuous spectrum because white light is composed of all colors Now if we pass a light through a prism that is not white we get a line spectrum which shows a line for each color that is contained in the light we used When an atom is exposed to light some of its electrons may absorb some of the light s energy As an electron absorbs energy it becomes excited and moves from its ground state to an excited state When the light is removed the electron falls back to its ground state and is no longer excited As it falls it emits some energy in the form of light And if this light energy is not completely removed the electron may fall down to a lesser excited state You should know about the quantum theory The quantum theory states that quanta are the smallest unit of energy that can be absorbed or emitted at a particular frequency An important concept associated with this theory is this When we say that an electron absorbs light energy and gets excited the electron absorbs one or more quanta of energy not a half or two thirds of a quantum but a whole number of quanta at a time As a result electrons exist in the atom only at certain energy levels After absorbing a certain number of quanta of energy the electron now has the exact amount of extra energy to jump to an available energy level or subshell A nucleus is held together by a force called the nuclear force which is non electrical force between protons and neutrons Some nuclei are stable and some are unstable The unstable nuclei have a tendency to break apart This tendency to break apart is what makes some nuclei radioactive Unstable nuclei try to become more stable by altering their content of protons and neutrons This process is called nuclear decay If in the process of nuclear decay a nucleus emits small particles then that nucleus is radioactive There are three kinds of radioactive decay Alpha decay An alpha particle consists of two protons and two neutrons Alpha decay reduces atomic number by two and reduces the mass number by four Beta decay If a nucleus wants to become more stable by reducing its neutron to proton ratio it converts a neutron to a proton The atomic number goes up by one but the mass number remains the same When this occurs a beta particle is emitted The beta particle is identical to an electron Positron emission If a nucleus wants to become more stable by increasing its neutron to proton ratio it converts a proton to a neutron The atomic number goes down by one but the mass number remains the same When this occurs a positron is emitted A positron is a particle that has the same mass as an electron but carries a positive charge Gamma rays 2 A gamma ray is electromagnetic radiation Gamma rays are often emitted together with alpha particles beta particles and positrons Electron capture Some nuclei convert a proton to neutron by capturing an electron from the first electron shell of their own atom Somehow the capture of the electron allows the nucleus to convert a proton to neutron Electron capture causes the atomic number to be reduced by one while the mass number remains the same Remember that when a radioactive particle is emitted it is lost and we must subtract it from our original atom In alpha decay subtract four from mass number subtract two from atomic number In beta decay no change in mass number add one to the atomic number In positron emission no change in mass number subtract one from atomic number In gamma rays no change in mass number or atomic number In electron capture no change in mass number subtract one from atomic number Radioactive particles have energy and this is their order 1 Gamma rays highest energy 2 Beta decay and positron emission equal energy between gamma and alpha 3 Alpha decay lowest energy Radioactive substances decay with specific half lives Every radioactive substance has a decay constant and the larger the decay constant the faster the radioactive substance will decay Rule number 3 l The half life of a radioactive substance is the time it takes for one half of some sample of the substances to undergo radioactive decay 2 The amount of radioactive substances that remains after a period of decay will decrease exponentially with increasing decay time Nucleons are more stable when they are all put together in the nucleus than when they were broken apart as separate protons and neutrons Every nucleus has what is called a nuclear binding energy Nuclear binding energy is equal to the energy that would be required to break a nucleus up into individual nucleons Some nuclei have higher binding energies than others A nucleus weighs less than the sum of its protons and neutrons individually The difference is what we call the mass defect The reduction in energy that s achieved when a nucleus is assembled involves a loss of mass Einstein formulated a relationship between the nuclear binding energy and the mass defect Rule number 4 For any nuclear binding energy energy equals the mass defect times the square of the speed of electromagnetic radiation or E MC2 The Periodic Table As you know each horizontal row on the periodic table is called a period And each vertical column on the periodic table is called a group In any group on the periodic table all of the elements have the same number of electrons in their outermost shell Electrons that are located in an atoms outermost shell are called valence electrons These electrons are most responsible for an atoms properties and chemical behavior Rule number 5 With progression from left to right across a period valence electrons are more tightly bound to the atom With progression down a group valence electrons are less tightly bound to the atom Five important Periodic Trends 1 As we move from left to right across a period or up a group ionization energy increases because the atoms positively charged nucleus tends to hold the electrons in the atom It takes energy to remove an electron The amount of energy necessary to take one electron out of an atom is called the ionization energy 2 As we move from left to right across a period electron negativity increases As we go down a group electron negativity decreases because valence electrons are less and less tightly bound Electron negativity refers to the degree of pull that an atom exerts on another atom s electrons when it is involved in a bond 3 As we move from left to right across a period atomic radius decreases As We go down a group the increased atomic radius increases Every atom has a radius And the radius is called atomic radius 4 As we move from left to right across a period electron affinity increases As we move from the top to the bottom of a group the electron affinity decreases or becomes more positive Electron affinity of an element is the amount of energy either released or absorbed when an electron is added to a neutral atom A negative electron affinity value means that energy is released when an electron is added A positive electron affinity value means that energy is absorbed in order to take an extra electron 5 As we move from left to right across a period metallic character decreases As We move down a group metallic character increases Metallic character requires a high degree of mobility of valence electrons Rule number 6 Try to remember these periodic trends As you move from left to right across a period atomic radius decreases ionization energy increases electron negativity increases electron affinity increases metallic character decreases As you move down a column atomic radius increases ionization energy decreases electron negativity decreases electron affinity decreases metallic character increases Rule number 7 An atom s chemical behavior is largely determined by the number of electrons in its highest energy shells The more Valence electrons or electrons in the outer shell the more stable an atom is When we see an element symbol with dots around it we are looking at a Lewis Dot Structure Molecular Shapes You may be asked to figure out the shape of certain molecules so this is how you go about finding them Count the number of atoms to which the central atom of the molecule is bound 1 If the number is 2 then the shape of the molecule is usually A Linear if the Lewis dot structure shows no unshared pair of atoms on the central atom but it is B Bent if the Lewis dot structure does show one or more unshared pairs of electrons on the central atom 2 If the number is 3 the molecule is A Trigonal planar if the Lewis dot shows no unshared pair of atoms on the central atom but it is B Trigonal pyramidal if the Lewis dot structure does show an unshared pair of electrons on the central atom 3 If the number is 4 the molecule is usually tetrahedral The Mole The mole is a unit One mole of anything is 60 x 1023 of that thing This number is also called AVogadro s number Units of Temperature Temperature is measured in either degrees Celsius or degrees Kelvin Degrees in Kelvin K degrees in Celsius C 273 Remember that absolute zero is 0 KelVin which corresponds to 273 Celsius
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