 General Chemistry
Ever wonder exactly how those reactions work? Or just want to investigate it's background a little further? Read my new series on general chemistry. This is the first part of several.
General Chemistry From Madchemist
(mostly from General Chemistry Interactive 4.0, the rest from various sources)
I. Atoms, Molecules, and Ions
All elements are composed of atoms (as you already know). All atoms of a given element are identical. Chemical compounds form when atoms combine, and the atoms themselves are not changed in a chemical reaction but rather the way they are grouped.
Atoms consist of a nucleus containing nuetrons and protons, with electrons that orbit the nucleus. Electrons have a negative charge (-), protons have a positive charge (+), and nuetrons are nuetral (). Isotopes are atoms that are the same element (still contain the same amount of protons), but the amount of nuetrons has been changed. They have the same atomic number but different mass numbers.
Atoms can combine to form molecules by sharing electrons to form a covalent bond. A molecule can be described in either two different formulas, empirical (sp?), or structural. The empirical formula of say dimethylamine is C2H4NH2 (or I think (CH2)2NH2 is empirical to), but the structural formula would be like CH2CH2NH2.
When an atom looses one or more electrons, it becomes a positive (+) ion, or a cation. If an atom gains electrons, it becomes a negative ion, or a anion. Of course opposites attract so cations will react with anions, to form ionic bonds.
All compounds are named using a system of rules. These rules vary depending on the type of compound being named. For binary ionic compounds (those that contain a metal and a non-metal), the metal, of course is alwys named first ([Na]OH,[ K]Br,[Li]I), followed by a name derived from the root name of the nonmetal (Na[OH], K[Br],Li[I]). In a compound (Type 1) containing a metal that always forms the same cation (sodium always forms Na+, magnesium Mg2+, and so on), the name of the metal is sufficient. In a compound (Type 2), that contains a metal that can form more than one cation (iron can form Fe2+, and Fe3+, and so on), the metal name is followed by a roman numeral that indicates the cation chrage (ex:Fe(III)Cl,Cu(II)Cl).
Polyatomic ions have special names that must be memorized along with their net charges, in compounds containing the entire element name and the second element named as if it were an anion. Prefixes are used to indicate the numbers of atoms present.
II. Stoichiometry
Stoichiometry deals with the amounts of chemicals needed in a reaction, and the amount produced. It's like chemical mathematics. A mole is a unit of measure equal to exactly 12 carbon atoms in 12C. This has been determined by an eqation that looks like this:
6.02214 x 10(to the 23rd). This is called Avogadros number. One mole of any substance contains Avogadros number of units. One mole of an element hass a mass equal to the elements molecular weight (ex:the MW of MD-P2P is 178.1872grams/mol). Molecular weights can be figured by finding the atomic mass of each atom in the empirical formula, and adding them all together.
A chemical equation represents the chemical reaction showing the reactants (or reagents) on the left side, and products on the right. A balanced equation gives the relative numbers in both sides.
Amounts of reagents used and amount of product produced can be calculated from the balanced equation for a reaction by using the mole ratios relating the reagents and products. The limiting reagent determines how much product can be formed (like MD-P2P in Al/Hg). Yeilds can be calculated using simple math from the limiting reagent and products to form a percentage. This must always be done in molar terms (meaning use moles when scaling, or figuring yeilds dumbass, then convert to grams).
The theoretical yeild of a product is the maximum amount that can be produced from a given limiting reagent. The actual yeild, the amount of product actually obtained in a given experiment, is always less than the theoretical yeild. That is your percent yeild.
III.Conserving Mass and Balancing Equations
If we were to examine the chemical reacton between hydrogen and chlorine gas:
1H2(g)+1 Cl2 (g)--> 2HCl(g)
reactants products
Here we see that 1 mole hydrogen (g means gaseous)+1 mole chlorine(g)-->2 moles hydrogen chloride(g).
H and H + Cl and Cl = HCl and HCl.
or
1 CH4 + 2O2 --> 1CO2 + 2H2O
C and H O and
and H and H + O and O }C+O+O=CO2, and H+H+H+H+O+O=2H2O.
and H and O
IIII. Solution Stoichiometry and Types of Reactions
Many reactions occur in aqueous solutions. Water is a good solvent for ionic solids becuase the polar water molecules interact with the ionic molecules. This helps the dissolving process. One characteristic property of a solution is it's electrical conductivity. Solutions of strong electrolytes conduct a lot of current. Weak electrolytes only dissociate slightly and form low conductivity solutions. Non-electrolytes produce no current and or ions when dissolved.
The Arrhenius definition regards an acid as a substance that produces H+ ions (protons) in solution; a base is a substance that produces OH- ions. The Bronsted-Lowry concept defines an acid as a proton donor and a base as a proton exceptor. A strong acid dissociates completely in water, and a weak acid dissociates only very slightly.
One means of describing a solution concentration is molarity:
moles of solute
Molarity (M)= ------------------ (I'm sure you've seen something like this before "add 3M HCl
liters of solution [not 3N, thats a whole other type of solution]", thats what this
pertains to)
Since molarity relates the volume of the solution and the moles of solute, it allows the principals of stoichiometry to be applied to reactions in solutions. A standard solution is one which the molarity is accurately known. When a solution is diluted, only solvent is added, which means that:
Moles of solution after dilution= moles of solute after dilution
Which leads to the relationship
M1V1=M2V2
Three types of equations describe reactions in solution: molecular equations, which show reactants and products as if they were molecules: complete ionic equations, in which all reactants and products that are strong electrolytes are represented as ions; and net ionic equations, in which only those solution components undergoing are represented.
Ex:
Water as solvent, and NaCl as solute
1H2O + 1NaCl ----> Na(+) + Cl(-) + H2O
Spectator ions (those ions remaining unchanged in a reaction) are not included in the net ionic equation.
General rules of soluability help to predict the conclusion of precipitation reactions. Volumetric analysis involves the titration of a solution containing the substance to be figured out. One type of volumetric analysis utilizes nuetralization reactions; the unkown concentration of an acid is determined by titration with a base of known concentration, or the other way around (one type of titration is say putting a drop of reaction mixture into a solution made from red cabbage; the red cabbage will change color, and it can be compared to a pH chart). The point in the titration where just enough titrant has been added to react exactly with the substance being determined is called the stoichiometric (equivalence) point.
Some types of reactions are reductions and oxidations. Also there are redox (oxidation-reduction) reactions, that envolve a transfer of electrones. One method for keeping track of electrons is to use oxidation states for atoms in compounds. A series of rules is used to assign these oxidation states. Oxidation is an increase in oxidation state (a loss of electrons). Reduction is a decrease in oxidation states (a gain in electrons). An oxidizing agent accepts electrons, and a reducing agent donates electrons.
Equations for these reactions occurring in aqueous solution can be balanced using the half-reaction method, which splits the reaction into two parts, the oxidation half, and the reduction half.
V. Precipitation Reactions
Reactions that involve the formation of a solid that will be insoluable to the solvent used are called precipitation reactions. The solid that separates is called a precipitate.
Ex:
Silver nitrate and sodium chloride in water
AgNO3(aq) + NaCl (aq) ---> AgCl (s) + NaNO3 (aq)
Ag(+) + NO3(-) + Na(+) + Cl(-) ----> AgCl + NaNO3
When one solution is poured into the other, a white precipitate forms, which is silver chloride, this can be filtered.
Solubility Rules
1. Most nitrate (NO3-) salts are soluble.
2. Most salts containing the akali metal ions (Li+,Na+,K+,Cs+,Rb+) and the ammonium ion (NH4+) are soluble.
3. Most halogen (chloride, bromide, and iodide) salts are soluble. Notable exceptions ar the salts containing the ions Ag+,Pb2+, and Hg2(2+).
4. Most sulfate salts are soluble. Notable exceptions are salts containing Ba2+,Pb2+,Hg2+, and Ca2+.
5. Most hydroxide salts are only slightly soluble. The important soluble hydroxides are NaOH and KOH. The compounds Ba(OH)2, Sr(OH)2 are Ca(OH)2 are only marginally soluble.
6. Most sulfide (S2-), and carbonate (CO3(2-)), chromate (CrO4(2-)), and phosphate (PO4(3-)) salts are insoluble or very slightly soluble.
VI. Acid Base Reactions
According to the Bronsted-Lowry model of acids (no not LSD dipshit) and bases, an acid is a proton (hydrogen ion) donor and a base is a proton exceptor. We usually work with these substances in aqueous solution, where an acid will donate a proton to water to form the hydronium ion,
H3O+. For example, when gaseous HCl dissolves in water, the HCl molecules dissociate into ions:
HCl(g) + H2O(aq) -----> H3O+(aq) + Cl- (aq)
However the hydronium ion in aqueous solution is, on average, hydrated by three water molecules to give an average compisition of H9O4+.
When a base is dissolved into water, it will dissociate into sodium and hydroxide ions:
NaOH(s) + H2O ----> Na+ (aq) + OH-(aq)
The aqueous hydroxide ion is also hydrated by an average of three water molecules to give an average compisition of H7O4-.
Thus, a solution of HCl (or other strong acid) in water consits of hydrated hydronium ions and hydrated chloride ions. A solution of NaOH (or other strong base) in water consits of hydrated sodium ions and hydrated hydroxide ions. The reactive species in these solutions are the hydronium ion and hydroxide ion; it is these ions that will react if we mix a solution of HCl with a solution of NaOH.
The sodium ion and the chloride ion will not precipitate in aqueous conditions, so they remain unchanged, they are spectator ions.
When an acid and base are mixed, they form water.
H3O+(aq) + OH-(aq) -----> 2H2O(l)
Only very small quantities of each ion can coexist without reacting to form water. These reactions are all called neutralization reactions.
Ex:Neutralization between acids and bases
NaOH + H2O ---> Na+ + OH- + H2O
When HCl is added:
HCl + H2O ----> H30+ + Cl-
net ionic:
Na+ + H3O+ + Cl- + OH- ---> 2H2O + NaCl
or
Al(OH)3 + H2O ---> Al3+ + OH- + H2O
When HBr is added:
HBr + H2O ---> H3O+ + Br-
net ionic:
H3O+ + Al3+ + OH- + Br- ----> 2H2O + Al(Br)3
or
HI + H2O ---> H30+ + I-
when base is added:
Na+ + OH- + H3O+ + I- ----> NaI + 2H2O
VII. Oxidation-Reduction Reactions
Reactions that involve the transfer of one or more electrons are called oxidation-reductions, or redox reactions. We can keep track of electrons in redox reactions by means of oxidation numbers. An element that loses electrons increases it's oxidation numer and is said to be oxidized, while one that gains it's electrons gecreases its oxidation number and is said to be reduced. A substance that causes another substance to be oxidized is called an oxidizing agent. And a reducing agent for reduction.
One type of redox reaction is a single displacement reaction, in which a more active element from it's compound. For example, zinc metal displaces tin metal from a solution of tin(II) chloride.
Zn(s) + SnCl2(aq) ---> ZnCl2(aq) + Sn(S)
This reaction can be represented as a net ionic equation, showing only the substances that are undergoing a change.
Zn(s) + Sn2+(aq) --> Zn2+(aq) + Sn(s)
In this reaction, Zn is the ruducing agent, and Sn2+ is the oxidizing agent.
Another ex:
Zn(s) + CuSO4(aq) ---> ZnSO4(aq) + Cu(s)
the net ionic equation is:
Zn(s) + Cu2+(aq) --> Zn2+(aq) + Cu(s) (the other product would be Zn(SO4)2)
If we place a piece of copper in a solution of zinc sulfate The copper will not displace the zinc.
Only certain elements will displace other ones. This series of elements is called the Activity Series. An element high in the series will displace lower ones, from a solution of it's compound.
Activity Series: Li, K, Ba, Ca, Na, Mg, Al, Zn, Fe, Cd, Ni, Sn, Pb, H, Cu, Hg, Aq, Au
Coming next:
Gases
Thermochemistry
Atomic Structure and Periodicity
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2001-04-02, 13:08
