Chem 2110 Test 3
-a solution is a homogeneous mixture of two or more substances; these can be all gases, all liquids, all solids, or a mixture of substances in difference physical states
-common ones consist of a solute (solid substance) dissolved into a solvent (liquid)
-if it is dissolved, the solute is soluble
**when the solvent is water, the solution formed is called an AQUEOUS solution
|Solubility in water|
MISCIBLE – liquids that are soluble in each other (called hydrophilic)
IMMISCIBLE – liquids that are not soluble in water (called hydrophobic)
-the amount of a substance that is dissolved in a given amount of water
-DILUTE – low concentration
CONCENTRATED – high concentration
SOLUBILITY – is the maximum amount of a solute that can be dissolved in a given amount of water
***it usually increases with temperature
SATURATED – a solution that contains the maximum amount of a solute at a given temperature
RECRYSTALLIZATION – a laboratory technique where temperature is lowered in a saturated solution to acquire crystals
|Three Types of Crystals|
-solids made of covalent molecules held together by intermolecular forces
-in the case of ice, the formation of H-bonds is the key factor for its structure
-molecular solids melt at very low temperatures because of the relatively weak strngth of the intermolecular forces
-three commonly seen molecular solids at room temperature and pressure are the iodine (I2), sulfur (S8), and phosphorus (P4) crystals, each having a formula that represents the smallest structural unit in the molecular solid.
3D NETWORK SOLIDS
-solids within which the regular arrangement of ions extends indefinitely in three dimensions and no individual molecules can be recognized. The ions are held together by IONC bonds in a fashion that ensures that each ion is bonded to as many other ions of opposite charge as possible.
-such solids have very high melting points
-also called ionic network solids
-melt at 3600 degrees and boil even higher… they do not dissolve in water.
-is another type of 3D network crystals
–the smallest unit is the carbon atoms, and the atoms are held together by covalent bonds
-also called covalent network solids
Concentration of commercial products vs lab
-usually indicated by mass or volume percentage
*Mass % = Mass solute/(mass solute+mass solvent) X100
*Volume % = Volume solute/(Volume solute+volume solvent) X100
Molarity = Moles of solute/Liters of Solution
|Solubility in water depends on:|
-The substance must be able to attract the dipoles of the water molecules. This attractive force must be great enough to overcome the original forces that hold the molecules, or ions, or atoms of the substance together.
-in other words, only polar substances are soluble in water
Compounds that have N-H, O-H, and F-H bonds are particularly hydrophilic because they can form H-bonds with water.
-a solution made up of ions, such as a NaCl solution, is a charged solution, meaning that it can conduct electricity, even though the net charge is zero
ELECTROLYTES – substances that dissolve in water and result in charged solutions
-NaCl is a strong electrolyte because it ONLY exists as ions in water
**Not all compounds that dissolve in water are strong electrolytes, some are weak electrolytes and some are non-electrolytes
**a polar covalent compound that dissolves in water but does not ionize completely is called a weak electrolyte… ie HF
**If strong electrolytes ionize completely in water, weak electrolytes ionize only slightly in water, and non-electrolytes are those that do not ionize at all
-shows how a compound ionizes in something
HCl + H2O > H3O + Cl-
-HCl is here a strong electrolyte, as it ionizes completely in water.
-used to indicate that the forward and reverse reactions are taking place simultaneously.
-eventually the rate of the reverse reaction becomes the same as the rate of the forward reaction, and the system is then said to be in a state of equilibrium
*********this is dynamic equilibrium… reactions in both direction are constantly occuring… it is just that the same proportion are changing in each direction, so the concentrations remain the same.
– the name for water’s self-ionization
-a very small proportion ionizes, as pure water does not conduct electricity.
equilibrium constant/reaction quotient
-Any chemical reaction can be treated as a system that eventually reaches equilibrium
-this is where concentrations of the products and reactants remain constant
;K; is used as the symbol for the equilibrium constant
***0 in general, the equilibrium constant for any reaction is written as the product of the concentrations of all products divided by the product of the concentrations of all reactants. If there is a coefficient, it should be to the power.
For example (water):
K = [H30][OH]/[H2O]2
**********the concentrations are those found at equilibrium*******
We can write a similar relationship for the concentrations of all participants at any given time of the reaction. Such a relationship is called the reaction quotient, Q. Q=(same formula)
*********by comparing the value of the reaction quotient with the equilibrium constant, we can predict the direction of a reaction.
|ionic product constant of water|
-based on the fact that, in any dilute solution (including pure water), the concentration of water can be considered as nearly constant….
-therefore, we can multiply K by [H2O]2 to get a new constant Kw.
Kw= ionic product constant of water = (K)([H2O]2) = (1.00 times ten to the negative 7 M)2
= 1.00 x 10-14 mol2L-2
-This equilibrium constant, Kw, applies not only to pure water but also to the self ionization of water in any acqueous solution. That is, the product of the hydronium ion and the hydroxide ion is always the same at 25C in any aqueous solution that has reached equilibrium.
|Relationship between Q and K|
-In general, if we know the initial cncentrations of the reactants and the products, we can predict the direction in which a reaction will proceed, as follows:
**If Q;K, the concentrations of the products will increase and the concentrations of the reactants will decrease until equilibrium is reached.
**If Q;K, the concentrations of the reactants will increase and the concentrations of the products will decrease until equilibrium is reached
**If Q=K, the reactants and the products are at equilibrium, and there will be no change in the concentrations of the reactants or the products.
|The pH scale/litmus paper|
-Acids are sour and bases are bitter.
-pH paper strips are used to identify if a solution is acidic or basic(alkaline).
;RED litmus paper and BLUE litmus paper are two exambles…. a basic solution will turn red litmus paper blue, whereas an acidic solution will turn blue litmus paper red. Some pH paper can be ssed to determine the pH of a solution to at least two significant figures.
pH = -log[H3O]
ie. for pure water:
pH= -log[1.oo x10-7] = 7.000
****Note that pH has no unites. Also note that because pH is a logarithmic scale, the number before the decimal point should not be included in counting significant numbers. Thus the pH value usually has one more digit that the concentration value.
*see example page 96
|What is pOH and what is the relationship between pH and ppOH?|
pOH = -log [OH]
pKw = -log [Kw]
***pKw = pH + pOH = 14 at 25C
Ka = the acid dissociation constant
-these constants, like any others, change only with temperature
***The larger the Ka, the stronger the acid***
Ka = [H3O][Conjugate Base]/[Acid]
**The larger the pKa, the weaker the acid is**
**When comparing the strngth of acids, it is easier to compare the pKa values than the Ka values, for the same reason as the use of pH values to compare the acidity of aqueous solutions.
|6 strong acids|
HCl (hydrochloric acid)
HBr (hydrobromic acid)
HI (hydroiodic acid)
HNO3 (nitric acid)
H2SO4 (surfuric acid)
HClO4 (perchloric acid)
B-L acids are those compounds that donate one or more protons in a reaction. (ie. HCl is a proton donor and therefore a B-L acid)
-the general symbol is HA
B-L bases are those compounds that accept one or more protons in a reaction. They are proton acceptors
-the general symbol is B
Lewis acids are any compounds that accept a pair of unshared electrons in a reaction. Ie. HCl is an electron pair acceptor, and therefore a Lewis Acid
Lewis baare any compounds that donate a pair of unshared electrons in a reaction. They are electron pair donors
– a substance, such as water, that can act as both an acid and a base
-When a base reacts with water, the base is said to be protonated. The production of hydroxide ion is the general result of a base dissolving in water. This causes an increase of [OH] in the solution and consequently a decrease of hydronium.
The strngth of bases can be quantitatively measured by the equilibrium constants for the reaction of the base with water.
Kb = K(of base) X [H2O] = ([BH][OH]/[B])
-The equilibrium constant Kb is called the base dissociation constant of a base. The larger the Kb, the stronger the base is.
pKb = -logKb
**The larger the pKb, the weaker the base is.
Alkali- metals formed between hydroxide and group one elements.
-these are the strongest inorganic bases
ie. NaOH, KOH
;-they dissolve completely
Alkaline Earth Metals – group II metals form 2+ ions so the bases formed are such as Be(OH)2, Ca(OH)2, etc
-** most of these reactions are highly exothermic, and in all cases hydrogen gas is produced. These metals are usually kept in oil solvents that can keep the moisture away
**The hydroxides can also be formed from the reaction of the metal oxides with water. Hydrogen gas is not produced in these reactions.
|conjugate acid base pair|
-Shows that there is a base in any acid an, for any base, there exists a corresponding acid.
Ka(HA) X Kb(A–) = [H3O]x[OH] =Kw = 1.00×10-14 mol2L-2
pKa(HA) + pKb(A–) = pKw = 14
****THE LARGER THE pKa OF AN ACID, THE STRONGER THE CONJUGATE BASE***
***ie. THE WEATER THE ACID, THE STRONGER THE CONJUGATE BASE.
;Stronger acids will react with stronger bases to produce weaker acids and weaker bases
|The Leveling effect of water|
-Strong acids and strong bases are all completely ionized in dilute aqueous solutions.
whatever the strong acid or base is, it is inevitably converted into either hydronium or hydroxide..
*****IN aqueous solutons, there is no base stronger than OH– and no acid stronger than H3O+
-this ;wiping out all difference; action is called the leveling effect of water. Certain bases such as NH2 and H– are much stronger bases than OH, however if water is chosen as the solvent, they will react with water first and will not be able to serve as the inteded Lewis base.
|Two major categories of chemical reactions:|
1)When one or more electrons are lost by one reactant and gained by another reactant, the reaction is a redox reaction
2)When one pair of unshared electrons is donated by one reactant and accepted by another reactant, the reaction is an acid-base reaction. Most acid base reactions involve proton transfer.
|When an acid reacts with a base…|
-salt(dissolved) and water are formed
-the result of an acid-base reaction is salt and water
The reactions between any B-L acids and bases are simpy the reaction of H30 and OH to form water.
Any B-L acid-base reactions can be represented by:
H30 + OH = 2H20
**reactions between strong B-L acids and bases are often called neutralization reactions.
-general name for compounds formed from acid-base reactions.
-ie. NaCl is the produce of the reaction between HCl and NaOH… water is the other product
-all salts are ionic compounds composed of as teast one cation and one anion
-the cation generally comes from the base, and the anion from the acid
-when naming, the cation always comes before the anion
|Estimating pH when salts dissolve in water|
1)salts formed from strong acids and strong bases give neutral solutions
2)Salts formed from strong acids and weak bases give acidic solutions
3)Salts formed from weak acids and strong bases give basic solutions
* the stronger one controls the pH of the salt water.
* For salts formed from weak acids and weak bases,; we will have to compare the pKa value of the acid with the pKb value of the base to make a prediction
pH = pKa + log([base]/[acid])
-the most important application of the equation is in making buffer solutions.
BUFFER SOLUTION – a solution made by mixing a weak base and its conjugate acid.
-the closer the concentrations of base and acid, the closer the pH gets to equalling the pKa
pH=pKa + log ([base]/[acid]) = pKa + log(1) = pKa
-this means we can make buffer solutions of any desired pH simply by choosing the conjugate weak acid-base pair that has the right pKa
-a buffer solution with equal base and acid (1) will be most eficient in resisting changes in pH
-is sometimes added to a solution to signal the end of an acid base reaction
-to function as an indicator, the compound must be a weak acid of an easily detectable color. It’s conjugate base must have a different color so that a color change can be observed as pH changes
-Litmus is one of the most commonly used indicators.
-Base form is blue, acid form is red.
-another commonly used indicator is phenolphthalein.
-basic form is pink, and acidic is colorless.
We write general acid-base equilibrium for any indicator HIn:
HIn + H2) <> In– + H30
If using Hasselbalch’s equation, we can see that when concentration of indicator acid and conjugate base are equal, it yields one, and the middle of the indicator’s color change, at this point we have a pH=pKa.
-This means we can chooose a suitable indicator for a desired pH range by looking at the pKa of the indicator.
-Universal indicators are a mixture of several indicators that have several color changes within the defined pH range.
-an acid that can donate more than one proton to a base.
-for strong acids (H2SO4) the dissociation of both protons occurs instantly.
-for weak polyprotic acids, (H3PO4), the dissociation of protons is stepwise and each step is an equilibrium
-according to the equilibrium constants, the dissociation of the second step is more difficult, and the 3rd is even more difficult.
-the more negatively charged an ion, the harder it is to give up a proton.
-dilution is a frequently used technique in most laboratories. Often is means adding water to a concentrated solution to make it less concentrated.
The number of the solute molecules stays the same before and after dilution
ppm means parts per million, more specifically, grams per one million milliliters.
M1V1 = M2V2
M=concentration (1 is before, 2 after)
V = volume (1 before, 2 after)
-use this expression for caculations of any dilution.
%1V1 = %2V2 (%=any percent concentration)
-a technique frequently used to accurately determine the concentration of one solution by reacting it with another solution for which the concentration is accurately known.
-In colorless compounds, an indicator must be used to signal the arrival of the equivalence point, which is the point during the titration at which just enough of one compound has been added to completely react with the other compound.
**at the equivalence point of any acid-base titration, e have Moles OH = Moles H3)
-CbaseVbase = CacidVacid
-can be used to calculate the concentration of the unknown acid or base
In the case of NaOH and H2SO4, it takes two NaOH to neutralize every H2SO4, the molar relationship between the acid and the base is 1:2, and the follow expression can be used to calculate the concentration of the unknown acid or base:
CbaseVbase = 2CacidVacid
titration curve (also pH curve)
-another way to determine the equivalence point of an acid-base reaction
-require a pH meter to keep track, rather than using color change
-curve is constructed by plotting pH readings vs the volume of NaOH added.
**When a weak acid is titrated with a strong base or a weak base with a strong acid, the titration curves look similar to the typical titration curve. The only difference is that the pH jumps are not as sharp and the pH at the equivalence point is not 7.0. This is because at the equivalence point the salts formed are no longer neutral salts.
|Amino Acids and pH|
-amino acids are the building blocks of proteins… they all consist of an amine group (NH2), a central carbon with hydrogen and side chain, and a carboxylic acid groub (COOH)
-glycine is the symplest, where R=H
-amino acids are amphoteric
-in any amino acid, the carboxylic group is a weak acid and the amine group is a weak base. The strength of each can be measured by their pKa values.
-when an amino acid dissolved in solution, the groups are either protonated or deprotonated, depending on the pH of the solution
see Q 92-94
ZWITTERION – is a dipolar amino acid ion structure… the net charge is zero
-some amino acids have an acidic side chain or a basid side chain. In those cases, a pI calculation can help decide at what pH they exist as dipolar ions. pI is called the isoelectric point, it is the pH at which the amino acid has zero net charge.
pI = (pKa1 + pKa2)/2
-for glycine this is about 6
-for acidic side chains, about 3
-for basic side chains, greater than 7
electrophoresis – makes use of this principle to achieve the separation of amino acids and protein fragments in biochemical research. Proteins are long chain molecules made up of amino acids and have similar structureal dependency on pH
|titration curves for amino acids and buffer regions|
-the structural dependency of an amino acid of pH can be visually observed from the titration curve for the amino acid.
is the horizontally flart part of a titration curve
-within the buffer region the pH remains relatively constant even though more and more naOH is being added
|Writing the Equilibrium Constant Expression|
-For any chemical equilibrium we can write a balanced general equation as this:
aA + bB + cC <> pP + qQ + rR
where coefficients are represented by lower case letters.
The equilibrium constant expression is written as:
K = ([P]p[Q]q[R]r)/([A]a[B]b[C]c)
* products over reactants *
-Sometimes one observed overall reaction is the final outcome of several reactions taking place at the same time.
-The overall reaction is the sum of the component reactions.
***When we add several reactions to obtain an overall reaction, we must multiply their equilibrium constants to obtain the equilibrium constant for the oberall reaction***
|The Position of Equilibrium|
-The value of equilibrium constant can be used to predict the position of equilibrium. We use pKa values to measure the strength of acids and bases becasue Ka is the equilibrium constant for the dissociation equilibrium of a weak acid or that of the conjugate acid of a weak base.
***The equilibrium constant for a reaction as writtten from right to left is the reciprocal of the equilirbium constant for the reaction written from left to right**
equilibria that involve solids and gases, solids and liquids, or liquids and gases… these do not involve water.
**For solids the term concentration does not apply…. the number is fixed and cannot vary. The concentration of a pure solid is constant and is independent of the amount of pure solid that is present.
***The pure solids and liquids taking part in heterogeneous equilibria are not included in the equilibrium constant**
|Le Chatelier’s Principle|
-The example of the decomposition of CaCO3 into CaO and CO2 in the open air illustrates one of several ways to shift the position of a chemical equilibrium towards producing more products….. by reducing the concentration of the products
If we want to shift the equilibrium to producing more products, we should make sure that Q;K. If we want to shift the equilibrium towards producing more reactants, we should make sure that Q?K.
In other words, there are two ways to shift the position of equilibrium forward:
1)add more reactants
Similarly, there are two ways to shift the position of equilibrium backward:
1) reduce the amount of reactants
If any of the conditions affecting a system at equilibrium are changed, the position of the equilibrium shifts so as to minimize the change.
in other words…
When the balance of an equilibriated system is disturbed, the system reacts to restore the balance
This is Le Chatelier’s principle
Can be restated:
When the concentrations of any of the reactants or products in a reaction at equilibrium are changed, the position of the equilibrium shifts so as to reduce the change in concentration that was made.
|Temperature and equilibrium|
-Equilibrium constant is dependent on temperature… changing the temperature at which to carry out a reaction can shift the equilibrium.
-If we know the equilibrium constants at both temperatures, before and after the change, we can quantitatively predict the direction of the shift.
An exothermic reaction gives out heat, whereas an endothermic reaction takes in heat.
|-volume of gas inversely varied with pressure|
x = -b+/- SQRT(b2-4ac)
|6 strong bases|
|Acetic Acid/Acetate/Formic Acid/Formate/Benzoic Acid/Oxalic Acid|
Formic – carboxylic with hydrogen
acetic acid – carboxylic with 1+ carbon and hydrogen
formate/acetate = conjugate bases
Benzoic acid – benzene with carboxylic acid
Oxalic acid – 2 carboxyl groups
|Two criteria contribute to the occurence of a reaction:|