Vivian Del Cid
Chemistry 101
Professor Rhani
Chapters 9 & 10
Chapter 9
Section 9.1
Kinetics is the field of chemistry that deals with the rates of chemical reactions and gives us the factors that affect rates and tells us how they work. In a homogenous reaction the reactants are in the dissolved state.
In a Heterogeneous reaction at least one reactant is not mixed with the other reactants. The particles of at least one reactant consist of big clumps of atoms, ions, or molecules. These reactions depend on the kinetic motion of the reactant particles but can only get at each other efficiently in the three states of matter. The higher the concentration of oxygen the higher the acceleration of combustion will be.
The following is a formula that expresses the relationship between rate and speed, this relationship is always expressed as a ratio:
Speed of travel = rate of vehicle motion = change in position = miles
time hour
The following formula is similar to the one above but this one expresses the rate of a chemical reaction:
Rate of reaction = change in concentration = mol/L
times
The concentration in this formula refers to the concentration of one of the reactants. The rate of the reaction is the rate at which the molarity of the reactant decreases. The rates of chemical reactions are not constant, a simple way of explaining this is by using X and Y:
X Y
The reaction begins at X (which is at its highest) and Y is zero. As you move to the right the concentration in X decreases and the concentration in Y increases. This causes a very steep curve if we were to draw one. As the reaction proceeds the curves flatten out and the concentration takes more time. The end result is a decrease in the rate of a reaction. The rate at which [X] decreases and [Y] increases is the same. In a more complex way to explain this relation the following example can be used:
X + Y Z*Bracket denote moles per liter concentration
in which,
rate **** [X]x [Y]y
The exponents can be computed by doing experiments. For example, if we find out the concentration of [X] while holding the value of [Y] constant doubles the reaction rate, the exponent of x must be 1. Temperature that by doubling is a factor that affects reaction rates, as of a reaction mixture by only 10 Celsius and can be expected to double or triple a reaction rate.
Catalysts also accelerate reactions, these are outside agents that in small concentrations accelerate reactions without themselves being changed and the rate of acceleration is called catalysis. Reactions in plants and animals require special catalysts called enzymes.
When you have a cut and use hydrogen peroxide the blood contains catalase, an enzyme hat accelerates the decomposition of hydrogen peroxide causing a fermentation of gaseous oxygen that helps to disinfect the wound. The reason why catalase is present is because hydrogen peroxide forms naturally in certain reactions. Since it is toxic our body protects us by destroying H2O2 as it forms.
A Catalyst works in one of two ways. It can accelerate a reaction at the same temperature, or it can cause a reaction to take place at the same rate at a much lower temperature.
The following is an illustration of the decomposition of potassium chlorate into potassium chloride and oxygen. The temperature varies with the presence of manganese dioxide.
Without MnO2, the2KClO3 + heat 2KCl + 3O2
temperature has to be 420 Celsius
With MnO2, the2KClO3 + heat 2KCl + 3O2
temperature need be only 270 Celsius
*The catalyst permits the decomposition to happen at a lower temperature.
Section 9.2
In order for electrons and nuclei to become reorganized, reactant particles have to collide. Collision frequency is defined as the total number of collisions occurring between the reactant particles per unit of volume per second. The only way the collision frequency can be increased without changing the temperature is to increase the concentration of the reactant particles. An increase in temperature will also increase collision frequency because the increase in speed must result in more frequent collisions.
Kinetic energy becomes chemical energy during collisions between reactant particles. The minimum collision energy for a reaction is called the energy of activation symbolized as Eact The reactant particles have a wide rage of speeds ranging form low (can even be zero) to very high speeds, this causes some collisions to be either a tap of a violent collision.
Some collisions are so insignificant that no distortion of electron clouds occur. This means little kinetic energy changes into potential energy. For every reaction there is a collision energy that provides the exact amount of energy to make an electron-nuclei rearrangement of the chemical reaction. This is the energy of activation.
Not only sufficient energy is needed, but particles must hit each other just right to pass the baton correctly.
The rate of reaction is the number of collisions that are successful, each second in each unit of volume of the reaction mixture. A high energy of activation means a small fraction of successful collisions and a slow rate of reactions. A small energy of activation would be the opposite; it would have a large fraction of successful collisions and a high rate of reactions. This all means that the rate of reaction depends on its energy of activation.
The heat reaction of an element is the net energy dismissed in other words the difference between the reactants and the products of a reaction.
This is an example of the heat reaction of hydrogen and oxygen:
This reaction occurs when the conversion of some of the chemical energy in the electron-nuclei arrangements of carbon and oxygen into the molecular kinetic energy of CO2 molecules. This is an exothermic reaction, and either a rapid exothermic reaction or a slow exothermic reaction can be producing a heat of reaction.
An endothermic reaction does not release any energy, instead there is a net absorption of energy.
Section 9.3
The number that lets us compare acid strengths is called an acid ionizationconstant, which is derived from another number an equilibrium constant.
Gulburg and Waage found that the molar concentration at equilibrium are related by an equilibrium law which is the equation:
Keq= [C]c [D]d
[A]a [B]b
*The products appear in the numerator and the reactants in the denominator.
Keq is called the equilibrium constant and it is calculated from the molar concentration found to exist at equilibrium. Keq has a different value for each equilibrium. A small value of Keq means that the reactants are favored at equilibrium.
Keq 1, reactants are favored at equilibrium
Keq 1, products are favored at equilibrium
We find the equilibrium law with the known value of Keq to be:
Keq = [H3O+][C2H3O2-] = 3.2 X 10 -7 (25 Celsius)
[H2O][HC2H3O2]
This can help us determine how weak acetic acid is as an acid. The Keq always remains a constant, this is why the it is called the equilibrium law.
Section 9.4
If the following equation is multiplied by a constant value of [H2O], a new expression and a new constant is obtained.
Keq X [H2O] = [H+][OH-] X [H2O] = a new constant
[H2O]
he new constant obtained is called the ion product constant of water and the symbol that represents this is Kw. Therefore, Kw = [H+][OH-] and no matter whether we change [H+] by adding acid or base to a solution, the value of [OH-] will adjust and the products will remain constant, the only way Kw will change is by changing the temperature.
Section 9.5
To make the comparisons between an acid-base balance, the Danish biochemist S.P.L. Sorenson came up with the concept of pH. Two ways of defining pH are:
[H+] = 1X 10-pH which tells us that the pH of a solution is the negative power to which the number 10 must be raised to express the molar concentration of a solution’s hydrogen ions ( Holum, 259).
The second way to define pH is pH= -log [H+] which is the result of taking the logarithms of both sides of the equation above, and relocating the minus sign (Holum, 259). It takes a pH value less than 7.00 for a solution to be acidic and a value more than 7.00 for it to be basic.
At 25 degree Celsius
Acidic solution pH < 7.00
Neutral solution pH = 7.00
Basic solution pH > 7.00
(Holum, 260)
An important thing to keep in mind is that the hydrogen ion concentration changes dramatically by a factor or 10 for each change of only one unit of pH.
PH refers to the molar concentration of hydrogen ions as opposed to the molar concentration of the solute contributing these ions.
A good way to get an idea of the pH of a solution is to use an acid-base indicator or a combination of them. Litmus is blue above a pH of 8.5 and red below a pH of 4.5.
Commercial papers are another way to test the pH of a solution; their containers provide a color code so you can match the color produced by a drop of solution. If a solution tested produces a highly colored result we cannot use indicators. For this type of situation pH meters are needed, they come specially equipped with electrodes that can be dipped into the solution to be tested. With this the pH values ca be read to the second decimal place.
Section 9.6
For a weak acid, an acid ionization constant Ka is needed and is derived form the equilibrium constant.
The equilibrium law for all monoprotic Bronsted acids is:
Ka = [H+][A-]
[HA]
The above is the Bronsted acids equilibrium equation
This is an equation for the ionization of a weak acid. The weaker the acid, the smaller is its Ka. The stronger the acid, the greater is its Ka. Examples of acids that have high Ka values are hydrochloric acid or nitric acid. The hydrolysis of the cation occurs when the reaction of a cation with water happens to generate a hydronium ion.
Section 9.7
Strong bases separate completely in water to release OH-, an example are sodium hydroxide. Strong bases separate completely in water to release OH-, an example are sodium hydroxide.
Weak bases react with a small percentage of water, an example of a weak base is ammonia and bicarbonate. They both react with water to make some OH-. To compare the strengths of weak bases, the base ionization constant, Kb is used. Any base is presented by B.
The base ionization constant is defined by the following equation:
Kb = [BH+][OH-]
[B]
The smaller the Kb the weaker the base is going to be.
Most of the anions in salts are Bronsted bases, and their Kb values are greater than 10-13. Anions whose conjugate acids are weak acids hydrolyze in water and tend to make the solution basic. On the other hand, Anions of strong acids do not hydrolyze enough to affect litmus.
Section 9.8
The pKais the negative logarithm of Ka, and the pKbis the negative logarithm of Kb. Both pKa and pKb are negative logarithms, which means the larger the value of pKa, the weaker the acid and the larger the value of pKb, the weaker the base. The following is the relationship that exists between Ka and Kb when we are working with a conjugate acid-base pair:
KaKb = Kw
This equation makes it easy to calculate Kb when we know Ka for its conjugate acid or it lets us calculate Ka for an acid when we know Kb for its conjugate base.
PKa + pKb = 14.00 (25 degrees Celsius)
Section 9.9
Maintenance of the pH of the blood is very critical, if the pH becomes lower, this means that the acidity of the blood is increasing the condition is called acidosis.
If the pH of the blood increases, which means that the blood is tending to become more basic or alkaline, the condition is called alkalosis.
These are both medical emergencies and they interfere with the working of respiration. Buffers prevent changes in pH when strong acids or bases are added to an aqueous solution. The way it does this is by neutralizing H+ because it is a base, and its conjugate acid neutralizes OH-.
The principal buffer at work inside cells is the phosphate buffer which consists of a pair of ions.
The principal buffer in blood is the carbonate buffer which is a carbonic acid in blood which is almost entirely in the form of CO2(aq). Ventilation is the circulation of air into and out of the lungs (Holum, 277). Hyperventilation is whenthe CO2 level increases, the respiratory center instructs the breathing apparatus to breath more rapidly and deeply (Holum,277). Hypoventilation is slow or shallow breathing. People with emphysema hypoventilate
Section 9.10
One important thing to remember about buffers is that is that they hold a pH steady but not necessarily at pH 7.
The following is the Henderson-Hasselbalch equation:
When [anion] = [acid].
Log [anion] = log 1 = 0
[acid] 1
So then pH = pKa + 0
Another important point is to note that buffers minimize pH changes but they do not completely prevent them. When dealing with the carbonate buffer in real life the Henderson-Hasselbalch equation is modified.
Section 9.10
The neutralization capacity is the capacity a solution has to neutralize a strong base. Tritration is the procedure used to measure the total acid or base of a solution neutralizing the capacity of the solution. A standard solution is one whose concentration is accurately known. The color change in an acid-base tritration happens when all the available hydrogen ions have reacted with all the possible proton acceptors. This is called equivalent point.
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