Dr. Rahni Chemistry 101

Chem101- 9

Tom Douwes 7/3/04

Dr. Rahni Chemistry 101

Chapter nine of the book goes into greater detail about chemical reactions and their mechanics. This chapter like chapter two is split up into nine separate sections. These sections are “reaction kinetics, equilibrium laws, ion-product constant of water, the pH concept, acid ionization constants, base ionization constants, pKa and pKb concepts, chemical reaction buffers, and acid-base titration,” (1, pg. 286-288). The chapter begins with the section dedicated to features that change the speed of a chemical reaction.

The sub-type of chemistry that “deals with the rates of chemical reactions,” (1, pg 247) is called kinetics. The name implies that the study of chemical reactions is really the study of some form of movement. The movement of the particles and energies involved in the chemical reaction is the most likely subject of study. The rate of a chemical reaction is essentially the speed at which it takes place. This means that the rate of a chemical reaction is “always expressed as a ratio,” (1, pg 247). The ratio for the rate of reaction is,

Change in Concentration / Time

The change in concentration referring to the concentration of one of the reactants involved in the chemical reaction. In a laboratory, however, this ratio is not exact enough, so scientists use the molarity of the reactant in the ratio and use the seconds unit of time. The ration then appears thusly,

mol/L

______

s

There are four things that affect the rates that reactants affect one another. These four things are whether a catalyst is introduced to the reaction or not, the nature of the reactants, the temperature where the reaction is occurring, and the concentration of the reactants that are going through the change into another substance. The way these different factors cause a change in the rate of chemical reaction is by changing the way that the particles of the reactants “collide” with one another.

One of the “major theories in kinetics,” (1, pg 251) is the collision theory. The central tenant of the collision theory is that through kinetic movement particles of a reactant collide. If the collision is successful it causes “electrons and nuclei to become reorganized relative to one another,” (1, pg 251). This reorganization is what causes the product to have different properties then those of the reactants.

A catalyst is an “outside agent that in small concentrations accelerates reactions without themselves being changed,” (1, pg 250). A catalyst does this by “lowering the activation energy,” (lecture) of the chemical reaction. The activation energy of a chemical reaction is the energy necessary to begin the reaction. In all living things catalysts are necessary to perform life sustaining processes. These catalysts are referred to as enzymes. Enzymes allow the organism to perform necessary chemical reactions that would other wise require too much energy to perform. The level of energy necessary to begin a chemical reaction is decided by the nature of the reactants being used.

The state of matter that a reactant is in affects the rate of its chemical reaction. This is because “reactions depend on the natural kinetic motions of the reactant particles,” (1, pg 247). For a chemical reaction to occur the separate particles of the reactants must collide. This means that they have to intermingle freely together. The more easily they can intermingle and collide the faster the reaction. The particles of a reactant that are involved in a chemical reaction are “atoms, molecules, or ions,” (1, pg 247). These particles when in a solid state are more tightly locked together and so it is more difficult for them to collide with other particles. The particles in a liquid or gas, however, are much freer and can easily collide with one another. Also the manner in which the reactants are combined with one another effects the rate of reaction. If the reactants are in a homogenous solutions then they are “in a dissolved state either in a liquid solvent or gaseous state,” (1, pg. 247).

As stated above this allows them to move easily collide since they are in a gas and liquid state. It also means that they two reactants are closer to being evenly dispersed then if they were in a heterogeneous reaction. In a heterogeneous reaction “at least one reactant is not intimately mixed with the others,” (1, pg. 247). This means that the particles are not evenly disburse with one another. It also means that they are in different states making it more difficult for the particles to interact.Finally the sizes of the particles of the reactants affect the rate of reaction. The smaller the individual particles of reactant the largerthe total surface area the reactant has. This means that it is easier for the different particles to collide. This makes the chemical reaction occur at a faster rate. An example of this is when one tries to combust magnesium. If you burn it in a solid strip or wire it takes much longer to burn then when you try to burn it in a powdered state.

Particle interaction is key to the rate of a chemical reaction. This is why the concentration of the reactants involved affects the reaction rate. The higher concentration of each of the different reactants of the chemical reaction in the system the faster it is likely to occur. This is again because of the need for the particles to strike or collide with one another. If the concentrations of the reactants are higher then it is more likely that the particles of the substances will collide.

Since the kinetic motion of the particles is what drives the chemical reaction then we can say that the higher the motion the higher the rate of reaction. This is how temperature affects the reaction rate. When you increase the heat of a substance you increase the particle movement in it. This means that if you increase the heat of two reactants you force the particles to move faster. This increases the probability that the particles will collide with one another, which increases the rate of chemical reaction. Temperature does not just increase the number of collisions that occur. It also affects the number of successful collisions.

For a chemical reaction to occur there must be a collision of the particles of the reactants in the reaction. But not all collisions will cause a chemical reaction. A collision must have enough force in it to cause a “new chemical bond in the products,” (1, pg 252). The level of force or energy necessary to cause these new bonds to form is called the energy of activation. The symbol for the energy of activation is Eact. When the energy of activation is reached successful collisions can occur, which means that the chemical reaction can start. This means that a more refined definition of a rate of a reaction is not simply the speed of the reaction. Rather it is “the number of successful collisions that occur each second in each unit of volume of the reacting mixture,” (1, pg 252). The idea of energy of activation also helps us understand exothermic and endothermic reactions better.

Exothermic reactions give off heat while endothermic reactions require constant input of energy to continue their reaction. In a reaction that is exothermic the energy of activation is such that it forces the particles of the reactants to form new chemical bonds that are not stable. This unstable product must then go through another change to become a more stable substance. The transition from unstable to stable causes more kinetic energy. This kinetic energy is, of course, the heat we experience from an exothermic reaction. In an endothermic reaction there is no release of kinetic energy. Instead the energy that must be constantly fed into the reactants is changed into potential chemical energy.

The next section of this chapter revisits chemical equilibrium. When a chemical reaction reaches chemical equilibrium “the opposing reactions take place at identical rates,” (1, pg 255). . It was discovered that “how the molar concentrations of species at equilibrium interact,” (1, pg 256). The “equilibrium equation,” (1, pg. 256) can be represented in an equation where A and B are the reactants and C and D are the products. The letters a, b, c, and d are the coefficients of the substances involved in the reaction.

aA + bB \===\ cC + dD

This equation shows that the rates of chemical reaction going both “forward and reverse,” (1, pg. 256) between the reactants and the products are equal. It was discovered that “molar concentrations are related by an equilibrium law,” (1, pg. 256). This law is expressed through an equation.

Keq = [C]c [D]d / [A]a [B]b

In the equation above Keq is the “equilibrium constant,” (1, pg. 256). It can be used to tell “whether the products or the reactants are favored,” (1, pg. 256) at equilibrium. Which group is favored is easily figured out by the below equation.

Keq < 1, reactants are favored at equilibrium

Keq > 1, products are favored at equilibrium

Keq is important because it is a constant. This is important because a constant is something “in nature,” (1, pg. 257) that does not change its value regardless of the changes that occur to the value of things connected to it. The concentration of chemical reaction that is in equilibrium can change but the equilibrium constant retains the same value. Each chemical equilibrium has a equilibrium constant. Certain equilibrium constants are very important in chemistry. One such equilibrium constant is the “the self-ionization of water,” (1, pg. 257). The chemical equation for this process is as follows.

H2O \==\ H+ (aq) + OH-(aq)

The equilibrium law for waster self-ionization is

Keq+ = [H+] [OH-] / [H2O]

By multiplying both sides by H2O we create a new constant the “ion product of water,” (1, pg. 257). This new constant is represented by the equation below.

Kw= [H+] [OH-]

This constant can be referred to as the ion product of water because it is no longer technically a equilibrium law because it no long has in it the H2O the reactant. It does however still behave like one.

The next section of the chapter deals with the concept of pH. A value of pH is used to describe hydrogen cations because it is easier to use for these cations then molar concentrations. The concept of pH can be defined in one of two ways.

[H+] = 1x 10-pH

Or

pH = - log [H+]

These equations are “completely equivalent,” (1, pg. 259). They both state that pH is “the negative power to which the number 10 must be raised to express the molar concentration of a solution’s hydrogen ions,” (1, pg 259). There is also a series of equations that describe pOH.

[OH-] = 1 x 10-pOH

And

pOH = - log [OH-]

These equations refer to the negative power that 10 have to be raised to express the molar concentration of hydroxide anions in a solution. By adding up the values ofpositive pH and positive pOH you get fourteen. This is why the pH scale goes from one to fourteen. The way the pH scale works is the lower the pH level the higher the acidity of the solution. This is because “pH occurs as a negative exponent,” (1, pg. 260) in the equations that define it. A good way to quantify a solutions acidity at twenty five degrees Celsius is shown below.

Acidic solution has a pH < 7.00

Neutral solution has a pH = 7.00

Basic solution has a pH > 7.00

The ph concept is very helpful in measuring the acidic or basic nature of a substance. It lets scientists know the general strength of an acid or base. Another way to tell the strength of an acid or base is by looking at the ionization constant for that particular acid or base.

Acids and bases both have ionization constants each has its own equation describing it. The equation for acid is below. In it Ka stands for the acid ionization constant HA stands for a acid and A- stand for the substance that ionizes from H+.

Ka = [H+] [A-] / [HA]

The equation for the base ionization constant is,

Kb= [BH+] [OH-] / [B]

In this equation Kb stands for the base ionization constant, OH- stands for hydroxide BH+ stands for the conjugate acid, and B stands for any base. The acid ionization constant and the base ionization constant tells us the relative strength of the acid or base being observed. The smaller the acid ionization constant the weaker the acid. Which means the less is disassociates into ions when put into water. The same thing holds true when comparing the value of the base ionization verses the base’s strength. Ka and Kb also have other similarities and uses. When the base and acid ionization constants of “conjugate acid-base pair,” (1, pg. 272) are multiplied together they equal the water ionization constant. The formula for this is simple.

KaKb = Kw

This means that when we know the value of an acid’s ionization constant we can calculate the ionization constant of its base conjugate. Another concept used to measure the strength of acids and bases is that of pKa and pKb. “The pKa is the negative logarithm of Ka and the pKb is the negative logarithm of Kb,” (1, pg. 271). In formula from they appear like below.

pKa = - log Ka

and

pKb = - log Kb

The principal for the ionization constants of bases and acids occurs for there negative logarithms in reverse. The higher the value of pKa the weaker the acid and the higher the value of pKb the weaker the base. Also because pKa and pKb are logarithmic functions of Ka and Kb they have the same function of equaling fourteen when summed and at twenty five degrees Celsius. The equation illustrating this is below.

pKa + pKb = 14.00

While the pH and Ka of a solution can tell us how acidic a solution is by giving the value of the hydrogen ions in it neither of these concepts gives all the information about a solutions chemical properties. For example neither of these tells us a solutions neutralizing capacity. “The neutralizing capacity of a solution is its capacity to neutralize a strong base,” (1, pg 284). There is a procedure that is used to discern the neutralizing capacity of a solution. The procedure is called titration. When a scientist performs this procedure he “compares the volume of a solution with an unknown concentration to the volume of a standard solution that exactly neutralizes it,” (1, pg 284). A standard solution being a solution that has a known concentration.

It does not take much for a strong acid or base to change the pH balance of a solution. If such a radical change in the pH balance occurs in a living thing then the life form perishes. We can see this in nature when plants pull in water that is polluted or affected by acid rain. The plant withers and dies. Our internal chemistry is very important to our health. There are terms to describe when our body chemistry shifts to far on either side of the pH scale. When our bodies become to acidic, that is shift to a value on the pH scale that is too low to be healthy a person is said to have acidosis. If instead the body’s chemistry shifts to a value on the pH scale that is too high then it is suffering from alkalosis. When either of these medical conditions become severe “they interfere with the smooth working \of respiration, thy physical and chemical act that brings oxygen in, uses it and removes waste carbon dioxide,” (1, pg. 275). This is particular bad in the case of acidosis as “the ability to breath out carbon dioxide is essential to the control,” (1, pg 276) of this condition. The body does however have a protection against sudden changes in its pH level. It uses buffers to block these sudden shifts in acidity.

A buffer is a “combination of solutes,” (1, pg 277) that is made up of a base and its conjugate acid. These two substances work together to neutralize other acids or bases in the bodies fluids that might shift the pH balance of the body to far in either direction on the pH scale. When a fluid contains a buffer in it, that fluid is “said to be buffered against changes in the pH,” (1, pg 276). The body’s cells and the body’s blood both have there own principal buffer. The body’s cells use a phosphate buffer. The phosphate buffer is made up of a “pair of ions, HPO42- and H2PO4-, the monohydrogen and dihydorgen phosphate ions,” (1, pg. 276). The buffer of the blood is called the carbonate buffer. The buffer is composed of the “conjugate pair, H2CO3 and HCO3-,” (1, pg. 276). It is this buffer that allows the body to prevent acidosis. When we breathe we make the neutralization of the acid in our blood permanent. The process by which we do this is expelling the carbon dioxide brought to our lungs by the blood. The process of “circulation of air into and out of the lungs,” (1, pg. 277) is called ventilation. When there is too much carbon dioxide in out blood the body goes into hyperventilation. This means we breathe in and out faster in an effort to pull in more oxygen and expel carbon dioxide. The opposite of this process is called hypoventilation; this is where we breathe slowly and shallowly. This prevents us from exchanging oxygen and carbon dioxide at the correct rate and may lead to acidosis.

The system of a buffer is that of a weak acid and its conjugate base. Further the base can be represented as an anion. So a buffer can be represented by the following equation.

[H+] = Ka x [acid]/ [anion]

By using the [H+] we can discern the pH of a buffer solution. By reworking the equation we can turn it into Henderson-Hasselbalch equation. This is down by taking the logarithms of both sides of the above equation and turning it into.

pH = pKa = log [anion]/ [acid]

By doing this conversion we can easily gage the level of acidity in the buffers that we carry around with us every day.

Bibliography

1)Fundamentals of General, Organic, and Biological Chemistry. Holum, John R. John Wiley & Sons, Inc. 1997.

2)Lecture. Dr. Rahni. Chemistry 101. 06/30/04.

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