Molecular As Well As Ionic Biology Essay

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Acids are substances which are kind of like molecular as well as ionic. They are ionic because once they are dissolved into water, they dissociate into H+ ions. They look like they have molecular formulas. The H+ ions in the acid dissociate into water therefore causing the acidity of the solution. Acids are seen everywhere around us. This can be seen in lemons (citrus acid) as well as coke (carbonic acid). Some acids have the property of being strong electrolytes and therefore once they are put into a solution of water, they dissociate into ions. Some aspects of the behavior of acids and bases can be explained adequately with the theory developed by Arrhenius as part of his studies of electrolytic dissociation. They react with metals to form hydrogen gas. All acids taste sour. Their pH is less than 7. They turn blue litmus paper turn red. Bases are also electrolytes. They turn red litmus paper blue. This can be seen in soap that is used in daily life. All bases taste bitter. Arrhenius proposed that in aqueous solutions, strong electrolytes exist only in the form of ions, whereas weak electrolytes exist partly as ions and partly as molecules. The theory basically stated that once acids were dissolved in water, they would break up into H+ ions and that when bases were dissolved in water, they would break up into OH- ions. For example, when hydrochloric acid dissolves in water, the molecules ionize completely, yielding hydrogen ions, H+, as one of the products. The equation of the following reaction can be described as follows.

Even the bases when dissolved in water, dissociate into ions. For example, if sodium hydroxide is taken and dissolved in water, they dissociate into Na+ and Cl- ions. The equation for the following reaction can be described as follows.

Neutralization reactions are reactions in which acid and base yields water and salt. The neutralization reaction of HCl and NaOH can be represented with the ionic equation

The basic idea of the Arrhenius theory is that in neutralization reactions, the hydrogen ions and hydroxide ions combine to form water. The Arrhenius theory became successful, but there were some limitations to this theory. The first limitation of the Arrhenius theory is that the acids and bases were limited to water solutions. Secondly, the theory does not explain why the bonds break in the reactants to product H+ ions. Thirdly, the theory does not include certain compounds that have the properties of bases. For example, ammonia, NH3, is a base, but there is no OH- ions present in ammonia therefore serving as a limitation for this theory. Due to the difficulty, ammonia was assumed to be aqueous and the following reaction was assumed.

Although the following chemical reaction can be hypothesized, there is no proof that the following equation exists.

The Arrhenius theory of acids and bases can be described using acid-base neutralization reaction and metal reactions. Any metal that is a reducing agent (reductant) that is below the hydrogen ion will react spontaneously with that hydrogen ion and that metal will break down and that acid will turn into hydrogen gas. Certain examples include:

Since there were limitations to the Arrhenius theory, a new theory was made. In 1923, J.N.Brønsted in Denmark and T.M Lowry in Britain came up with a new theory. Their new theory was expanded from the Arrhenius theory. The new theory was improved and now worked for acids and bases independently of how they behave in water. This theory made sense because there were no OH- ions involved as a part of it. The theory stated that an acid was a proton giver and so it donated H+ ions and that a base was an acceptor of protons and so it accepted an H+ ion. If the old example of ammonia gas is revisited, it was hard to explain with the Arrhenius theory. But it is convenient to explain this with the Brønsted-Lowry theory.

This reaction can now be described using the new theory. In this reaction, H2O acts like an acid because it gives a proton, H+, which the base ammonia accepts. Due to this transfer, new ions are formed on the products side of the equation. The same reaction can also be written in reverse.





o show that the one acid of the reactant correspond to one base of the product and that they are related in terms of H+ ions, there is a term used known as conjugate bases and conjugate acids. In this case, the ammonia molecule, it is possible to say that the NH4+ is the conjugate acid of the base. The equilibrium constant can also be written for the following reaction.

However, the term [H2O] is not included because the water molecules are present in excess and therefore it is considered as a solvent and the concentration of water is essentially a constant. Therefore the new equilibrium constant for this reaction is as follows. This constant of equilibrium is known as base ionization constant.

Later, there was a more general theory about the acid-base theory developed by G.N.Lewis. "The Lewis model of acids and bases proposes that an acid is an electron pair acceptor while a base is an electron pair donor". In the above following reaction, water is considered to be a Lewis acid since it gives electrons to the ammonia molecule. One interesting thing about this theory is that compounds/elements which do not have hydrogen in them can still be acids. The molecules which need lone pairs to complete octets are the strong Lewis acids because they accept the lone pairs from Lewis bases. From the knowledge about period tables, it is possible to say that the elements which are found below period two are generally good Lewis Acids because from top to bottom on the periodic table, the atomic size increases and therefore the elements have more capacity to hold valence shells. The theory focuses on gases and solids. It is important in describing reactions between organic molecules. The product that is formed from a Lewis acid and a Lewis base is called as an adduct. One species donates pair of electrons to form a covalent bond which is known as coordination. When the bonds of the Lewis acid join the bonds of the Lewis base, it is known as a coordinate covalent bond. Species with incomplete valence shells are Lewis acids. The octet rule is fulfilled when a coordinate covalent bond is formed.

Lewis acids can also occur among multiple bonds as in the case of water and carbonic acid. The water molecule gives a proton to the carbonic acid and the acid accepts the lone pair of water. These reactions can be indicated using arrows to indicate the transfer of lone pairs.

Water is amphiprotic according to the Brønsted-Lowry theory. This means that water can either act like an acid or a base. This was not mentioned in the Arrhenius theory. When an acid reacts with a base, the reaction is favored in the forward direction of the weaker base of a conjugate acid-base pair. For example, if the reaction of HCl is taken and broken down, the HCl would become a part of the Cl- ion. There are two categories of bases under the Brønsted-Lowry theory. They are anions and amines. The anions are like bases because their negative charges act like bases. The amines are compounds which are related to ammonia.

HCl(aq) + OH- (aq)→ H2O(l) +Cl-(aq)

The reverse would also be favorable. Therefore, it is possible to conclude that the stronger the acid, the weaker its conjugate base. An acid is strong when the acid accepts protons and dissociates completely, 100%. In some cases, water can behave like an acid or a base in a chemical reaction. Due to this event, a hydronium ion, H3O+, and a hydroxide ion, OH- is formed. The reaction is reversible. From the previous knowledge, it is known that during chemical reactions, molecules collide and that the reaction is effective if the collisions are effective. This concept could be related to the formation of the hydronium ions. The equation for the following reaction can be described as follows:

It is also possible to write the equilibrium constants for this equation. The equilibrium constant for this equation would be:

Strong acid reactions always go to completion because they dissociate. Regarding bases, the stronger ones are the hydroxides. They are considered as strong because they also dissociate into OH- ions when dissolved in water. On the other hand, the thing that makes acids and bases weak is the ambiguity whether the acid will dissociate completely or not. Examples of strong acids include hydrochloric acid (HCl), hydrosulfuric acid (H2SO4) etc. Strong bases:- NaOH (sodium hydroxide) etc. Weak acid:- Acetic acid. The roles of hydronium ions are very important. This is because during the course of the reaction, they are covalently bonded by hydrogen and water molecules. The H+ ion is very small and has a positive charge during the reaction and it "wishes" to bond with the oxygen atom which has a negative charge. When these two bond, a hydronium ion is formed.

Strong Acid Example

The below reaction is the reaction between water (H2O) and hydrochloric acid (HCl). The reactants react to yield the product, hydronium ion, and a chlorine ion. The reaction dissociates into ions 100% because the products are much weaker than the reactants. (i.e The hydronium ion is weaker than HCl and the chlorine ion is weaker than the water ion). Therefore the reaction proceeds to completion. Therefore the hydronium ion plays a huge role in this reaction.

Weak Acid Example

The below reaction is the reaction between acetic acid and water. The reactants react to produce hydronium ion and a C2H3O2- ion. The most important thing about the reaction is that it is reversible reaction. In this case, the hydronium ion is stronger than the acetic acid and the acetic acid ion is greater than the water ion, the reaction is reversible and can go either way. This results in an ambiguity because once the acetic is dissolved in water, it may or may not dissociate 100% in water due to the fact that one of the acid may be stronger than it's conjugate base and that the conjagate base may be stronger than it's acid resulting in equilibrium. Equilibrium (reversible) reactions cannot be predicted because they may go either way.

The above constant is the equilibrium constant. The Ka is called the acid ionization constant.

It is known that by now, water can collide with water, and produce hydronium ion and a hydroxide ion in solution. Therefore, in a chemical reaction the products will always differ to match up the equilibrium constant. In spontaneous conditions (298 K), the concentration of a hydronium ion is 1.00 X 10-7 M. Therefore, the equilibrium constant for that reaction is

Keq = [H3O+][ OH-] = 1.0 X 10-14

Because that number is very small, a convenient way was developed to represent these hydroxide and hydronium ions.

In 1909, a biochemist named Søren Sørenson named a scale called the pH scale which measured the potential of a hydrogen ion. The pH scale was measured on a logarithmic basis, i.e. in the powers of ten. "The pH scale measures how acidic or basic a substance is.". Acidic solutions have the property of having a pH that is less than 7. On the other hand, basic solutions have the property of having a pH number that is greater than 7. Neutral solutions possess a pH scale of exactly 7. Since, in the reaction the H+ ion is considerably small, the pH scale expresses the hydrogen ion in terms of this and therefore it is easy and convenient to use. The equation for the pH scale is:

pH =

Each unit on the pH scale represents the concentration of the hydronium ions. The concentration of the hydronium ion is 1.0 X 10-7. pH = -1og(1.0 X 10-7) = -(-7.00) = 7.00.

This is the pH for a neutral solution. It is always 7. While calculation the pH, the number of significant figures tells how much to numbers to keep after the decimal. In a lab, there are two ways to measure the pH of a solution. For accuracy, a pH meter can be used. An alternative way is to use an indicator which changes its color to know whether the given solution is an acid or a base. For acidic solutions, the blue litmus paper turns red as the reverse for basic solutions. Examples of indicators include phenolphthalein, methyl orange, bromthymol blue etc.

Buffers are types of mixtures which take in or release hydrogen ions, therefore making the pH of a solution constant. Examples of buffer include acetic acid and the acetate ion. Since, everything has limitations even buffers have limitations. They have a specific amount known as the buffer capacity. If the hydronium or hydrogen ions are added to excess to the buffer, the pH scale changes. "Hydrolysis is a chemical reaction during which molecules of water (H2O) are split into hydrogen cations (H+) (conventionally referred to as protons) and hydroxide anions (OH−) in the process of a chemical mechanism.". This is related to the pH scales of salt solutions.

Strong acids with Strong bases

The reaction for strong acids and strong bases are neither acidic nor basic, which means that they are neutral.

Strong acids with weak bases

The reaction between strong acids and weak bases are slightly acidic.

Strong bases with weak acids

The reaction between weak acids and strong acids produce slightly basic products.

Weak Base

2NaOH(aq) + H2CO3 (aq) → Na2CO3 (aq) + 2H2O(l)

Slightly basic

Strong Acid

As learnt earlier, the water can act as a base or an acid. Sometimes the water ionizes itself (self-ionization), one part of H2O molecules act like an acid while the other part of water molecules, act like a base known as amphiprotic. According to the equation, when water reacts with water to produce a hydronium ion and a hydroxide ion, the equilibrium constant can easily be determined because the reaction is a reversible reaction. At SPONTANEOUS conditions (25° C or 298 K), the ionization constant for water is 1.0 X 10-7 M. This is symbolized by Kw. This is related to the acid and base dissociation constants because water dissociates into hydroxide ion and a hydronium ion once dissolved. The acids and bases constants also involve the hydroxide ions in the products side. "The acid-base ionization/dissociation constant, pKa, is a measure of the tendency of a molecule or ion to keep a proton (H+) at its ionization center(s), and is related to the ionization ability of chemical species.". If the general equation is taken,

This means that the larger the Keq, the more the acid reacts to produce hydronium ions. In other words, the constant explains as to how strong the acid is. If the dissociation constant is greater than 1, then the given acid is a strong acid whereas if the constant is less than 1 then the given acid is a weak acid.

For a diprotic or a triprotic acid, the dissociation takes place in many steps. For example, the dissociation of carbonic acid takes place in two steps.

Neutralization reactions are reactions in which acid and base combine together to yield salt and water. These reactions have some properties or characteristics. At a certain point in time during the lab, the moles of acid equal the moles of base which is referred to as the equivalence point for the reaction. At this point, neither the products nor the reactants are in excess. One common example of neutralization reaction includes the reaction between hydrochloric acid and sodium hydroxide. The chemical equation for this is:

One such example of a base include CH3NH2. The chemical equation for the following reaction is as follows.

Like the acid dissociation constant, there is also a base dissociation constant. It is derived from the equilibrium constant which is also like the acid constant. The stronger the base, the more hydroxide ions produced. Like acids, this constant measures the strength of the bases or how strong the given base is.

The neutralization reactions play an important role in this lab because the lab mainly involves the reaction of potassium acid with sodium hydroxide to produce water and a salt as its product. The chemical reaction for the following reaction is:

During a lab, the solution that is poured inside the buret is known as the titrant. The titrant is essentially the base. The titrant is added to the acid, drop-wise, until the moles of acid are equal to the moles of base. In other words, the equivalence point is reached. During this process, an indicator is used. A special process known as titration is used during this process.

"Titration is a common laboratory method of quantitative chemical analysis that is used to determine the unknown concentration of a known reactant." The titration is performed for qualitative analysis in experiments to determine the concentration of the reactant through this process. For a titration, there is another substance involved during the lab which holds the unknown concentration of the acid or the base. This substance is known as the standard solution. During titration, more frequently, an acid-base indicator is added.

A graph which represents the pH scale versus the volume of the titrant used is called the titration curve. Titration curves are drawn by measuring the pH of the substance using a pH meter and then plotting the data, using a recorder.

Titration of a strong acid with a strong base

It is started with a low pH because of the presence of the hydronium ion in the bottom of the flask. And then as the titration process starts the point of equivalence is slowly reached. The point of equivalence contains water and ions. Recommended indicators are bromothyolol blue indicator. As the titration curve begins the color of this indicator stays yellow for a certain time and then turns green when the point of equivalence of reached and then it reaches blue. Reaching the point of equivalence is hard to get because if one small drop is added to the solution, it immediately changes its color to blue. The pH starts off with a low value in the beginning of the graph. The pH slowly changes until just before the equivalence point. Once, the equivalence point is reached, the pH increases drastically. The pH keeps rising slowly after the equivalence point. For this type of titration, a suitable indicator would lie within the range of 4 to 10.

Titration of a weak acid with a strong base

In this case, the pH value in the equivilance point will greater than 7. It levels in the beginning because it forms a buffer. During this type of graph, the weak acid becomes a part of the conjugate base and the weak acid becomes a part of the conjugate acid. There is a region formed which is known as a buffering region. The starting/initial pH value is grater than that of a strong acid. There is a sharp change in the beginning of the graph. The anion produced by the neutralization of the weak acid is a common ion that reduces the extent of ionization of the acid). Because [HA] = [A-] at the point of half-neutralization, pH = pKa. After the equivalence point, the titration curve is similar to the curve of the strong acid with a strong base. The steep portion of the titration curve at the equivalence point has a pH range of about 7 to 10. A good indicator for this type of scenario would be phenolphthalein.

Titration of a Weak Polyprotic Acids and Strong Bases

There are more than one equivalence points for this scenario because the acid continuously ionizes during the course of the chemical reaction. This scenario is like titrating two various acids at once. The first part is similar to other graphs where in the beginning it behaves like a normal strong acid with a strong base graph. In the second part, when the second acid ionizes, the curve levels off and repeat this pattern again. Phenolphthalein is a good indicator to detect the equivalence point.

Some bases have the ability to accept more than one proton. The carbonic ion is mainly dibasic (which comes under the category polybasic) because it has the ability to allow two protons in a reaction. Some acids can also donate more than one proton. In these types of dibasic solutions, there are much dissociation that occur along the way of the curve and therefore the curves have more than one equivalence points.

Figure - Titrating a Strong Acid into a Weak Dibasic species.

The point during the process of titration in which the given indicator changes its color is known as the endpoint. The important thing is that the endpoint must match the equivalence point of the neutralization. In other words, if the indicator's endpoint is very close to the equivalence point, the color will stay for some time to indicate that the point of equivalence is very close. Thus, this is achieved if a suitable indicator that is over the equivalence point is reached. Therefore, at the endpoint (where the given indicator changes its color) it can be assumed that the given chemical equation is in the state of equilibrium, meaning that the mole ratios on both the products and the reactants are the same. The important thing to remember is that there can be man endpoints but there can be only one equivalence point because its only at one point where the total mole ratios of the products is equal to the mole ratio of the reactants.

All this knowledge relates to the lab in many ways. Firstly, all three definitions and examples of acids and bases apply to this lab. The Arrhenius theory can be used to explain the chemical reaction. The potassium acid dissociated in water to produce the hydrogen ions and the sodium hydroxide also dissociated into water to produce the hydroxide ions as all acids and bases dissociate in water to produce the hydrogen and hydroxide ions as per the Arrhenius theory. Secondly, it can also be explained using the Bronsted-Lowry theory because the KHP acid donated a hydrogen ion and the sodium hydroxide accepted the ion. Also, the sodium hydroxide was the base (NaOH) and became the conjugate acid and the acid (KHP) became the conjugate base as NaP. Finally, it can also be explained using the Lewis theory because the potassium acid donated lone pairs to the sodium hydroxide to complete its octet. During this process, the water acted as an acid due to its amphiprotic nature. The hydronium ion was also formed during this process since the water molecule consists of a hydronium/hydrogen ion. In other words, water consists of hydrogen and a hydronium ion and therefore since water was formed, it consisted of those two ions. The potassium acid was considered to be as a weak acid since when dissolved in water it did not conduct electricity. Also, the lab was the reaction between a strong base and a weak acid. The reaction between this reactants produced slightly basic solutions since the reaction of strong bases with weak bases produced slightly basic solutions. Due to equilibrium, a stress was applied to the system therefore bringing the Le Chateleir's principle into place. In addition, since the reaction was in a state of equilibrium, the conjugate bases and acids was applied to the reaction. The acid did not dissociate 100% because it was a weak acid. Therefore the electro negativity of the base was greater than the acid, making the reaction proceed.

During the lab, the pH of the solution was found by adding an acid-base indicator to the water which contained the acid. The indicator that was used during this process was phenolphthalein. The given solution turned pink if the base was present and turned clear if the acid was present. Therefore the concept of indicators could be used during this lab. The acid and the base dissociation constants could also be found out using the acid and base formulas. But the main part of the lab was the titration process. The titration of this lab was performed for finding the concentration of acid in the unknown substance. Therefore, the titration process was performed using a standard solution. The titration curve for this reaction was a strong base with a weak acid and therefore the titration curve was very similar curve to the curve of a strong acid with a strong base. Therefore a suitable indicator for this type of reaction was phenolphthalein. This could be observed as three drops of this indicator was added to each trial and the given acid in the water turned pink or stayed clear to represent whether there was more acid or base in the given solution. This could also be observed as the indicator turned pink and was clear as it reached closer and closer to the endpoint. During the lab, the endpoint was reached when the solution stayed pink for around 30 s or more and then faded and turned clear. This was when the endpoint was reached.