Strength and Concentration – Acid-Base Reactions

by Adam Le Gresley, PhD

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    00:00 So, note the difference between strength and concentration. Strength relates directly to Ka, the acidity constant for a given dissociation. Concentration, on the other hand, relates purely to the amount per... per litre of a given ion. Strength refers to Ka and concentration refers to the amount of acid actually in solution. So, therefore, it's possible to have a concentrated solution of a weak acid or a dilute solution of a very strong acid. Concentration is measured in moles per litre or moles per dm3. And what you often find is it's referred as mol dm-3 and, sometimes, just as M (molar).

    00:54 Acid strength can be measured using Ka, as we've already indicated. The larger the Ka, the greater the degree of dissociation of the acid into the conjugate base and, of course, the important H+. But, these are not generally convenient ways of measuring it because you have to use, again, standard form in this case. 1.8 ×10 to the -5 is a little bit more difficult to work with, especially if you're looking at it from a biological perspective.

    01:18 And a lot of the calculations may appear to be confusing and may even result in some errors 15 00:01:24,510 --> 00:01:24,110 as a consequence.

    01:24 So, as a consequence of this, pKa is used with the small letter p denoting negative log (-log). That's all p means in this context. So, we take whatever the value of Ka is and then we carry out the negative log to the base 10 of that value.

    01:45 By carrying out the negative log to the base 10, we get a value which is a bit more easy to deal with. Usually, a value running from 1 through to 14 or, at the very least, a decimal value which is easy to deal with.

    01:58 If we look at this equation, what we mean is, by taking pKa, we are taking the negative log to the base 10 of the concentrations of H+ multiplied by a conjugate base A- divided by the non-dissociated concentration of our acid HA.

    02:15 So, if, for example, we were to look at ethanoic acid, we see that the pKa of this calculation results in a value of 4.77. The stronger acid, trichloroethanoic acid, results in a pKa of 0.70. And, as we actually go down in terms of pKa value, we're increasing the degree of dissociation and increasing the concentration of H+ that is actually formed.

    02:48 When an acid or a base is added to water, the concentration of H+ will change. And this is the origins of the term pH. pH, like pKa in this case, just means that we're taking the negative log to the base 10 of the concentration of H+ in solution. Hence, the term pH derived from the German expression potenz Hydrogen.

    03:14 Pure water has a pH of 7. And there's a relatively easy rule of thumb to bear in mind with this. When the concentration, for example, is approximately 1 × 10 to the -7 in terms of moles per litre, then you can actually just remove that -7 and remove the negative value. So, in other words, if I have 1 × 10 to the -7 moles of H+ in a litre of water, I will have a pH of 7. So, sometimes, there's a general rule of thumb which doesn't require you to use a calculator. Therefore, if we had a concentration of 1 × 10 to the -2, we can just take that power, remove the negative value and we end up with a pH of 2, which you should appreciate is an acidic pH.

    04:06 Remember, pure water has a pH of 7 and it also dissociates to a small extent. Acidic solutions will always give a greater concentration of H+ that exists in water and have a pH of less than 7. Basic solutions will have a smaller concentration of H+ free in solution and have a pH greater than 7, always.

    04:31 Typically, physiological conditions result in a pH of around 7, although obviously, that varies depending on whether or not you're talking, for example, about urine samples, saliva samples or indeed plasma blood samples.

    About the Lecture

    The lecture Strength and Concentration – Acid-Base Reactions by Adam Le Gresley, PhD is from the course Ionic Chemistry.

    Included Quiz Questions

    1. 11.33
    2. 2.12
    3. 2.67
    4. 8.92
    5. 22.6
    1. C2H2O4
    2. HClO4
    3. HI
    4. H2SO4
    5. HBr
    1. Benzoic acid is a strong acid which gets fully ionized in a solvent at room temperature.
    2. Benzoic acid, a weak organic acid, dissociates incompletely to H+ and C6H5CO2- ions.
    3. Benzoic acid has a smaller Ka than the sulfuric acid.
    4. C6H5COOH has a higher pKa value than the nitric acid.
    5. The pKa values for benzoic acid in water and DMSO at 25°C are 4.2 and 11.1, respectively.
    1. An acidity constant gives a qualitative measure of an acidic reaction at a particular temperature.
    2. An acidity constant gives a quantitative measure of the strength of an acid in the solution.
    3. pKa = - log Ka
    4. The value of the acidity constant gives a measure of the extent of dissociation of acids.
    5. The structural factors have a pronounced effect on the value of acidity constant.
    1. ...of a chemical reaction in a closed system is a state when the rate of a forward reaction equals the rate of backward reaction with no changes in the concentrations of reactants and products.
    2. …represents a continuous variation in the reactant concentration during an irreversible reaction.
    3. …represents a regular shift in the product concentration during an irreversible reaction.
    4. …exists when the system is in an ever-changing state.
    5. …represents a state of the system when the rate of a forward reaction is more than the rate of backward reaction.
    1. ...with the alteration in the concentrations of products or reactants, temperature, and pressure of the system.
    2. …with the addition of a hydrolytic enzyme to the system.
    3. …with the addition of a ligase enzyme to the system.
    4. …with the addition of a metal catalyst to the system.
    5. …with the addition of a nanomaterial-based catalyst to the system.
    1. …the given acid is strong and dissociates into ions easily.
    2. …the given acid is weak and dissociates into ions with ease.
    3. …the given acid is strong and does not dissociate into ions with ease.
    4. …the given acid is weak and partially dissociate into ions.
    5. …the given acid is weak and does not dissociate into ions even at elevated temperatures.
    1. K = {[H+] × [C3H7COO-]} / [C3H7COOH]
    2. K = {[H+] × [C3H7COOH]} / [C3H7COO-]
    3. K = [H+] / {[C3H7COOH] × [C3H7COO-]}
    4. K = {[C3H7COOH] × [C3H7COO-]} / [H+]
    5. K = [C3H7COOH] / {[H+] × [C3H7COO-]}
    1. The thermodynamics are dependent on the reaction mechanism; whereas the kinetics of a chemical reaction are independent of the reaction mechanism.
    2. The thermodynamics pertain to the equilibrium, whereas kinetics relate to the rate of the reaction.
    3. The thermodynamics remains unaffected by the presence of catalysts; on the other hand, the addition of catalysts alters the kinetics of the reaction system.
    4. The thermodynamics are independent of the reaction mechanism; whereas the kinetics of a chemical reaction depends upon the reaction mechanism.
    5. The thermodynamics deal with equilibrium constant (K), while kinetics cover the rate constant (k).
    1. Carboxylic acids are stronger acids than the mineral acids like HCl or HNO3.
    2. Weak acids dissociate to a small extent in an aqueous solution.
    3. Due to the generation of small amounts of H+ during dissociation, the weak acids possess less acidic character than H2SO4.
    4. Ethanoic acid possesses a low acidity constant (Ka = 1.8 × 10-5).
    5. Oxalic acid gives a less stable conjugate base, hence it undergoes partial dissociation.
    1. The larger the size of an electronegative atom, the lower the acidic strength of the molecule will be.
    2. The acidity increases with an increase in positive charge on an atom.
    3. The electronegativity of an atom or conjugate base directly reflects the acidic character of a molecule.
    4. The delocalization of negative charge through delocalization contributes to the acidic trait of an acid.
    5. Inductive effect of electronegative atoms enhances the acidity of acids through stabilization of the conjugate base by attracting electrons towards themselves.
    1. The chlorine helps stabilize the conjugate base by causing the delocalization of electron density because it pulls the electrons towards itself away from the carboxylate group via the electron withdrawing inductive effect.
    2. The chlorine atom destabilizes the conjugate base through the electron releasing inductive effect.
    3. The chlorine atom helps in concentrating the electron density on the carboxylate group.
    4. The chlorine atom contributes to delocalizing the resonance effect in acetic acid derivatives.
    5. The chlorine atoms possess a pKa value elevating effect.

    Author of lecture Strength and Concentration – Acid-Base Reactions

     Adam Le Gresley, PhD

    Adam Le Gresley, PhD

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