Table of Contents
Calculating pH of Strong Acids
There are multiple definitions of acids and bases. The first definition was worded by Arrhenius.
The Arrhenius definitions:
Arrhenius Acid = a substance, that when dissolved in water, yields hydrogen ions. (H+)
Arrhenius Base = a substance, that when dissolved in water, yields hydroxide ions (OH-)
Let's note that Arrhenius definitions are restricted to aqueous (solute dissolved in water) solutions.
Bronsted-Lowry definitions:
Bronsted-Lowry Acid = a proton (H+) donor
Bronsted-Lowry Base = a proton (H+) acceptor
A conjugate acid-base pair consists of an acid and a base that differ in one hydrogen only. For example: H3O+ and H2O, HF and F-. Notice, that every conjugate acid-base pair differs in one hydrogen only. An acid will have one more hydrogen than the base. If the difference is more than one hydrogen, we do not have a conjugate acid-base pair.
Example: Identify conjugate acid-base pairs
To do this problem correctly, we need to go from left to right (reactants to products). HF lost a hydrogen to become F- . It is a proton donor and therefore, an acid. On the other side, F- is its conjugate base. HCO3- gained a hydrogen to become H2CO3. HCO3- is a base and H2CO3 is its conjugate acid.
The final definition of acids and bases was proposed by G.N. Lewis.
Lewis Definitions
Lewis Acid: a molecule that accepts a pair of electrons
Lewis Base: a molecule that donates a pair of electrons
PS: Oftentimes in a Lewis acid-base reaction, we see two molecules coming together to form one molecule
Example:
Nitrogen in NH3 gives its electrons to boron in BF3. NH3 is the Lewis base (electron pair donor). BF3 is an electron acceptor and Lewis base.
Strengths of Acids and Bases
A strong acid completely ionizes in water.
Strong acids that need to need to be memorized: HCl, HBr, HI, H2SO4, HNO3, HClO4
Strong Bases: Group 1 or 2 metals (Except Be) + OH
Example: NaOH, Mg(OH)2
Strongest acids have weakest conjugate bases
How to Decide Whether Reactants or Products Are Favored in an Acid–Base Reaction
The direction of the acid-base reaction is always favored towards the side with the WEAKER acid.
HClO4 >H2SO4 >HI >HBr >HCl >HNO3 >H3O+> HSO4- >H2SO3 >H3PO4 >HNO2 >HF >HC2H3O2 >H2CO3 >H2S >HClO >HBrO >NH4+>HCN >HCO3- >H2O
Example: For the following reaction, decide whether reactants or products are favored at the completition of the reaction.
The weaker acid is on the reactant side in this reaction. Therefore, reactants will be favored at the completion of the reaction.
How To Determine Acid Strength
Binary Acids (HX)
Binary acid refers compounds that contain H+nonmetal
Rules for strength of binary acids:
Acid strength increases as we go down a group (column) on the Periodic Table. Explanation: As we go down a group, the radius of the nonmetal increases and the bond strength H-X decreases, resulting in a stronger acid. Example:HF<HCl<HBr<HI
Acid strength increases going from left to right across a row(period) on the Periodic Table. Explanation: As we go left to right across a period , the electronegativity increases, making H-X bond more polar. Example: CH4<NH3<H2O<HF
Oxoacids
Oxoacids contain H+ Polyatomic ion that has an oxygen Ex: H3PO4, H2SO4
Rules for strength of Oxoacids:
Acids strength increases as the electronegativity of the central atom (not H or O) increases. For example, let's compare HClO and HBrO. Since Br is more electronegative than Cl, HBrO will be a stronger acid then HClO
When acids have the same central atom, the more oxygens there are, the more acidic they are. Example: HClO4>HClO3>HClO2
Self-Ionization of Water and pH scale
Two water molecules can come together to make ions in a reaction called self-ionization of water.
H2O(l) + H2O(l) <--> H3O+(aq) +OH- (aq)
If we write Kw (equibrium constant) for this reaction, we get
Kw= [H3O+][OH-]
This is a very important equation that will use throughout current and next chapters. It allows us to calculate the concentration of OH- when H3O+ is known and vice versa.
PS: H3O+ and H+ are used interchangeably. H3O+ is created from H+ bonding to a water molecule.
pH scale
pH scale is a logarithmic scale that describes acidity of a solution. The pH scale is from 0 to 14 at 25°C.
pH values from 0 to 7 describe an acidic solution where [H3O+]>[OH-]
pH of 7 describes a neutral solution where [H3O+]=[OH-]
pH values from 7 to 14 describe a basic solution where [OH-]>[H3O+]
the higher the pH the more basic the solution is and the lower the pH the more acidic the solution is
We can use these equation to find any unknown as long as we have one known. For example, if we have the concentration of OH- ([OH-]), we can find pH, pOH and [H3O+].
Example 1: The pH of human arterial blood is 7.40. What is the hydronium-ion concentration?
Solution: [H3O+] = 10^-pH = 10^-7.40 = 4.0*10^-8
Example 2: Calculate the pH of solution (at 25°C) that has an OH- concentration of 1.2*10^-6
Solution: There are multiple ways to do this and we will show one of the ways.
[H3O+][OH-]= 1.0*10^-14
[H3O+](1.2*10^-6) = 1.0*10^-14
[H3O+] = 8.3333*10^-9
pH = -log[H3O+] = -log(8.3333*10^-9) = 8.08
How to Calculate pH of Strong Acids
Recognize that you have a strong acid. It has to be one of the following: HCl, HBr, HI, H2SO4, HNO3, HClO4.
The concentration of H3O+ is equal to the concentration of the acid (except for H2SO4 which gives 2H+ ions) because the acid dissociates completely.
pH= -log[H3O+]
Example: Calculate the pH at 25C for a 0.15 M HNO3
Solution: HNO3(aq) ---> H+(aq) + NO3-(aq)
[HNO3] = [H+] = 0.15M
pH = -log(0.15) = 0.82
How to Calculate pH of Strong Bases
Recognize that you have a strong base. It has to be Group 1 or 2 metal (except Be) with OH. Example: NaOH, KOH, Mg(OH)2
Use simple stoichiometry to calculate the concentration of OH-. It will be equal to the concentration of base for those bases that have one OH such as NaOH or twice the concentration for the bases that have two OH- ions such as Mg(OH)2
pOH = -log[OH-]
pH= 14-pOH
Example: Calculate the pH at 25C for a 0.020M solution of Ba(OH)2
Solution:
Ba(OH)2(aq) ---> Ba^2+(aq) + 2OH-(aq)
[OH-]= 2*0.020M = 0.040M (because there are two OH- ions in one Ba(OH)2 formula unit)
pOH= -log(0.040M)= 1.3979400
pH = 14-1.3979400 = 12.60
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