Bronsted-Lowry acids and bases. The difference between dissociation and ionisation Dissociation...
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![Page 1: Bronsted-Lowry acids and bases. The difference between dissociation and ionisation Dissociation refers to a reaction where a molecule or substance breaks.](https://reader034.fdocuments.in/reader034/viewer/2022050721/56649e665503460f94b60fce/html5/thumbnails/1.jpg)
Bronsted-Lowry acids and bases
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The difference between dissociation and ionisation
• Dissociation refers to a reaction where a molecule or substance breaks apart into smaller units.
• The units are not necessarily ions, although this is often the case.
• Ionization generally refers to a reaction which forms ions from an uncharged species.
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Defining acids and bases
• In chemistry, the Brønsted-Lowry theory is an acid-base theory, proposed independently by Johannes Nicolau Brønsted and Thomas Martin Lowry in 1923
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Bronsted-Lowry definition of acid• A substance behaves as an acid when it:
1.donates a proton (H+) to a base.
2.Acids are proton donors.
3.When acids react with water, hydronium
(H3O+) ions are produced. H+ ions cannot exist by themselves
• HCl + H2O H3O+ + Cl-
acid base
H+ is attracted to the negative end of H2O to become H3O+
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Bronsted-Lowry definition of a base
• A substance behaves as a base when it:
1. accepts a proton from an acid (Bases are proton acceptors.
2. When bases react with water, hydroxide (OH−) ions are produced.
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Acids and bases• HCl is an acid because it donates H+ • NH3 accepts H+ and therefore is the base • (NH4
+ and Cl– then form an ionic compound)
• Lewis acid: electron pair acceptorLewis base: electron pair donor
H
H
H
H
HH
H
HN NCl Cl+
+-
+
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Acid/base conjugate pairs• Conjugate means joined together
• When an acid and a base react together a conjugate acid and base are formed
• HNO3 + H2O H3O+ + NO3-
acid base acid 2 base 2
• The conjugate pairs are (HNO3 /NO3-)
(H2O / H3O+)
• They differ by a H+
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Acid/base conjugate pairs
HCN(l) + H2O CN–(aq) + H3O+(aq)
• HCN is acid, H2O is base• H3O+ is acid, CN– is base• A conjugate acid-base pair are two
substances that differ from each other by just one proton (H+)
• HCN and CN– and H2O and H3O+ are conjugate acid-base pairs
HCN(l) + H2O CN–(aq) + H3O+(aq)
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Questions
• Pg 245 Q 1,2,3
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Hydrolysis• Hydrolysis is a chemical reaction during
which an anion reacts with water to produce OH– or a cation reacts with water to produce H3O+
• H2O + Cl- OH- + HCl
• H2O + H+ H3O+
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Acid and base strength• Acids and bases have different strengths
• Some acids donate protons more readily than others
• The strength of an acid is its ability to donate hydrogen ions to a base.
• The strength of a base is its ability to accept hydrogen ions from an acid.
• Strength is different from concentration (pg 248 Figure 14.9)
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Strong acids and bases • A strong acid will completely ionise in
solution (producing many ions)
• A strong base will accept protons (H+) easily
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Weak acids and bases• A weak acid does not ionise to any great
extent and so contains a larger number of molecules compared with the number of ions produced in solution.
Completely ionised
Partially ionised
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Reversible reaction• Reversible arrows in an equation show that the
products on the right can react together and produce the left hand side.
• A chemical equation without a double arrow isn't reversible and can only go in one direction.
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Polyprotic acids
• Polyprotic acids are acids capable of donating more than one proton (H+).
1.Monoprotic
HCl
2. Diprotic
H2SO4
3. Triprotic
H3PO4
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Amphiprotic substances
• Some substances act as acids and bases
• They can donate or accept protons and are called amphi (meaning both) protic (hydrogen ions)
• Example water, ammonia and amino acids
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• Write the formulae of the conjugate bases of the following acids:
• H2SO4 H2S HS- NH4+
• Write the formulae of the conjugate acids of the following bases:
• OH- HCO3- H2O CN-
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pH and pOH• pH stands for the potential of hydrogen or
concentration of H+• pOH is a measure of the hydroxide ion
concentration • The acidity of a solution is a measure of the
concentration of H+• In a neutral solution there is the same
concentration of H+ or H3O+ and OH-
• Basic solutions have a lower concentration of H3O+ than OH-
• Acidic solutions have a greater concentration of H3O+ than OH-
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Ionic product• [H3O+ ] [H+ ] represents the concentration of H3O+ or H +
• [OH-] represents the concentration of OH-
• Experiments show that all aqueous solutions contain H + and OH- that the product of their molar concentrations is 10-14M2 at 25C
• The ionic product is:
[H3O+ ]x[OH-] =
In pure water [H3O+ ]=[OH-]
10-7M = 10-7M
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At 25C a solution is:
• Acidic if [H3O+ ] > 10-7M [OH-] < 10-7M
• Neutral if [H3O+ ][OH-]
• Basic if [H3O+ ] < 10-7M [OH-] > 10-7M
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Calculating pH and pOH
• pH is calculated using the following formula:
pH = -log [H+]
• pOH is calculated using the following formula:
pOH = -log [OH-]
• pH + pOH = 14.0
• [H+] = 10 -pH
• [OH-] = 10 -pOH
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pH calculations for weak acids
• Can we use the pH calculation for weak acids?
• No
• Why?
• Because weak acids have not fully ionized so we do not know the H+ concentration
• You have to wait till year 12 and you do the equilibrium constant
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Calculating pH and pOH
• Find the pH of a solution of sodium hydroxide that has a pOH of 2
• pH = 14 – pOH
• pH = 14 - 2 = 12
• Find the pOH of a solution of hydrochloric acid that has a pH of 3.4
• pOH = 14 – pH
• pOH = 14 - 3.4 = 10.6
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Questions• Find the pH of 25.0 mL of a 0.045 M
(mol/L) solution of HCl. What is the pOH?• Note that HCl is a strong monoprotic acid
which means that...[HCl] = [H+]= 0.045 MpH = -log [H+]pH = -log 0.045pH = 1.35
• The pOH is given by pH + pOH = 14We substitute in the pH of 1.35 and get:1.35 + pOH = 14So, pOH = 12.65
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Questions• a) Find the pOH of 0.000685 M solution of NaOH.• b) What is the pH of the solution?
a)Note that NaOH is strong and monobasic which means that...
• [NaOH] = [OH-] = 0.000685 Mhence, pOH = -log [OH-]pOH = -log 0.000685pOH = 3.164
b) pOH is 3.164.The pH can be found by using pH + pOH = 14.Substituting in gives us pH + 3.164 = 14So, pH = 10.836
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Question
• What is the H+ concentration in the solution of pH 3.47
• [H+] = 10-pH
= 10 -3.4
= 3.39x10-4 mol/L or M
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Questions• What is the concentration OH- ions in a
solution of pH of 10.47• Find concentration of H+ ions[H+] = 10-pH = 10-10.4
= 2.51x10-11
• Find OH- concentration[H+ ] x [OH-] = 10-14
[OH-] = 10-14
2.51x10-11
= 3.98 x 10-4 mol/L or M
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Find the pH of a 0.2 mol L-1 (0.2M) solution of H2SO4
• Write the balanced equation for the dissociation of the acid
• H2SO4 2H+(aq) + SO42-(aq)
• Use the equation to find the [H+]: 0.2 mol L-1 H2SO4 produces 2 x 0.2 = 0.4 mol L-1
• Calculate pH:
pH = -log[H+]
pH = -log [0.4] = 0.4
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• Pg 254 Q 9 a,b,c, 10 a,b 11