Acids and Bases

Learning outcomes

Acids and Base effect on Litmus Acid turns litmus red Base turns litmus blue

Acids Sour taste Turn blue litmus RED (ABR) Neutralise bases React with metals to produce Hydrogen

What do they all have in common? Acids Common lab acids Hydrochloric acid: HCl Sulphuric acid: H2SO4 Nitric acid: HNO3 Everyday acids Ethanoic acid (Vinegar): CH3COOH Carbonic acid (Fizzy drinks): H2CO3 Citric acid: C6H8O7 Dil. sulphuric acid in car batteries H2SO4 *(need to know 2 for exam) What do they all have in common?

 They all contain Hydrogen. Acids  They all contain Hydrogen. Classification of acids Acids are classified as Monobasic Dibasic or Tribasic depending on the number of hydrogen atoms per molecule that are removed by reaction with a base

Acids Donates Example Monobasic Dibasic Tribasic

Acids Donates Example Monobasic Dibasic Tribasic One H+ Two H+ Three H+

Acids Donates Example Monobasic Dibasic Tribasic One H+ HCl Two H+ H2SO4 Tribasic Three H+ H3PO4

Bases Bitter taste Slippery feel Turn red litmus blue (BRB) Neutralise acids A base that dissolves in water is called an alkali.

What do most of these have in common? Everyday bases Milk of Magnesia neutralises stomach acid: Mg(OH)2 Sodium Hydroxide in oven cleaners: NaOH Calcium Hydroxide in limewater: Ca(OH)2 Calcium carbonate: CaCO3 Soaps * (need to know 2 for exam) What do most of these have in common?

Defining Acids and bases Arrhenius vs Bronsted-Lowry

Arrhenius: Definition of Acids An acid is: a substance that dissociates in water to produce H+ ions. HX  H+ + X- HCl  H+ + Cl- H2SO4  2H+ + SO42-

Arrhenius: Strong and Weak Acids Arrhenius’ defined some acids as strong and others as weak.  A strong acid is one that fully breaks up (dissociate) in water to give hydrogen ions = HA = H3O+ + + - - + - = A- + - + - + - + -

Arrhenius: Strong and Weak Acids A weak acid only slightly dissociates in water to give hydrogen ions = HA + - = H3O+ + + + - = A- - -

Summary of Arrhenius’ Acids Strong/ Weak Dissociation Classification Hydrochloric Strong HCl → H+ + Cl- Monobasic Nitric HNO3 → H+ + NO3- Sulfuric H2SO4 → 2H+ + SO42- Dibasic Phosphoric Weak H3PO4 → 3H+ + PO43- Tribasic A strong acid is one that fully dissociates in water to give hydrogen ions A weak acid only slightly dissociates in water to give hydrogen ions

Arrhenius: Definition of Bases An base is: a substance that dissociates in water to produce OH- ions (hydroxide ion). XOH  X+ + OH- NaOH  Na+ + OH- Mg(OH)2  Mg2+ + 2OH-

Arrhenius: Strong and Weak Bases  A strong base is one that fully dissociates in water to give hydroxide ions (OH-)  A weak base is one that only partially dissociates in water to give hydroxide ions (OH-)

Summary of Arrhenius’ Bases Strong/ Weak Dissociation Sodium Hydroxide Strong NaOH → Na+ + OH- Magnesium Hydroxide Weak Mg(OH)2 → Mg2+ + 2OH- Calcium Hydroxide Ca(OH)2 → Ca2+ + 2OH- A strong base is one that fully dissociates in water to give hydroxide ions (OH-) A weak base only slightly dissociates in water to give hydroxide ions (OH-)

Which represents a weak acid dissolved in water?

Which is the strongest acid?

HW Show using a diagram how these would dissociate in water: Strong base Weak base

Problems with Arrhenius’ Theory of Acids and Bases 1. The H+ ion produced when an acid dissociates reacts with the water molecule to form the H3O+ (hydronium ion). This is what exists in solution and not H+. HCl + H2O → H3O+ + Cl- 2. His theory only holds for reactions occurring in water. No other solvents are included.

Formation of Hydronium Ions An acid is a substance that dissociates in water to produce H+ ions. 1+ 1+ 1+ + H+ H2O H3O+ hydrogen ion water hydronium ion (a proton)

Formation of Hydronium Ions An acid is a substance that dissociates in water to produce H+ ions. HCl + H2O → H+ + Cl- + H2O HCl + H2O → H3O+ + Cl-

Bronsted and Lowry (1923) They continued on Arrhenius’ work and redefined Acid and Bases.

Bronsted and Lowry (1923) ACID = DONOR  An acid is a substance that donates H+ ions (proton) in solution . ACID = DONOR A base is a substance that accepts H+ ions (proton) in solution

Bronsted and Lowry (1923) HCl + H2O → H3O+ + Cl- Consider: HCl + H2O → H3O+ + Cl- The molecule of HCl has given its H+ ion to water. Since HCl has donated its proton it is called an acid. The molecule of H2O has accepted the H+ ion from HCl and is therefore a proton acceptor - a base.

Bronsted and Lowry (1923) A strong acid is a good proton donor A weak acid is a poor proton donor A strong base is a good proton acceptor A weak base is a poor proton acceptor

Advantages of Bronsted-Lowry Theory over Arrhenius’s. This theory does not require water as a solvent. It has a broader range of species that can be defined as an acid and a base which may not have been classified under the Arrhenius theory. Conditions: The B/L theory will only work if there is an acid present to donate a proton but also there must be something to accept this proton. Similarily a base will only accept an H+ion if there is an acid there to donate it.

HCl + NH3 → NH4+ + Cl- What am i? Donates Proton (Acid) Accepts a Proton (Base)

NH3 + H2O → NH4+ + OH- What am i? Here the H2O molecule dissociates into H+ and OH- The NH3 accepts the H+ ion to give NH4+ and so under Bronstead- Lowry rules is a base. The water donates the proton and is therefore the acid.

Water can act as an acid and a base?? HCl + H2O → H3O+ + Cl- NH3 + H2O → NH4+ + OH-

Amphoteric  A substance that can act as an acid and a base is called an amphoteric substance. Water can act as either an acid or a base....

Conjugate Acid-Base Pairs Conjugate pair: Two substances which differ by a H+ An acid changes into its conjugate base when it donates a H+. HCN  CN- A base changes into its conjugate acid when it accepts a H+. HCO3-  H2CO3

Exercise #1 HCN ----------- CN- HClO3 ----------- ClO3- What are the conjugate bases of each of the following Bronsted- Lowry Acids: HCN ----------- CN- HClO3 ----------- ClO3- NH4+ ----------- NH3 H2S ----------- HS- HF ----------- F- H2O ----------- HO- H2SO4 ----------- HSO4-

Exercise #2 HCO3- ----------- H2CO3 NH2- ----------- NH3 What are the conjugate acids of each of the following Bronsted- Lowry Bases: HCO3- ----------- H2CO3 NH2- ----------- NH3 Cl- ----------- HCl SO42- ----------- HSO4- HF ----------- H2F+ NH3 ----------- NH4+

Conjugate Acid-Base Pairs Accepts/adds a H+ H2SO4 + NH3 ↔ NH4+ + HSO4- H2SO4 donates a proton and so is an B/L acid. Its conjugate base is HSO4- as it differs by one proton. NH3 accepts a proton and so is a B/L base. Its conjugate acid is NH4+ as it differs by one proton. Donates/ removes a H+ Conjugate acid-base pair

Conjugate Acid-Base Pairs ↔ This symbol means a reaction can take place in both directions CH3COOH + H2O ↔ CH3COO- + H3O+ Conjugate acid-base pair Conjugate acid-base pair

Conjugate Acid-Base Pairs For the forward reaction: CH3COOH + H2O → CH3COO- + H3O+ B/L Acid B/L Base For the reverse reaction: CH3COOH + H2O ← CH3COO- + H3O+ B/L Base B/L Acid

Exercise HCl + OH- ↔ Cl- + H2O H2O + H2SO4 ↔ HSO4- + H3O+ Label the acids, bases and conjugate acid-base pairs present in each of these equations: HCl + OH- ↔ Cl- + H2O H2O + H2SO4 ↔ HSO4- + H3O+

Exercise NH3 + H2O ↔ NH4+ + OH- HNO2 + H2O ↔ NO2- + H3O+ Label the acids, bases and conjugate acid-base pairs present in each of these equations: NH3 + H2O ↔ NH4+ + OH- HNO2 + H2O ↔ NO2- + H3O+

Label the acid /bases and conjugate acid/bases Answer

Label the acid /bases and conjugate acid/bases Answer

Label the acid /bases and conjugate acid/bases Answer

Label the acid /bases and conjugate acid/bases Answer

Label the acid /bases and conjugate acid/bases Answer

Strength of Conjugate acids/Bases Strong acid  Weak conjugate base Weak acid  Strong conjugate base Same applied for bases

Neutralisation Acid + Base  Salt + Water Neutralisation is a reaction between an acid and a base to form a salt and water HCl + NaOH  NaCl + H20 The metal from the base replaces the H+ in the acid Acid + Base  Salt + Water

Everyday examples of Neutralisation Sodium Hydrogen Carbonate NaHCO3 in Alka Seltza is taken to neutralise excess HCl in the stomach Lime (Calcium hydroxide) is added to soil or water to neutralise it Toothpaste contains a base to neutralise the acids in food