TOPIC 8 ACIDS AND BASES 8 1 Theories

TOPIC 8 ACIDS AND BASES 8. 1 Theories of Acids and Bases

ESSENTIAL IDEA Many reactions involve the transfer of a proton from an acid to a base. NATURE OF SCIENCE (2. 5) Falsification of theories – HCN altering theory that oxygen was the element which gave a compound its acidic properties allowed for other acid-base theories to develop. NATURE OF SCIENCE (1. 9) Theories being superseded – one early theory of acidity derived from the sensation of a sour taste, but this had been proven false. NATURE OF SCIENCE (5. 5) Public understanding of science – outside of the arena of chemistry, decisions are sometimes referred to as the “acid or litmus test”.

INTERNATIONAL-MINDEDNESS Acidus means sour in Latin, while alkali is derived from the Arabic word for calcined ashes. Oxygene means acid-forming in Greek, and reflects the mistaken belief that the element oxygen was responsible for a compound’s acidic properties. Acid-base theory has been developed by scientists around the world, and its vocabulary has been influenced by their languages.

THEORY OF KNOWLEDGE Acid and base behavior can be explained using different theories. How are the explanations in chemistry different from explanations in other subjects such as history?

UNDERSTANDING/KEY IDEA 8. 1. A A Bronsted-Lowry acid is a proton (H+) donor and a Bronsted-Lowry base is a proton (H+) acceptor.

APPLICATION/SKILLS 8. 1. A Be able to deduce the Bronsted. Lowry acid and base in a chemical reaction.

GUIDANCE Be familiar with the representation of a proton in aqueous solution as either H+ or H 3 O+.

THREE MAIN TYPES n n Arrhenius Acid – produces H+ ions Arrhenius Base – produces OH- ions n n n This is an early theory. Bronsted-Lowry Acid – proton donor Bronsted-Lowry Base – proton acceptor Lewis Acid – electron pair acceptor Lewis Base – electron pair donor

HA + H 2 O ↔ A- + H 3 O+ n HA is the acid because it has a proton to donate. What other acid is present? n H 2 O is the base because it can accept a proton. What other base is present? n HA ↔ A- + H+ n

UNDERSTANDING/KEY IDEA 8. 1. B A pair of species differing by a single proton is called a conjugate acid-base pair.

APPLICATION/SKILLS 8. 1. B Be able to deduce the conjugate acid or conjugate base in a chemical reaction.

GUIDANCE Be able to clearly show or indicate the location of the proton transfer. For example: CH 3 COOH/CH 3 COOrather than C 2 H 4 O 2/C 2 H 3 O 2 -

CONJUGATE ACIDS & BASES The conjugate acid/base pair consists of the substances related to each other by the donating and accepting of a proton. n Remember that Bronsted-Lowry acids are proton (H+) donors and bases are proton acceptors. n The acid and base in a conjugate acid-base pair differ by just one proton. n

FIGURING OUT FORMULAS n 1. What are the conjugate bases of the following compounds? n n n NH 3 and H 2 CO 3 When you need the base – remove one hydrogen and adjust the charge down by 1. NH 2 - and HCO 3 -

n 2. What are the conjugate acids of the following? n n n OH- and CO 32 To make the conjugate acid, add one H+ and adjust the charge up by one. HOH and HCO 3 -

CONJUGATE ACID/BASE PAIRS n HA + B ↔ A- + BH+ “HA AND A-”ARE A CONJ ACID/BASE PAIR HA IS THE ACID BECAUSE IT HAS AN “H+” TO DONATE SO A- IS ITS CONJUGATE BASE. “B AND BH+” ARE A CONJ ACID/BASE PAIR B IS THE BASE BECAUSE IT ACCEPTS AN “H+” SO BH+ IS THE CONJUGATE ACID.

ANOTHER EXAMPLE HC 2 H 3 O 3 + H 2 O ↔ C 2 H 3 O 2 - + H 3 O+ Here water acts as the base because it accepts the H+ from acetic acid. n NH 3 + H 2 O ↔ NH 4+ + OH Here water acts as the acid because it is donating the H+ to ammonia to form the ammonium ion. n

Examples of Bronsted-Lowry Acid Base Systems Note: Water can act as acid or base Acid Base Conjugate Acid Conjugate + H 2 O H 3 O + + Cl- H 2 PO 4 - + H 2 O H 3 O + + HPO 42 - NH 4+ H 2 O H 3 O + + NH 3 HCl + Base : NH 3 OHPO 43 - + Acid + H 2 O Conjugate Acid H 2 O HPO 42 - + Conjugate NH 4+ + OH- 18

UNDERSTANDING/KEY IDEA 8. 1. C Amphiprotic species can act as both Bronsted-Lowry acids and bases.

GUIDANCE Know the difference between the terms amphoteric and amphiprotic.

Amphiprotic vs Amphoteric Amphiprotic substances can act as either proton donors or proton acceptors. n Amphoteric substances can act as either acids or bases. n All amphiprotic substances are amphoteric - since when they donate a proton they are acting as an acid, and when they accept a proton they are acting as base. n

n Polyprotic species are frequently involved in the reactions where they behave amphiprotically

Amino Acids can act as amphiprotic

n Show by means of equation how the anion in K 2 HPO 4 are amphiprotic?

Citations International Baccalaureate Organization. Chemistry Guide, First assessment 2016. Updated 2015. Brown, Catrin, and Mike Ford. Higher Level Chemistry. 2 nd ed. N. p. : Pearson Baccalaureate, 2014. Print. Most of the information found in this power point comes directly from this textbook. The power point has been made to directly complement the Higher Level Chemistry textbook by Catrin and Brown and is used for direct instructional purposes only.