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Ch. 18: Acids & Bases Sec. 18.1: Acids & Bases: An Introduction.

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Presentation on theme: "Ch. 18: Acids & Bases Sec. 18.1: Acids & Bases: An Introduction."— Presentation transcript:

1 Ch. 18: Acids & Bases Sec. 18.1: Acids & Bases: An Introduction

2 Objectives §Identify the physical & chemical properties of acids & bases. §Classify solutions as acidic, basic, or neutral. §Compare the Arrhenius, Brønsted-Lowry, and Lewis models of acids & bases.

3 Properties Acids §Taste sour §May sting or burn on contact §Turn blue litmus paper pink §Are electrolytes §React with most metals to form H 2 gas* §React with carbonates to form CO 2 gas** Bases §Taste bitter §Feel slippery §Turn pink litmus paper blue §Are electrolytes

4 Metal* & Carbonate** Acid Reactions §Recall that acid and metal reactions are single replacement reactions: l Mg + HNO 3  ? l Al + H 2 SO 4  ? §Recall that acid and carbonate reactions are double replacement reactions with the immediate decomposition of H 2 CO 3 : l HBr + CaCO 3  ? l HCl + KHCO 3  ?

5 Aqueous Solutions §All aqueous solutions contain H + and OH - ions because water does dissociates slightly. The process is called self-ionization. H 2 O H + and OH - §If [H + ] = [OH - ], the solution is neutral. It is not acidic or basic. Pure water is neutral because [H + ] has to equal [OH - ]. §In an acidic solution, the [H + ] > [OH - ]. §In a basic solution, [OH - ] > [H + ].

6 The Hydronium Ion §Since H + ions will bond to water molecules in a solution, the solution does not really contain H + ions. It contains H 3 O + ions. §We will use the symbols H + and H 3 O + interchangeably to represent a hydrogen ion in a solution.

7 The Arrhenius Model §An acid is a substance that contains hydrogen and produces H + ions in aqueous solution. §A base is a substance that contains a hydroxide group and produces OH - ions in aqueous solution.

8 Examples An Arrhenius acid HCl --> H + (aq) + Cl - (aq) HCl is an acid. When dissolved in water, the resulting solution is acidic. A Arrhenius base NaOH --> Na + (aq) + OH - (aq) NaOH is a base. When dissolved in water, the resulting solution is basic.

9 Brønsted-Lowry Model Two chemists independently proposed a new model of acids & bases - one that would recognize that some bases (like NH 3, for example) did not contain hydroxide yet produced OH - ions in water.

10 Brønsted-Lowry Model According to Bronsted-Lowry, ammonia was considered a base: NH 3 + H 2 O  NH OH -

11 Brønsted-Lowry Model §An acid is a hydrogen ion (proton) donor. §A base is a hydrogen ion (proton) acceptor. Look at this general equation: HX (aq) + H 2 O H 3 O + + X - the acid the base

12 Practice Problems §Identify the acid and base in the following reactions: 1 H 3 O + + OH -  H 2 O + H 2 O 2 HCl + NH 3  NH Cl - 3 S -2 + H 2 O  HS - + OH - 4 HS - + H 2 O  S -2 + H 3 O + 5 H 2 O + HC 2 H 3 O 2  C 2 H 3 O H 3 O + 6 C 2 H 3 O H 3 O +  H 2 O + HC 2 H 3 O 2

13 Brønsted-Lowry Model HX (aq) + H 2 O H 3 O + + X - §On accepting the H + ion, H 2 O becomes H 3 O + (which is an acid). WHY?? Because in the reverse reaction, the H 3 O + donates its H + ion to X -. §On donating its H + ion, HX becomes X - (which is a base). WHY?? Because in the reverse reaction, the X - accepts the H + ion.

14 Brønsted-Lowry Model §The forward and reverse reactions are BOTH reactions of an acid and base. §The acid and base that react in the reverse reaction are called the conjugate acid and base. §The conjugate acid is the substance produced in the forward reaction when the base accepts H + from an acid. §The conjugate base is the substance leftover after the acid has donated H + to a base in the forward reaction.

15 Practice Problems §Identify the conjugate acid and the conjugate base in the reactions from before: 1 H 3 O + + OH -  H 2 O + H 2 O 2 HCl + NH 3  NH Cl - 3 S -2 + H 2 O  HS - + OH - 4 HS - + H 2 O  S -2 + H 3 O + 5 H 2 O + HC 2 H 3 O 2  C 2 H 3 O H 3 O + 6 C 2 H 3 O H 3 O +  H 2 O + HC 2 H 3 O 2

16 Brønsted-Lowry Model HX (aq) + H 2 O H 3 O + + X - §The hydronium ion is the conjugate acid of the base water. §The X - ion is the conjugate base of the acid HX. §A conjugate acid-base pair are two substances that are related to each other by the donating & accepting of a H + ion.

17 Examples HCO 3 - & H 2 CO 3 are a conjugate acid-base pair. H 2 O & OH - are also a conjugate acid-base pair.

18 Examples

19 Water §Water (& other substances) that can act as both an acid & a base are said to be amphoteric. §HF + H 2 O H 3 O + + F - BASE §NH 3 + H 2 O NH OH - ACID

20 Practice Problems Identify the conjugate acid-base pairs in the following reactions: 1.HSO H 2 O H 3 O + + SO CO H 2 O HCO OH - 3.NH OH - NH 3 + H 2 O 4.OH - + HC 2 H 3 O 2 C 2 H 3 O H 2 O

21 An acid that can donate only one hydrogen ion is a monoprotic acid. Acids that can donate more than one hydrogen ion are polyprotic acids. See Table 1 on p Which acids are monoprotic? polyprotic? Monoprotic and Polyprotic Acids

22 The Lewis Model According to the Lewis model, a Lewis acid is an electron-pair acceptor and a Lewis base is an electron pair donor. The Lewis model includes all the substances classified as Brønsted-Lowry acids and bases and many more.


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