Bronsted-Lowry Theory of Acids and Bases

5.1.4 Brønsted-Lowry Theory of Acids & Bases That lack of an OH- group in ammonia is a bit of a problem at first glance, but let's take a closer look at what happens when NH3 is added to water: NH3 (g) + H2O(l) NH4+(aq) + OH–(aq) This is known as the Brønsted-Lowry theory of acids and bases can be stated as follows:

Brønsted-Lowry theory of acids and bases Acids are substances that produce a hydrogen ion (or a proton donor) Bases are substances that can accept a hydrogen ion (or a proton acceptor)

Using hydrochloric acid as an example, consider how HCl acts as an acid: HCl (g)→ H+(aq) + Cl-(aq) or HCl(g) + H2O(l) → H3O+(aq) + Cl-(aq) When something is donated, something else will receive it. It's not obvious in the top reaction, but it is water, H2O, that accepts the hydrogen. This is how an acid can act as a proton donor - it gives away it's hydrogen ion to a water molecule. View an animation of how HCl acts as an acid

Let's take a closer look at how a base can "accept a hydrogen ion" Let's take a closer look at how a base can "accept a hydrogen ion". Our ammonia example does a good job of showing this: NH3 (g) + H2O(l) NH4+(aq) + OH–(aq) The ammonium ion, NH4+, was formed when ammonia accepted a hydrogen ion. This makes ammonia a base. The +1 charge results because the ammonium ion has lost one electron. The H2O changes to OH– after giving up it's H+ View an animation of how NH3 acts as a base

5.1.5 Conjugate Acid-Base Pairs Conjugate acid-base pairs differ from each other by the presence or absence of a single hydrogen ion (proton). Every acid has a conjugate base, and every base has a conjugate acid. The conjugates will always be listed on the product side of the reaction.

5.1.6 Amphoteric Substances Substances that can act as an acid is one reaction and as a base in another are called  amphoteric substances Example: In the first reaction the bisulfate ion, HSO4– acts as a base. In the second reaction it acts as an acid: 1. HSO4– + H3O+ ↔ H2SO4 + H2O HSO4– accepts a proton from H3O+ 2. HSO4– + OH– ↔ H2O + SO42- HSO4– gives (donates) a proton to OH –

Practice Problems 5.1.6

5.1.7 Polyprotic Acids Are acids that contain more than one hydrogen ion that can be lost. Example: Sulfuric acid, H2SO4 has two hydrogen ions that it can give up. The first hydrogen ion is released as: H2SO4 (aq) → H+(aq) + HSO4-(aq) The second hydrogen will be more difficult to remove because it must now be removed from a negative ion, HSO4-. HSO4-(aq) → H+(aq) + SO42-(aq) Acids that can donate more than one hydrogen ion are called polyprotic

How many H+ can citric acid, H3C6H5O7, release? What do you think the equations describing the release of the H+ will be? Citric acid can release three H+. The reactions would be: 1.    H3C6H5O7 (aq) → H+(aq) + H2C6H5O7-(aq) 2.    H2C6H5O7-(aq) → H+(aq) + HC6H5O72-(aq) 3.    HC6H5O72-(aq) → H+(aq) + C6H5O73-(aq)

Assignment 5.1.7