Acid & Bases

The aim of this blog post is to give you some introductory notes to the acids and bases unit. I have just been covering this online with my IB2 class so I thought it was worth sharing what we have been looking at. You could copy and paste these into a word document and have your own set of introductory notes.

Before we even start looking at acids, bases and pH, let us think about logs, as we use these to measure pH. Logs allow us to put a wide range of numbers into a meaningful scale.

Eg, 0.0000000000001 to 10

By logging these numbers we get -12 to 1

This seems far easier to deal with!

By powerbiker1 (Thomas Kees) – selbst photographiert (eigenes Archiv), CC BY-SA 3.0 de, https://commons.wikimedia.org/w/index.php?curid=9597380

 

But how does this relate to pH?

Well, the pH scale is a log scale.

The pH scale measures the concentration of H⁺ ions ([H⁺])

 

Acid / Base theory

What is an acid? One theory says an acid is an H⁺ donor (or a proton donor).

But, Fe²⁺ is considered an acid …. And there are no protons here

The theories are:

 

1. The Bronsted – Lowry theory

A Bronsted Lowry acid is a proton donor.

A Bronsted Lowry base is a proton acceptor

Eg, H₂SO₄ ⁺ → H₂NO₃⁺ + HSO₄^–

You will come across this reaction in the organic chemistry unit. Two acids are reacting together but according to our theory, nitric acid (HNO₃) is the proton acceptor so it is actually thought of as being a Bronsted Lowry acid.

 

2. Lewis Acid Base Theory

The Lewis theory puts another spin on things. In this theory:

A Lewis acid is an electron pair acceptor (in organic chemistry it is a electrophile, Eg, H⁺)

A Lewis base is an electron pair donor (in organic chemistry it is a nucleophile, Eg OH^–)

 

Calculating the pH of strong acids

Strong acids completely dissociate in water

HNO₃ → H⁺ + NO₃^–

[Weak acids, partially dissociate in water eg, CH₃COOH ⇌ CH₃COO^– + H⁺ ]

Calculation for pH:

pH = – log ₁₀ [H⁺]

This means that for a strong acid, the [H⁺] = [Strong acid]

If a pH is given, we can also work out the [H⁺] which is the [Strong acid]

[H⁺] = 10 ^–pH

 

Here are a few questions for you to try to check your understanding of things. Be careful with Q3 & 6 as there are two protons for every acid molecule. What will you need to do in your calculations to deal with this?

Calculate the pH of:

1. HCl 0.08 mol dm^-3
2. HNO₃ 1.5 mol dm^-3                                
3. H₂SO₄ 0.034 mol dm^-3

 

Calculate the concentration of these acids:

1. HCl                    pH 0.0056
2. HNO₃                pH -0.32
3. H₂SO₄               pH 3.66