## O-Level Chemistry: Strength versus Concentration of Acids

In the previous blog post, we have watched a video on the difference between Strength of Acids and Basicity of Acids.

If you have missed it, you can click HERE to watch it.

Today, we are going to discuss another misconception by many students studying basic Chemistry.

It is very important not to confuse the Strength of Acids with the Concentration of Acids.

I have observed that many of my GCE O-Level and IGCSE Sec 3 and Sec 4 Pure Chemistry Tuition Class students are confused by what they have learned from their school’s Chemistry teachers or previous home-based Chemistry tutors. It usually take me some effort to re-align their concepts and associated keywords.

As we have learned in the previous blog post, Strength of Acids refers to the extent of dissociation (or ionisation) of an acid. We will see words like “STRONG” and “WEAK“.

Concentration of Acids is different. It refers to how much of an acid (the solute) is dissolved in the solution. We will see words like “CONCENTRATED” and “DILUTE“.

Note that the strength of an acid is not affected by its concentration.

A dilute acid solution is not the same as a weak acid solution, and a concentrated acid solution is not the same as a strong acid solution.

i.e Dilute Acid $\neq$ Weak Acid

i.e Concentrated Acid $\neq$ Strong Acid

A strong acid will dissociate completely regardless of its concentration (whether it’s dilute or concentrated).

A weak acid will dissociate partially regardless of its concentration (whether it’s dilute or concentrated).

The term concentration is actually a concept covered in the topic of Mole Concepts or Mole Calculations.

The concentration of a solution is given by the amount of a solute dissolved in a unit volume of the solution.

Recall that a solution is made up of a solute and a solvent i.e. Solution $\equiv$ Solute + Solvent

As such, concentration of an acid can be easily changed!

You can decrease the concentration of an acid solution by adding more solvent to it.

You can also increase the concentration of an acid solution by adding more solute to it.

However, the strength of an acid cannot be changed!

A strong acid like HC(aq) will always dissociate completely to produce a high concentration of hydrogen, H+ ions, regardless of whether the solution is dilute or concentrated.

A weak acid like CH3COOH(aq) will always dissociate partially to produce a low concentration of hydrogen, H+ ions, regardless of whether the solution is dilute or concentrated.

Hope the above explanation is clear to you.

Together with the previous blog post, i have discussed on the difference between these three common terminologies used in the topic of Acids and Bases, namely:

## Strength of Acids $\neq$ Basicity of Acids $\neq$ Concentration of Acids

I hope you find the content easy for your understanding and if you have any questions, leave me a comment below.

PS: You can do a search on the top right hand corner for more discussions and questions on “Acids and Bases” or select from the suggested blog posts under “Related Articles” below.

PPS: If you are based in Singapore and want to master Acids, Bases and Preparation Salts, i will be conducting an annual Topical Chemistry Revision Workshops during the June School Holidays. You can get in touch with us at PassWithDistinction.com to find out more.

## O-Level Chemistry Video: Strength versus Basicity of Acids

We can find acids everywhere around us. They are in the air, in water, in fruits and even in humans and animals.

To study acids in more details, chemists would need to classify them in order to know them better.

Strength of Acids and Basicity of Acids are two very common but important ways to classify all the acids in this world. It is very important to know them well so that you are not confused by their close similarity.

Many of our students who join my weekly Sec 3 and Sec 4 GCE O-Level Pure Chemistry Tuition Classes (including IP, IB and IGCSE students) always commented that they are very confused with these two concepts when they first heard it from their secondary school teachers. Their keywords are all messed up and when it comes to Chemistry Exams, they tend to get zero or very low marks for this section on acids. So do beware!

Let’s take a look at the video …..

Length of video: 9.90 mins

Let’s re-cap what you have learned in the video…….

First, it is very important to know the formal definition of acids. Acids are substances which dissociates (or ionises) in water to produce hydrogen, H+ ions.

• Strength of Acids

The term strength of an acid refers to how easily an acid dissociates (or ionises) when dissolved in water. We can explain the strength of acids in terms of their extent of dissociation (or ionisation).

Strong Acid: An acid that dissociates completely in water to produce a high concentration of hydrogen, H+ ions.

e.g. HCl(aq) $\rightarrow$ H+(aq) + Cl (aq)

Take note of the $\rightarrow$ used in the dissociation equation for strong acids.

Weak Acid: An acid that dissociates partially in water to produce a low concentration of hydrogen, H+ ions.

e.g. CH3COOH(aq) $\rightleftharpoons$ CH3COO(aq) + H+(aq)

Take note of the $\rightleftharpoons$ used in the dissociation equation for weak acids.

• Basicity of Acids

The term basicity of acid refers to the number of moles of H+ ions that are being dissociated per mole of an acid.

Monobasic Acid: 1 mole of H+ ions are dissociated per mole of acid.

e.g. HCl(aq) $\rightarrow$ H+(aq) + C(aq)

Dibasic Acid: 2 moles of H+ ions are dissociated per mole of acid.

e.g. H2SO4(aq) $\rightarrow$ 2H+(aq) + SO42-(aq)

I hope you find the content easy for your understanding and if you have any questions, leave me a comment below.

PS: You can do a search on the top right hand corner for more discussions and questions on “Acids and Bases” or select from the suggested blog posts under “Related Articles” below.

## O-Level Chemistry Video: Energy Changes – Exothermic and Endothermic Reactions

Energy cannot be created or destroyed. However, it can be changed from one form to another. This is what you learned from basic Physics syllabus.

All chemical reactions involve energy changes.

Some physical changes, such as boiling or dissolving a ionic solid in water, also involve energy changes.

Two keywords in this topic called Energy Changes: Exothermic and Endothermic Changes

• An exothermic change is a change in which heat is given out to the surroundings
• An endothermic change is a change in which heat is absorbed from the surroundings

Let us take a look at a video to have a better understanding of both Exothermic and Endothermic Changes. I will be using physical state changes of solids, liquids and gases to introduce the concepts involved.