## 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.

Length of video: 8.37 mins

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 “Energy Changes” or select from the suggested blog posts under “Related Articles” below.

## O-Level Chemistry Video: Use of Catalyst on Rate of Reaction

In the previous blog post, we have watched a video on 5 factors that can affect the rate of a chemical reaction.

Today, we shall discuss the role of a suitable catalyst and how it increases the rate of a chemical reaction.

Many of my students in our Sec 4 GCE O-Level Pure Chemistry tuition classes always share with us that their friends have difficulty to score full marks in Chemistry Examinations whenever they were being asked about how a catalyst work to increase the rate of reaction.

Length of video: 7.41 mins