A Level Chemistry Lessons

How to deduce Nature of Salt in Salt Hydrolysis

A salt can be acidic, neutral or alkaline.

We can deduce the nature of the salt from its constituent ions, namely:

1. ion formed from a weak acid will be a conjugate base, eg CH3COO- (from weak acid CH3COOH)

2. ion formed from a strong acid will be neutral, eg Cl- (from strong acid HCl)

3. ion formed from a weak base will be a conjugate acid, eg NH4+ (from weak base NH3)

4. ion formed from a strong base will be neutral, eg Na+ (from strong base NaOH)

This can be summarised as shown:

salt hydrolysis nature of ions summary

Next we can move on to deduce the nature of a salt.

Let's have some examples.

1. Sodium chloride NaCl is neutral
What we do is simply just deduce the nature of each constituent ion based on the acid or base that forms that ion.

salt hydrolysis sodium chloride neutral

Na+ is neutral since it is formed from strong base NaOH

Cl- is neutral since it is formed from strong acid HCl

Since both cation and anion are neutral, NaCl must be neutral.

2. Sodium ethanoate CH3COONa is alkaline
salt hydrolysis sodium ethanoate is alkaline

Na+ is neutral since it is formed from strong base NaOH.

CH3COO- is alkaline since it is the conjugate base of weak acid CH3COOH.

Since we have a conjugate base, CH3COO- will dissociate in water to give OH-, hence the salt is alkaline.

3. Ammonium chloride NH4Cl is acidic
salt hydrolysis ammonium chloride is acidic

Cl- is neutral since it is formed from strong acid HCl.

NH4+ is acidic since it is the conjugate acid of weak base NH3.

Since we have a conjugate acid, NH4+ will dissociate in water to give H+, hence the salt is acidic.

Watch this video to learn an easy way to understand salt hydrolysis, a concept that students often find confusing in Ionic Equilibria!

Topic: Ionic Equilibria, Physical Chemistry, A Level Chemistry, Singapore

You can also view this video lesson with screenshots and detailed explanation

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Bronsted Acids and Bases: Types and Strength

According to Bronsted-Lowry theory of acids and bases,

- an acid is a proton, H3O+ or H+ donor

- a base is a proton, H3O+ or H+ acceptor

The strength of an acid or base is related to the extent of dissociation in solution:

- a strong acid or base is fully dissociated

- a weak acid or base is partially dissociated

So therefore we will have 4 possible types of acids and bases:

1. Weak Acid

2. Strong Acid

3. Weak Base

4. Strong Base

Also, each of these acids and bases will dissociate in solution to give different species.

1. Weak Acid Dissociation eg CH3COOH
bronsted theory weak acid CH3COOH dissociation

CH3COOH is a weak acid and dissociates partially in solution (as indicated with reversible arrow) to form H+ and CH3COO- ions.

Since this is a reversible process, CH3COO- can accept H+ to form back CH3COOH.

Therefore the nature of CH3COO- is basic and we call CH3COO- the conjugate base of CH3COOH.

2. Strong Acid Dissociation eg HCl
bronsted theory strong acid HCl dissociation

HCl is a strong acid and will dissociate fully (as indicated with full arrow) in solution to form H+ and Cl- ions.

Since this is an irreversible process, Cl- has no tendency to accept H+ and form back HCl.

Therefore the nature of Cl- is neutral.

3. Weak base dissociation eg NH3
bronsted theory weak base NH3 dissociation

NH3 is a weak base and dissociates partially in solution (as indicated with reversible arrow) to form OH- and NH4+ ions.

Since this is a reversible process, NH4+ can donate H+ to form back NH3.

Therefore the nature of NH4+ is acidic and we call NH4+ the conjugate acid of NH3.

4. Strong base dissociation eg NaOH
bronsted theory strong base NaOH dissociation

NaOH is a strong base and will dissociate fully (as indicated with full arrow) in solution to form OH- and Na+ ions.

Since this is an irreversible process, Na+ has no tendency to donate H+ and form back NaOH.

Therefore the nature of Na+ is neutral.

So in summary:

- weak acid will dissociate to give conjugate base

- weak base will dissociate to give conjugate acid

- strong acid will dissociate to give a neutral ion

- strong base will dissociate to give a neutral ion

bronsted theory dissociation summary

These concepts are fundamental and very important in Ionic Equilibria.

Check out this video lesson to learn more about acids and bases and their dissociation in solution!

Topic: Ionic Equilibria, Physical Chemistry, JC, H2, A Level Chemistry, Singapore

You can also view this video lesson with screenshots and detailed explanation.

Do check out the following for more video lessons:
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If you are looking for H2 Chemistry Tuition, do consider taking up my classes at Bishan or check out my online chemistry tuition lessons. 

Balance Redox Reaction via Half Eqn Method

Balancing Redox Equations via the Half-Equation Method can be done via the following systematic steps.

1. Balance Redox Equation in Acidic Medium
Let's have this reaction as an example:

balance redox reaction using half equation method example

We can determine which species is oxidised and reduced by comparing the oxidation number:

- Oxidation state of manganese decreases from +7 to +4 hence MnO4- is reduced

- Oxidation state of iodine increases from -1 to 0 hence I- is oxidised

We can then write down the half equations for oxidation and reduction:

balance redox reaction half equations

Next we need to balance each half equation.

For half equation in acidic medium, the steps are:

1. Balance elements oxidised or reduced

2. Balance oxygen using water

3. Balance hydrogen using H+

4. Balance charge using electron

So after applying these 4 steps for each half equation, we'll end up with these two balanced half equations:

balance redox reaction half equation balanced in acidic medium

Next we need to modify each half equation so that the number of electrons for both half equations are the same (lowest common multiple).

In this case the lowest common multiple is 6, so we need to multiply the reduction half equation by 2, and the oxidation half equation by 3.

We can then add the oxidation and reduction half equations together to get an overall redox equation in acidic medium.

balance redox reaction overall balanced in acidic medium

2. Balance Redox Equation in Alkaline Medium
If we want to balance the redox reaction in alkaline medium, an additional step is required which is to add OH- to neutralise the H+.

From the balanced redox equation in acidic medium that we have done previously, there are 8H+ on the left hand side of the equation.

Therefore we need to add 8OH- on the left side to neutralise the 8H+, forming 8H2O.

We also need to add 8OH- on the right side to keep the equation balanced.

Usually there will be H2O on both sides of the equation to cancel out.

This will give us the overall balanced redox equation in alkaline medium:

balance redox reaction balanced in alkaline medium

Watch this video tutorial to learn how to balance a redox equation step-by-step!

Topic: Redox Titrations, Physical Chemistry, JC, H2, A Level Chemistry, Singapore

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Compare Strength of 2 Acids with different pH

If there are two acids, acid A at pH1 and acid B at pH3, which acid is the stronger acid?

strong acid vs weak acid question - pH1 versus pH3

You might be thinking acid A is the answer.

Since it has a lower pH, therefore there are more H+ in solution A.

So A must be the stronger acid.

It's obvious right?

Well... not really.

The truth is we cannot determine the strength of an acid just by looking at its pH alone.

There are 2 instances where A is not the stronger acid.

1. Acid A is a weaker acid at higher concentration
Acid A can be a weak acid at a much higher concentration than Acid B which is a strong acid.

This means A can still dissociate to give more H+ in solution, hence its pH is lower.

strong acid vs weak acid scenario one - A is weak acid at higher concentration

2. Acids A and B are the same acid but acid A has a higher concentration
Now that we know that pH is dependent on concentration of the acid, we can also have the scenario that both acids A and B are the same strong acid, eg HCl.

If the concentration of acid A is higher, it will dissociate to give more H+ in solution, hence A will have a lower pH.

strong acid vs weak acid scenario 2 - same acid with different concentrations

Using pH alone to predict the strength of acids is a very common mistake among students for the topic of Ionic Equilibria.

Be sure to check this video out to clear this misconception once and for all!

Topic: Ionic Equilibria, Physical Chemistry, JC, H2, A Level Chemistry, Singapore

You can also view this video lesson with screenshots and detailed explanation at 

Do check out the following for more video lessons:
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If you are looking for H2 Chemistry Tuition, do consider taking up my classes at Bishan.

How to draw Amino Acids and Proteins

Learn how to draw Alpha Amino Acids and Primary structure of Proteins by Maverick Puah the Chemistry Guru.

Electrophilic Addition Mechanism for Alkenes

Step-by-step tutorial to learn how to draw the Electrophilic Addition Mechanism of Alkene by Maverick Puah the Chemistry Guru.

Do check out the following for more video lessons:
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How to compare Basicity of Amines, Phenylamines and Amides

Learn how to compare Basicity for Amines, Phenylamines and Amides using Lewis Acid-Base Theory by Maverick Puah the Chemistry Guru

Do check out the following for more video lessons:
Organic Chemistry Videos 
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Saytzeff Rule, Zaitsev Rule: How to predict Major Product for Elimination Reaction

Elimination Reaction for some alcohols and alkyl halides will result in different alkene products, and Saytzeff or Zaitsev Rule is used to determine the major product.

Listen to this podcast to learn how to apply Saytzeff Rule.

Maverick Puah is a top A Level Chemistry Tutor and YouTube Educator in Singapore. Check out his online video lessons. 

Markovnikov Rule and Predicting Alkene Major Product

When an asymmetrical reactant such as HBr, HCl and H₂O is added to an asymmetrical alkene, two possible products can be formed.

Listen to this podcast to learn how to predict the major product using Markovnikov Rule.

Maverick Puah is a top A Level Chemistry Tutor and YouTube Educator in Singapore. Find out more about his online chemistry lessons.