Past Zoom Webinar

Calculate pH of Weak Acid

In this JC2 webinar we want to learn how to calculate the pH of a weak acid.

Let's take a look at this question.

calculate weak acid pH question

We know that this acid is weak since its acid dissociation constant Ka is given.

Note that strong acids are fully dissociated hence do not have Ka values.

We can write down the dissociation of the acid via the ICE table.

calculate weak acid pH ICE table

ICE stands for Initial, Change and Equilibrium.

Since the degree of dissociation is not known, we can let it be x.

Hence we can determine the equilibrium concentrations of CH3COOH, CH3COO- and H+ in terms of x.

Next we can substitute these values into the Ka expression.

calculate weak acid pH Ka expression

Since Ka is known, we now have an equation in terms of x, and we can solve for it.

However the equation is quadratic so we have to do an approximation to simplify the calculation.

Note that for A Level Chemistry Syllabus, solving for quadratic equations is not required.

The approximation is pretty simple. Since the degree of dissociation is small, we assume the concentration of the acid at equilibrium is equal to its initial concentration.

calculate weak acid pH approximation

We can now rewrite the Ka expression into a non-quadratic equation.

calculate weak acid pH applying approximation

Now we can calculate x, which is the concentration of H+, and the pH.

calculate weak acid pH calculate pH

Most of the weak acids in Ionic Equilibria are monoprotic with small degree of dissocation.

Hence the ICE table and approximation mentioned here will be applicable most of the time.

This means we can just use the following formula to calculate H+ concentration directly.

calculate weak acid pH formula

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

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Can pH be Negative?

In this JC2 webinar we want to discuss if pH of an acidic solution can be negative.

Let's take a look at this question.

can pH be negative question

Calculating the pH of a strong acid is pretty straightforward.

We know strong acids are fully dissociated so the concentration of H+ and HCl are equal.

can pH be negative strong acid full dissociation

We can then calculate the pH from the H+ concentration to get a negative value.

can pH be negative calculate pH

This result might confuse some of us, as we usually think the lowest possible pH is either 1 or zero.

The reason why we have that concept is due to the pH scale that we are very familiar with.

can pH be negative pH scale

Notice the pH scale gives us the following impressions which are not always true!

- pH of neutral water is always 7
- the strongest possible acid has a minimum pH of 0 (or 1 depending on the textbook that you were using)
- the strongest possible base has a maximum pH of 14

Let's calculate the H+ concentration that corresponds to pH 0.

can pH be negative conc of H plus for pH 0

The concentration of H+ at pH 0 is only 1.0 mol dm-3.

It's definitely possible to increase the concentration of H+ to beyond 1.0 mol dm-3, and pH will become negative.

Similarly, we can calculate the OH- concentration that corresponds to pH 14.

can pH be negative conc of OH minus for pH 14

Again it's possible to increase the concentration of OH- to more than 1.0 mol dm-3 and pH will be greater than 14.

It's important to note that many ideas from the pH scale that we take for granted are actually misconceptions.

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

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Basicity of Acids and Dissociation of Diprotic Acids

In this JC2 webinar we want to explain basicity of acids and learn how to interprete the dissociation of diprotic acids.

1. Basicity of Acids

Basicity of acids is related to the number of protons an acid can donate.

A monoprotic acid donates 1 proton or 1 H+, eg strong acid HCl and weak acid CH3COOH

A diprotic acid donates 2 protons or 2 H+, eg strong acid H2SO4 and weak acid H2CO3

Similarly, a monobasic base accepts 1 proton and a dibasic base accepts 2 protons.

2. Dissociation of Diprotic Acid H2CO3

A diprotic acid will not donate both of its protons in 1 reaction.

The dissociation will occur in 2 stages:

basicity of acids dissociation of diprotic acid

Notice each step now becomes the dissociation of a monoprotic acid.

The acid in the first dissociation is H2CO3, while the acid in the second dissociation is HCO3-.

This means we need to be very familiar with the concept and dissociation of monoprotic acids, and apply them to diprotic or polyprotic species.

3. Comparing Acid Strength

For a polyprotic acid, the first acid is always the strongest and the strength decreases as the dissociation proceeds.

This means that the first acid is always stronger than the second acid, which is in turn stronger than the third acid (if triprotic).

For the H2CO3 example given earlier, H2CO3 is a stronger acid than HCO3-.

H2CO3 has more protons to donate than HCO3-, while HCO3- is negatively charged so it's harder to lose a positive H+.

Hence H2CO3 is a stronger acid than HCO3-.

For a polyprotic base, the trend is the same.

The first base is always stronger than the second base which is in turn stronger than the third base (for triprotic bases).

Understanding how we handle polyprotic acids and bases as a series of monoprotic species will be useful for subsequent concepts discussed in Ionic Equilibria, eg determine pH of diprotic acids.

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

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How to Calculate pH of Water

In this JC2 webinar we want to learn how to calculate the pH of water and deduce its nature at different temperatures.

1. Auto-ionisation of water

Pure water will undergo acid-base reaction with itself to a very small extent to form H+ and OH-.

This process is known as auto-ionisation of water and the concentration of H+ and OH- are equal since water will dissociate to give equimolar amounts of H+ and OH-.

calculate pH of water autoionisation of water

The equilibrium constant for this dissociation is known as the ionic product of water, Kw, where

Kw = [H+][OH-]

It has a given value of 10-14 mol2 dm-6 at 25oC or 298K.

We also need to know that auto-ionisation of water is an endothermic process.

2. Calculate pH of water at 298K

Since concentration of H+ and OH- in pure water are the same, we can calculate pH of water to be 7 via the following working:

calculate pH of water calculate pH of water at 298K

At first glance this seems hardly surprising, but pH of water is 7 only when Kw = 10-14 at 25oC.

When there is a change in temperature, Kw, H+ concentration and therefore pH will change.

3. Deduce pH of water when temperature is above 298K

Recall that auto-ionisation is endothermic.

When there is an increase in temperature, the position of equilibrium will shift right to favour endothermic reaction to absorb excess heat.

Since the forward reaction is favoured, more H+ and OH- is formed and their concentrations will increase.

This means H+ concentration will be greater than 10-7 mol dm-3, and the corresponding pH will be less than 7.

calculate pH of water deduce pH when temp above 298K

Hence pH of water will be less than 7 at temperatures above 298K, and greater than 7 at temperatures below 298K.

Does this mean that water becomes acidic when temperature is above 298K, and water becomes alkaline when temperature is below 298K?

4. Deduce nature of water at different temperatures

It is not possible to change the nature of water by changing its temperature.

At any temperature, regardless of the extent of dissociation, the concentration of H+ and OH- will always be the same.

This means for pure water, [H+] = [OH-] will always be true and water will always be neutral regardless of temperature.

calculate pH of water deduce nature of water at different temp

The reason why students might find this confusing is because we tend to use pH to deduce whether a solution is acidic, neutral or alkaline.

The concept that a solution is acidic when pH is less than 7, neutral when pH is 7, and alkaline when pH is greater than 7 is only true at 25oC or 298K.

When dealing with non-standard temperatures, we have to compare H+ and OH- concentrations to deduce the nature of solution:

Acidic when [H+] > [OH-]
Neutral when [H+] = [OH-]
Alkaline when [H+] < [OH-]

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

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Theoretical vs Experimental Lattice Energy

In this JC2 webinar we want to compare theoretical and experimental lattice energy.

Let's take a look at this question.

theory vs expt LE qn

We are asked to explain the differences between the experimental and theoretical values of lattice energy for AgF and AgI.

Experimental or actual lattice energy is determined from the Born-Haber cycle of an ionic compound.

Check out this video if you are interested to know how to draw Born-Haber cycle.

Theoretical or calculated lattice energy is based on the following formula assuming a highly ionic compound.

theory vs expt LE compare theoretical and experimental lattice energy

A highly ionic compound has only electrostatic attraction between cations and anions.

There is no distortion of electron clouds of the anions and no covalent character.

Therefore the difference between theoretical and experimental lattice energy tells us the extent of covalent character in that ionic compound.

If LE(experimental) is very close to LE(theoretical), the assumption that the ionic compound is highly ionic is true.

Therefore the ionic compound is highly ionic with little or no covalent character.

Ionic compounds that are highly ionic consist of:
- cations with low charge density and polarising power
- anions that are small and non-polarisable

If LE(experimental) is significantly different from LE(theoretical), the assumption that the ionic compound is highly ionic is false.

Therefore the ionic compound has significant covalent character.

Ionic compounds that have covalent character consist of:
- cations with high charge density and polarising power
- anions that are large and polarisable

Comparing AgF and AgI, larger iodide ion is more polarisable and there is a greater distortion of electron cloud of iodide.

AgI will have more significant covalent character than AgF hence a greater difference between theoretical and experimental lattice energy.

theory vs expt LE explaining AgI more covalent character

Topic: Energetics, Physical Chemistry, A Level Chemistry, Singapore

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