A-Level H2 ChemistryAdvanced Chemistry Made Easy

In the previous blogpost, we have discussed about the differences between Real Gas and Ideal Gas.

Hope you are learning something. Drop me a line in the ”Leave a Comment” section below. I would love to hear from you.

Today, we shall look at why Different Real Gases deviate to a different extent from the Ideal Gas behaviour.

Recall from the Basic Assumptions of Kinetic Theory as applied to Ideal Gas, you will realised that Intermolecular Forces of Attraction plays a big part in the deviation of real gas from ideal gas behaviour.

Polar molecules have stronger forces of attraction between molecules and so, they will deviate much more from ideality as compared to non-polar molecules.

Example:

Let’s compare 3 gases (HCl, NH₃ and H₂) and see how we can apply the concepts we have just discussed:

This means that polar molecules such as NH₃ and HCl (both are polar molecules) will deviate more from ideality as compared to H2 (non-polar molecule).

Further comparing NN₃ and HCl, we will expect NH₃ to deviate more than HCl from ideality due to Hydrogen-Bonding between NH₃ molecules as compared to pd-pd forces between HCl molecules. (REVISE on the topic of Chemical Bondings)

As such, deviation from ideal gas behaviour increases as intermolecular forces increase.

H₂ (weak van der Waals forces) < HCl (pd-pd) < NH₃ (strong hydrogen-bonding)

Hope the above discussion is clear for your understanding.

In the next blog post, we will discuss on exam-based questions that are related to this concept.

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In earlier blogpost, we have mentioned on the basic assumptions of kinetic theroy as applied to Ideal Gas.

Today, we shall continue to discuss on what really is a Real Gas and how it differs from the Ideal Gas (and thus its assumptions).

A Real Gas has the following features:

1. Gas particles have a certain volume and size – the particles cannot just move anywhere in the container

2. Intermolecular forces of attraction exist between the gas particles, though they are usually weak – the particles tend to stick together and thus reduces the pressure of the container slightly

Now, so when can a Real Gas behaves like an Ideal Gas?

A Real Gas is most like an Ideal Gas under the following conditions:

1. At Low Pressures:

- Gas particles are widely spaced apart and thus they have negligible size (and volume)

- Intermolecular forces of attraction between gas particles are virtually zero

2. At High Temperature:

- Negligible intermolecular attractions since the gas particles have sufficient kinetic energy to overcome it

In another way of putting it, this means a

Real Gas will shows the Biggest Deviation from Ideality:

1. At High Pressures

- Gas particles are packed so close together, and the size of a gas molecule cannot be assumed to be negligible

2. At Low Temperature

- Low kinetic energy and the intermolecular forces of attractions between gas particles are significant

The above concepts will be required for A-Levels H2 Chemistry Exams, but many students do not know the key concepts itself, and how to apply them to solve questions.

This is what many of my JC students attending the H2 Chemistry Weekly Coaching Class told me. Many of their lecturers actually classify this topic as ‘E-learning’ or ‘Self-Learning’ and ask the students to read through the notes on their own. Beware!

In the next post, i will share with you how different Real Gases deviate from Ideal Gas behaviour to a different extent.

Stay tuned to it!

PS: How do you find the Chemistry Tips so far in this blogpost? Leave me a comment, i would love to hear from you =)

(Photo Credit Coconino)

In GCE A-Levels, one of the topics that is always neglected by most Pre-U (JC) lecturers and tutors is Ideal Gas Law, or sometimes we call it Gaseous State.

It is a ’small topic’ in terms of content, but it is ‘HUGE’ when it comes to student’s ability to handle the questions in Promo Exams and GCE A-Levels H2 Chemistry papers.

I realised that many JC1 and JC2 students are very afraid of this topic when it comes to their H2 Chemistry exams.

In Basic Chemistry syllabus such as GCE O-Level Chemistry (refer to SimpleChemConcepts.com for key chemistry concepts), we understand Gases are based on Kinetic Particle Theory, by assuming the following:

1. There are no forces between the gas particles
2. Particles are very far apart, and have alot of empty spaces
3. Low densities and can be easily compressed
4. Particles move in random motion and can move any direction
5. Pressure of the gas is due to the particles hitting and bouncing off the walls of the container
6. They have no shape

Now, the above is always true when it comes to Basic Chemistry, but for Advanced Level Chemistry such as A-Levels H2 Chemistry, we meed to understand more about concepts regarding Gases.

We need to realise that the above assumptions is hypothetical and gases are not always ideal – they are known as Real Gas.

The 4 basic assumptions of the Kinetic Theory as applied to Ideal Gas are:

1. Gas particles (atoms or molecules) are of negligible size and volume
2. Gas particles have negligible intermolecular forces of attraction
3. Gas particles are in constant random motion
4. Gas particles collide with each other which are elastic i.e. no loss in kinetic energy

The above is important as A-Levels H2 Chemistry students are required to be able to state the assumptions of the kinetic theory as applied to ideal gas.

Look out for the next post where i will share with you that most gases are Real Gas instead of Ideal Gas, and thus the limitations of ideality of gases.

Stay tuned for it!