A Level H2 Chemistry Tuition by 10 Year Series Author

Chemical Energetics: Exam-Based MCQ Question on Hess’ Law & Energy Cycle

June 12, 2019 By Sean Chua 5 Comments

In the previous blog posts, we have talked about the different Standard Enthalpy Changes of Reactions as well as look into using Energy Cycles with Hess’ Law to solve Chemistry questions related Chemical Energetics.

Today, i am going to re-post a question for discussion which i have blogged about several years back. This questions is evergreen and you will be seeing such questions again in your upcoming A-Level H2 Chemistry examinations.

The following question was sent by one of our A-Level JC1 H2 Chemistry students in Singapore.

It is a very is simple yet interesting MCQ question and requires the students to be very good in drawing the Energy Cycle Diagram, in order to solve the question.

In fact, you can also solve this question using Algebraic Method.

Using Energy Cycle and Hess Law to solve Chemical Energetics question in H2 Chemistry Tuition Class in Singapore — MCQ question on Chemical Energetics

My student thinks that the answer should be option “B”‘. Do you agree with her?

Show your working or brief explanation on arriving at your answer.

“80% of the Success is about taking the Necessary Actions”

I hope you find the content easy for your understanding and if you have any questions, leave me a comment below. Feel free to share this blog post with … Continue reading ...

Chemical Energetics: Application of Hess’ Law & Energy Cycle Diagram

June 7, 2019 By Sean Chua 8 Comments

Chemical Energetics (also known as Thermochemistry) is a very important topic in Thermodynamics and is a highly application topic in A-Level H2 Chemistry Examinations.

In it, Hess’ Law and the use of Energy Cycle Diagrams is of utmost importance. Let’s take a look at some of the key concepts of Hess’ Law.

Hess’ Law of Constant Heat Summation:

It is defined as the enthalpy change (ΔH) of a particular reaction is determined only by the initial and final states of the system regardless of the pathway taken.

H2 Chemistry Tuition showing Hess Law in Chemical Energetics — Hess’ Law of Constant Heat Summation

By Hess’ Law,

Enthalpy change for Pathway 1 = Enthalpy change for Pathway 2

ΔH (for A → D) = ΔH (for A → B) + ΔH (for B → C) + ΔH (for C → D)

Uses:

Application of Hess’ Law is particularly useful when the enthalpy change of a reaction cannot be determined directly by experiment. In this cases, we will use Hess’ Law to calculate the enthalpy change from other data that is available (individual steps) and can be experimentally determined. A very common application of Hess’ Law is the Born-Haber Cycle which is used to determine the lattice energies of ionic … Continue reading ...

Chemical Energetics: Definitions of Standard Enthalpy Changes of Reactions

June 3, 2019 By Sean Chua Leave a Comment

A-Level H2 Chemistry students in junior colleges (JC) in Singapore always complain about this topic called Chemical Energetics (also known as Thermochemistry) and how difficult the questions are set in their schools. This topic is usually covered in term 1 or term 2 in JC1.

There are 3 sections in Chemical Energetics, namely:

Enthalpy Changes, ΔH
Entropy Changes, ΔS
Gibbs Free Energy, ΔG

After teaching batches of students in the last 16 years, i realised those students who are weak in this topic tends to take short-cuts. They are too “lazy” to understand and remember the key definitions of each of the different Standard Enthalpy Changes of Reactions. No wonder they are struggling with this topic. If you want to master this topic, you cannot take short-cuts and expect yourself to do well.

Students need to have a good grasp of all these fundamentals before they can answer application questions on Hess’ Law, Born Haber Cycle, Energy Level Diagram, etc.

Enthalpy Change of Reaction, ΔH

To start, let’s look at Enthalpy Change of Reaction, ΔH, which is defined as the heat change (heat energy absorbed or evolved) when the reaction takes place between the reagents as … Continue reading ...

Organic Chemistry: Constitutional (Structural) Isomerism

May 21, 2019 By Sean Chua Leave a Comment

First of all, re-cap that isomerism can be broadly classified into 2 types:

Constitutional isomerism, in which atoms are linked together in different ways
Stereoisomerism, in which atoms have the same connectivity but different arrangements in space.

Today, we will be talking about constitutional isomerism.

Constitutional isomers have the same molecular formula but different structural formulae. This simply means that the atoms are linked together in different ways. They are also known as structural isomers.

There are three types of constitutional isomerism:

Positional Isomerism
Chain Isomerism
Functional Group Isomerism

1. Positional Isomerism

Positional isomers are molecules with the same molecular formula and same functional group but differ in the position of the functional group.

Positional isomers are also commonly known as compounds having the same substituents at different positions on the same carbon skeleton.

Positional isomers have similar chemical properties but different physical properties.

Examples of positional isomers:

2. Chain Isomerism

Chain isomers refers to organic compounds with the same molecular formula and same functional group, but different carbon skeleton.

Chain isomers have similar chemical properties but different physical properties.

Examples of chain isomers:

3. Functional Group Isomerism

Organic Chemistry: Total Number of Stereoisomers

May 17, 2019 By Sean Chua Leave a Comment

Stereoisomers of organic molecules includes both cis-trans (geometric) isomers and enantiomers (optical isomers) of a particular substance.

Stereoisomerism occurs when organic compounds have the same molecular formula and the same structural formula, but the atoms in the molecules have different spatial arrangements.

So, how do we calculate and determine the maximum number of stereoisomers a molecule can have?

Provided there is no internal plane of symmetry in the structure of a molecule, and chirality occurs only due to chiral centres,

the maximum number of stereoisomers a molecule can have = 2^m+n

whereby:

m = number of chiral centres; and

n = number of double bonds that can give rise to cis-trans isomers

Let’s take methoprene molecule as an example.

*Methoprene molecule has one chiral centre (C*) and two C=C double bonds. It exhibits both enantiomerism and cis-trans isomerism.*

There is only one chiral centre in a methoprene molecule.

Both the C=C double bonds also satisfy the criteria for cis-trans (geometric) isomerism.

As such, the maximum possible number of stereoisomers for methoprene is 2^{m+ n} = 2⁽¹⁺²⁾ = 2³ = 8.

I hope the above discussion is clear for you.

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