A Level H2 Chemistry Tuition by 10 Year Series Author

Chemical Energetics: Experimental Method to Determine Enthalpy Change of Combustion

July 20, 2019 By Sean Chua Leave a Comment

In the previous blog post, we have defined the Enthalpy Change of Combustion as well as other enthalpy changes.

For a quick recap, Standard Enthalpy Change of Combustion is defined as:

Energy released when 1 mole of a substance is completely burned in oxygen under standard conditions

e.g. C₂H₄ (g) + 3 O₂ (g) → 2 CO₂ (g) + 2 H₂O (l) ΔH_c^θ(C₂H₄)

Experimental Determination of Enthalpy Change of Reaction

The First Law of Thermodynamics states that energy cannot be created or destroyed; it can only be converted from one form into another. Based on this law, we can determine the enthalpy change of a reaction.

System is defined as the reaction (part of the universe which is being studied).
Surrounding is defined as the environment of the reaction (rest of the universe that is not part of the system).

The enthalpy change of a reaction is heat (at constant pressure). The heat given out (or taken in) by a reaction is transferred to the surrounding). The surrounding is often some water, a solution containing the reactants or products, or even the air.

Experimental Determination

Chemical Energetics: Application of Gibbs Free Energy in Thermodynamics

July 17, 2019 By Sean Chua Leave a Comment

In the previous post, we have look into how Gibbs Free Energy change, ΔG is used to predict the spontaneity of a chemical reaction.

Today, we shall look into the application of this important concept and see how we can apply it to solve GCE A-Level H2 Chemistry exam-based questions. The following type of question is commonly tested in JC1 Promotional Examination (Promos) as well as JC2 Preliminary Examination (Prelims).

Question:

When 1 mole of carbon dioxide gas solidifies as dry ice, 25.2 kJ of heat energy is evolved. The sublimation temperature of carbon dioxide is -78.5 °C. What is the entropy change when 132 g of carbon dioxide gas solidifies at this temperature?

Thought Process:

Recall that ΔG is:

< 0 for any spontaneous reaction
> 0 for any non-spontaneous reaction
= 0 for any reaction in equilibrium

2. Recall the formula which links ΔG to ΔH, ΔS and T:

ΔG = ΔH – TΔS

3. Recall the units involved for the different thermodynamics terms:

Units of ΔG and ΔH are kJmol^-1 or Jmol^-1.
Units of ΔS are kJmol^-1K^-1 or Jmol^-1K^-1.
Units of T is K.

Suggested Solution:… Continue reading ...

Chemical Energetics: Gibbs Free Energy in Thermodynamics

July 14, 2019 By Sean Chua Leave a Comment

Spontaneous process of water flowing down a waterfall

In order to predict whether a reaction is spontaneous, we need to apply the Second Law of Thermodynamics which states that the entropy of the universe increases for a spontaneous change. Technically, we should consider the entropy change that takes places in both the system and the surrounding. This is given as:

ΔS_universe = ΔS_system + ΔS_surrounding

In Chemistry, the focus is always on the system. Thus, to simplify discussion, it is more convenient if ΔS_system + ΔS_surroundingare combined together in a single function known as the Change in Gibbs Free Energy, ΔG.

Change in Gibbs Free Energy, ΔG can be expressed in terms of Enthalpy Change, ΔH and Entropy Change, ΔS of the system.

ΔG = ΔH – TΔS

Do note the importance of the units involved in the three different thermodynamics terms.

Units of ΔG and ΔH are kJmol^-1 or Jmol^-1.
Units of ΔS are kJmol^-1K^-1 or Jmol^-1K^-1.
Units of T is K.

We can use the sign of ΔG to predict if a reaction is spontaneous.

Three Scenarios for ΔG:

1.

Chemical Energetics: Entropy (S)

July 11, 2019 By Sean Chua Leave a Comment

Entropy, S, is defined as the measure of the disorder or randomness in a system.

The more “mixed up” or “disorderly” or “messy” a system becomes, we say the entropy of the system increases.

4 Factors that affects the Entropy of a chemical system:

1) Change in Phase (Physical State)

S_gas >> S_liquid > S_solid

A solid has the lowest entropy because their particles are closely packed together and held in regular pattern. Thus, it’s structure is highly ordered.

A liquid has higher entropy than solid because its particles are arranged in an irregular pattern, although the particles are still quite close together.

The entropy of a gas is much greater than that of a liquid because gaseous particles are very far apart from each other and can move freely at great speeds. They are not constrained to be close to each other.

2) Change in Temperature

As temperature increases, the entropy of a system also increases.

As temperature increases, the particles undergo greater vibration (in solids) and more rapid movement (in liquids and gases). This causes more disorderliness. As such, entropy increases.

3) Change in Number of Particles (especially for Gases)

As the … Continue reading ...

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 ...