Commenting on enthalpy change of neutralisation.

The definition of the enthalpy change of neutralisation is the heat evolved when one more of water is formed between an acid and an alkali. Essentially, it is the reaction between 1 mole of H⁺ and 1 mole of OH^- to give 1 mole of H₂O.

Typically, enthalpy change of neutralisation between a strong acid and a strong base is -57 kJ mol^-1. While, if it is between a weak acid and a strong base, or a strong acid and a weak base, the enthalpy change of neutralisation will be slightly less exothermic than -57 kJ mol^-1. This is largely because some amount of heat is taken in my the molecule to allow for complete acid (or base) dissociation.

1. Interesting question:

However, when we react 1 mole of H₂SO₄ with 2 moles of NaOH, do we get a more exothermic enthalpy change of neutralisation? The answer is NO.

This is because the reaction between sulfuric acid and sodium hydroxide produces 2 moles of water. This implies that the enthalpy change for this reaction is twice of that of enthalpy change of neutralisation; since the latter is defined to be per mole of water formed.

Therefore, when we obtain -114 kJ mol^-1 (which is more exothermic than -57 kJ mol^-1) for the reaction between 1 mole of sulfuric acid and 2 moles of sodium hydroxide, this value refers is the enthalpy change of reaction. Hence, when this number is divided by two because two moles of water is obtained, we will get enthalpy change of neutralisation.

2. Application - in terms of planning an experiment:

In conclusion, we often use calorimetry method to typical to determine the enthalpy change of neutralisation of an acid and a base. However, in using energetics in determining strength of acid, we are essentially trying to determine enthalpy change of reaction.

For example, when the reaction between sulfuric acid and sodium hydroxide is compare it with ethanoic acid and sodium hydroxide (and both acids are equimolar), it is important to ensure that the amount of hydroxide used is in excess. It is also good to ensure the volumes of hydroxide used are the same - this facilitates the comparison.

The former will produce an enthalpy change of reaction that is slightly more than twice of that of the latter; resulting in the former to have a temperature change that is twice as much as the latter.

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^{Article written by Kwok YL 2010.}

^{Disclaimer and remarks:}

• ^{If you would like to use this source, kindly drop me a note by leaving behind a comment with your name and institution. I am all for sharing as the materials on this blog is actually meant for the education purpose of my students.}
• ^{This material is entirely written by the author and my sincere thanks will be given to anyone who is kind, generous and gracious to point out any errors.}

Chemical Energetics - Applying Gibbs Free Energy

From the chart below, we can observe that there are certain chemical reactions whose change in Gibbs Free Energy change with temperature. Hence, we are able to perform mathematical calculations to predict at which temperatures the reaction will happen, become spontaneous and cease to occur.

Table to show change in Gibbs Free Energy and T
(1) Predict the temperature for reactions to happen or become spontaneous.

The difference between the two terms is quite simple. Spontaneous chemical reaction implies that the change in Gibbs Free Energy is smaller than 0. Reaction whose change in Gibbs Free Energy is equals to 0 can happen; they are just reversible. Hence, for reactions to happen, the change in Gibbs Free Energy has to be smaller or equals to 0.

Steps to predict Temperature
(2) Predict the temperature for reactions to become non-spontaneous.

In non-spontaneous reactions, we are implying that we need to determine temperatures where the reaction's change in Gibbs Free Energy becomes greater than 0.

Steps to predict Temperature
(3) Units.

The temperature used in the formula to calculate change in Gibbs Free Energy is in Kelvin (K). While the unit for change in Enthalpy is in KJ mol^-1 and the unit for change in Entropy is J mol^-1K^-1. Hence, you need to do the necessary conversion and exercise care when apply the formula.

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Article written by Kwok YL 2009.}

^{Disclaimer and remarks:}

• ^{If you would like to use this source, kindly drop me a note by leaving behind a comment with your name and institution. I am all for sharing as the materials on this blog is actually meant for the education purpose of my students.}
• ^{This material is entirely written by the author and my sincere thanks will be given to anyone who is kind, generous and gracious to point out any errors.}

Chemical Energetics - Gibbs Free Energy

With just the knowledge of enthalpy changes does not explain why certain reactions happen and others do not; as we have seen both exothermic and endothermic reactions happening.

With the introduction of entropy; which indicates the distribution of particles in a system does not help us to predict whether a reaction occurs or not. This is because we do observe reactions which show an increase in disorder (positive entropy change) and reactions which show a decrease in disorder (negative entropy change) happen.

However, we do observe that certain reactions, which has a certain enthalpy change and certain entropy change, can take place at certain temperature but will be unable to do so at other temperatures.

This observation allows for the introduction of Gibbs free energy. The change in Gibbs free energy accounts for the change in enthalpy and the change in entropy. It is this term, when negative in value, tells us that the reaction will be spontaneous, while when it is positive, tells us that the reaction is non-spontaneous. This term is a direct application of 2nd Law of Thermodynamics.

In addition, it is vital that we are able to appreciate that by changing temperature, it can sometimes cause a chemical reaction which was initially spontaneous to become non-spontaneous as we can see from the diagram below.

It is good to take note that when temperature change, when we apply the above formula we assume that the enthalpy change and entropy change remains the same despite there is a change in temperature. (Or at least, that the change in entropy and change in enthalpy due to temperature cancels each other out.)

However, the change in temperature may lead to the substances (in the chemical equation) to be in a different phase at the new temperature as compared to the old one (An example is shown below). This phase change results in a significant change in the entropy and hence we cannot assume that the change in entropy remains constant at a different temperature.

While at the "A" level Chemistry syllabus, we do assume that enthalpy change remain the same despite temperature change. This is because the assumption is that the enthalpy change of the reaction is affected solely by the chemical bonds present in the substances and that these bonds remain of the same strength even at a higher temperature. - (Although, I must add that this assumption is used to simplify the calculation of enthalpy change.)

In conclusion, reaction that is spontaneous will happen on its own in nature, while reactions that are not spontaneous will need an external supply of energy so that it can take place - for example photosynthesis is one such process, thus planets make use of light energy to enable this reaction to take place.

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Article written by Kwok YL 2009.}

^{Disclaimer and remarks:}

• ^{If you would like to use this source, kindly drop me a note by leaving behind a comment with your name and institution. I am all for sharing as the materials on this blog is actually meant for the education purpose of my students.}
• ^{This material is entirely written by the author and my sincere thanks will be given to anyone who is kind, generous and gracious to point out any errors.}

Chemical Energetics - Definitions

In this entry, I will be elaborating about the different definitions of enthalpy changes. In each set, there is an illustration and a subsequent set of annotations to give further elaboration.

(A) First set:

(1) In simple terms, the enthalpy change of reaction is per mole of reaction. What this implies is that the thermochemical equation is balanced and all the stoichiometric coefficients are intergers.

(2) In the enthalpy change of formation, do note that the non-metal substances which exist as simple discrete molecules such as hydrogen and oxygen, exist as H₂ and O₂ respectively.

(3) As combustion are exothermic reactions, we will expect all enthalpy change of combustion to be exothermic.

(B) Second set:

(4) The enthalpy change of atomisation stresses on the formation of one mole of gaseous atoms. Please do not be confused with this definition and that of enthalpy change of formation.

(5) The second ionisation energy simply refers to the second electrons removed.

(6) While the second electron affinity refers to the second electron added.

(7) Lattice energy describes the strength of an ionic bond. It is directly proportional to the product of the charges of the cation and the anion and it is inversely proportional to the sum of the respective radius of the cation and anion. As lattice energy talk about formation of the ionic bond only, it is an exothermic process.

(C) Third set:

(8) Enthalpy change of solution is the enthalpy change when one mole of substance dissolves in water. We cannot assume that dissolving substance into water does not produce or require heat.

(9) Enthalpy change of neutralisation stresses on the formation of one mole of water in a reaction between an acid and a base. Between a strong acid and a strong base, the enthalpy change of neutralisation is -57 kJ mol^-1.

However, if a weak acid and a strong base is used (or vice versa), the enthalpy change of neutralisation is less exothermic than -57 kJ mol^-1. This is because some amount of heat is required to dissociate the weak acid (or weak base) completely.

(10) Bond energy - do take note that it is defined to be the breaking of one mole of a given covalent bond to give gaseous products.

Hence, in order to ensure it is the energy that breaks the covalent bond, the substance must exist as a gas first. Generally, substances that have covalent bonds exist as simple discrete molecule. Hence, if liquid state or solid state is used, added energy is required to break the intermolecular forces. Therefore, the gas state is used as the intermolecular forces between gas molecules are the weakest.

(D) Final set: This provides additional information.

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Article written by Kwok YL 2009.}

^{Disclaimer and remarks:}

• ^{If you would like to use this source, kindly drop me a note by leaving behind a comment with your name and institution. I am all for sharing as the materials on this blog is actually meant for the education purpose of my students.}
• ^{This material is entirely written by the author and my sincere thanks will be given to anyone who is kind, generous and gracious to point out any errors.}

Chemical Energetics - More Energy Cycles

In this post, I will be sharing with you how to construct (i) Born Haber Cycle and (ii) Energy Level Diagram. In principle, both are considered energy cycles but these cycles are usually used to calculate enthalpy change of formation of the ionic compound. These cycle also can help to calculate lattice energy, should the value of enthalpy change of formation of the ionic compound can be obtained. (You can read about how to construct a generic energy cycle over here.)

Like in the earlier post, the state symbols of all the substances have to be shown.

My example which I will use to illustrate how to draw the Born Haber cycle is the standard enthalpy change of formation of CaF₂. As standard conditions are employed, the enthalpy change of atomisation of Ca and F are also in standard conditions. (You would expect enthalpy change of atomisation of solid F₂ to be more endothermic than that of gaseous F₂.)

It is interesting to note that it is quite impossible to obtain a standard condition for ionisation energy (I.E.) and electron affinity (E.A); we shall however assume that these will be constant at all temperatures.

(i) Born Haber Cycle:

In addition, I will use the standard enthalpy change of formation of CaO to demonstrate how to construct energy level diagram. The considerations are the same as earlier mentioned; with enthalpy change of atomisation of Ca and O to be at standard conditions.

(ii) Energy Level Diagram:



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Article written by Kwok YL 2009.}

^{Disclaimer and remarks:}

• ^{If you would like to use this source, kindly drop me a note by leaving behind a comment with your name and institution. I am all for sharing as the materials on this blog is actually meant for the education purpose of my students.}
• ^{This material is entirely written by the author and my sincere thanks will be given to anyone who is kind, generous and gracious to point out any errors.}

Chemical Energetics - Construction of cycle

The drawing of the energy cycle is essential in trying to calculate the enthalpy change of a reaction. This value is an unknown, thus in order to enable you to calculate it, you will be given at least two know enthalpy values and their respective thermochemical chemical equation (note that such equations, state symbols are essential).

In this entry, there are two videos available. The first video provides a simplified guide to the rules of constructing an energy cycle. While the second video shows how these rules are applied to construct the energy cycle.

The first video: A simplified guide to drawing an energy cycle.

The second video: Application of rules to an actual example.




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Article written by Kwok YL 2009.}

^{Disclaimer and remarks:}

• ^{If you would like to use this source, kindly drop me a note by leaving behind a comment with your name and institution. I am all for sharing as the materials on this blog is actually meant for the education purpose of my students.}
• ^{This material is entirely written by the author and my sincere thanks will be given to anyone who is kind, generous and gracious to point out any errors.}

Chemical Energetics - Calorimetry

Almost all chemical reactions produce heat. In the production of heat, experiments are conducted to investigate the amount of heat the reaction produced per mole of substance reacted. However, there are experiments which are conducted to investigate the amount of heat the reaction produce per mole of product formed.

Enthalpy change of reaction is defined as the amount of heat produced by the reaction per mole of substance reacted. Specific chemical reactions have their own enthalpy change definition such as Enthalpy change of neutralisation and enthalpy change of combustion. These definitions will be highlighted in another entry.

Hence, this post aims to explain how an experiment is done so that we can investigate the amount of heat the reaction produced per mole of substance reacted. These experiments are know as calorimetry. There are three different situations that can occur in a common laboratory.

(1) Using a water-beaker calorimeter.

In this experiment, a water-beaker calorimeter is set up as shown below. The combustion reaction occurs below the calorimeter and heat is transferred from the reaction to the calorimeter. The temperature of the solution in the calorimeter increase and using the formula shown below, we can obtain the enthalpy change of combustion. Do note that c = specific heat capacity of water, m = mass of water in the beaker and the negative sign implies the reaction is exothermic (releases heat).

Another formula is in the above picture. This formula uses C, which heat capacity of the calorimeter. The capital C is independent of the mass of the calorimeter. Hence, in this situation, it is the temperature change of the calorimeter which we measured.

(2) Mixing two solutions together.

The diagram below illustrates the steps taken. Usually, this method is done when the two reactants are in aqueous solutions. When these reactants are mixed together, the chemical reaction will transfer or absorb heat from the solution. When the reaction absorbs heat, such reaction is endothermic and the enthalpy change has a positive sign.

(3) Adding solid into an aqueous reactant.

The illustration below illustrates the steps taken. This method is used when one of the reactant is a solid and the other is an aqueous solution. From the formula below, note that V = volume of the solution, while c = specific heat capacity of the solution.

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Article written by Kwok YL 2009.}

^{Disclaimer and remarks:}

• ^{If you would like to use this source, kindly drop me a note by leaving behind a comment with your name and institution. I am all for sharing as the materials on this blog is actually meant for the education purpose of my students.}
• ^{This material is entirely written by the author and my sincere thanks will be given to anyone who is kind, generous and gracious to point out any errors.}