5.2 Enthalpy cycles (SL)

Summary

This video, presented by M Sjcam, explains Hess's Law and its application in enthalpy cycles. The law states that the enthalpy change of a chemical reaction is independent of the path taken. The video demonstrates this concept using examples involving the formation and combustion of compounds like benzene and ethene. By comparing different routes, the video shows how to calculate enthalpy changes using Hess's Law, highlighting key concepts such as the enthalpy change of formation and combustion in an easy-to-understand manner.

Takeaways

• 📚 Hess's law states that the enthalpy change for a chemical reaction is independent of the route taken.
• 🔄 In an enthalpy cycle, there are multiple routes from reactants to products, but the enthalpy change remains the same regardless of the route.
• ➡️ The equation ΔH3 = ΔH1 + ΔH2 represents the relationship between different routes in an enthalpy cycle.
• 🧪 An example involving carbon and hydrogen shows the enthalpy change for forming benzene from its elements.
• 🔥 The enthalpy change of combustion for benzene (ΔH2) is -3268 kJ/mol, and for the sum of 6 moles of carbon and 3 moles of hydrogen (ΔH3) is -3222 kJ/mol.
• 📝 Using Hess's law, the enthalpy change of formation for benzene (ΔH1) is calculated as +46 kJ/mol.
• 📉 An enthalpy level diagram for the combustion of ethene (C2H4) shows an enthalpy change of -1411 kJ/mol.
• ⚖️ The enthalpy change of formation for ethene can be determined by adding the enthalpy changes of carbon and hydrogen combustion and reversing the sign.
• 🟢 The enthalpy change of formation for ethene is calculated as +51 kJ/mol.
• 🔁 Hess's law is crucial in calculating unknown enthalpy changes by comparing different reaction routes.

Q & A

What is Hess's Law and how does it relate to enthalpy changes in chemical reactions?

Hess's Law states that the enthalpy change for a chemical reaction is the same, regardless of the route by which the reaction occurs. It means that the total enthalpy change for a reaction is independent of the number of steps or the intermediates involved.

How is the enthalpy change for a reaction represented mathematically according to Hess's Law?

According to Hess's Law, the enthalpy change for a reaction can be represented as ΔH3 = ΔH1 + ΔH2, where ΔH3 is the direct enthalpy change from reactants A to products C, and ΔH1 and ΔH2 are the enthalpy changes for the intermediate steps from A to B and from B to C, respectively.

What is the significance of the two possible routes in an enthalpy cycle?

The two possible routes in an enthalpy cycle demonstrate that the overall enthalpy change for a reaction is the same, regardless of whether the reaction proceeds directly from reactants to products or through one or more intermediate steps.

What is the enthalpy change of formation for benzene, as mentioned in the script?

The enthalpy change of formation for benzene, as calculated using Hess's Law, is found to be +46 kilojoules per mole.

How is the enthalpy change of combustion for benzene represented in the script?

The enthalpy change of combustion for benzene is represented as ΔH2 and is given a value of -3268 kilojoules per mole.

What is the significance of the negative value for the enthalpy change of combustion for benzene?

The negative value for the enthalpy change of combustion for benzene indicates that the reaction is exothermic, meaning it releases energy in the form of heat.

What does the enthalpy change of -3222 kilojoules represent in the script?

The enthalpy change of -3222 kilojoules represents the sum of the enthalpy changes for the combustion of 6 moles of carbon and 3 moles of hydrogen.

Can you explain the concept of an enthalpy level diagram as presented in the script?

An enthalpy level diagram is a graphical representation of the enthalpy changes for different steps in a chemical process. It helps visualize the energy changes associated with each step and the overall process.

What is the enthalpy change for the combustion of ethane (C2H4) as given in the script?

The enthalpy change for the combustion of ethane is given as -1411 kilojoules per mole.

How is the enthalpy change for the formation of ethene (C2H4) from its elements calculated in the script?

The enthalpy change for the formation of ethene from its elements is calculated by adding the enthalpy changes for the combustion of carbon and hydrogen and then reversing the sign, resulting in a positive value of 51 kilojoules per mole.

Why does the script mention reversing the sign of the enthalpy change when calculating for step Y?

The sign of the enthalpy change is reversed when calculating for step Y because the calculation is going against the direction of the enthalpy level diagram arrow, which indicates an endothermic process (positive enthalpy change).

Outlines

00:00

🔍 Introduction to Enthalpy Cycles and Hess's Law

This paragraph introduces the concept of enthalpy cycles and Hess's Law. It explains that according to Hess's Law, the enthalpy change for a chemical reaction is independent of the path taken. The paragraph discusses a specific enthalpy cycle where there are two possible routes from reactants A to product C—either via an intermediate B or directly. The enthalpy change for both routes is the same, which can be expressed mathematically as ΔH3 (direct route) equals ΔH1 (A to B) plus ΔH2 (B to C).

05:01

💡 Example: Enthalpy Cycle with Benzene Formation

This paragraph provides an example of an enthalpy cycle involving the formation and combustion of benzene. It describes two routes to go from carbon and hydrogen (reactants) to carbon dioxide and water (products). Route one involves the formation of benzene (ΔH1), followed by its combustion (ΔH2). The enthalpy change for the entire process (ΔH3) can be calculated using Hess's Law, leading to the determination of ΔH1 as the enthalpy change of benzene formation.

📉 Energy Level Diagram: Ethene Combustion

This paragraph discusses an energy level diagram showing the combustion of ethene (C2H4). It explains the enthalpy change for the combustion of one mole of ethene to form carbon dioxide and water. The paragraph also covers the enthalpy changes for the combustion of carbon and hydrogen to form their respective products. Using Hess's Law, the enthalpy change for the formation of ethene from its elements is calculated, highlighting the importance of reversing the sign when going against an arrow in the diagram.

🔬 Final Enthalpy Calculation for Ethene Formation

This final paragraph concludes the calculation of the enthalpy change for the formation of ethene. It summarizes the calculated value as +51 kJ/mol, derived from combining the enthalpy changes of carbon and hydrogen combustion with the enthalpy change of ethene combustion. This value represents the enthalpy change of formation for ethene.

Keywords

💡Hess's Law

Hess's Law states that the enthalpy change for a chemical reaction is independent of the route by which the reaction occurs. This law is crucial in understanding how different reaction pathways leading to the same product will yield the same total enthalpy change. In the video, Hess's Law is used to explain how the enthalpy change from reactants A to product C can be calculated through different intermediate steps, illustrating its fundamental role in thermochemistry.

💡Enthalpy

Enthalpy is a measure of the total energy of a thermodynamic system, including internal energy and the energy required to make room for it by displacing its environment. The concept is central to the video as it focuses on enthalpy changes during chemical reactions. The script frequently refers to the enthalpy changes (ΔH) in reactions, such as the formation and combustion of compounds, which are vital for understanding energy transformations in chemistry.

💡Enthalpy Cycle

An enthalpy cycle is a diagram that represents the enthalpy changes involved in a chemical reaction, showing different possible pathways from reactants to products. In the video, the enthalpy cycle is used to visualize Hess's Law, demonstrating that regardless of the pathway taken—from reactants A to intermediate B to product C, or directly from A to C—the total enthalpy change remains the same. This concept helps in calculating unknown enthalpy changes in reactions.

💡Enthalpy Change of Formation

The enthalpy change of formation (ΔH_f) is the heat change that occurs when one mole of a compound is formed from its elements under standard conditions. This concept is applied in the video when discussing the formation of benzene (C6H6) and ethene (C2H4). The video calculates the enthalpy change for these formations, demonstrating how Hess's Law can be used to find these values when direct measurement is not feasible.

💡Enthalpy Change of Combustion

The enthalpy change of combustion (ΔH_c) refers to the heat released when one mole of a substance is completely burned in oxygen under standard conditions. The video details the enthalpy change of combustion for benzene, carbon, and hydrogen. It shows how these values are used in conjunction with Hess's Law to determine the enthalpy change of formation for other compounds like benzene and ethene.

💡Chemical Reaction Pathways

Chemical reaction pathways are the various steps or routes a reaction can take from reactants to products. The video discusses two pathways: a direct route from A to C and an indirect route via B. The significance of these pathways in the context of Hess's Law is that they demonstrate the principle that the total enthalpy change is independent of the pathway, which is a core message of the video.

💡Reactants and Products

Reactants are the starting materials in a chemical reaction, while products are the substances formed as a result of the reaction. In the video, reactants such as carbon and hydrogen are mentioned, which react to form products like carbon dioxide, water, benzene, and ethene. Understanding the nature of reactants and products is essential for applying Hess's Law and calculating enthalpy changes.

💡Standard Conditions

Standard conditions refer to a set of predefined conditions, usually 298 K (25°C) and 1 atmosphere of pressure, under which enthalpy changes are measured. In the context of the video, these conditions are implied when discussing the enthalpy changes of formation and combustion. Knowing that these measurements are under standard conditions allows for consistent comparisons between different reactions.

💡Energy Level Diagram

An energy level diagram is a graphical representation of the energy changes during a chemical reaction, showing the relative enthalpy of reactants and products. The video presents an energy level diagram for the combustion of ethene, helping to visualize the enthalpy changes involved. These diagrams are key in understanding how different steps in a reaction contribute to the overall enthalpy change.

💡Kilojoules per Mole (kJ/mol)

Kilojoules per mole (kJ/mol) is a unit of measurement used to express the amount of energy involved in a chemical reaction per mole of substance. The video uses this unit when discussing the enthalpy changes of various reactions, such as the formation and combustion of benzene and ethene. This unit is essential for quantifying and comparing the energy changes in different reactions.

Highlights

Introduction to Hess's Law and its significance in determining enthalpy changes for chemical reactions.

Explanation that the enthalpy change for a chemical reaction is independent of the route taken, according to Hess's Law.

Description of an enthalpy cycle with two possible routes: from reactants A to product C directly, or via an intermediate B.

Introduction of the equation: ΔH3 (direct route A to C) = ΔH1 (route A to B) + ΔH2 (route B to C).

Application of Hess's Law to a specific example involving the formation and combustion of benzene (C6H6).

Calculation of the enthalpy change for the formation of benzene (ΔH1) using Hess's Law: ΔH1 = ΔH3 - ΔH2.

Explanation of how to rearrange the equation to solve for ΔH1, leading to the value of 46 kJ/mol.

Introduction to an energy level diagram for the combustion of ethene (C2H4).

Explanation of the enthalpy change of combustion for ethene, with a detailed description of the energy levels involved.

Description of the upward arrow representing the positive enthalpy change for the formation of ethene from carbon and hydrogen.

Calculation of the enthalpy change for the combustion of carbon and hydrogen to form carbon dioxide and water.

Demonstration of the use of Hess's Law to calculate the enthalpy change for the formation of ethene.

Clarification that the enthalpy change sign must be reversed when going against the direction of the arrow in the energy level diagram.

Final calculation of the enthalpy change of formation for ethene as +51 kJ/mol.

Summary of the application of Hess's Law to both enthalpy cycles and energy level diagrams to determine enthalpy changes in chemical reactions.

Transcripts

this is M sjcam in this video I'll be

looking at enthalpy cycles we'll start

by looking at Hess's law

Hess's law states that the enthalpy

change for a chemical reaction is

independent of the route by which the

chemical reaction occurs in this

enthalpy cycle there are two possible

routes to get from reactants a to

product c one route is to go from a via

B to C the other route is to go directly

from A to C so according to Hess's law

the enthalpy change for the reaction

will be the same if we go from A to B to

C or if we go from a directly to C so

this can be represented in equation form

as Delta H 3 which is the direct route

from A to C is equal to Delta H 1 which

is the route from A to B plus Delta H 2

which is the route from B to C so

regardless of the route that we take to

get from reactants a to product C the

enthalpy change will be the same next

we'll have a look at an example in this

enthalpy cycle there are two possible

routes to go from the reactants which

are carbon and hydrogen to the products

which are carbon dioxide and water the

first route starts with the reaction of

carbon and hydrogen to form one mole of

C 6 H 6 which is benzene this represents

the enthalpy change of formation of

benzene and we'll call this Delta H 1

next we have the combustion of one mole

of benzene to form carbon dioxide and

water which we will call Delta H 2 the

value of Delta H 2 which is the enthalpy

change of combustion for benzene is

negative 3268 kilojoules per mole Delta

H 3 which is the sum of the entropy

change of combustion for 6 moles of

carbon and 3 moles of hydrogen is

negative 3222 kilojoules

according to Hess's law the

to be changed for the reaction is

independent of the route taken so an

equation form Delta H 3 is equal to

Delta H 1 plus Delta H 2 if we rearrange

the equation we get Delta H 1 equals

Delta H 3 minus Delta H 2 this gives us

Delta H 1 equals negative 3222

minus negative 3268 which gives us an

entropy change for Delta H 1 of positive

46 kilojoules per mole which is the

entropy change of formation of benzene

in our next example we look at an

enthalpy or energy level diagram so

we'll start with this downwards arrow on

the right which is for the enthalpy

change of combustion of C 2 H 4 which is

e thin so in the reaction 1 mole of

ethane reacts with 3 moles of oxygen to

form two moles of carbon dioxide and 2

moles of water and the enthalpy change

for this reaction is negative 1411

kilojoules per mole next we look at the

enthalpy change given the letter Y this

enthalpy change represents the formation

of one mole of ethene from its elements

carbon and hydrogen which is the

enthalpy change of formation note that

this arrow is pointing in the upwards

direction which means it has a positive

enthalpy change next we look at the two

arrows pointing downwards the first

arrow represents the combustion of 2

moles of carbon to form two moles of

carbon dioxide to calculate this

enthalpy change we multiply the enthalpy

change of combustion of carbon by 2 to

give us negative 788 kilojoules the next

arrow represents the combustion of 2

moles of hydrogen to form two moles of

water to calculate this we multiply the

enthalpy change of combustion of

hydrogen by 2 to give us negative 572

kilojoules a-kor

Hess's law the entropy change for

reaction is independent of the route

that is taken so to calculate the ant be

change for step y we can add together

the ant be changes for these three steps

note that because we are going against

this arrow we need to reverse the sign

of the enthalpy change so the enthalpy

change for step y equals negative 788

plus negative 572 Plus 1411 which gives

us positive 51 kilojoules per mole this

is the enthalpy change of formation

value for e thene