Calculating Enthalpy Changes from Bond Enthalpies

Summary

This educational video script delves into calculating enthalpy changes (ΔH) using bond enthalpies. It explains the concept of bond enthalpy, which represents the energy required to break a bond, exemplified by the endothermic processes of breaking H-H and H-Cl bonds. The script introduces average bond enthalpies, crucial for estimating ΔH in reactions involving common bonds like C-H and C=O. Using the combustion of methane as a case study, the video illustrates how to calculate ΔH by subtracting the energy released from the energy inputted, highlighting the exothermic nature of the reaction. The script also guides through a similar calculation for the combustion of methanol, emphasizing the practical application of bond enthalpies in thermodynamic assessments.

Takeaways

• 🔥 Bond enthalpy represents the energy required to break one mole of a specific bond in a gaseous state.
• 📚 The script explains the endothermic process of breaking bonds, such as the H-H bond in hydrogen gas and the H-Cl bond in HCl gas.
• 🌐 Average bond enthalpies, also known as mean bond enthalpies, are used when specific bond energies vary across different molecules.
• 🔍 The script uses the combustion of methane as an example to demonstrate how to calculate enthalpy change using bond enthalpies.
• ⚖️ The process involves calculating the energy required to break bonds in reactants and the energy released when new bonds are formed in products.
• 🔢 The 'in minus out' method is used to determine the enthalpy change, where 'in' is the energy required to break bonds and 'out' is the energy released when bonds are formed.
• ⏳ The script provides a step-by-step calculation for the combustion of methane, showing how to use average bond enthalpies to find ΔH.
• 📉 The calculated value for ΔH in the combustion of methane is compared to the standard enthalpy of combustion from data books, highlighting the use of averages versus actual bond energies.
• 🧪 The script also demonstrates how to calculate the enthalpy change for the combustion of methanol, emphasizing the importance of writing out the reaction in displayed formula format.
• 📊 The final calculated ΔH values for the reactions are used to confirm whether the reactions are exothermic or endothermic based on the energy balance.

Q & A

What is the significance of bond enthalpy in the context of the video?

Bond enthalpy represents the energy required to break one mole of a specific chemical bond into its gaseous atoms. It is an essential concept for understanding the energy changes during chemical reactions.

Why are bond enthalpy values always endothermic?

Bond enthalpy values are always endothermic because energy must be supplied to break the attractive forces between covalently bonded atoms.

What is the average bond enthalpy for the CH bond mentioned in the video?

The average bond enthalpy for the CH bond is +413 kJ/mol.

How does the video explain the calculation of enthalpy change using bond enthalpies?

The video explains the calculation of enthalpy change by subtracting the energy released when new bonds are formed from the energy required to break the original bonds, using the 'in minus out' method.

What is the purpose of using average bond enthalpies instead of specific bond enthalpies?

Average bond enthalpies are used because it's impractical to have data tables for every possible bond in different molecules. They provide a generalized value for the energy needed to break a bond.

What is the role of bond enthalpies in calculating the enthalpy change for the combustion of methane?

Bond enthalpies are used to calculate the energy required to break bonds in the reactants and the energy released when new bonds are formed in the products. The difference between these values gives the enthalpy change for the combustion of methane.

How does the video demonstrate the calculation of the enthalpy change for the combustion of methane?

The video demonstrates the calculation by multiplying the average bond enthalpies by the number of bonds broken and formed, then subtracting the energy required to break bonds from the energy released by bond formation.

What is the difference between the calculated enthalpy change for the combustion of methane using bond enthalpies and the data book value?

The calculated value using bond enthalpies is -816 kJ/mol, while the data book value is -890 kJ/mol. The difference arises because the calculation uses average bond enthalpies, whereas the data book uses specific bond enthalpies.

Why is it important to draw the chemical equation in displayed formula format when calculating enthalpy changes?

Drawing the chemical equation in displayed formula format helps visualize all the bonds involved in the reaction, making it easier to identify which bonds are broken and formed, and thus accurately calculate the enthalpy change.

What is the enthalpy change calculated for the combustion of one mole of methanol using bond enthalpies?

The enthalpy change calculated for the combustion of one mole of methanol using bond enthalpies is -678 kJ/mol.

Outlines

00:00

🔍 Introduction to Bond Enthalpies

This paragraph introduces the concept of bond enthalpies, specifically focusing on how to calculate the enthalpy change (ΔH) using bond enthalpies. It explains that bond enthalpy is the energy required to break one mole of a specified bond into its gaseous atoms. The process is endothermic, meaning energy must be supplied to break the bond. The paragraph provides examples of bond enthalpies for the H-H bond in hydrogen gas and the H-Cl bond in HCl gas, with values of 436 kJ/mol and 432 kJ/mol, respectively. It also discusses the use of average bond enthalpies, or main bond enthalpies, for bonds that occur in many different molecules, such as the C-H bond. The paragraph sets the stage for using these values to calculate the enthalpy change of reactions, starting with the combustion of methane.

05:02

🔥 Calculating Enthalpy Change for Methane Combustion

This paragraph delves into the calculation of the enthalpy change for the combustion of methane using bond enthalpies. It details the process of breaking bonds in the reactants, which requires energy input, and forming new bonds, which releases energy. The paragraph outlines the calculation by breaking four C-H bonds and two O=O double bonds in the reactants, using their average bond enthalpies to determine the energy input. It then calculates the energy released when new bonds are formed in the products, specifically the C=O bonds in carbon dioxide and the O-H bonds in water. The 'in minus out' method is used to determine the net enthalpy change, resulting in an exothermic reaction with a ΔH of -816 kJ/mol. The paragraph also contrasts this calculated value with the standard enthalpy of combustion for methane from data books, noting the difference due to the use of average bond enthalpies versus specific bond values.

10:04

🌡️ Enthalpy Change Calculation for Methanol Combustion

The final paragraph extends the concept of enthalpy change calculation to the combustion of methanol. It emphasizes the importance of writing out the balanced chemical equation in displayed formula format to visualize all the bonds involved. The paragraph then proceeds to calculate the energy required to break the bonds in the reactants, including three C-H bonds, one C-O single bond, and one O-O bond, using their respective average bond enthalpies. For the energy released upon bond formation in the products, it accounts for the creation of two CO bonds in carbon dioxide and four O-H bonds in water. The calculation again uses the 'in minus out' method, resulting in an exothermic reaction with a ΔH of -678 kJ/mol. The paragraph concludes by highlighting the exothermic nature of methanol combustion and the practical implications of these calculations in understanding the energy changes in chemical reactions.

Keywords

💡Enthalpy Change (ΔH)

Enthalpy change, denoted as ΔH, is a measure of the heat absorbed or released in a chemical reaction at constant pressure. It is a key concept in thermodynamics and is central to the video's theme of calculating energy changes during chemical reactions. The video uses ΔH to explain the combustion of methane and methanol, showing how bond enthalpies are used to calculate the overall enthalpy change for these reactions.

💡Bond Enthalpy

Bond enthalpy refers to the energy required to break one mole of a specific chemical bond. In the video, bond enthalpies are used to calculate the energy needed to break bonds in reactants and the energy released when new bonds are formed in products. This concept is essential for understanding the calculations of ΔH for the combustion of methane and methanol.

💡Endothermic Process

An endothermic process is one that absorbs energy from its surroundings. The video explains that breaking chemical bonds is an endothermic process, as energy must be supplied to overcome the forces holding the atoms together. This is illustrated by the bond enthalpies of hydrogen gas and HCl gas, which are positive values indicating energy input.

💡Exothermic Process

An exothermic process is one that releases energy to its surroundings. The video discusses combustion reactions, such as the burning of methane, as exothermic processes. These reactions release more energy in the form of new bonds than was required to break the original bonds, resulting in a negative ΔH value.

💡Average Bond Enthalpies

Average bond enthalpies, also known as mean bond enthalpies, are used when the exact bond enthalpy for a specific bond in a molecule is not available. The video mentions that these values are used to estimate the energy changes in reactions involving bonds like the C-H bond, which can vary in energy depending on the molecule.

💡Combustion Reaction

A combustion reaction is a chemical reaction between a substance and an oxidant, usually oxygen, that releases energy in the form of heat and light. The video focuses on the combustion of methane and methanol, using bond enthalpies to calculate the enthalpy change for these reactions, which are exothermic and release energy.

💡Methane

Methane (CH4) is a simple hydrocarbon and the main component of natural gas. In the video, the combustion of methane is used as an example to demonstrate how bond enthalpies can be used to calculate the enthalpy change of a reaction. The calculation involves breaking the C-H bonds in methane and forming new bonds in the products.

💡Methanol

Methanol (CH3OH) is an alcohol that serves as a fuel and solvent. The video includes a calculation of the enthalpy change for the combustion of methanol using bond enthalpies. This example shows how the method can be applied to more complex molecules than methane.

💡In Minus Out Method

The 'in minus out' method is a technique used in the video to calculate the enthalpy change of a reaction. It involves subtracting the total energy released when new bonds are formed (out) from the total energy required to break the original bonds (in). This method is used to determine whether a reaction is endothermic or exothermic.

💡Standard Enthalpy of Combustion

The standard enthalpy of combustion is the change in enthalpy when one mole of a substance is burned in excess oxygen at standard conditions. The video compares the calculated enthalpy change for the combustion of methane using bond enthalpies to the standard enthalpy of combustion value found in data books, highlighting the use of average bond enthalpies in calculations.

Highlights

Introduction to the concept of bond enthalpy and its significance in calculating enthalpy changes.

Explanation of bond enthalpy as the energy required to break one mole of a specific bond into gaseous atoms.

Demonstration of bond enthalpy using the example of hydrogen gas (H2) and its endothermic process.

Illustration of bond enthalpy with the process of breaking HCL gas into hydrogen and chlorine atoms.

Discussion on the endothermic nature of bond-breaking processes and the energy input required.

Introduction of average bond enthalpies and their use in organic chemistry for common bonds like CH, CC, and OO.

Explanation of how to calculate the enthalpy change for a reaction using bond enthalpies, exemplified by the combustion of methane.

Detailed calculation of the energy required to break bonds in the reactants of methane combustion.

Calculation of the energy released when new bonds are formed during the combustion of methane.

Comparison of the calculated enthalpy change with the standard enthalpy of combustion from data books.

Introduction to the calculation of enthalpy change for the combustion of methanol using bond enthalpies.

Step-by-step calculation of the energy required to break bonds in the reactants for methanol combustion.

Calculation of the energy released during the formation of new bonds in methanol combustion.

Final calculation of the enthalpy change for methanol combustion using the 'in minus out' method.

Highlighting the exothermic nature of methanol combustion and its practical implications.

Conclusion on the importance of bond enthalpies in predicting and calculating enthalpy changes in chemical reactions.

Transcripts

in this video I'm going to look at how

Delta H or eny change can be calculated

from Bond

enthalpies so of course we better start

off by explaining what is meant by Bond

eny so I've written down two equations

here so in the first one we've got

hydrogen gas H2 gas one mole of this and

we are turning it into two hydren

atoms and the eny change for this

process is plus so it's endothermic

436 KJ per

mole in the second equation I've got one

mole of HCL

gas and I'm turning that into a hydrogen

atom in the gas phase and a chlorine

atom in the gas phase and that is again

endothermic and it's it's now 432 K per

mole so if you look at those two

equations what's happening is we are

breaking one mole of a specified

Bond so a bit like the taking the pen

top off the pen requires energy

so so to get the pen top off I had to

put in energy and essentially that's

what the these equations are telling us

how much energy is required to break one

mole of a specified Bond into its

gaseous

atoms and of course these are always

going to be endothermic processes you

always have to put energy in to break

the attraction between the two atoms

that are covalently bonded together so

always

endothermic now the two examples have

just shown you the HH bond between those

two hydrogen atoms and the HCL bond

between hydrogen and chlorine atoms they

only occur or that Bond only occurs in

those specific

molecules when you get to something like

the CH Bond so I've drawn up the

equation there that would represent the

bond enthalpy for the CH Bond now that

can occur in lots and lots of different

molecules so if you think about methane

has four CH Bonds

in and in um ethane there are six CH

Bonds in and so on it actually requires

a slightly different amount of energy

each time to break one mole of that Bond

so we can't have data tables with every

single possible um CH bond in it so we

need to use what's called average bond

enthalpies or they're also known as main

Bond

enthalpies so this value that I've

written up here this +

413 is actually the

average bond

enthalpy that's sometimes referred to as

the mean Bond

eny so there's five typical bonds that

you would see in um organic chemistry

we've got the CH Bond the oo Bond o

CC and the HH Bond and these are the

average bond enthalpies to break one

mole of this specified Bond into the Gus

atoms

so what we're going to do then is we're

going to use these average bond

enthalpies these main bond enthalpies to

calculate a value for an enthalpy change

for a particular reaction so the

reaction I'm going to start with is the

combustion of

methane so we're going to use bond

enthalpies to work out a value for Delta

H and obviously this is an exothermic

reaction action It's the reaction we use

every day when we turn on the gas the

gas cooker um and so on and when you do

calculations using Bond enthalpies it's

always a good idea to draw the equation

out if it hasn't already been done for

you in the exam draw the equation out in

the displayed formula because now you

can see all the bonds that are involved

in the

reaction so the first thing we're going

to do is we're going to work out how

much energy is required to break the

bonds in the reactants so remember

energy has to go

in to break coal bonds and we're going

to use the average bond enthalpies the

main bond enthalpies to give us a value

for

that so in this reaction we are breaking

four CH bonds 1 2 3 4 so that's going to

require four times the average bond

enthal be for one mole of CH bonds so

that's 4 * 413 K per mole and we're also

breaking two moles of the o double bond

so we need to double the value for this

and of course you would get all of this

data these Bond enthalpy values they

would be supplied in the question and

that's giving us a total amount of

energy that has to go in to to break the

bonds of

2,646

K remember to break the bond we had to

supply energy now it's obvious that when

we make a coent bond energy is going to

be

released so the second part of this

calculation is we're going to calculate

the energy that's given out when these

new bonds have been made so if we have a

look again at the display formula we can

see that the bonds that are formed we've

got one 2 C

O's in one mole of carbon

dioxide and we've got 1 2 3 4 so that's

the O Bond *

4 I've looked up the average bond

enthalpies for the c bond o and the O

and I've multiplied them by the correct

number

and that's coming out at a total value

for the energy

out at

3,462

K so we'll just take stock of the

information at this point we had to

supply

2,646 K to break these bonds but we're

getting out

3,462 K when these bonds are made so

obviously this reaction is giving out

more more energy than had to be supplied

so we know anyway that this is an

exothermic reaction this is the reason

why the way I convert this to an

enthalpy change I take the energy in and

from that I subtract the energy out so I

call this the in minus out method so e

in is

2646 minus the E out so that that's -

3462 so that's giving us a value of -

816 K per mole that's what we would

expect minus because it's

exothermic now again you might be us to

compare one value calculated by one

method you might have to compare that to

a value calculated by another method or

even a Data Book value so the Data Book

value for the standard enthalpy of

combustion of methane is minus

890 KJ per

mole we're getting a different value for

this method because we are using average

or mean Bond enes whereas when this is

calculated for the data book it's act

the actual values for those bonds are

dictating the enthropy change for the

reaction so we'll we'll finish with this

equation so we're going to try and

calculate using Bond enthalpies a value

for the enthalpy change for the

combustion of one mole of

methanol there's the balanced chemical

equation and remember the first thing I

said that we should do is write it out

in displayed formula if it hasn't

already been done for you in the

exam so there's the equation in

displayed formula format and in green

I've written down the Bond enthalpies

the main bond enthalpies for all of the

bonds involved in the equation we've got

the CH Bond we've got some here the C

single Bond o there is there we've got

the o o obviously between the oxygen

atoms in O2 in the atmosphere the o

double bond C so the bond in carbon

dioxide and the ho Bond so we've got

this one here and we've got some in

water as

well these are all measured in K per

mole so we're going to use these to

calculate the value for Delta H for this

reaction using the in minus out

method so we've obviously got 3 * 413 3

CH bonds

broken 1 C single Bond or so that's 1 *

336 1 one o Bond needs to be broken so

that's 1 *

463 and just be careful with the

fraction here so we've got 1 and2 moles

of o2 required to balance the equation

so we therefore need 1 and 1/2 times the

497 remember this is per mole we've got

1 and A2 moles here 3 over2 moles and

that's why we need to multiply that by

1.5 so if you want to work that out I'll

do the same and that gives us an answer

of

27835

K that has to go in to break all of the

bonds and now for the energy we're going

to get out when these new bonds are

formed so we're making 2 moles of cou

bond o so that's 805 * 2 we're making

two water molecules so in each water

molecule we have two o bonds so if we're

making two moles of water we're

obviously making four moles of O bonds

so that's why we need to multiply the

463 by 4 so again work it out and I'll

do the same and that comes out at

3462

K so you can see we're getting more

energy out than we had to put in

obviously this is an exothermic

reaction what you combust me ethanol it

obviously does give out heat and you can

see from the the in and the out values

we're getting more energy out and that's

going to give us a negative answer when

we come to the Delta R calculation and

feeding those numbers into the Delta Ral

e in minus E out we get a value of minus

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