Calculating Enthalpy Changes from Bond Enthalpies
Summary
TLDRThis 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
🔍 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.
🔥 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.
🌡️ 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.
Mindmap
Keywords
💡Enthalpy Change (ΔH)
💡Bond Enthalpy
💡Endothermic Process
💡Exothermic Process
💡Average Bond Enthalpies
💡Combustion Reaction
💡Methane
💡Methanol
💡In Minus Out Method
💡Standard Enthalpy of Combustion
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
678.com
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