Hess's Law
Summary
TLDRThis video explains Hess's Law and its application in determining the enthalpy change (Delta H) of a reaction when it's not directly available. The instructor demonstrates how to manipulate thermochemical equations to calculate Delta H by adding and adjusting other known reactions. The example provided involves calculating the Delta H for sulfur trioxide formation from sulfur and oxygen, showing how to reverse and scale reactions. The video concludes with a discussion of formation reactions, emphasizing the importance of having one mole of the product in such calculations.
Takeaways
- 📉 Hess's Law helps calculate the enthalpy change (ΔH) of a reaction by adding up the ΔH values of multiple related reactions.
- 📘 If ΔH for a reaction is unknown, you can either measure it using a calorimeter or use Hess’s Law with known reactions.
- 🔁 Hess's Law states that you can reverse or multiply chemical reactions to match the desired overall reaction.
- ⚖️ When multiplying a reaction, the ΔH value must also be multiplied by the same factor.
- ↔️ Reversing a reaction changes the sign of its ΔH; an exothermic reaction becomes endothermic and vice versa.
- 🧪 In an example with sulfur and oxygen forming sulfur trioxide, the reactions are manipulated to match the overall reaction.
- ✖️ Intermediate products, like sulfur dioxide in this case, can cancel out when added across reactions.
- 🔢 The total ΔH for the overall reaction is the sum of the ΔH values of the individual reactions.
- 📊 In formation reactions, it’s common to have fractional coefficients, which is acceptable as long as there’s one mole of product.
- 🌡️ Hess's Law is highly useful for calculating ΔH for reactions where direct experimental data is unavailable.
Q & A
What is the main method discussed in the video to calculate Delta H when it is not known?
-The video discusses using Hess's Law to calculate Delta H by manipulating other known reactions that add up to the overall desired reaction.
What is Hess's Law?
-Hess's Law states that it is possible to add two or more thermochemical equations to produce a final equation for a reaction, and the sum of the enthalpy changes for the individual reactions is the enthalpy change for the final reaction.
What example reaction is used in the video to explain Hess's Law?
-The reaction used is sulfur (S) plus oxygen gas (O2) yielding sulfur trioxide (SO3), where the Delta H of the reaction is initially unknown.
How is the first reaction modified to match the desired reaction?
-The first reaction, which has one mole of sulfur (S), is multiplied by 2 to have two moles of sulfur, matching the reactant side of the target reaction.
What happens to the Delta H when the reaction is multiplied by 2?
-The Delta H is also multiplied by 2. For example, if the original Delta H was -297 kJ, it becomes -594 kJ after multiplying the reaction by 2.
Why is the second reaction flipped in the explanation?
-The second reaction is flipped because sulfur dioxide (SO2) is on the product side, but it is needed on the reactant side to match the desired reaction. Reversing the reaction also changes the sign of the Delta H.
How do the sulfur dioxide (SO2) molecules get canceled out?
-The two moles of sulfur dioxide (SO2) on the reactant side of the flipped reaction cancel out the two moles of sulfur dioxide on the product side of the first reaction.
What is the final Delta H for the overall reaction after using Hess's Law?
-The final Delta H for the reaction is -792 kJ after adding the modified Delta H values from the two reactions.
Why is the equation divided by 2 at the end?
-The equation is divided by 2 to convert it into a formation reaction, which involves forming one mole of the product, sulfur trioxide (SO3), from its elements. Formation reactions require one mole of the product.
What is the final Delta H per mole for the formation reaction?
-After dividing by 2, the final Delta H for the formation of one mole of sulfur trioxide (SO3) is -396 kJ per mole.
Outlines
🧪 Understanding Hess's Law: Thermochemical Equations
The video begins by introducing the challenge of calculating the enthalpy change (Delta H) of a reaction, especially when it's not readily available. Various methods, such as using a calorimeter or leveraging other known reactions, are introduced. The focus then shifts to Hess's Law, which allows for the summation of enthalpy changes from individual reactions to calculate the enthalpy for a final reaction. This concept is simplified by using a practical example involving sulfur, oxygen gas, and sulfur trioxide to illustrate the application of Hess's Law.
🔄 Manipulating Reactions Using Hess's Law
In this section, a specific reaction involving sulfur and oxygen is used to demonstrate how Hess's Law works. The example begins by multiplying a reaction to balance the sulfur atoms on both sides, ensuring the thermochemical equations add up correctly. The process of adjusting the enthalpy values according to the manipulation of the reactions is explained step-by-step, highlighting how reversing reactions changes the sign of Delta H. This process ultimately leads to determining the overall Delta H for the desired reaction.
➗ Simplifying to Formation Reactions
The video further refines the example by introducing the concept of formation reactions, where sulfur trioxide is formed from its elemental components. To simplify the reaction into a formation reaction, everything is divided by two, including the enthalpy, yielding a more standard format with one mole of product. It also explains that in formation reactions, fractional coefficients are acceptable as long as there's one mole of the product. The section concludes by emphasizing the usefulness of Hess's Law in finding Delta H for complex reactions.
Mindmap
Keywords
💡Thermochemical equations
💡Delta H (ΔH)
💡Hess's Law
💡Calorimeter
💡Sulfur trioxide (SO3)
💡Formation reaction
💡Reverse reaction
💡Exothermic reaction
💡Endothermic reaction
💡Fractional coefficients
Highlights
Hess's law allows you to calculate the Delta H of a reaction by adding up the enthalpy changes of multiple reactions.
If you don't know the Delta H of a reaction, you can measure it with a calorimeter or use Hess's law to calculate it using other reactions.
Hess's law states that the sum of the enthalpy changes of individual reactions equals the enthalpy change of the final reaction.
When applying Hess's law, you can manipulate reactions by multiplying them to match the desired reaction.
If you flip a reaction, you also need to flip the sign of the Delta H.
To match the desired equation, sulfur needed to be multiplied by two, and the corresponding Delta H was doubled.
Reversing the reaction to swap the sides of sulfur dioxide and sulfur trioxide requires reversing the sign of Delta H.
The sulfur dioxide molecules cancel out when combining two reactions to form sulfur trioxide, leaving the final reaction balanced.
Adding the Delta H values of both reactions gives the final Delta H for the reaction, which is -792 kilojoules.
For formation reactions, it’s important to reduce the reaction to produce one mole of the product, which involves dividing the entire equation by two.
Formation reactions often involve fractional coefficients to ensure that the product has one mole.
When dividing the reaction for a formation process, the enthalpy change also needs to be halved.
Hess's law is useful when the Delta H of a reaction is unknown and allows you to calculate it easily by using related reactions.
Formation reactions focus on creating one mole of a product from its elements, and fractional coefficients are acceptable in these equations.
Using Hess's law simplifies finding the enthalpy changes without performing direct experimental measurements.
Transcripts
[Music]
all right sometimes when you're dealing
with thermo chemical equations you might
not know the Delta H of your reaction so
how there's different ways you can go
about getting the Delta H of the
reaction you can actually do the
reaction itself and use a calorimeter
and figure out the Delta H or that you
can actually use other reactions they
might add up to your particular action
what does that mean okay there's this
law called Hess's law
Hess's law states that it is possible to
add two or more thermo chemical
equations to produce a final equation
for a reaction and the sum of the
enthalpy changes for the individual
reactions is is the enthalpy change for
the final reaction now this is
definitely a mouthful it's much easier
to explain if you actually do it so
let's actually go over here and let's
say I had this reaction sulphur pulse
oxygen gas yields me sulphur trioxide
and I have no idea what the Delta H is
okay well there are many many many
reference books that tell you what the
Delta H is for other reactions but this
one for some reason I couldn't find or
my reference book didn't have it or I
didn't want to do the particular
reaction in lab or something along those
lines so how can I find this well Hess's
law says that I can manipulate these
reactions in order to get this overall
reaction and that way I can figure out
my Delta H if I add up the two H's so
first I have to make sure that I have
these reactions actually do add up to
the final reaction so let's look at this
first thing I need to sulfurs on my
reactant on my reactant side well here's
sulfur but I only have one so I'm going
to multiply this whole reaction by 2 so
I can get two sulfur's on my reactant
side so I'm going to multiply this by 2
so that makes two sulfur plus two oxygen
gases yields me two sulfur dioxides and
because I multiplied the reaction by two
this also must be multiplied by two
because this this negative 297
kilojoules is telling me how much
energies can be released for one mole of
sulfur but I'm now doing two moles of
sulfur so I'm going to multiply this by
two so my Delta H is now going to be
what is it negative 594 kilojoules okay
great
so now I have my two moles of sulfur on
my reactant side that's awesome but now
I have sulfur dioxide on my product side
and I don't want it i want sulphur
trioxide so I'm gonna look at this
reaction
here wellhere's call for dioxide on the
product side and sulfur trioxide on the
reactant side so I actually want to flip
those around so I'm going to do the
reverse reaction so I'm going to say
oxygen gas I'm going to make sure I note
that oxygen gas plus two moles of sulfur
dioxide gas is going to give me two
moles of sulfur trioxide gas okay and
because I flipped it this the only this
forward direction is going to be an
endothermic reaction but if I reverse it
I mean it's going to becoming an
exothermic reaction so this is going to
be negative so my Delta H is going to be
negative 198 kilojoules okay so let's
make sure this works so if I were to add
these two reactions together I note that
I have two moles of sulfur dioxide here
and two moles of sulfur dioxide here I
can cross those out because this is a
product and this is the reactant so I
can just cross those out okay
and now if I add everything up I can't
nothing else crosses out so I'm going to
add everything up so I have two moles of
sulfur solid plus two moles of oxygen
which should be oxygen to sorry
Oh to gas plus another bulb
Oh two guest yields two moles of so3 gas
okay so what if I add these together two
plus one is three second say three moles
of oxygen gas so two sulfur atoms are
bold two moles of sulfur plus 3 moles of
oxygen yields two moles of sulfur
trioxide which is exactly my original
equation is so yes I did this properly I
can check it off and so I want to find
my Delta H I'm just going to add these
guys up
so negative 594 plus negative 198 is
going to give me negative seven ninety
two so negative 792 kilojoules so that
actually now I could know this is
negative 792 kilojoules because houses
law says that I can do this okay so this
is basically houses all on a nutshell
you might have several different
reactions you're going to manipulate but
actually once you get the hang of it
it's actually kind of fun and pretty
easy
but most going to do one left thing this
is a reaction it is a formation reaction
meaning we're taking or forming sulfur
trioxide from its elements so in order
to do formation reactions the best way
is we want to get this to be one mole so
I'm going to divide the whole thing by
two because I don't like information
reactions if you want to learn more
about them and there's a video on that
as well but a formation reactions have
one mole of the product so we're
dividing everything by two which ends up
was sulfur solid plus three-halves
oxygen gas yields sulfur trioxide gas
and then my Delta H is also going to be
divided by 2 oops
which is going to be negative 396
kilojoules and we can actually say
kilojoules per mole because we have one
mole of this so this is a formation
reaction and notice this is the fraction
in it but in formation reactions is
totally okay we are allowed to have
fractional coefficients and our
formation reactions because our main our
main important thing is that we have one
mole of product so this is has its law
in action and then we put a little bit
of formation Delta H of formation in
there as well so Hess's law is really
really useful when dealing with
reactions where you do not know our
Delta HS and we need to find it pretty
easily
[Music]
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