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Course: Class 10 Chemistry (India) > Unit 1
Lesson 4: Endothermic and exothermic reactionsExothermic and endothermic reactions
Energy is released in exothermic whereas absorbed in endothermic reactions. Created by Ram Prakash.
Want to join the conversation?
- How is it possible that some decomposition reactions are exothermic? What is the reason behind it?(5 votes)
- Decomposition reactions can be exothermic or endothermic, depending on the chemical energy of the substances.
If the chemical energy of reactants are greater than products that time the reaction is exothermic with negative ( ΔH) and at that time, the product is more stable than reactant.
Consider the example of most nitrates and explosive compounds in general -
After melting (endothermic) and sometimes phase changes (also endothermic), decomposition occurs and is very exothermic.
This may occur each time the resultant compounds after decomposition are more stable than the initial compound.(6 votes)
- At3:18, it is said that if the energy released is greater than the energy taken in, it is known as a exothermic reaction and vice versa, it's known as an endothermic reaction. But what if a reaction has an equal amount of energy input and output. What do you call that kind of a reaction (if it does exist!!)?(3 votes)
- it's called a thermoneutral reaction(3 votes)
- At3:18, it is said the if the released energy is more than the energy taken in, then it is called an exothermic reaction. But if this happens,won't "Energy input < Energy output, and so isn't energy created in this exothermic reaction and vice versa in endothermic reactions?(1 vote)
- In chemical equation nothing new is created in generally. The energy release is due to loss of energy by the electron while changing its shell to achieve stability(3 votes)
- At4:42, burning Magnesium or Carbon(Coal) is an Exothermic reaction since it produces a lot of heat. But to start the reaction we do provide heat to it, then it must also be called an Endothermic reaction right?(0 votes)
- Good question. Actually, burning is an exothermic reaction as it releases heat. The energy which is first given to the system is known as activation energy, the minimum energy required to start the reaction. For a reaction to occur, particles much have the energy to break away from each other or to make bonds. They must have the energy to collide with each other, also known as collision theory. Hope its clear(1 vote)
Video transcript
- [Instructor] It gets
very cold in winter. And that's why I like to use my heater. It gives out a lot of heat. Whereas in summers, it gets very hot. And that's when I like to use my AC. It takes in a lot of heat. So just like these devices,
we also have certain chemical reactions that
give out a lot of heat or take in a lot of heat. For example, if you burn coal. Coal is mostly made up of carbon. So when you burn it, you get a
lot of heat and light energy. And another example is if
you burn a magnesium ribbon, it will burn with a dazzling white flame, giving out a lot of heat and light energy. So these kind of reactions
that give out a lot of energy are called exothermic reactions. And you can guess the meaning of it by knowing the meaning of the words. See, exo, it means external or out. And thermic, this means
something to do with heat. Meaning the reactions that give out heat, they will be called exothermic reaction. Whereas, we have some
other kind of reactions. Like burning of iron sulfate. Now when you burn this, there will be a color change happening,
there will be some gases that will be released. So definitely, this is
a chemical reaction. But in this reaction, we are not getting a lot of heat or light energy. In fact, we have to supply
a lot of heat energy to initiate this chemical reaction. Now, this kind of chemical
reactions are called endothermic chemical reactions. Now, the meaning of this is,
endo means internal or in and thermic means heat,
something to do with heat. So the reactions in
which heat is taken in. They are called as endothermic reactions. Now, before we look at other examples, let me first tell you why
certain reactions give out heat and why certain other
reactions take in heat. Now let's look at the atomic picture. So if you have some atoms
that are sticking together in a molecule, and you
want to pull them apart. So for this, you need
to provide some energy. Energy is required to pull
away the atoms in a molecule. And we can understand
this from magnet analogy. If there are two magnets
that are attracting and are stuck together. So to pull them apart, we
need to provide some energy. Now on the contrary,
if there are two atoms that are attracting each other. And if we let go of them,
then they will come together and in this process,
energy will be released. Now we can understand this
also from a magnet's analogy. If there are two magnets that
are attracting each other and we let go of them, then they will come together and stick. There will be sound energy. Tok, right? And sound is also a form of energy. So in this example, if
two magnets are attracting and we let go, they
come and stick together. And sound energy is released. Similarly over hear, when
two atoms are attracting each other and we let go of them, they come together and energy is released. Now see, in any chemical reaction, there will be a rearrangement
of atoms happening. Few atoms will be going
away from each other and few atoms will be coming together. Now, whenever two atoms are going away, we will need to provide energy for this. And if two atoms are attracting and they are coming together,
then in this process, energy will be released. Now, if in total, a lot of
energy is being released in the chemical reaction, we
call it exothermic reaction. Whereas if in total, a lot
of energy was required, then we call that reaction
as endothermic reaction. You can understand this more clearly from an example of buying (mumbles). Imagine that you are buying a phone, and you bought it for rupees 100. Rupees 100. So you invested 100 rupees in buying it. But you sold this phone and
you were able to sell it for rupees 200. So in total, you make
profit in this transaction. This transaction give out more money. So you can call that, this transaction was an exo money process. It gave out more money. Whereas, imagine you had bought this phone for the same price, rupees 100. But you were able to sell
it for a lower price. Say, rupees 50. In this case, you lost
money in this transaction. This transaction took away money from you. So we'll call this process
as endo money process. So similarly, you can see
that process that give out in total more energy,
are called exothermic whereas, chemical reactions
that take away more energy are called endothermic. Now that you know a lot about
exo and endothermic reactions, let's look at some examples of that. So here, I have a magnesium ribbon. And I'm going to burn it. Now, you see that it's burning
with a dazzling white flame. It's giving a lot of
light and heat energy. So clearly, this is an
exothermic reaction. It gives out a lot of heat energy. Now let's write down
its chemical equation. So first, we took magnesium
ribbon and we burn it in air. When something burns in
air, it actually reacts with the oxygen in the air. Because oxygen gas is very reactive. Now this, these two reactants,
they are going to combine to give me one single product. They are going to give
me magnesium oxide, MgO. And this is the white residue
that we see in the end. Now let me quickly balance this equation. Here, we have 2Mg and we'll get here 2MgO. Now, this reaction is balanced. Now see, in this reaction,
we are getting a lot of heat. This is an exothermic reaction. So lot of heat is being generated
along with that product. So that's why I will write plus heat on the product side to
represent that a lot of heat is being generated. Or instead of heat, you
can write plus delta. Delta is a Greek letter that is used to represent heat in short form. So this is how you write
an exothermic reaction. Let's look at another example. If you burn coal, something
very similar happens. This coal has carbon and
when it burns in air, it reacts with the oxygen
to form carbon dioxide. And in the process, a lot
of heat is being released. And its chemical reaction
looks something like this. Carbon plus oxygen gives carbon dioxide. Plus heat on the product side. Let's look at another example. So here, I have a white
powder calcium oxide. Now, if I add water to it,
we will get a white solution. Milky white solution. Now, the fun part over here is, that if you touch the container
in which this reaction is happening, that can
container will feel warm to you. That means in this reaction, a lot of heat is being generated. That's strange, right? You're adding water to something and instead of getting
cold, it's getting hotter. So let's write down its chemical reaction. So here, we took calcium oxide. And we combine this with water, H2O. And in the product side, we
are getting calcium hydroxide. Calcium CaOH, whole two. And along with this also, a
lot of heat is being generated. Heat plus delta. So these are some examples
of exothermic reactions. And you also saw how do we represent them in chemical equations. We write plus heat on the product side. Because along with the product,
we get a lot of heat energy. Now one more thing I want you to focus on is that see, in all of
these three reactions, there are reactants that are combining to give me one single product. Reactants are combining to
give me one single product. Same this is happening here. Now see, these kind of reactions are called combination reactions. And we will talk about them
in detail in a separate video. But over here, we can see that in general, combination reactions
are exothermic in nature. They release heat. So combination reactions
in general are exothermic. Not always, there are
some exceptions to it, but we can say that in
general, they are exothermic. Now with this, let's move on to examples of endothermic reactions. So here, I have light grain
crystals of iron sulfate. And I'm going to heat them. Now when I do that, I notice
a color change happening. In fact, there's also some
smelly gas that is evolving. So definitely, there is
some chemical reaction that is going on but see, in this process, I'm not getting light or heat energy. In fact, I had to provide
a lot of heat energy for this reaction to start. That means this reaction
is an endothermic reaction. It requires a lot of heat energy. It takes in a lot of heat energy. Let's write down its chemical reaction. So iron sulfate, when
heated, it breaks down. It decomposes into iron
oxide, sulfur dioxide and sulfur trioxide. And in this process, not a lot
of heat is being generated. In fact, a lot of heat is being required to initiate this process. Now, to represent that, we write plus heat on top of this arrow. Plus heat, or we can even write delta. The Greek letter delta. Now, you might be wondering
why on top of this arrow and why not on the reactant side, well, I don't know. Maybe this is a
combination that we follow. So this represents that heat was required to initiate this reaction. Now let's look at another example. I have a white powder of silver chloride. And I want to leave it
outside under the sun or any light for that matter. After some time, I see that there's a color change over here. The white powder is
turning grayish in color. So what's happening over here is, that silver chloride is
taking in energy from sunlight and it's breaking down into
silver and chlorine gas. So this is also an example
of endothermic reaction. Here, the compound takes in energy from, in the form of sunlight. So to represent that,
I will write sunlight on top of this arrow. Now, let's look at another example. Here, when I pass
electricity through water, I can see that some gases are evolving. Now what's happening
over here in this case is that when we pass electricity, if the water takes in
energy from the electricity and breaks down, decomposes into hydrogen gas and oxygen gas. And this is also an example
of endothermic reaction. Here, the energy is taking in
in the form of electricity. So I will write that on top of
the arrow, electricity, yes. So these are some examples
of endothermic reaction. Here in all of these examples,
energy is being taken in. Now, you can see one common
thing that is happening in all of these three reactions. There's one single reactant
that is breaking down or decomposing into multiple product. Now, such kind of reactions are called decomposition reactions. And we have talked about that
in detail in a separate video. But over here, I want to
point out that in general, decomposition reactions
are endothermic in nature. They require more energy
for their reactions. So we can say decomposition
reactions in general, not always, there are some exceptions, but in general, are endothermic in nature. Okay, now let's summarize the video. We saw that in exothermic
reactions, heat is given out. Whereas in endothermic
reactions, heat is taken in. We also saw that to represent
exothermic reaction, we write plus heat or
delta on the product side. Whereas to represent
endothermic reactions, we write heat or energy
on top of the arrow.