Chemical Equilibrium Grade 12 Chemistry
Summary
TLDRThis chemistry lesson introduces the concept of chemical equilibrium, focusing on how reactions can be reversible and not always go to completion. The instructor explains that equilibrium is reached when the forward and reverse reactions occur at the same rate, creating a dynamic balance. Key terms like 'closed system' and 'reversible reaction' are defined, and the conditions necessary for equilibrium are discussed. The lesson also touches on how equilibrium can be disturbed by factors like temperature, concentration, and pressure, setting the stage for further exploration in upcoming classes.
Takeaways
- 🔬 **Chemical Reaction Rates**: The lecture introduces the concept of reaction rates, explaining that reactions continue until one reactant is completely used, known as the limiting reagent.
- 🔄 **Reversible Reactions**: It's highlighted that not all reactions go to completion; many are reversible, meaning the products can be converted back into reactants.
- ⚖️ **Chemical Equilibrium**: The concept of chemical equilibrium is introduced, where the forward and reverse reactions occur at the same rate, resulting in a dynamic balance that never finishes.
- 🌡️ **Effect of Temperature**: The lecture suggests that temperature can influence the rate of evaporation and condensation, affecting the equilibrium state.
- 💧 **Water Evaporation Example**: An example of water in a sealed container is used to illustrate how evaporation and condensation can lead to a dynamic equilibrium.
- 📉 **Graphical Representation**: The lecture explains how to graph the rates of forward and reverse reactions, showing how they start high and low respectively, then equalize at equilibrium.
- 🔍 **Microscopic vs. Macroscopic Changes**: It's emphasized that at equilibrium, there are no observable changes macroscopically, but microscopically, the forward and reverse reactions continue at the same rate.
- 📋 **Requirements for Equilibrium**: A closed system is necessary for chemical equilibrium, where no mass can be transferred into or out of the system.
- 📈 **Concentration Changes Over Time**: The lecture describes how the concentration of reactants decreases and products increases over time, leveling off at equilibrium.
- 🔧 **Disturbing Equilibrium**: The script mentions that equilibrium can be disturbed by changing factors such as temperature, pressure, concentration, or by adding a catalyst.
Q & A
What is the main topic discussed in the script?
-The main topic discussed in the script is chemical equilibrium, specifically focusing on the rates of reaction and how reversible reactions can lead to a state where the forward and reverse reactions occur at the same rate.
What is a limiting reagent and how does it affect a reaction?
-A limiting reagent is a reactant that is completely consumed during a chemical reaction, thus determining the amount of product formed. When the limiting reagent is used up, the reaction stops, and the reaction is said to go to completion.
What is the difference between a reversible and an irreversible reaction?
-A reversible reaction is one where the products can be converted back into reactants, and the reaction can proceed in both the forward and reverse directions. An irreversible reaction, on the other hand, goes to completion and does not have a reverse reaction under the same conditions.
What is meant by 'chemical equilibrium'?
-Chemical equilibrium is a state in a reversible reaction where the rates of the forward and reverse reactions are equal, and there is no net change in the concentrations of the reactants and products over time.
Why is it important for a system to be closed to achieve chemical equilibrium?
-A closed system is important for achieving chemical equilibrium because it prevents the loss or gain of mass, allowing the system to maintain the necessary conditions for the forward and reverse reactions to occur at the same rate.
How does the rate of the forward reaction change as a reaction progresses towards equilibrium?
-As a reaction progresses towards equilibrium, the rate of the forward reaction decreases. This is because the concentration of reactants decreases as they are converted into products, leading to a slower rate of the forward reaction.
How does the rate of the reverse reaction change as a reaction progresses towards equilibrium?
-The rate of the reverse reaction increases as a reaction progresses towards equilibrium. Initially, there may be little to no reverse reaction, but as products accumulate, the reverse reaction rate increases until it matches the rate of the forward reaction at equilibrium.
What is dynamic equilibrium and how is it different from a static equilibrium?
-Dynamic equilibrium is a state where the forward and reverse reactions occur at the same rate, but the reactions are still happening, so there is a continuous process of conversion between reactants and products. Static equilibrium, on the other hand, would imply no reaction is occurring at all, which is not the case in a dynamic equilibrium.
What factors can disturb an existing chemical equilibrium?
-Chemical equilibrium can be disturbed by changing the temperature, pressure, concentration of reactants or products, or by introducing a catalyst. These changes can cause the equilibrium to shift to re-establish a new balance according to Le Chatelier's principle.
What is Le Chatelier's principle and how does it relate to chemical equilibrium?
-Le Chatelier's principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of the equilibrium will shift to counteract the change. This principle is used to predict the direction in which a chemical equilibrium will move in response to changes in temperature, pressure, or concentration.
Outlines
🔬 Introduction to Chemical Equilibrium
The script begins with an introduction to a new section on chemical equilibrium for grade 12 chemistry students. It explains that reactions can be reversible, with products turning back into reactants, leading to a state of equilibrium where the forward and reverse reactions occur at the same rate. The concept of a limiting reagent is introduced, where the reaction stops once the limiting reagent is used up. The script emphasizes that not all reactions go to completion and that reversible reactions are denoted by a double arrow in the reaction equation. An example of water in a closed container is used to illustrate the process of reaching equilibrium through evaporation and condensation.
🌡️ Dynamic Equilibrium and Closed Systems
This section delves into the concept of dynamic equilibrium, where the rates of the forward and reverse reactions are equal, and no net change is observed. The importance of a closed system for achieving equilibrium is highlighted, with examples of open and closed systems provided. The script explains that in a closed system, mass cannot be lost, which is essential for reaching equilibrium. The instructor uses the example of water evaporation and condensation to illustrate how dynamic equilibrium is reached when the rates of these processes become equal. The section ends with a summary of the requirements for reaching chemical equilibrium.
📈 Graphing Reaction Rates and Equilibrium
The script continues with a discussion on how to graph the rates of forward and reverse reactions over time. It explains that initially, the forward reaction rate is high due to the high concentration of reactants, but it decreases as the reactants are consumed. Conversely, the reverse reaction rate starts low and increases as products are formed. The point at which both rates are equal is identified as the dynamic chemical equilibrium. The instructor emphasizes that at equilibrium, the concentrations of reactants and products remain constant, even though they may not be equal, and there is no observable change macroscopically. The section includes a detailed explanation of how to interpret graphs representing reaction rates and concentrations over time.
🌟 Factors Influencing Equilibrium
This part of the script introduces the factors that can influence the position of an equilibrium, such as temperature, concentration, pressure, and the presence of a catalyst. It briefly mentions Le Chatelier's principle, which will be discussed in more detail in the next lesson. The instructor provides a preview of the upcoming topics and emphasizes the importance of understanding how equilibrium can be disturbed by changes in these factors. The section concludes with a reminder for students to record the key definitions and concepts discussed in the lesson.
📚 Recap and Preview of Future Lessons
The script concludes with a recap of the key definitions and concepts covered in the lesson, including open and closed systems, reversible reactions, and chemical equilibrium. It also previews the topics for the next lesson, which will focus on Le Chatelier's principle and the factors that influence the position of equilibrium. The instructor encourages students to ensure that all the information from the lesson is recorded in their chemistry books for future reference.
Mindmap
Keywords
💡Chemical Reaction Rates
💡Limiting Reagent
💡Reversible Reaction
💡Chemical Equilibrium
💡Closed System
💡Open System
💡Dynamic Equilibrium
💡Le Chatelier's Principle
💡Concentration
💡Macroscopic Level
Highlights
Introduction to the concept of chemical equilibrium and reaction rates.
Explanation of limiting reagents and reactions going to completion.
Definition of reversible reactions and the symbol for reversible processes.
Illustration of chemical equilibrium with the example of water evaporation and condensation.
The requirement for a closed system to achieve chemical equilibrium.
Dynamic equilibrium and its visual representation with arrows indicating rates.
The difference between open and closed systems in the context of chemical reactions.
Graphical representation of how reaction rates change over time to reach equilibrium.
Description of how the concentrations of reactants and products change as equilibrium is approached.
The microscopic view of a dynamic equilibrium where forward and reverse reactions occur at the same rate.
Introduction to the concept of Le Chatelier's principle and its implications for equilibrium.
Factors that can disturb an equilibrium state: temperature, concentration, pressure, and catalysts.
Summary of key definitions: open system, closed system, reversible reaction, chemical equilibrium.
Emphasis on the importance of understanding the factors that influence the position of an equilibrium.
Instruction to students to record the definitions for future reference.
Anticipation of future lessons discussing the detailed mechanics of Le Chatelier's principle.
Transcripts
good morning grade 12s welcome back we
are doing a new section today starting a
new section so you can stay in your
chemistry book it's chemistry
rule a line of the rates of reaction
and make a new big hitting chemical
equilibrium
okay
so
let's get started okay so as we know
this is a reaction equation we have
reactants on the one side arrow and
products on the other side you don't
need to take this down
i'll tell you when to pause and take
something down
okay so
many reactions continue until one of the
reactants are used up remember the
limiting reagent is used up and then the
reaction stops
and we say that these reactions go to
completion in other words they finish
and they finish because one of the
reactants is just
used up literally used up
and so the reaction cannot continue so
it finishes and it's done and it's over
okay
however
not all reactions go to completion and i
don't know if you knew this but a lot of
reactions are
reversible so you may have heard me say
that before but we can have a reversible
reaction and i'm sure you guys have
known have seen this before especially
in the fertilizer section
we have a double error like that
so it's half arrow pointing to the right
half arrow pointing to the left
so if i have a reaction that goes to
completion it's just a single arrow from
reactants to products that's what we've
been doing up till this point but now
we're talking about
chemical equilibrium
which is something that is reached when
we have the forward reaction and the
reverse reaction
taking place at the same rate so in
other words the reaction never finishes
a plus b
which origins
break down and create c plus d
and then it can go the other way so c
plus d can break down again and create a
plus b so it goes backwards and forwards
so here it says this is known as a
reversible reaction
a reaction is reversible when products
can be converted back to reactants thus
a chemical equilibrium exists okay so
firstly i want you to pause the screen
and take down this
slide
okay here's another slide
all chemical reactions happening in a
closed system are reversible
so take down that line please and you
already said that a reaction is
reversible when the products can be
converted back into reactants you did
write that down right let's just check
yes a reaction is reversible when
products can be converted back into
reactants so all i need you to take down
here is the purple line it needs to
happen in a closed system so let me show
you an example
okay
here we have water in a sealed container
to close system
what's going to happen here we're going
to have evaporation
you can see there
the little water molecules there's
arrows pointing upwards you know
whatever evaporation is right it's when
we go from a liquid form to a gas form
and the particles escape and
kind of you know gather here
and then what happens is okay so what
happens is the forward reaction
is greater than the reverse reaction in
other words evaporation is more than
condensation
then eventually you're going to have a
lot of water vapor molecules up here in
this space
and they are going to react
and they're going to condensate so you
know what condensation is it's when it
forms back into water droplets so
eventually the rate of condensation will
equal the rate of evaporation and that's
where we get dynamic equilibrium let me
use another
picture to illustrate
okay so here
in a water has been poured into a beaker
and the system is definitely not in
equilibrium if you can see the left
arrow which indicates evaporation is
greater than the right arrow over here
which indicates condensation
eventually
the two arrows are are equal because
evaporation so the fact that the water
is turning from liquid into gas water
vapor and condensation the fact these
gas
gaseous water particles are condensating
and forming liquid again that is
happening at an equal rate
and that's when equilibrium is reached
and this they say equilibrium has
reached a lower temperature we can also
increase the temperature then it's just
equilibrium will be reached at a higher
temperature so more
water will evaporate
but that means more condensation will
occur okay so i'm going to either this
one i want you to take down to pause the
screen
or you can take down
this one
okay
okay no matter which picture you took
down i need you to copy down this that
i've highlighted so this is the
explanation
if
your system has a lid
water will evaporate and then it'll
condense again evaporation and
condensation that's happening at the
same time eventually when they occur at
the same rate
dynamic equilibrium occurs so remember
evaporation
is greater initially because there's
more liquid than gas so the forward
reaction is greater but then as time
goes on and there's more gas
the reverse reaction which is
condensation is going to be the same
they're going to be equal and that's
when we've reached dynamic equilibrium
so we're going from liquid to gas and
from gas to liquid at the same rate the
reaction does not stop
it evaporates and condenses at constant
rates
and so the oval effect appears to be no
change so if we stare at this glass or
the speaker with a lid on
we're not going to see any difference
the water level isn't going to fall or
rise that's because evaporation is
happening at exactly the same rate
as condensation so to our eye it looks
like nothing's happening but there is
something happening
okay so i hope you took that down let's
move on
here's another explanation of the same
thing no need to write this down
just i want you guys to understand this
so initially the water evaporates and
you can see there's a little number of
molecules in the gas phase here we go
they're little blue circles they
increase and then when there's a lot of
them they start to collide
and condense so condensation happens
and when the rate of evaporation so then
floating into the air and turning into
little gas
particles
when that equals to the rate of
condensation then we'll have an
equilibrium and the level of water will
remain constant but the two processes
are still happening
okay again not necessary to write if you
want to of course go ahead so this is a
reversible change occurs does a
reversible change go to completion no
just remember that it doesn't finish
it occurs both in the forward and
reverse direction so it goes from the
one to the other from the one to the
other and it says here this is what an
equation will look like
um there's liquid to gas then gas to
liquid and the double arrow indicates
that it's reversible but we prefer to
show it like this half arrows
so reversible reactions are arduino
denoted by a double arrow pointing in
both directions and if a chemical is in
a state of dynamic equilibrium so that's
when the forward rate is equal to the
reverse rate then we show the double
arrow like this little half arrows okay
so
this i need you to write down
requirements to reach chemical
equilibrium so remember chemical dynamic
chemical equilibrium is where the rate
of the forward reaction is equal to the
rate of the reverse reaction and the
reaction does not
go to completion why does this look so
stretched
ah try to fit it in okay
anyway so what do we need to ensure that
a chemical equilibrium is reached we
need a closed system
so here's the definition of a closed
system
the system is isolated from its
surroundings in such a way that there's
no mass transferred into or out of the
system so
in the case of the water example where
we go from liquid water to gas if it was
open beaker or open test tube or
whatever the gas particles would be
allowed to escape
and if they escape how are they going to
condense again and how are we ever going
to reach equilibrium so this is what we
need a closed system take this down
please pause the screen
here's an example of a completely
isolated system so the sun can't even
come in but we just need a closed system
so no mass can be lost so it's like a
putting a transparent lid on
so sunlight can still get in which will
encourage evaporation to take place
open system as you can see the mouse is
just being lost and it's kind of flying
everywhere
okay what is an open system please take
the sun as well so it's where mass or
energy can be transferred into or out of
the system
so if we're cooking and we have um
your little saucepan saucepan without a
lid that's an open system
so
water vapor can escape into the air
that's why when you cook it gets very
like wet and the condensation can happen
on your i don't know stove
i don't know how your kitchen is you
know what i mean though
so in open beaker equilibrium can never
be reached that's why if you open
your kettle it'll never boil grade 12.
okay here's a little summary open system
continuously interacts with its
environment and a closed system is
isolated from its surroundings what i'm
going to ask you guys to do at the end
of this lesson is to copy down these
five definitions which we can ask you in
this section and which they probably
will ask you we will get to all of them
we haven't touched on all of them yet
but i'll come back to that at the end of
this lesson
so we understand all of that if you want
you can take down the things you don't
have
and now what i want you to do is i want
us to
consider this situation
so you are going to write this down
eventually and i have a graph
this little graph here
that matches the description that i've
written
over here
okay so
first i suggest you listen
and then you can go back pause and take
down
so it says consider the following
reversible reaction in a closed system
cool we have a closed system and cool
our reaction is reversible see the
double half arrows there which means
we're going to reach a dynamic
chemical
equilibrium but this is how it happens
so
we start a reaction we add a and b
into the closed system
it says initially the concentration of
the reactants a and b is high obviously
we've just started the reaction we just
did rates of reaction in the previous
section you guys should know
we have a lot of reactants high
concentration and so the forward
reaction is high in other words the one
that goes
this way
because we're going to want to make
products there's a lot of a and b we
want to make products so the one the
reaction that's going to the right in
other words the forward reaction is
going to be very high
and as the reaction goes on as time goes
on the concentration of a and b is going
to decrease
obviously because they they're busy
making products
and that means the rate of the forward
reaction is going to go a bit slower
so let me show you
in terms of the graph
so initially the concentration of a and
b is high because we've just started the
reaction
so that means the rate this graph is
rate
this isn't a concentration graph this is
rate
so the rate of the forward reaction is
going to start off high there's the
forward reaction in red
maybe i should color code it with my
arrows
forward reaction is red
and reverse reaction is blue
okay
so initially the rate of the forward
reaction is high then as the forward
reaction is going we're going to produce
more
of c and d so this is going to become
less and less which means the rate of
the forward reaction is going to
decrease as you can see the curve is
going down as time goes on let's read
further
initially there were no products
obviously because we just started
but
are more what but more
products were formed
and the reaction the reverse reaction is
starting to get progressively faster
yeah i said broken line that was
obviously a different graph that i was
showing okay
so initially
there's no reverse reaction because we
just started the reaction
so the rate was nothing
then as we producing more products the
rate is going to increase because
remember when we produce these products
c and d can be like okay cool now we can
do the reverse reaction
because we can also break down and form
a and b so the reverse reaction is going
to increase
at
after time t1 both reactions proceed at
the same rate and the system has reached
let's say dynamic
chemical
equilibrium
so let's look at t1 there's t1
you see where both of them are at the
same rate
so after a bit of time
the rate of the forward reaction and the
rate of the reverse reaction will be the
same that's why they are along the same
rate okay
and that means that we've reached
dynamic chemical equilibrium
and that what that means is the product
c and d
are being formed as fast as they break
down into a and b
so it's happening at the same rate and
if we look at the macroscopic level so
you sitting there staring at the beaker
you're not going to see anything so no
observable change
and
the concentrations of all the substances
remain constant
not necessarily equal
and i say they sketch two i will draw
another graph for that now for the
concentration you need to be careful
because this is rates so the rate of the
forward reaction was high then it
decreases rates as the reverse reaction
was low then it increases then they at
the same rate that is a completely
different graph to the concentration
graph
at the microscopic level both forward
and reverse reactions proceed at the
same rate and the equilibrium is dynamic
i hope you understand that so what i
want you to do now
is to pause the screen and take that
down there's your explanation pause now
and there's your graph
please take down the full graph and
color code it like i have
okay this is not necessary to write down
but i just want to show you the length
of the arrows indicate the rates of the
reaction so initially the forward
reaction is high and the reverse
reaction is
non-existent or very small and then as
time goes on the rate of the forward
reaction see the length of the arrow
becomes shorter and the rate of the
reverse reaction becomes bigger so that
the arrow becomes longer
okay
so that's how it starts initially
forward reaction is big reverse reaction
is small and then as time goes on
forward reaction the arrow shrinks a
little bit and the reverse one gets a
bit bigger and then eventually at
equilibrium the two arrows are the same
length okay
so the length of the arrows are also
important
okay and then i want to show you sketch
two so
if you look here it says at the
microscopic level no observable changes
take place the concentrations of all the
substances in the reaction mixture
remember brackets means concentration
concentration of a and a b and of c of d
they remain constant but not necessarily
equal so this is just a very um
simple sketch i i didn't break it up
into concentration of a b c and d
i just group the reactants together and
the products together
initially we have a lot of reactants and
then as time goes on the reactants
decreases not necessarily to zero
because the reaction isn't going to
completion products start at zero and
then they go up so you can see here when
the graph starts to level off that's t1
that's when equilibrium has been reached
and there's no change in the amount
so the amounts aren't going to go up and
down
but
they remain constant you see and
the amount of product and the amount of
reactants won't necessarily be the same
but they what what they do have in
common is that they don't it's not
changing
i hope that makes sense
here's another quick sketch that i want
you to take down this is using an actual
chemical equation so it is a reversible
reaction here we go
hydrogen and iodine produce hi
hydrogen iodide
and you can see it's reversible
so the black line indicates the forward
reaction where h2 and i2
break down or decompose to produce hi
the reverse reaction is the other way
around so h i
breaking down to produce h2 and i2 and
eventually the rate of the forward
reaction is high because we have a lot
of h2 and i2 but then it slows down as
they get used up
initially the rate of the reaction of
the reverse is 0 but then as the product
is being produced which is hi
we're going to get an increase in the
rates of reaction and where they level
or fear where they have the same rate is
where dynamic chemical equilibrium has
been reached
okay and then second last thing before
we end of the lesson this is what we'll
be doing tomorrow but i just want to
briefly mention it once we reach an
equilibrium so once we get to this point
where we've got a nice equilibrium and
the rate of the forward reaction and the
rate of the reverse reaction are equal
we can disturb this equilibrium so we
can cause this equilibrium to be
disturbed by doing
one of a few things so
we can either change the temperature
increase or decrease the temperature
we can change the pressure and that's if
we have gas
gaseous reactants
or we can change the concentration
of one of the products or reactants
and catalyst is another one
so tcp
temperature concentration pressure
catalyst is a little bit of a different
story we will get there
temperature concentration pressure
please write this down and this is what
we'll be doing tomorrow so pause the
screen and then lastly
for this lesson i'm going to scroll to
the important definitions so first of
all this is from the exam guidelines it
says this is what you must be able to do
explain open and close system we did
that what is a reversible reaction we
did that what is chemical equilibrium we
did that and we just listed the factors
now that influence the position of an
equilibrium
so tomorrow we're actually going to
discuss this in detail and that is when
we'll get into this bottom
section here but we've done the first
little chunk of work today
and i want you to write down these
definitions so
there are five
that are very important for the section
open system
closed system
reversible reaction chemical equilibrium
and the thing in brackets is important
please
le chatelier's principle which we'll get
to tomorrow okay so pause the screen and
take that down and then i'll see you
guys tomorrow and please make sure that
everything is in your book there
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