Chemical Equilibrium Grade 12 Chemistry

Summary

This 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

00:00

🔬 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.

05:01

🌡️ 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.

10:02

📈 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.

15:04

🌟 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.

20:06

📚 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.

Keywords

💡Chemical Reaction Rates

Chemical reaction rates refer to the speed at which reactants are converted into products in a chemical reaction. In the video, the instructor discusses how the rate of a reaction is initially high when there is a high concentration of reactants, but it decreases as the reaction progresses and the reactants are consumed. This concept is crucial for understanding the progression towards chemical equilibrium.

💡Limiting Reagent

A limiting reagent is the reactant that is completely consumed when the reaction goes to completion, thus determining the maximum amount of product that can be formed. The script mentions that many reactions stop when the limiting reagent is used up, which is a fundamental concept in stoichiometry and reaction yield calculations.

💡Reversible Reaction

A reversible reaction is one in which the products can be converted back into the reactants. This is a key concept in the video, as it contrasts with reactions that go to completion. The instructor uses the example of a double arrow in a chemical equation to denote a reversible reaction, indicating that the reaction can proceed in both the forward and reverse directions.

💡Chemical Equilibrium

Chemical equilibrium is a state in which the rates of the forward and reverse reactions are equal, and there is no net change in the concentrations of reactants and products over time. The video script explains that this state is dynamic, meaning that the reactions continue to occur, but the system appears unchanged macroscopically. This is a central theme of the lesson, as it introduces the concept of dynamic equilibrium in chemical reactions.

💡Closed System

A closed system is one where no mass can be transferred into or out of the system. The video emphasizes that for chemical equilibrium to occur, the reaction must take place in a closed system. An example given is a sealed container where water can evaporate and then condense back, allowing the system to reach equilibrium without losing water vapor to the environment.

💡Open System

An open system is one where mass or energy can be transferred into or out of the system. The instructor contrasts this with a closed system, explaining that in an open system, such as an uncovered pot of boiling water, equilibrium cannot be reached because water vapor can escape, and the system continuously loses mass.

💡Dynamic Equilibrium

Dynamic equilibrium refers to the state where the forward and reverse reactions occur at the same rate, leading to no net change in the system. The video uses the example of water evaporation and condensation to illustrate how, over time, the rates of these opposing processes equalize, resulting in a dynamic equilibrium where the macroscopic appearance of the system does not change.

💡Le Chatelier's Principle

Le Chatelier's Principle, although not fully explained in the script, is mentioned as a concept to be covered in the next lesson. It states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change. The video script alludes to this principle as a method to disturb equilibrium by changing temperature, pressure, or concentration.

💡Concentration

Concentration in the context of the video refers to the amount of a substance in a given volume. It plays a critical role in determining the rate of a chemical reaction. The instructor explains that as the concentration of reactants decreases, the rate of the forward reaction slows down, while the reverse reaction, initially slow due to the lack of products, speeds up as more products are formed.

💡Macroscopic Level

The macroscopic level is the scale at which observable changes can be seen without the aid of magnification. In the video, the instructor contrasts the macroscopic appearance of a system at equilibrium, where no net change is observed, with the microscopic level where individual molecules are reacting and transforming. This distinction helps students understand that equilibrium does not mean that reactions have stopped at the molecular 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