IGCSE CHEMISTRY REVISION [Syllabus 7]- Chemical Reactions

Summary

This IGCSE Chemistry revision video covers key concepts in chemical reactions, including the difference between chemical and physical changes, collision theory, and factors affecting reaction rates such as concentration, temperature, pressure, and particle size. It also explains reversible reactions and equilibrium, emphasizing how changes in conditions shift the position of equilibrium. The video introduces catalysts and their role in reactions, and concludes with an overview of redox reactions, explaining oxidation and reduction in terms of oxygen and electron transfer. The content is designed to aid exam preparation.

Takeaways

• 🔍 Chemical reactions differ from physical changes, where a physical change involves state changes like ice to water, while chemical changes occur at a molecular level.
• ⚡ The collision theory explains that for a chemical reaction to happen, particles must collide and have sufficient energy for the reaction to proceed.
• 🌡️ The rate of chemical reactions is influenced by factors like concentration, pressure, temperature, and particle size, each affecting collision rates and energy levels.
• 🔬 Higher concentration increases collision rates due to more particles being closer together, speeding up the reaction.
• 💨 Pressure affects gases by increasing particle collisions in a confined space, leading to faster chemical reactions.
• 🔥 Temperature impacts reaction rates by increasing particle energy and speed, leading to more frequent and forceful collisions.
• 🧊 Smaller particle sizes provide larger surface areas for collisions, which enhances reaction speed, especially in solids.
• 🔗 Catalysts, such as enzymes, speed up chemical reactions without being consumed, affecting both forward and reverse reactions without altering the reaction equilibrium.
• ⚖️ Equilibrium in reversible reactions occurs when the rate of the forward reaction equals the rate of the reverse reaction, stabilizing the concentrations of reactants and products.
• 🔄 Changes in conditions like concentration, temperature, and pressure can shift the equilibrium, favoring either the forward or reverse reaction depending on the situation.

Q & A

What is the difference between a chemical change and a physical change?

A chemical change occurs at a molecular level when two or more molecules interact and form new substances, while a physical change involves changes in the state of matter without altering the chemical structure, such as ice turning into water.

What are the two key conditions for a chemical reaction to occur according to the collision theory?

The two conditions are: 1) particles must collide, and 2) they must have sufficient energy during the collision for the reaction to occur.

How does concentration affect the rate of a chemical reaction?

Higher concentration increases the number of particles in a given volume, leading to more frequent collisions, which increases the rate of the chemical reaction.

What is the effect of pressure on the rate of chemical reactions involving gases?

Higher pressure increases the number of gas particles in a given space, leading to more collisions and an increased rate of reaction. Pressure only affects reactions involving gases.

How does temperature affect the rate of a chemical reaction?

Higher temperature increases the energy and speed of particles, leading to more frequent collisions and higher energy, thus increasing the rate of reaction.

Why does smaller particle size increase the rate of chemical reactions in solids?

Smaller particle size increases the surface area available for collisions, allowing more particles to react simultaneously, thus increasing the rate of the chemical reaction.

What is the role of catalysts in chemical reactions?

Catalysts speed up chemical reactions without being consumed in the process. They do this by lowering the activation energy required for the reaction to occur.

What is a reversible reaction, and what happens at equilibrium?

A reversible reaction is when the products of a reaction can react to form the original reactants. At equilibrium, the forward and reverse reactions occur at the same rate, so the concentrations of reactants and products remain constant.

How does the position of equilibrium shift when the concentration of reactants or products is changed?

If the concentration of reactants increases, the equilibrium shifts to the right, producing more products. If the concentration of products increases, the equilibrium shifts to the left, producing more reactants.

What is a redox reaction, and how can it be defined in two ways?

A redox reaction involves both reduction and oxidation. It can be defined by oxygen transfer (oxidation is the gain of oxygen, reduction is the loss of oxygen) or by electron transfer (oxidation is the loss of electrons, reduction is the gain of electrons).

Outlines

00:00

📘 Introduction to Chemical Reactions

This section introduces the basics of chemical reactions, distinguishing between chemical and physical changes. It explains that a physical change involves changes in the state of matter, like ice melting into water, whereas a chemical change happens at the molecular level when two or more molecules interact. The collision theory is introduced, emphasizing that particles must collide with sufficient energy for a reaction to occur. Factors like temperature, pressure, and concentration affect the collision rates and therefore the rate of reaction.

05:02

📈 Factors Influencing Reaction Rates

This paragraph delves into how different factors impact the rate of a chemical reaction. Concentration is explained as the number of particles in a given volume, with higher concentrations increasing the likelihood of particle collisions. Similarly, high pressure increases the number of particles in a given space, raising collision rates. Temperature is also a key factor, as higher temperatures increase both particle energy and collision rates. Additionally, smaller particle sizes provide larger surface areas for collisions, speeding up reactions. The concept of catalysts, which speed up reactions without being consumed, is briefly introduced.

10:03

🔄 Reversible Reactions and Equilibrium

Reversible reactions are introduced, where products can revert to reactants. The concept of equilibrium is explained as the point where the rate of the forward reaction equals the rate of the reverse reaction, causing concentrations of reactants and products to remain constant. The position of equilibrium can shift depending on environmental conditions. A shift to the right means more reactants are forming products, while a shift to the left indicates more products are breaking down into reactants. This paragraph sets the stage for exploring how conditions like concentration, temperature, and pressure affect equilibrium.

⚖️ Shifting Equilibrium: Effects of Concentration, Temperature, and Pressure

This section details how changes in conditions can shift the equilibrium of a reaction. An increase in product concentration shifts the equilibrium to the left, while a decrease shifts it to the right. Temperature changes are crucial—an increase favors the endothermic reaction (absorbing heat), while a decrease favors the exothermic reaction (releasing heat). Pressure affects equilibrium in gaseous reactions: higher pressure shifts equilibrium towards the side with fewer gas particles, and lower pressure shifts it towards the side with more particles. Catalysts do not affect the equilibrium position but speed up both the forward and reverse reactions.

🔋 Introduction to Redox Reactions

Redox (reduction-oxidation) reactions are introduced, highlighting two ways to define them: by oxygen transfer and electron transfer. Oxidation is defined as the gain of oxygen or the loss of electrons, while reduction is the loss of oxygen or the gain of electrons. An example involving copper oxide and hydrogen illustrates how reduction and oxidation occur simultaneously. Another example shows magnesium losing electrons (oxidation) and chlorine gaining electrons (reduction) in the formation of magnesium chloride. This section provides a foundational understanding of redox reactions, essential for further exploration of electrochemistry.

Keywords

💡Chemical Change

A chemical change occurs when the molecular structure of substances changes, resulting in the formation of new substances. In the video, the presenter emphasizes that chemical changes involve interactions at a molecular level, such as when two or more molecules interact to create a new substance, unlike physical changes that only affect the state of matter.

💡Physical Change

A physical change refers to changes in the physical states of matter without altering the chemical composition of the substance. The video contrasts this with chemical changes, using the example of ice melting into water, where the molecular structure remains the same but the state shifts from solid to liquid.

💡Collision Theory

Collision theory explains that for a chemical reaction to occur, two conditions must be met: particles must collide, and they must do so with sufficient energy. The video highlights this theory as fundamental to understanding reaction rates and explores how factors like temperature and pressure influence particle collisions and energy.

💡Rate of Reaction

The rate of reaction refers to how quickly a chemical reaction takes place. The video discusses factors that affect reaction rates, such as temperature, concentration, pressure, and surface area. Each factor influences the collision rate of particles, which in turn affects the speed at which a reaction occurs.

💡Concentration

Concentration refers to the number of particles in a given volume. The video explains that higher concentrations lead to more frequent collisions between particles, thus increasing the rate of reaction. This concept is key to understanding why reactions happen faster in more concentrated solutions.

💡Catalysts

Catalysts are substances that increase the rate of a chemical reaction without being consumed in the process. The video explains that catalysts work by providing an alternative reaction pathway with lower activation energy. Biological catalysts, such as enzymes, are highlighted as examples of catalysts in living organisms.

💡Reversible Reaction

A reversible reaction is one where the reactants form products, but the products can also react to reform the reactants. The video explains this concept with the example of A + B forming C, while C can break down back into A and B. Reversible reactions are important in understanding equilibrium in chemistry.

💡Equilibrium

Equilibrium occurs in reversible reactions when the rate of the forward reaction equals the rate of the reverse reaction, resulting in constant concentrations of reactants and products. The video emphasizes that equilibrium can shift based on environmental factors like temperature and pressure, affecting the concentrations of the substances involved.

💡Le Chatelier's Principle

Le Chatelier's Principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium will shift to counteract the change. The video illustrates this by showing how increasing or decreasing concentrations, temperature, or pressure can shift equilibrium to either the left or right, depending on the conditions.

💡Redox Reaction

Redox reactions involve both reduction and oxidation processes, where oxidation is the loss of electrons, and reduction is the gain of electrons. The video explains redox reactions using examples, such as copper oxide being reduced to copper and hydrogen being oxidized. These reactions are fundamental to many chemical processes, including combustion and corrosion.

Highlights

Differences between chemical and physical changes

Chemical reactions occur at the molecular level

Collision theory for chemical reactions

Two conditions for a chemical reaction to occur

Importance of collision rates and energy levels

How concentration affects the rate of chemical reactions

Pressure's role in chemical reaction rates

Temperature's impact on particle energy and collision rates

Particle size and surface area in chemical reactions

Catalysts and their effect on reaction rates

Reversible reactions and their characteristics

Equilibrium in reversible reactions

Position of equilibrium and its dependence on environmental conditions

Shift in equilibrium due to changes in concentration

Effect of temperature on equilibrium shifts

Impact of pressure on the position of equilibrium

Catalysts do not affect the position of equilibrium

Redox reactions defined by oxygen gain or loss

Redox reactions can also be viewed as electron gain or loss

Transcripts

hey guys welcome to another revision

video in IGCSE chemistry today we're

gonna be covering the fundamentals of

chemical reactions so without further

ado we'll begin so the first thing you

have to understand is the differences

between a chemical change in a physical

change a physical change relates to the

changes in the physical states of matter

for example eyes turning into water and

a chemical change occurs more molecular

level when two or more molecules

interact with one another taking a look

at chemical reactions in a bit more

detail we need to think about the

collision theory which states that two

conditions have to be met for a chemical

reaction to occur one of which being two

or more particles must collide the other

being that the particles when they do

collide have to have sufficient energy

in order for the reaction to occur so in

other words when two particles collide

but they don't have enough energy then

nor the reaction won't occur and

oppositely when they even if they have

enough energy when they don't collide

then of course then that's just not

going to work either so these two

conditions have to be met and this is

actually quite important when we take a

look at the rate of a chemical reaction

because a lot of conditions like

temperature pressure and all that sort

of stuff actually affect things like

collision rates of particles and you

know the energy levels and therefore

affecting how quickly the reaction

happens or in other words the rate of a

chemical reaction so this is exactly

what we're gonna be looking at now the

sir there's a couple of things the

concentration is basically how many

particles there are in a given volume

right so a higher concentration suggests

that there are more particles that are

closer to each other so therefore by

chance alone the rate of collision will

be higher and that means the rate of a

chemical reaction will thus be increased

as well pressure works exactly the same

way except that it's you know it's only

related to gases only so a high pressure

actually suggests that there are more

particles in a given space and therefore

again the collision rate will be higher

and therefore increasing the rate of a

chemical reaction the temperature ignore

the road size or in the temperature

if you think about it a higher

temperature will actually mean that the

particles the individual particles have

more energy but not only that because

they're traveling at a higher speed the

collision rate will also be higher as

well so both factors will be elevated

therefore increasing the rate of a

chemical reaction as well when we look

at particle size we are really only

referring to solids and it suggests that

a smaller particle size actually has a

lot larger surface area for collisions

to happen and therefore achieving a

higher rate of chemical reaction so I've

sort of diagrammatically represented

that here if you can see that the red

particles are trying to decompose the

the blue this blue block of solid into

its individual you know particles as

opposed you've got two options this big

block you can see that it's actually

quite inconvenient for the red ones to

act on because it cannot access the

middle atoms because it's sort of

surrounded by other blue particles

whereas if you were to separate the blue

particles into separate pieces you've

got a lot more area well the red

particles have a lot more area to work

on now to decompose this is the solid

and therefore the the rate of chemical

reaction will be a lot higher now when

we take a look at the at something

called catalysts so these are substances

that basically don't get used in the

chemical reaction but are there to

increase their rate and a bio enzymes

found in our bodies and that's an

example of a biological catalyst so

let's take a look at the concept of

reversible reactions so this is when

reactants form products so this is made

to be reactants sorry and the products

actually react to form back the

reactants for example it gives B to give

a a so a plus B gives C but C reacts

back to or decomposes back to give a and

B so in other words we can sort of

exemplify the reaction like so with this

double headed error so at some point in

a reversible reaction what will happen

as the rate of the forward reaction will

actually begin to equal exact

the rate of the reverse reaction so this

means that C is being formed at the same

rate as a and B is being formed because

a and B make C but C is informing a and

B at exactly the same rate what that

means therefore is that the

concentrations of all three of these

things ie the reactants and the products

will all remain constant and when this

happens this is called an equilibrium so

the concentrations of these individual

you know reactants and products and an

equilibrium is called the position of

equilibrium and we'll talk about that in

a bit more detail now so let's put that

up here again so the position of

equilibrium can actually shift okay

depending on how we alter the conditions

of the environment so when we talk about

a shift if we talked about it being a

shift to the right

it suggests that more of a and B as

getting used to make C this means that

the equilibrium concentrations of a and

B will decrease whereas this the

equilibrium concentration of C will

actually increase oppositely when we

talk about a shift to the lift it's

suggesting that more C is getting broken

down to make a and B so therefore the

concentration of products C goes down

where the concentrations of a and B will

inevitably elevate so depending on the

change in you know conditions the

equilibrium can either shift to the

right or to the left and we're gonna be

looking at that and a bit more detail

and this slide so for the sake of you

know the argument we're gonna say that a

plus B gives C again I've added some

extra information will say that the

forward reaction is exothermic whereas

the reverse reaction is endothermic and

this is important you'll you'll see why

later

but what's what's really really really

important to understand here is that the

position of equilibrium will always

always always shift in the direction to

oppose the change being imposed so you

know you'll understand what this means

as we go through each of these examples

for example concentration when we

increase the product concentration right

for example if we increase the

concentration of products C that will

shift the equilibrium to the lift

because

what so remember we're if we increase

the concentration of C the equilibrium

will try and oppose that change to

reduce the concentration and the way

that it can do that is shift the

equilibrium to the lift by producing

more a and B from C therefore decreasing

concentration of C exactly the same

thing if we were to forcefully decrease

the concentration of C and manipulate

the environment that way then the

equilibrium will actually shift to the

right as if to make more C so I hope

that makes sense and similar thing if we

were to increase the reactant

concentration for example the

concentration of a you would assume that

the equilibrium will actually shift to

the right in order to remove or reduce

the concentration of a and because we

just increased it and wants to oppose

that change and decrease it so that's

sort of how this whole thing works when

we talk about the temperature exactly

the same thing that's why I gave you

this extra information above if we were

to increase the temperature what the you

know what the reaction will do we'll try

to you know decrease the temperature so

it's so therefore it will favor the

endothermic reaction which means that

the reaction is taking in heat so it

will shift in this example it will shift

the equilibrium to the left if we were

to decrease the temperature it will

actually favor the exothermic reaction

because it's gonna try to increase the

temperature and you know by up it's

obviously opposing what we're doing so

it'll shift the equilibrium equilibrium

to the right if we were to do that now

when we talk about pressure I want you

to take a look at this diagram to the

right here because it's actually quite

important pressure can be pressure can

be basically defined by the the

individual gas molecules hitting against

the walls of the container and which

hits in so if you have more particles

and this given space then all there'll

be more collisions with the wall and

therefore the pressure will be higher if

you've got a smaller amount of particles

then there'll be less particles hitting

the walls and therefore less pressure so

picture really only is to do

gasses right so a high pressure suggests

that there is a lot more particles in

that given space compared to you know

lower pressures so if we were to

increase the pressure again the

equilibrium will shift to the side or it

will shift towards trying to decrease

the pressure and the way it can do that

is to favor the side that has lower

number of particles if you look at it

here the left left hand side in this

example has two particles but the

right-hand side only has one so

therefore it will actually shift the

equilibrium to the right in this

instance towards the side with a low

amount of particles because that gives

lower pressures if you were to decrease

the pressure it will be the opposite

it'll try to increase the pressure in

the way that it can do that is shift the

equilibrium position towards the side

that has more particles because more

particles means more particles hitting

against the walls of the container as

postie erratically and that gives rise

to higher pressures so in this case

decreasing the pressure will shift the

equilibrium position to the left and a

catalyst actually doesn't affect the

equilibrium at all so don't get tricked

by this and an exam it was only there

just increase the rate of reaction and

it increases the speed of both the four

and the reverse reaction it doesn't

affect the position so lastly we're

gonna be looking at the concept of redox

it's shortened full reduction in

oxidation and this can be demonstrated

in two different ways and births are

absolutely correct one is the oxygen

gained or lost right oxygen oxygen oxide

oxidation is the gain of oxygen whereas

reduction is the loss of oxygen so in

this example to the right here copper

oxide can be you can say that it's been

reduced because it goes from copper

oxide to copper therefore it's lost an

oxygen but if you think about it

hydrogen on the other hand has gained

oxygen so therefore it's you can say

that the hydrogen has been oxidized

another way of looking at redox is the

electron gain or loss so oxidation is

defined as the loss of electrons and

reduction is the gain of electrons so if

you look at this example here magnesium

at a2 chlorine giving magnesium chloride

you can see that there's no absolutely

no oxygen involved but there certainly

is some sort of electron transfer so

what we're gonna be looking at is how

magnesium right the the atom has evolved

or change itself into the ionic form in

this ionic structure of magnesium

chloride so if you take a look at it

separately magnesium has actually lost

two electrons are and to form the

magnesium cation and therefore because

it's lost electrons it's said to have

been oxidized

whereas chlorine on the other hand the

molecule has gained electrons to become

the iron therefore it's an example of

reduction because it's again gained

electrons so I hope they hope you guys

found that helpful please visit my

website doublet or free exam Academy com

for more detailed notes and kind on the

topic otherwise please like share and

subscribe and I know that it's fairly

close to exams and also if you have any

further questions just comment and I

will try to answer thanks and I'll see

you in the next video

[Music]