Lesson video

Hello, I'm Mrs. Adcock and today's lesson is on Burning Hydrocarbons.

We are going to be looking at the products made when we burn hydrocarbons in oxygen.

Today's lesson outcome is I can write balanced symbol equations for combustion reactions and describe the difference between complete and incomplete combustion.

Some keywords we will meet in today's lesson are combustion, complete combustion, hydrocarbons, and incomplete combustion.

Here you will find the keywords, each used within a sentence.

It would be a good idea to pause the video now and read through those sentences.

You might want to make a note of them so that you can refer back to them later in the lesson.

Today's lesson on Burning Hydrocarbons is split into three parts.

So first of all is combustion, followed by complete and incomplete combustion, and finally, we will be looking at writing balanced symbol equations for combustion.

Right, let's get started on combustion.

Natural gas, coal, and crude oil are all fossil fuels and fossil fuels are finite resources which means they are being used at a faster rate than they are produced.

Fossil fuels contain hydrocarbons which are molecules made of hydrogen and carbon atoms only.

Examples of hydrocarbons include methane which has the molecular formula CH4.

So you can see methane contains hydrogen and carbon atoms only.

Ethane which has the molecular formula C2H6 and propane which is a larger hydrocarbon.

It has three carbons and it has the molecular formula C3H8.

Humans burn fossil fuels to provide energy.

Some examples of where we burn fossil fuels include in cars, so we burn diesel and petrol, to cook food, and to generate electricity.

Can you think of any other examples of where we burn fossil fuels? Let's have a go at answering this question to check you have understood the learning so far.

Which of the following are hydrocarbons? Is it A, methane which has the molecular formula CH4, B, ethane which has the formula C2H6, C, propanol which has the formula C3H7OH, or D, butane, C4H8.

Now don't be put off by any unfamiliar names or formula there.

Just think about what you know about hydrocarbons and decide if each of those are hydrocarbons.

There may be more than one correct answer.

The correct answer is A, B, and D.

So well done if you chose A, B and D.

They are all hydrocarbons as they are molecules made up of hydrogen and carbon atoms only.

Burning is scientifically known as combustion and combustion requires three things which are shown in the fire triangle below.

So for combustion, we need heat, fuel, and oxygen.

Now heat could be the sun, or friction, or a spark.

They're all examples of a heat source, and then we need oxygen and fuel.

All three of those are required for combustion to occur.

Combustion is an example of an exothermic reaction and exothermic reactions are chemical reactions in which thermal energy is given out, and there's an image there showing combustion of wood.

The general equation for combustion is fuel and oxygen react together to produce carbon dioxide and water.

So if our fuel is a hydrocarbon, then the hydrogen in the fuel will react with oxygen to produce water and the carbon in the fuel will react with oxygen to produce carbon dioxide.

Now we can test for the presence of oxygen and carbon dioxide in the following ways.

So the test for oxygen is oxygen will relight a glowing splint and the test for carbon dioxide is that carbon dioxide, if you bubble it through limewater, then the limewater will turn cloudy.

We have a couple of video clips we can watch to show these tests for gases.

The first one is oxygen relighting a glowing splint.

Now we are going to watch a clip showing a test for carbon dioxide.

Time for a check.

What three things are required for combustion? Is it A, fuel, heat, and oxygen, B, fuel, heat and carbon dioxide, or C, fuel, water, and oxygen? The correct answer is A.

Well done if you chose A.

Fuel, heat, and oxygen are all required for combustion to take place.

Oxygen gas will A, make limewater turn cloudy, B, turn white and hydrous copper sulphate blue, or C, relight a glowing splint? Well done if you chose option C, oxygen will relight a glowing splint.

Option A is a test for carbon dioxide and option B is not one we've covered in today's lesson, but option B is a test for water.

We can identify the products of combustion by using the following lab equipment.

We've got a burning fuel.

In this case it's a candle.

Some iced water, so the tubes go from the burning fuel, pass through the iced water, and into some limewater.

Then at the top there, we have a tube that goes to a vacuum pump.

The vacuum pump will draw the products of combustion through the apparatus.

Let's look at the setup a little bit more closely.

The burning candle produces chemical products as it reacts with oxygen in the air.

So the candle will react with the oxygen to produce carbon dioxide and water.

As the products pass through the apparatus, any water vapour will condense here.

So we can see as the water vapour reaches that iced water, it will cool down and condense and then liquid water will be left in the bottom of that U-shaped tube.

As the products pass through this part of the apparatus, any carbon dioxide present will turn the limewater cloudy.

To summarise, this apparatus allows us to identify the products of combustion which are water and carbon dioxide.

For our first practise task, you need to first of all, label the diagram of the equipment used to show the products of combustion, and then secondly, describe a chemical test for both oxygen and carbon dioxide.

If you pause the video now and have a go at those questions, and I'll see you when you're ready to go over the answers.

The labels you should have added include burning fuel, iced water, and limewater.

The chemical test for oxygen is if you place a glowing splint in the oxygen gas, it will relight.

And the test for carbon dioxide is that if you bubble carbon dioxide through limewater, then limewater will turn cloudy.

So well done if you've got those tests correct.

It's now time for us to move on to the second part of our lesson on complete and incomplete combustion.

When hydrocarbons combust in a plentiful supply of oxygen, the products are carbon dioxide and water and because there is a plentiful supply of oxygen, this means that the fuel can fully combust.

So this is known as complete combustion.

An example of complete combustion of a hydrocarbon is shown there below.

So methane which is a hydrocarbon can react in a plentiful supply of oxygen to produce carbon dioxide and water as the products.

Time for a check.

Butane gas is a fuel used in camping stoves.

Which is the correct equation for the complete combustion of butane? And butane is a hydrocarbon.

Is it A, butane plus oxygen react to make hydrogen and water, B, butane and oxygen react to make carbon dioxide and hydrogen, or C, butane and oxygen react to make carbon dioxide and water? Well done if you chose option C.

The products of complete combustion are carbon dioxide and water.

During complete combustion, a large amount of energy is released.

When a Bunsen burner is used, the combustion of methane gas releases significant amounts of heat and light energy into the surroundings, and we can see in the image there that we've got complete combustion of methane gas producing a blue Bunsen burner flame.

Why does opening the air hole result in complete combustion? Incomplete combustion of fuels occurs when there is a limited supply of oxygen.

Incomplete combustion releases less energy than complete combustion.

This time we've got a picture of a Bunsen burner with an orange flame showing incomplete combustion of the methane gas.

Why does closing the air hole on a Bunsen burner result in incomplete combustion of methane gas? When the air hole is open, there is more oxygen available for the fuel to react with so we have complete combustion of air.

This time we've closed the air hole, so there is a limited supply of oxygen so we have incomplete combustion.

During incomplete combustion, a fuel only partially reacts with oxygen producing water along with carbon dioxide, carbon monoxide, and soot and soot is unburnt carbon.

Examples of equations for incomplete combustion are fuel reacting with a limited supply of oxygen to produce carbon monoxide and water or fuel reacting with a limited supply of oxygen to produce carbon, soot, and water.

These are examples of some equations for incomplete combustion.

There are many possible product combinations that you could have and you could make carbon monoxide, and carbon, and water.

Time for a check.

Is this statement true or false? Complete combustion is when a fuel burns in a limited supply of oxygen.

So that statement is false.

So can you work out why that statement is false and justify your answer? Is it A, complete combustion is when a fuel burns in a plentiful supply of oxygen or B, incomplete combustion is when a fuel burns in a plentiful supply of oxygen.

Well done if you chose A.

Which equation or equations are possible for the incomplete combustion of ethane? Is it A, ethane plus oxygen react to make carbon dioxide and water, B, ethane plus oxygen react to make carbon monoxide plus water, or C, ethane plus oxygen react to produce carbon plus water? Remember we are looking for which of those equations are possible for incomplete combustion.

So the correct answers are B and C.

They are both possible equations for incomplete combustion of ethane.

A shows the products for complete combustion.

For the second practise task of the lesson, decide if each of the statements refers to complete or incomplete combustion.

So if you pause the video here, read through those statements, decide if they refer to complete or incomplete combustion, and then I'll see you back in a moment when you're ready to go over the answers.

Right, let's go over the answers.

A, when a fuel burns in a plentiful supply of oxygen, that will be complete combustion.

If it burns in a limited supply of oxygen, that's incomplete combustion.

The products are carbon or carbon monoxide and water, that's incomplete combustion.

The products are carbon dioxide and water, that will be complete combustion.

Releases large amounts of energy, that's complete combustion, and releases smaller amounts of energy will be incomplete combustion.

Well done if you correctly matched whether those statements were complete or incomplete combustion.

For question two, you need to write word equations for the following reactions.

Now be careful to identify first of all, if they are complete or incomplete combustion.

So have a go at writing your word equations and I'll see you when you're ready to go over the answers.

So 2A is complete combustion of petrol, so the petrol reacts with oxygen to produce carbon dioxide and water.

Question 2B is incomplete combustion.

So you should have methane reacting with oxygen, but this time some possible answers you may have given include giving carbon monoxide and water as the products, or carbon, soot, and water as the products, or maybe you put carbon plus carbon monoxide plus water as the products for incomplete combustion.

2C, the burning of diesel in a plentiful supply of oxygen.

Now because it's plentiful supply of oxygen, this will be complete combustion.

So you should have diesel plus oxygen react to form carbon dioxide and water and 2D is the reaction of butane with oxygen in a limited supply of oxygen.

So that will be incomplete combustion and possible answers you may have given include butane plus oxygen reacting to produce carbon monoxide plus water or butane plus oxygen reacting to produce carbon, soot, plus water.

So well done if you were able to correctly identify the products for complete and incomplete combustion and write those word equations.

We are ready to move on to the third part of our lesson which is balancing symbol equations for combustion.

When balancing equations, we make sure that there are the same number of atoms of each element in the reactants and the products and this is because mass must be conserved.

So we've got an example of a reaction there and we've got methane reacting with oxygen to produce carbon dioxide and water.

So this is an example of a complete combustion reaction.

There are not the same number of atoms of each element in the reactants and products in the above reaction.

So you can see that we've written out the molecular formula, so we've got CH4 plus O2 react to produce CO2 plus H2O.

But if you add the number of carbon atoms in the reactants and products, you will see that you've got one carbon in the reactants and one carbon atom in the product.

So they are balanced.

However we need all of the atoms of each element to be balanced.

And if you look in our reactants we've got four hydrogen atoms, but in our product we've only got two hydrogen atoms, and we've got two atoms of oxygen in our reactants and three atoms of oxygen in our product.

So these atoms for these elements are not balanced, so overall this equation is unbalanced.

To balance these equations, we cannot change the molecular formula.

So for example, H2O and CH4, these are fixed formula.

So you can't change the formula of a molecule, but we can change the number of molecules.

So for example, you could say we've produced two molecules of H2O.

So we've got our equation written out again and this time we've put spaces in the front where we could write numbers to balance this equation.

To balance the equation, we need to adjust the number of each molecule by placing numbers in front of the molecular formula.

So if we put a two in front of an O2 and two in front of the H2O, this equation is now balanced.

So we have one atom of carbon in our reactants and in our products.

We have four atoms of hydrogen in the reactants and again, in the products, and we've got four oxygen atoms in the reactants and we've got four oxygen atoms in our product so this equation is now balanced.

Which equation shows the correct balanced equation for the complete combustion of propane? And the formula for propane is C3H8.

Well done if you chose A.

A is a correctly balanced equation for the complete combustion of propane, B is incorrect as the formula do not contain subscript numbers, and C is incorrect as it is unbalanced.

For a final practise task of today's lesson, you need to have a go at balancing these equations.

So if you balance the equations A to E and then come back in a few minutes when you're ready, and we can go over the answers.

Have a look through carefully now and see if you balanced those equations correctly.

You should have not changed the molecular formula, but added numbers in front to change the number of molecules if required to make sure that you have the same number of atoms for each element in the reactance and the products.

So well done if you got those correct.

Let's recap the key points we have covered in today's lesson on Burning Hydrocarbons.

Combustion is the chemical combination of a substance with oxygen involving the production of heat and light.

Complete combustion of fuels like hydrocarbons involves the production of carbon dioxide and water and the chemical test for oxygen is a combustion reaction involving relighting a glowing splint.

Incomplete combustion produces carbon particulates, soot, and carbon monoxide due to reduced oxygen levels.

Well done for working hard in today's lesson on Burning Hydrocarbons.

I have enjoyed the lesson and hope that you have too, and I hope you're able to join me for another lesson soon.