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Gases, Reversible reactions and Ammonia C3.4 Love, Heba
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### Transcript of Gases, reversible reactions and ammonia - C3.4 Heba Saey Gases, Reversible reactions and AmmoniaC3.4

Love, Heba Calculating Volumes Important fact- ( AVOGADRO’S LAW) get this stuck in your head! One mole of any gas always occupies at room temperature and atmospheric pressure.

In other words – One mole of anyyyyy Gas – even one mole of fart gas… will

always have a volume of at room temperature which is usually 25 degrees,

and 1 atmosphere! ( got it?)

I REMEMBER HIS NAME BY : AVOCADO’S LAW ( OBV DON’T WRITE THAT IN THE EXAM) Volumes of Gases – 2 formulas

Moles in Volumes of Gases

Volume

Moles 24

Volume of Gas =

x 24

Don’t worry.. It will make sense in the next few slides… Note…

When dealing with Calculating Volumes questions , you need to realise

That you may be required to know the following: - Calculating Masses ( like in C2) - Use the Mass/ Mr = no. of moles Example 1 What’s the volume of 4.5 moles of chlorine at

RTP ( ROOM TEMP)

Volume

Moles 24

Step 1) Know which formula your usingStep 2) Volume = moles x 24Step 3) Moles given = 4.5Step 4 ) 4.5 x 24 = volumeStep 5 ) Answer = 108

Volume = Example 2How many moles are there in 8280 of

hydrogenGas? ( beware – look at the volume…)

Volume

Moles 24

Step 1) Know which formula your usingStep 2) Moles = volume/24Step 3) Volume given = 8280/1000= 8.28 Step 4 ) 8.28/24= molesStep 5 ) Answer = 0.345 moles

Volume = Volumes in reactions( don’t freak out! )

x 24

Two stages 1) Find reacting mass ( like in C2 )

2) Convert mass into volume using formula below

Example : Find the Volume of Carbon dioxide produced at RTP when 2.7g of carbon is completely burned in oxygen.

To do this : 1) Find the reacting mass of

carbon dioxide 2) Use the formula on the side( ill show you on the next slide) Example 1 Example : Find the Volume of Carbon dioxide produced at RTP when 2.7g of carbon is completely burned in oxygen.

FIRSTLY : Write balanced equation:

Only use what's required – in this case we only need Carbon and Carbon dioxide NOT Oxygen.

C

Mr / Ar 12 12+ ( 16 x 2) = 44

Divide by 12 1 3.666666

Multiply by 2.7 g 2.7g 9.9 g

So 2.7 g of Carbon gives you 9.9 g ( got it ? )

x 24 Volume of Carbon dioxide = x 24 Example 2 ( using Moles and Volumes )

Example How much carbon dioxide is formed when 30 of Oxygen reacts with carbon monoxide?

FIRSTLY : Write balanced equation:

Only use what's required – in this case we only need Oxygen and Carbon dioxide NOT Carbon monoxide.

2

How many moles in reaction?

1 2

Volume ( always equal to no. of moles in equation )

1 2

Volume in reaction 30 2 x 30 = 60

So of Oxygen gives you 60 of Carbon dioxide ( got it ? )

x 24 Reversible Reactions A Reversible reaction is one where the products of the reaction can react together and convert back to the original reactants

( in other words it can go both ways)

𝐴+𝐵𝐶+𝐷 All reversible reactions will reach a Dynamic Equilibrium …..

DYNAMIC EQUILBRIUM

Reactions in a closed system ( where no reactants can escape ) always reach a state of equilibrium. Equilibrium basically means that the amounts of reactant and products will reach a certain balance and stay there..

Happening in both directions, but the overall effect is nil….This is because the forward and reverse reactions cancel each other out b/c their Happening at exactly the same rate in both directions. ( DON’T confuse yourself

by visualising it..JUST LEARN IT… ) Effect of temperature on an equilibrium

Remember Endothermic and Exothermic? If more energy in the form of heat is required to break old bonds then the reaction is

endothermic. If less energy in the form of heat is required to break old bonds, but new bond forming

produces more heat energy – this is exothermic..

+ HEAT

ENDOTHERMIC

EXOTHERMIC+

+ FORWARD ( RIGHT) REACTION = EXOTHERMIC

BACKWARD ( LEFT ) REACTION = ENDOTHERMIC

MORE VOLUME = LEFT

LESS VOLUME = RIGHT

( this will make sense later ) What if we raised the temperature?

ENDOTHERMIC reaction will be favoured

What the F**K does that mean??? ( CGP ARE SHIT AT EXPLAINING SOMETIMES)

Remember how we said that when old bonds are broken, if they use a lot of energy in the form of heat , then the reaction is endothermic. Therefore, if we raise the

temperature, the reaction will use up the extra heat to break old bonds.

So… therefore the position of equilibrium will be towards the left because the products on the right are broken into the two reactants HEAT is required to do

this… This reduces the amount of product produced … What if we reduced the temperature?

EXOTHERMIC reaction will be favoured

What the F**K does that mean??? ( CGP ARE SHIT AT EXPLAINING SOMETIMES)

Exothermic produce heat when new bonds are formed. They require much less Heat energy to form these new bonds, therefore if the temperature is lowered,

the bond forming part of the reaction ( usually to the right) will be favoured.. DUH! Usually this would be the “ products” section of the equation. So if there is a reduce

in temperature, the amount of products produced will increase….

Still confused.. There's a picture on the next slide… Endothermic reaction Will result in producingThese two reactants as bonds are broken - Raising

temperature

will lead to equilibria moving To the left ( less product)

Exothermic Reaction would produce a product ( new bonds ) and will also produce heat. Less heat is required for this process to occur. So lowering the temperature wouldn't bother the exothermic section of the reaction Pressure and position of Equilibrium

To start off with…

You need to know how to know which side has the most volume

N2(g) + 3H2(g) 2NH3(g)

1+3 = 4 2HIGHER VOLUME What if we raise the pressure?

From the previous slide, we realised that there is more volume on the left side of the reaction compared to the right…

If we increase the pressure OBVIOUSLY the side making less volume will be favoured, this will help make MORE volume. The side with the

less volume is favoured as it has much less pressure/volume. What if we reduce the pressure?

From the previous slide, we realised that there is more volume on the left side of the reaction compared to the right…

If we decrease the pressure OBVIOUSLY the side making more volume will be favoured, this will reduce the volume produced . The side with the

more volume is favoured as it has much more pressure/volume. The whole reason for the differences in equilibrium position is to maintain a balanced reaction overall and toreach equilibrium …. Catalyst effects… Increasing temperature and pressure will only increase the

rate of reaction – and so you reach equilibrium faster. However it will result in less product being made…

The catalyst does not affect the yield, but it does increase the rate of the reaction. This allows a lower temperature to be used (increasing temperature is one

way of speeding up a reaction) and using a lower temperature allows the yield to be maximised as well as

saving money on energy costs. To wrap up …

Look at the graph. You can see that for any given temperature the yield of ammonia increases as the pressure increases. You can also see that, for any given pressure, the yield goes down as the temperature increases. This is because the forward reaction is exothermic. The Haber process This process produces AMMONIA

N2(g) + 3H2(g) 2NH3(g)Nitrogen – extracted from air

Hydrogen- obtained from natural gas

This is a reversible reaction , occurring in both directions, not all the hydrogen and nitrogen will be converted to ammonia.The reaction reaches dynamic equilibrium where the amount of reactants will be balanced with the amount of product produced. Conditions requiredin industry to manufacture

Ammonia:

Pressure : 200 atmospheres Temperature : 400˚c

Catalyst : Iron The process – not as hardas you think…

- High pressure is used as the forward reaction is favoured- therefore pressure is set high as possible to produce the highest % yield of ammonia

- The forward reaction is exothermic ( reducing temp = increase yield) , however you must remember that the high temperature supplied in this process causes equilibrium to move the wrong way – towards the backward reaction.

- We know lower temperatures produce higher yields , but its very slow, so they increase the temperature anyways to get a much faster rate of reaction

- 450 degrees would produce less yield than in lower temperature, but its best to gain yield at a faster rate

- Ammonia is formed as gas , but as it cools in the condenser it liquefies and is removed

- Iron catalyst makes reaction go faster, but remember it doesn’t affect the %yield High pressure favours 2NH3 causing higher % yield of 2NH3

N2(g) + 3H2(g) 2NH3(g)

1+3 = 4 2HIGHER VOLUME In summary . . .

High temperatures makes reaction reach equilibrium quicker. The

reason why low temperatures aren't used is because its slower despite it producing more. The temp used still

produces yield because temp is moderately high.

High pressure is used as it favours the forward reaction ( the one with less volume) ,

causing a higher yield

Iron catalyst speeds up reaction but doesn’t affect position of

equilibrium. 