Embedded Files
objectives
objectives
1. Describe an experiment to show that a gas will diffuse faster than another gas that has heavier particles
2. Say how, and why, the temperature affects the rate at which a gas diffuses
Gases
Gases
When you blow up a balloon, you fill it with air particles. They collide with each other. They also hit the sides of the balloon, and exert pressure on it. This pressure keeps the balloon inflated.
In the same way, all gases exert a pressure. The pressure depends on the temperature of the gas and the volume it takes up, as you’ll see below.
Heating Gases
Heating Gases
The same happens with all gases:
When you heat a gas in a closed container, its pressure increases.
That is why the pressure gets very high inside a pressure cooker.
The particles in this gas are moving fast. They hit the walls of the container and exert pressure on them. If you now heat the gas . . .
. . . the particles take in heat energy and move even faster. They hit the walls more often, and with more force. So the gas pressure increases.
▲In a pressure cooker, water vapour (gas) is heated to well over 100°C. So it is at high pressure. You must let a pressure cooker cool before you open it!
Diffusion Activity
Diffusion Activity
I am going to spray some deodorant in the corner of the room
As soon as you can smell it stand up
You have 2 mins as a pair to explain what just happened!
Diffusion
Diffusion
Diffusion is the spreading out of particles in a gas or liquid from an area of high concentration to low concentration
Particles in gas and liquid are free to move
In gas, particles are moving very fast
For diffusion to happen though particles must bounce off each other to go in all directions – that’s why it takes a while to smell the deodorant!
Experiment on Rate of Diffusion
Ammonia + Hydrogen Chloride → Ammonium Chloride
NH3(g) + HCl(g) → NH4Cl (s)
Watch the demo done by your teacher
Describe what you saw, use a labelled diagram to help
Bonus: can you explain what’s happening?
What do you think could affect the rate of diffusion for this experiment?
Why does the NH4Cl form closer to the HCl?
Rate of Diffusion
Rate of Diffusion
gases do not all diffuse at the same rate, every time. It depends on these two factors:
1. Mass of Particles
1. Mass of Particles
1.The particles in hydrogen chloride gas are twice as heavy as those in ammonia gas. So which gas do you think will diffuse faster? Let’s see:
- Cotton wool soaked in ammonia solution is put into one end of a long tube (at A below). It gives off ammonia gas.
- At the same time, cotton wool soaked in hydrochloric acid is put into the other end of the tube (at B). It gives off hydrogen chloride gas.
- The gases diffuse along the tube. White smoke forms where they meet:
- The white smoke forms closer to B. So the ammonia particles have travelled further than the hydrogen chloride particles – which means they have travelled faster.
The lower the mass of its particles, the faster a gas will diffuse.
That makes sense when you think about it. When particles collide and bounce away, the lighter particles will bounce further.
The particles in the two gases above are molecules. The mass of a molecule is called its relative molecular mass. So we can also say:
The lower its relative molecular mass, the faster a gas will diffuse.
2. Temperature
2. Temperature
When a gas is heated, its particles take in heat energy, and move faster. They collide with more energy, and bounce further away. So the gas diffuses faster. The higher the temperature, the faster a gas will diffuse.
activity - consolidation
activity - consolidation
Part 1 - Watch the above videos and answer these questions
1.The particles in liquids and gases show random motion. What does that mean, and why does it occur?
2.Why does the purple colour spread when a crystal of potassium manganite (VII) is placed in water?
3.Bromine vapour is heavier than air. Even so, it spreads upwards in the experiment above. Why?
4.a What is diffusion?
b Use the idea of diffusion to explain how the smell of perfume travels.
Part 2 - Answer the following Questions
1.A large crystal of potassium manganate(VII) was placed in the bottom of a beaker of cold water, and left for several hours.
a Describe what would be seen:
i after five minutes
ii after several hours
b Explain your answers using the idea of particles.
c Name the two processes that took place during the experiment.
2.Use the idea of particles to explain why:
a solids have a definite shape
b liquids fill the bottom of a container
c you can’t store gases in open containers
d you can’t squeeze a sealed plastic syringe that is completely full of water
e a balloon expands as you blow into it.
3.Below is a heating curve for a pure substance. It shows how the temperature rises over time, when the substance is heated until it melts, then boils.
a What is the melting point of the substance?
b What happens to the temperature while the substance changes state?
c The graph shows that the substance takes longer to boil than to melt. Suggest a reason for this.
d How can you tell that the substance is not water?
e Sketch a rough heating curve for pure water.
4. A cooling curve is the opposite of a heating curve. It shows how the temperature of a substance changes with time, as it is cooled from a gas to a solid. Here is the cooling curve for one substance:
a What is the state of the substance at room temperature (20°C)?
b Use the list of melting and boiling points on page 9 to identify the substance.
c Sketch a cooling curve for pure water.
5.Using the idea of particles explain why:
a the smell of burnt food travels through the house
b when two solids are placed on top of each other, they do not mix
c pumping up your bike tyres gives a smooth ride
d smokers can cause lung damage in other people
e heating a gas in a closed container will increase its pressure
f a liquid is used in a car’s breaking system, to transfer the pressure from the brake pedal
g poisonous gases from a factory chimney can affect a large area.
6.a Which of these are examples of diffusion?
i.a helium-filled balloon rising in air
ii.a hydrogen-filled balloon deflating, due to gas passing through the skin
iii.the smell of perfume from a person standing on the other side of a room
iv.sucking a drink from a bottle, using a straw
v.an ice lolly turning liquid when it is left out of the freezer
vi.the tea in the cup changing colour when you add milk, without stirring
vii.a light, coloured gas, spreading down through a gas jar
viii.a blue crystal forming a blue solution, when it is left sitting in a glass of water
ix.spraying paint from a spray can.
b For one of the examples of diffusion, draw a diagram showing the particles before and after diffusion has taken place.
7. You can measure the rate of diffusion of a gas using this apparatus. The gas enters through the thin tube:
The measuring tube is sealed at the top with a plug of porous plaster. Air and other gases can diffuse in and out through the tiny holes in the plug.
The water rises in the measuring tube if the chosen gas diffuses out through the plug faster than air diffuses in. Air is mainly nitrogen and oxygen.
a When you use hydrogen gas, the water rises in the measuring tube. Why?
b What does this tell you about the rate of diffusion of hydrogen, compared to the gases in air?
c Explain your answer to b. Use the term mass!
d The molecules in carbon dioxide are heavier than those in nitrogen and oxygen.So what do you think will happen to the water in the measuring tube, when you use carbon dioxide? Explain your answer.
8.Look at the table:
a.Which two gases will mix fastest? Explain.
b.Which gas will take least time to escape from a gas syringe?
c.Would you expect chlorine to diffuse more slowly than the gases in air? Explain.
d.An unknown gas diffuses faster than nitrogen but more slowly than methane. What you can say about its relative molecular mass?
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