form 4 chem chapter 5

8/13/2019 Form 4 Chem Chapter 5

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Chemistry Form 4 Page 77 Ms. R. Buttigieg

11. The Gas Laws

Is a gas Denser or Lighter than Air?

To find whether a gas is denser or lighter than air, we find its

Relative Molecular Mass (RMM).

It its RMM is less than 29 (RMM of air average RMM of nitrogen and oxygen)

it is lighter.

If its RMM = 29, it has the same density of air.

If its RMM is greater than 29, then it is denser than air.

Make sure you can draw the following gas collecting Diagrams

o Over water (for gases that are not soluble in water)

o Downward delivery (for gases that are denser than air)

o Upward delivery(for gases lighter than air)

o Dry gas with anhydrous calcium chlorideUse of syringe or gas jar

o Dry gas with conc H2SO4

Error 1 _______________________________________________________________________

Error 2 _______________________________________________________________________

Error 3 _______________________________________________________________________

(b) Name another piece of

apparatus that could be usedto collect the dry gas

(instead of using a gas jar).

______________________

(4 marks)


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11.1 Volume changes due to physical factors

o Particles in a gas are very far apart.

o They have no forces between them (i.e. the particles in a gas do not attract each

other).o They move randomly (without any fixed direction) with high speed colliding with each

other and with the walls of the container.

o When they collide with the walls of the container they exert a pressure.

o The higher the temperature, the higher the kinetic energy and so the greater thespeed of the particles.

The assumptions that the kinetic theory makes about an ideal gas.

An ideal gas is defined as one in which all collisions between atoms or molecules are

perfectly eleastic and in which there are no intermolecular attractive forces. One can

visualize it as a collection of perfectly hard spheres which collide but which otherwise do

not interact with each other. In such a gas, all the internal energyis in the form of kinetic

energy and any change in internal energy is accompanied by a change in temperature.

An ideal gas can be characterized by three state variables: absolute pressure (P),

volume (V), and absolute temperature (T). The relationship between them may be

deduced from kinetic theoryand is called the

The conditions assumed for an Ideal Gas:

1. Molecules are perfectly elastic (no

STICKINESS)

2. Molecules are point masses (no SIZE)

3. Molecules move at random

o The system consists ofNdistinguishable

particles contained in a cubic box of volume V.

o The particles are always in motion, moving in

random directions with some distribution of speeds.

o Being ideal gas particles, they exert no forces on each other.


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o Collisions with the walls of the box are perfectly elastic - no energy is gained or lost in

such collisions.

The effect of change of temperature, and change of pressure, on gas volumes

explained in terms of the kinetic particle theory.

1. __________When the volume of a gas is made smaller, the particles have lessspace to move in, so they collide with the walls of the container more often,

exerting a greater pressure on it.

2. ___________When the temperature is increased (at constant volume, the

kinetic energy of the particles increases. So they move more quickly. Thus they

collide with the walls of the container more often and with more energy. So the

pressure increases.

3. ___________When the temperature of a gas is increased, its volume increases

(keeping the pressure constant).

Consider a gas in a container. The piston can move either way. If the temperature of the

gas is increased, the particles exert (make) a greater pressure, since they move more

quickly and collide with the walls of the container more often and with more energy.

Since the pressure on the inside of the piston is greater than on the outside, the piston

begins to move outwards (increasing the volume).

When the volume increases, the particles have more space to move in. So they collide

less frequently with the walls of the container. So the pressure of the gas begins todecrease.

When the pressure inside = pressure outside, the piston stops.

So P at the beginning = P at end, and T and V

gas piston

pistongas


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The GAS LAWS

Boyles law- The pressure of a fixed mass of gas is inversely proportional to the volumeof the gas at constant temperature.

P 1

V

Boyle used a simple apparatus to examine the relationship between pressure and volume. In

this apparatus, a small volume of air was trapped in a glass tube by mercury. A flexibletube connected this tube to a small reservoir of mercury that could be raised or lowered

relative to the trapped gas volume. The difference in height of the interior and exterior

mercury levels is, of course, the pressure relative to the pressure of the atmosphere.

Boyle was able to demonstrate that:

V x P = constant

or that the volume and pressure are inversely proportional. This relationship is calledBoyle's Law.

Charles law The Volume of a fixed mass of gas is directly proportional to the Kelvin

temperature at constant pressure.

V T


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Pressure law The Pressure of a fixed mass of gas is directly proportional to the Kelvin

temperature at constant volume.

P T

Combining the three: PV for a fixed mass of gas is constant.

T

So, for a fixed mass of gas, P1V1 = P2V2 (temperature in Kelvin)

T1 T2

Somewhat later on in 1848,the English scientist William

Thompson, who was later

raised to the Peerage as Lord

Kelvin, recognized that a

graph of volume vs.temperature for any gas

resulted in a straight line thatintersected the temperature

axis at -273.15oC.

This temperature later became

known as absolute zero and

was used as the basis for anabsolute temperature scale.

Temperature unitscorresponding to theCentigrade scale in this

measure are called Kelvins.


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Gay-Lussac's Law - Pressure Law

Simply stated, Gay-Lussac's Law indicates that for a fixed amount of gas (fixed number of

moles) at a fixed volume, the pressure is proportional to the temperature.

p/T = constant or p1/T1= p2/T2

In other words, as the temperature increases, the pressure increases. (When you put a pickle jarin the refridgerator, the drop in pressure from the trapped air becoming colder makes it hard to

open the jar later!)

Question: Consider a container with a volume of 22.4 L filled with a gas at 1.00 atm at 273 K.What will be the new pressure if the temperature increases to 298 K?

Solution:

Using Gay-Lussac's Law and solving for p2we get:

p1T2 (1.00 atm)(298 K)p2= ----- p2= -----------------

T1 (273 K) p2= 1.09 atm

Note: When the temperature increases, the pressure increases!

Also notethat it is essential to use temperature on an absolute scale (i.e. use Kelvin instead ofoC!

1. A container initially at 0.500 atm and 25oC. What will the pressure be at 125

oC?

2. A container is initially at 47 mm Hg and 77 K. What will the pressure be when the

container warms up to room temperature of 25o?

3. A gas thermometer measures temperature by tmeasuring the pressure of a gas inside the

fixed volume container. A thermometer reads a pressure of 248 Torr at 0oC. What is the

temperature when the thermometer reads a pressure of 345 Torr?


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Exam papers - JL 2002

7. (a) Use the simple kinetic particle theory to explain:

(i) what causes the pressure inside a car tyre;

______________________________________________________________________________

______________________________________________________________________________

(ii) why there is an increased pressure if the tyre becomes warmer.

______________________________________________________________________________

______________________________________________________________________________

(b) If a gas occupies 11.2dm3at 0C and 2 atm. pressure, what volume would it occupy at s.t.p?

JL 2000

The unbalanced equation for the reaction of iron with steam is

Fe + H2O Fe3O4 + H2

a) Balance the equation

b) If 4.2g of iron reacts completely with steam, calculate the maximum volume of hydrogen

which could be collected at standard temperature and pressure.

(3 marks)

JL 2001

50cm3

methane were mixed with 125cm3

of oxygen and the mixture was exploded. Thefollowing reaction occurred.

CH4(g) + 2O2 (g)CO2(g) + 2H2O(g)

Calculate the total volume of the gases remaining immediately after the explosion.

N.B. This should include the volume of any gas that remains unreacted. Show your

reasoning/working. (Assume that all volumes are measured under the same conditions oftemperature and pressure).

form 4 chem chapter 5

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