Chemistry Notes Form 4

Chemistry Notes Form 4

Chemistry Notes Form 4


Factors that affect the rate of evaporation

Evaporation occurs at all temperatures at the surface of the liquid

It happens more rapidly when:

i) The temperature is higher, since then more molecules in the liquid are moving fast enough to escape from the surface,

ii) The surface area of the liquid is large so giving more molecules a changes to escape because more are near the surface, and

iii) Wind or draught is blowing over the surface carrying vapour molecules away from the surface thus stopping them from returning to the liquid and making it easier for more liquid molecules to break free

Kinetic Theory and Gas Laws

Due to the kinetic theory we begin to understand why gases exert pressure

The molecules of a gas are far apart and in continuous random motion, colliding with each other and with the walls of the vessel in which the gas is held

The molecules have mass, so they have energy hence they exert force on each collision and hence pressure

If the temperature of the gas is increased at constant volume, the molecules gain more energy and move faster, hitting the walls with more force and exerting greater pressure

If the volume of the gas is increased at constant temperature, the molecules have more space in which to move

The frequency of collisions decreases reducing the pressure

Boyle’s law

The pressure of a fixed mass of gas is inversely proportional to its volume if its temperature is kept constant

Consider a gas trapped in a container as shown

The mass, hence number of moles are constant and do not change during the course of the investigation

The piston is frictionless and moves smoothly without allowing the gas to escape

When the pressure changes, the volume of the gas changes as shown

 

Graphical representation of Boyle’s law

Graph between P & V at constant temperature is a smooth curve known as “parabola”

 

Graph between 1/P & V at constant temperature is a straight line

 

If pressure, p is doubled, the volume is halved

That is, p is inversely proportional to V

In symbols

P ∝ 1/V or p = constant X 1/V

PV = constant

p1V1 = p2V2 = constant.This is Boyle’s law

Charles’ Law

The volume of a fixed mass of gas is directly proportional to its absolute temperature if the pressure is kept constant

 

We can then say that the volume V is directly proportional to the absolute temperature T, ie doubling T doubles V, etc Therefore

V ∝ T or V = constant X T

Or V/T = Constant

Volume V1 = 1dm3 V2 = 2dm3 V3= 3 dm3

Temperature (OC) 0 ºC 273 ºC 546 ºC

Temp (K) 273 K 546K 819K

Volume/Temp 1/273 2/546 3/819

Equation ∴ V1/T1 = V2/T2 = V3/T3

∴ V/T = constant

Graphical representation

Graph between Volume and absolute temperature of a gas at constant pressure is a “straight line”

 

Absolute scale of temperature or absolute zero

If the graph between V and T is extrapolated, it intersects T-axis at -273

16 0C

At -273

16 0C volume of any gas theoretically becomes zero as indicated by the graph

 

But practically volume of a gas can never become zero

Actually no gas can achieve the lowest possible temperature and before -273.16 0C all gases are condensed to liquid

This temperature is referred to as absolute scale or absolute zero

At -273.16 0C all molecular motions are ceased

This temperature is called Absolute Zero

Degrees on this scale are called Kelvin’s and are denoted by K while θ stands for a Celsius scale temperature

They are exactly the same size as Celsius degrees

Since –273 0C = 0K, conversions from 0C to K are made by adding 273 T = 273 + θ

0 0C = 273K

15 0C = 273 + 15 = 288K

The letter T represents Kelvin or absolute temperatures and θ stands for a Celsius scale temperature

Pressure law

The pressure of a fixed mass of gas is directly proportional to its absolute temperature if the volume is kept constant

p ∝ T or p = constant X T

Or p/T = Constant

The three equations can be combined giving

Pv = constant

T For cases in which p, V and T all change from say p1, V1 and T1 to p2, V2 and T2, then P1V1 = p2V2 T1 T2

Gases and the Kinetic Theory


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