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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