# Chemistry

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## Properties Of Matter 1 of 27

## Volume Of Gases

Standard Temperature and pressure (STP) is 0°C and 1 atmosphere (atm)

Volume Of Gas = number of moles x 22.4

V = n x22.4

n = v/22.4

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## Properties Of Gases

Gases all have similar properties

1. Low density compared with liquids or solids

2. They spread to fill the container in which they are placed

3. They exert pressure in all directions

4. They can diffuse easily and quickly

5. They are easily compressed

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## The Ideal Gas Law

For an Ideal Gas:  PV= nRT

P= pressure

V= Volume

n= number of moles

R= the universal gas content

T= temperature

The value of R depends on the units used for pressure:

R             Pressure

62.4         mmHg

0.082       atm

8.31         kPa

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## Dalton's Law Of Partial Pressure

The total pressure in a gas mixture is the sum of the partial pressures of each indiviual gas:

Ptotal= P1 + P2 + P3 ...

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## Graham's Law

Diffusion is the tendency of gases to spread out e.g spaying deodrant

Effusion is when a gas escapes through a tiny hole in its container e.g a ballon

Graham's Law of effusion shows that the lighter gases effuse faster, than heavier gases 6 of 27

## Gas Pressure

The pressure of a gas is due to the gas particles colliding with the walls of the container.

A barometer is a device used to measure atmoshperic pressure.

Pressure is measured in a number of units. The System International (SI) unit of pressure is the pascal (pa). Older units of pressure that are still used are millimetres of mercury (mm Hg) and atmospheres (atm).

1 atm = 760mmHg = 101.3 kPa

kPa = kilo pascals

= 1000 pascals

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## Boyle's Law

Increasing the pressure on a sample of gas will lead to a decrease in volume (keeping it the same temperature). At a constant temperature:

P1V1= P2V2

Intial     Final

Note: This only applies to an "ideal" gas, an ideal gas is one that follows the 5 assumptions of the kinetic molecular theory.

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## Kinetic Molecular Theory

1. Gases are made up of particles moving with rapid, constant random motion.

2. The higher the temperature, the faster the particles move. They have increased kinetic energy.

3. The forces of attraction and repulsion between the gas particles are pratically zero.

4. The particles are very far apart. The volume of the particles is very small compared with the volume that the gas occupies.

5. Particles collide with each other and the walls of the container exerting pressure. The collisions with each other are perfectly elastic. This means that no kinetic energy is lost when they collide with each other.

If all five basic assumptions are met, it is considered an "Ideal Gas"

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## Charle's Law

Changing the temperature of a gas directly changes the volume occupied. Pic On Right: Low temp = Small Volume

Pic On Left: High temp = Larger Volume

For  a fixed amount of gas at a constant pressure: Note: Temperature must be in Kelvin

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## Gay - Lussac's Law: Pressure and Temperature

If the volume of a gas is kept constant, as the temperature of the gas increases, the pressure increases. At a constant volume: Note: Temperature must be in Kelvin

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## The Combined Gas Law

The combined gas law combines Boyle's, Charles and Gay-Lussac's law's together.

The combined gas law allows you to do calculations when the amount (moles) of gas is constant: 12 of 27

## Manipulating the Formula's

The combined gas law can be altered if pressure, volume or temperature is kept constant.

Pressure is constant - Charles Law

Volume is constant - Gay Lussac's Law

Temperature is constant - Boyle's Law

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## Ideal Vs. Real Gases

An Ideal Gas is a gas that obeys all the gas laws perfectly at a particular temperatureand pressure. No gas behaves ideally, however, at all temperatures and pressures. Real Gases show large departures from ideality, particulary at high pressures and low temperatures. 14 of 27

## Enthalpy and Thermochemical Equations

Enthalpy (H) is the heat content of a system

The change in enthalpy ΔH is given by:                                        Δ = delta/change in

ΔH = H(products) - H (reactants)

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## Exothermic reactions

- Heat is released to the surroundings                                                                                   - ΔH is negative                                                                                                                  - It feels hot

Energy diagram 16 of 27

## Endothermic reactions

- heat is absorbed from the surroundings                                                                                 - ΔH is positive                                                                                                                      - feel cold

Energy diagram: 17 of 27

## Reversible Reactions

A reversible reaction is one in which the conversion of reactants to products, and the conversion of products to reactants occur simultanelously

When the rates of the forward and reverse reactions are equal, a state of balance called chemical equilibrium has been reached.

eg. 18 of 27

## Le Chatelier's Principle

Le Chatelier's Principle states that if a change is applied to a system at equilibrium, the system reacts in such a way as to counteract the change, in order to return to equilirbium.

- Increase in pressure favours side with fewer moles

- Decrease in pressure favours side with more moles

- When temperature is increased endothermic reaction is favoured

- When temperature is decreased exothermic reaction is favoured

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## Dynamic Equilibrium

- Rate Forward reaction  = rate reverse reaction                                                                        - No observable changes                                                                                                         - Both directions of reaction are occuring

Heat on left of equation = endothermic

Heat on right of equation = exothermic

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## Changing Equilibrium Position 21 of 27

## Acids

An acid is a substance which in solution produces hydrogen ions H+

Properties - sour to taste

- will dissolve in water

- they are neutralised by bases

Examples:

Acetic acid                 CH3COOH

Carbonic Acid              H2OCO3

Hydrochloric Acid         HCl

In solids acids Ionise:

HCl --> H+ + Cl-

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## Bases

A base is a substance which in solution produces hydroxide ions OH-

Examples:

Magnesium hydroxide Mg(OH2)

Sodium Hydroxide

NaOH --> Na+ + OH-

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## Strength and Concentration

Strong Acids are completely disassociated (100%). If the acid is less then 100% disassociated it is termed a weak acid.

For strong acids in water show a one way arrow.

Weak acids are partially ionized or disassociated and therefore form few ions. A double arrow is used to indicate their dissolving in water.

Concentration is a ration of the amount of solute to the amount of solvent in a solution - usually measured in moles/litre (molarity)

Concentrated acids or bases have high molarity usually in excess of 6mol/L

Dilute acids have smaller molaritites usually less than 6mol/L

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## Ka

The acid Ka is the ratio of the ionised

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## pH calculations

1. Calculate the pH of a 0.05M solution of hydrochloric acid

(H+) = 0.05M

pH= -log(H+)

pH = -log(0.05)

pH = -(-1.3)

pH = 1.3

2. The pH of a solution is 6.35. What is the H+ ion concentration?

pH = -log (H+)                                          (H+) = 10^-6.35

6.35 = -log (0.05)                                     (H+) = 4.5 x 10^-7 M

-6.35 = log (H+)

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## Neutralization Reactions

Neutralization Reaction = acid and base react in aqueos solution to produce a salt and water

Example

HCl  +   NaOH  -->  NaCl  +   H2O

acid      base          salt        water

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