Unit 1 The Particle Nature of Matter

Elements

Primary constituents of matter which cannot be broken down

Compounds

Consist of atoms of different elements chemically bonded together in a fixed ratio

Mixtures

Contain more than one element or compound in no fixed ratio

Homogeneous mixture

A mixture in which the composition is uniform throughout the mixture.

Heterogeneous mixture

A mixture which has a non-uniform composition and its properties are not the same throughout such as water and oil.

Examples of how to separate components in a mixture based on physical property

Sand and salt

• Difference: solubility in water

• Technique: Solution and filtration

Salt and water

• Difference: boiling point

• Technique: Distillation

Iron and sulfur

• Difference: magnetism

• Technique: Response to a magnet

Pigments in food colouring

• Difference: adsorption to solid phase

• Technique: paper chromatography

Filtration

Solid is separated from a liquid using a membrane.

• solid is collected on the membrane as the residue and the filtrate containing the solute passes through

Solvation

Substances of different solubilities in water can be separated by adding water. The resulting mixture can be filtered with the insoluble substance removed as the residue leaving the soluble substance and water solution as the filtrate.

Crystallisation

Separated by evaporation

Distillation

• Used to separate a solvent from a solute.

• Solvent has lower boiling point than the solute so it is collected as a gas

• The gas passes into the condensing tube which is surrounded by cold flowing water

• The gas is condensed into a pure solvent collected in the beaker at the bottom

• Example: separating water from seawater

Paper chromotography

• used to separate components with different affinities for the water in the paper

• they separate as the solvent moves up the paper

• can be used to investigate pigments in food colouring

State symbols

• Solid (s)

• Liquid (l)

• Gas (g)

• Aqueous (aq)

Vaporization

Liquid to gas

Condensation

Gas to liquid

Melting

Solid to liquid

Freezing

Liquid to solid

Sublimation

Solid to gas

Deposition

Gas to solid

Temperature (T)

A measure of average kinetic energy E_k of particles

Temperature change when a substance is heated

• Temperature is constant during changes of state (melting point/boiling point)

Mole

The SI base unit used to measure the amount of a substance.

Divide the number of particle by Avogadro constant to get the number of moles and the other way around.

Relative atomic mass

The ratio of the average mass of atoms of a chemical element to the atomic mass constant

Relative formula mass

Adding the relative atomic masses of all the atoms or ions present in its formula

Molar mass

The mass of one mole of a substance. Units are g mol^-1

Relationship between number of moles and mass

n = m/M where

n - number of moles

M = molar mass

m = mass in grams

Empirical formula

The simplest ratio of atoms of each element present in that compound

Molecular formula

The actual number of atoms of each element present in a molecule

Determining the empirical formula

• Convert the mass of each element to moles

• Divide by the smallest number to give the ratio

• Approximate to the nearest whole number

Converting percentage by mass to empirical formulas

• Divide the percentage mass of each element by its molar mass to convert it to moles

• Divide the moles of each element by the smallest number to give the ratio

• Approximate to the nearest whole number

Converting empirical formulas to percentage by mass

• Calculate the molar mass of the compound

• For each element, total the mass of its atoms and divide by the molar mass, then multiply by 100 to get a percentage.

• Check that the numbers add up to 100%

Avogadro’s law

Equal volumes of all gases measured under the same conditions of temperature and pressure contain equal numbers of molecules

Ideal Gas

Consists of moving particles with negligible volume and no intermolecular forces. All collisions between particles are considered elastic.

• negligible volume compared with the volume the gas occupies

• no intermolecular forces between the particles except when the molecules collide

• gas particles have a range of speeds and move randomly. The average kinetic energy of the particles is proportional to the temperature

• Collisions of the particles are elastic: kinetic energy is conserved

Pressure of a gas

The pressure of a gas is due to gas particles colliding with the walls of the container. When the volume of a gas increases, the gas particles collide less frequently with the walls as thy must travel greater distances between collisions. This decreases the pressure. When the temperature of a gas increases the gas particles have increased kinetic energy so the collisions with the walls are more energetic and frequent which increases pressure.

Real gases

Real gases deviate from the ideal gas model at low temperature and high pressure.

• volume of the gas particles is not negligible so they travel less distance between collisions with the wall making the collisions more frequent and pressure greater.

• there are attractive forces between the particles which reduces the speed of colliding particle making the pressure lower than for an ideal gas.

Molar volume of an ideal gas

The molar volume of an ideal gas is constant at. aspecific temperature and pressure.

Ideal gas equation

PV = nRT

SI units of pressure

Pa (Nm^-2, atm)

SI unit of volume

m³ (dm³, cm³)

Ion

Atom or molecule with a net electric charge due to loss or gain of one or more electron

Liquid vs aqueous

Liquid is the melted physical state of a substance

Aqueous means dissolved in water

Pressure

The amount of force exerted per unit area of a surface

Density

mass/volume

Value of PV/nRT if the volume of gas particles is not negligible

Greater than 1

Value of PV/nRT if there are attractive forces between the particles

Less than 1

STP Conditions

When the temperature is 0C 273K and the pressure is 100kPa, one mole of a gas has a volume of 2.27×10^-2m³mol^-1 or 22.7dm^3mol^-1

Relationship between volume and temperature

If the pressure is held constant, increasing the temperature increases its volume. When the straight line on a graph is extended backward it always crosses the temperature axis with the volume = 0 at -273C or 0K. (Charles law)

Relationship between pressure and temperature

If the volume is held constant, increasing the temperature of a fixed mass of gas increases the pressure. If temp is in kelvin it is a proportional relationship. This is because an increase in the temperature means the average kinetic energy of the particles will increase and thus there will be more collisions leading to a higher pressure.

Units when using pV=nRT

• pressures must be in Pa, if kPa are used multiply by 10³

• Volume must be in m³, if dm³ are given, divide by 10³, if cm³ are given, divide by 10^6

• Temperature must be in kelvin, if C is given, add 273.15.

Avogadro’s law

V/n = constant