chemistry chapter 1

what is an element, compound and mixture?

element - a substance made up of 1 type of atom (e.g magnesium is made up of only magnesium atoms and chlorine is made up of only chlorine atoms)

compound - a substance made up of 2 or more elements chemically combined in fixed proportions (have to use a chemical reaction to return them to the reactants)

mixture - different elements or compounds not chemically combined (use physical seperation techniques to separate them)

what happens to properties in elements, compounds and mixtures?

e - each element has it’s own unique properties

c - have completely different properties from the elements they’re made from

m - are a combination of the properties of their substances

what is filtration used to do?

separates an insoluable solid from a liquid

why do we have to be careful when heating during crystalisation?

some chemicals will break down when we heat them

what does crystaliation do?

separates a dissolved solid from a liquid

describe the method for simple distillation

• pour solution of liquid and dissolved solid into the flask

• which is connected to a continuous glass tube

• the tube is surrounded by a condenser, cold water runs through it to keep the glass tube cold

• there is a thermometer in the top of the glass tube

• heat liquid with bunsen burner so evaps

• turns into water vapour, heat until boils as shown on thermometer

• passes through condenser and condenses back into liquid in the beaker

• left with crystals of solid in flask and liquid in beaker

describe the method for fractional distillation

• separates a mixture of liquids with different bps

• gently heat

• both will evap but the one with the lower boiling point will rise quicker than the other

• they will enter the fractionating collum which is filled with glass beads that make the vapours condense into the flask again

• the repeated evap and condense increases the amount of the lower boiling point chemical in the fractionating collum

• they reach thermometer, temmp inc, means that a mixture of the 2 vapours is passing over it

• when the temp on the thermometer stops rising it’s the lower of the 2 boiling points

• we will now have mostly the lb chemical, which will condense and be collected in a fresh beaker

• when the thermometer is constant at the higher temp, we’re collecting a relativley pure sample of the second chemical

what was the first theory about atoms?

they are tiny spheres that can’t be divided

what was discovered in 1897?

electrons, told them that atoms must have an internal structure

what was done to challenge the plum pudding model?

ernest rutherford’s alpha particle scattering experiment

describe the experiment and what it showed

• they used a piece of gold foil (as gold can be hammered to be a few atoms thick) and fired positive alpha particles at it

• most passed straight through - told them that the atom was mostly made up of empty space

• some were deflected - told them that the center of the atom has a positive charge, as the alpha particles were positive, so they would repell when coming into contact with a positive nucleus

• some bounced straight back - told them that the center of the atom has a great mass, lead to the discovery of the nucleus and the nuclear model

describe the structure of the first nuclear model

• most of it empty spacre

• positive nucleus in the middle

• electrons around the edge

how was the nuclear model changed?

neils bhor introduced that electrons orbit the nucleus in energy levels/shells

james chadwick discovered neutrons

what do the numbers on an element tell us?

top — mass number - p+ n

bottom - atomic number - p and e

top - bottom = neutrons

what is meant by isotope abundance?

how common they are

what is the equation for calculating the relative atomic mass of an isotope?

(mass number of isotope 1 x percentage abundance of isotope 1) + (mass number of isotope 2 x percentage abundance of isotope 2) / 100

why are group 1 elements stored in oil?

to prevent them reacting with air or water

what physical properties do the group 1 elements have and how do they get them?

• soft - their outer electrons are loosley held and spread out, so the layers of atoms can slide over each other easily

• low melting and boiling points - they have weaker metallic bonding, as they can only provide 1 outer-shell electron to the delocalised ‘sea’ of electrons

• low density - they’re big atoms but they aren’t very heavy, as the atoms are spread out

• shiny when cut - fresh metal surface is exposed but reacts quickly with air and becomes dull again

describe how metallic bonding works

metallic bonding is the strong electrostatic force between positive metal ions and a sea of delocalised electrons - which is made up of the outer shell electrons that are free to move (in group one, metallic boding is weaker, as there is only 1 delocalised electron free to move)

why dont inner electrons take part in metallic bonding?

they’re held close tightly to the nucleus and unable to move

describe the physical properties of group 7 (halogens) and explain why they have them

• they’re diatomic - they’re grouped in 2s e.g Cl2

• low melting and boiling points, but they increase down the group

• reactivity decreases as you go down the group - more electron shells makes it harder to gain and electron due to weaker attraction

what is the reaction with a metal + oxygen? (important for group 1)

metal + oxygen = metal oxide

this is an oxidation reaction

what is the reaction with a metal + water? (important for group 1)

metal + water = metal hydroxide + hydrogen

what is the reaction for a metal + acid?

metal + acid = salt + hydrogen

what is a thermal decomposition reaction?

metal carbonate + heat = metal oxide + carbon dioxide

what are the 5 reactions for making salts?

acid + metal - salt + hydrogen

acid + metal oxide - salt + water

acid + metal hydroxide - salt + water

acid + metal carbonate - salt + water + CO2

acid + ammonia - ammounium salt

what are the reactions for the halogens?

halogen + metal - metal halide

displacement with a halogen + salt of a less reactive halogen

why are the group 1 metals called the alkaline metals?

when they react to form a metal hydroxide, they dissolve in water and make the solution alkaline

what do the halogens look like at room temp?

fluorine - pale yellow (g)

chlorine - green/yellow (g)

bromine - red/brown (l)

iodine - grey/black (s)

asatine - dark/metallic (s)

what are the properties of the noble gases?

• inert/unreactive - they have a full outer electron shell

• oudourless, colourless and tasteless

• low melting and boiling points - weak forces between atoms, increases as u go down

• poor conductors of heat and electricity - they don’t have delocalised electrons

• as you go down, atomic size increases

what was dobringers idea for the periodic table and why didn’t it work?

• grouped elements into triads based on similar properties

• the middle element had a mass roughly between the other 2

• it didn’t work for all elements

what was newland’s idea for the periodic table and why didn’t it work?

• ordered elemenyts by increasing atomic mass

• found that they made patterns of 8 (octaves)

• he tried to force them into these patterns even if they didn’t fit, so his work was mocked and rejected

what was medeleev’s idea for the periodic table and what did he change?

• he arranged elements by increasing atomic mass

• he left gaps for elements that hadn’t been discovered yet, which allowed him to predict properties based on the elements surrounding them through trends

• grouped elements with similar properties together even if that meant swapping them e.g Iodine (I) has a smaller atomic mass than tellurium (Te), but Mendeleev put iodine after tellurium because it fits better with the halogens

• elements like gallium and germanium had the properties he predicted

what did they change in the modern periodic table?

they arranged it by increasing atomic number rather than atomic mass as they discovered that atomic number defins what element an atom is, which explained why medeleev had to swap things around

what is the formula for an group 1 metal’s reaction with chlorine?

metal + chlorine = metal chloride

what are the observable changed during reactions with postassium, lithium and sodium

p - burns with a lilac flame, burns quicky, fizzes

s - orange flame, floats, melts into a ball

l - red flame, moves on surface

what is the half equation for oxidation?

Na - Na+ + e-

oxidation is loss of electrons, as it forms a neutral Na atom

what is the half equation for a reduction reaction?

Cl2 + 2e- - 2Cl-

reduction because electrons have been gained to make a negative ion

what is a simple rule for noticing if a reaction is reduction or oxidation?

on its own - in a compound = oxidised

in a compound - on its own = reduced