Transcript Chemistry C2 Part One

Chemistry C3
Part One
Revision PowerPoint - Big ideas
The Periodic Table
organising elements into groups
Newlands and Mendeleev
• Newlands listed the elements in order of
atomic weights. He spotted repeating
patterns (octaves) in behaviour
• Mendeleev arranged elements into
groups and periods to fit repeating
patterns. He left gaps for undiscovered
elements.
Modern Periodic Tables
• Modern Periodic Tables arrange
elements by increasing atomic
number.
• The number of electrons in the
highest energy level (outermost
shell) indicates the group number
of the element.
Group 1 - The Alkali Metals
• Elements in Group 1 (The Alkali Metals)
are soft, low density metals.
• Alkali metals react rapidly with water,
forming alkalis & hydrogen.
• Alkali metals form ionic compounds
with non-metals.
• Group 1 ions have the charge +1
Reactivity down the group
• Group 1 elements become more reactive
further down the group.
• Large atoms lose electrons from their
outer shells more easily because they
are further from the positive nuclear
charge force and shielded by more inner
shells of electrons.
Group 7 – The Halogens
• Elements in Group 7 (The Halogens) are
coloured non-metals.
• More reactive halogens displace less
reactive halogens from solutions
containing halide ions,
e.g. Cl2 + 2Br– → Br2 + 2Cl–
• Group 7 ions have a charge of –1
Chemistry C3
Part Two
Revision PowerPoint - Big ideas
Hard water
Causes of hard water
• Hard water is caused by calcium and
magnesium ions dissolving when acidic
rainwater flows through rocks.
• Temporary hard water is caused by
calcium hydrogencarbonate, can be
removed by boiling the water.
• Permanent hardness caused by calcium
sulphate isn’t removed by boiling.
Softening and desalinating
• Washing soda and ion-exchange
resins can soften hard water.
• Seawater can be desalinated by
distillation which needs a lot of
energy
Tap water treatment
• Water is filtered to remove solids
and sterilised with chlorine to kill
microbes.
• Dissolved substances are removed
by specialised filters or by ion
exchange.
Chemistry C3
Part Three
Revision PowerPoint - Big ideas
Chemical energy
and calorimetry experiments
Calorimetry
• In calorimetry the energy
released from a chemical reaction
is transferred to water.
• The energy transferred,
E = m x c x ∆T
Calorimetry
• Energy level diagrams show the
change in chemical energy as
reactants change into products.
• Energy is released when new
chemical bonds form.
• Energy is required to break
chemical bonds.
Chemical energy diagrams
• In an exothermic
reaction chemical
energy in the
reactants in
transformed into
thermal energy
• The surroundings
become warmer
Chemical energy diagrams
• In an endothermic
reaction thermal
energy is taken in
from the
surroundings and is
stored as chemical
energy in the
products
• The surroundings
cool down
Activation energy
• Catalysts reduce
the minimum
amount of energy
needed to start a
reaction
• This is called the
activation energy
Hydrogen fuel
• Hydrogen releases energy when it
reacts with oxygen in combustion
or in fuel cells.
2H2 + O2 → 2H2O
• Hydrogen burns exothermically
Chemistry C3
Part Four
Revision PowerPoint - Big ideas
Chemical Analysis
Flame tests 1
• Colours used in flame tests
are used to identify Group 1
ions.
• Each salt produces a
characteristic flame colour:
• Lithium (Li+) – magenta
• Sodium (Na+) – golden yellow
• Potassium (K+) – lilac
Flame tests 2
Other metal ions also produce
coloured flames.
• Calcium (Ca2+) – brick red
• Barium (Ba2+) – apple green
• Copper (Cu2+) - green with
blue flashes
Precipitation reactions
Precipitation reactions are used to identify other metal
ions. Test the solutions of the salts with sodium
hydroxide solution.
A coloured precipitate of a metal hydroxide forms.
light blue jelly ppt.
copper (II) ions, Cu2+
dirty green jelly ppt.
iron (II) ions, Fe2+
rusty red- brown jelly ppt.
iron (III) ions, Fe3+
White hydroxide precipitates?
When you test solutions of the salts with sodium hydroxide
solution, sometimes the precipitate is white.
This is how to distinguish between magnesium, calcium and
aluminium ions …
Add extra (excess) sodium hydroxide
solution to the white precipitate.
The precipitate dissolves
in excess sodium hydroxide
= aluminium ions
The precipitate does not
dissolve in excess sodium
hydroxide
= calcium or magnesium ions
Sulphate test
• Energy level diagrams show the
change in chemical energy as
reactants change into products.
• Energy is released when new
chemical bonds form.
• Energy is required to break
chemical bonds.
Carbonate test
• Energy level diagrams show the
change in chemical energy as
reactants change into products.
• Energy is released when new
chemical bonds form.
• Energy is required to break
chemical bonds.
Chloride bromide iodide test
• Test the solution of the salt with silver
nitrate solution, acidified by nitric acid
silver iodide ppt. is yellow
silver bromide ppt. is cream
silver chloride ppt. is white
Chemistry C3 Part Five
A = Acid
B = Base (alkali)
Revision PowerPoint - Big ideas
Acid-alkali titrations
volumetric analysis of acids & alkalis
Apparatus
• Titrations involve
reactions between
solutions of acids
and solutions of
alkalis
• Alkalis are delivered
using pipettes
• Acids are delivered
using burettes
Solution concentration
The concentration of a solution depends on
the mass of the solute dissolved in a certain
volume of water.
Mass is measure in grams, volumes in dm3
Concentration = Mass of solute, in g
in g/dm3
Volume of solution, in dm3
Molar concentration
The concentration of a solution depends on
the number of moles of the solute dissolved
in a one decimetre cubed of solution.
Concentration, c = m ÷ Mr
in mol/dm3
V
m = mass of solute in grams
Mr = molar mass of the solute in g/mol
V = volume of solution in dm3
How many moles are delivered?
The number of moles, nA, of acid delivered
during the titration depends upon:
• the volume of acid, VA added
to the flask
• the molar concentration
cA of the solution added
to the flask
nA = cA x VA
Titration question
20cm3
The concentration
of the acid is
unknown
25cm3
0.2mol/dm3
HCl + NaOH → NaCl + H2O
1 mole
1mole
Calculation to find acid concentration
ACID, A
nA = cA x VA
1 = cA x 20
ALKALI (Base), B
nB = cB x VB
1 = 0.25 x 25
20 x cA = 0.25 x 25
cA = 0.25 x 25
20
cA = 0.31 mol/dm3
Indicators and colour changes
Indicators switch their colours at certain pH’s
Match the type of titration with the most suitable
indicator using the equivalence point pH.
Equivalence points and pH
This titration graph shows
what happens to the pH
when a strong alkali is slowly
added to a strong acid. Use
methyl orange.
This titration graph shows
what happens to the pH
when a strong alkali is slowly
added to a weak acid. Best
indicator = phenolphthalein
Chemistry C3
Part Six
Revision PowerPoint - Big ideas
The Haber Process
making ammonia from its elements
Ammonia NH3
• Ammonia is manufactured when nitrogen
gas reacts with hydrogen gas – the Haber
Process
• Nitrogen - N2 is obtained from air
• Hydrogen is obtained from natural gas
(methane) reacting with steam
CH4 + H2O → CO + 3H2
Faster reaction
• Nitrogen and hydrogen gases are mixed
and passed over a catalyst of iron filings
• The catalyst speeds up this slow reaction
• Iron filings have a larger surface area
(better than a block of iron)
• A reversible exothermic reaction
takes place
N2 + 3H2 ↔ 2NH3 + heat ∆H = –92 kJ/mole
Conditions for the reaction
• Ammonia is being
produced by the forward
reaction
• Ammonia is broken down
by the backward reaction
into nitrogen and
hydrogen
• Conditions are chosen to
produce a reasonable
yield of ammonia as
quickly as possible
Temperature considerations 1
N2 + 3H2 ↔ 2NH3 + heat ∆H = –92 kJ/mole
• Raise the temperature of the reaction and
molecules of gas will have more kinetic
energy, move faster and collide more
violently.
• Violent collisions lead to old chemical bonds
breaking – but be careful, ammonia
molecules may decompose faster too!!
• 450℃ is a good compromise for this
exothermic reaction – you get a reasonable
yield of ammonia at a reasonable rate.
Temperature considerations 2
Fast rate
High yield
Slow rate
350
450℃
Low yield
450
Temperature in ℃
550
Rate of the ammonia reaction
Yield of ammonia
N2 + 3H2 ↔ 2NH3 + heat
∆H = –92 kJ/mole
450℃ is the optimum
temperature for a
reasonable rate and a
reasonable yield.
You can’t have a fast rate
and a high yield.
Use the Goldilocks
principle … not too hot …
not too cold … but just
right! 450℃
Pressure considerations 1
1N2 + 3H2 ↔ 2NH3 + heat
∆H = –92 kJ/mole
• Notice that the number of molecules on the
left hand side of the equation is 3+1 = 4 and
there are 2 molecules of ammonia on the
right.
• For all reversible reactions involving gases an
increase in pressure effects the equilibrium
position favouring the reaction that
produces fewer molecules
Pressure considerations 2
1N2 + 3H2 ↔ 2NH3 + heat
∆H = –92 kJ/mole
• So, an increase in the pressure shifts the
equilibrium to the right and increases the
yield of ammonia
• A high pressure of 100 - 200 atmospheres is
chosen for the Haber process
• Using higher pressures adds to building costs
(walls for reaction chambers will be thicker)
and running costs (faster electric pumps)
Chemistry C3
Part Seven
Revision PowerPoint - Big ideas
Reversible reactions
Reversible reactions 1
These are chemical reactions which can
proceed in two directions
• the forward reaction changes the
reactants into product
e.g. NH3 (g) + HCl (g) → NH4Cl (s)
• the product can break down to recreate the original reactants
e.g. NH4Cl (s) → NH3 (g) + HCl (g)
Reversible reactions 2
The equations for reversible reactions contains
the two-way arrow reaction sign
e.g. NH3 (g) + HCl (g) ↔ NH4Cl (s)
By changing the reaction conditions we can
favour either the forward reaction or the
reverse reaction.
Other reversible chemical reactions include the
decomposition of limestone:
CaCO3 (s) ↔ CaO (s) + CO2 (g)
Dynamic chemical equilibrium 1
Ammonia and hydrogen
chloride gases can react in
a closed system such as
the beaker in the diagram.
Nothing can get in &
nothing can escape.
Over time the reaction
reaches dynamic chemical
equilibrium. The forward
and backward reactions
are both happening.
Dynamic chemical equilibrium 2
At this point of dynamic
chemical equilibrium, the
rate of the forward
reaction and the rate of
the backward reaction are
exactly equal.
The amount of each
substance remains
constant and the forward
and backward reactions
continue to proceed.
Y
X
At X the heated ammonium
chloride solid decomposes.
NH4Cl (s) → NH3 (g) + HCl (g)
At Y the cooled gases
combine together to form
solid ammonium chloride.
NH3 (g) + HCl (g) → NH4Cl (s)
Chemistry C3
Part Eight
Revision PowerPoint - Big ideas
Organic Chemistry
alcohols, carboxylic acids and esters
Alcohols CnH2n+1OH
• Alcohols, such as ethanol, are a
family (homologous series) of
compounds containing the –OH
functional group
• They make excellent fuels (e.g.
bioethanol is a petrol substitute)
• They are used as solvents and fuels
• They mix with water easily (whisky)
Carboxylic acids (e.g. vinegar)
• Ethanol is oxidised to ethanoic acid.
This happens when we leave wine
open to the air.
• Carboxylic acids have a –COOH
functional group
• Carboxylic acids are weak acids.
Acid molecules are partially ionise.
They release small quantities of H+
ions into the water, pH = 4
Carboxylic acids are weak acids
• The vinegar = pH3 to 4
Vinegar is a weak, partially
ionised acid
• Hydrochloric acid = pH0 to 1
HCl is 100% dissociated into ions
and it is a strong acid
Carboxylic acid reactions 1
• Carboxylic acids react with
carbonates like marble chips to form
carbon dioxide.
CaCO3 + 2H+ → CO2 + H2O + Ca2+
• The reaction is slow because they
are weak acids.
Carboxylic acid reactions 2
• Carboxylic acids react with alcohols
in the presence of an acid catalyst to
form esters
• Esters are sweet-smelling liquids
found in perfumes and fruits like
oranges and strawberries
• Esters also give some fruits a special
flavour
Esters – nice niffs & flavours
• The ester ethyl ethanoate has the
smell of pears
• It is made when ethanol reacts with
ethanoic acid in the presence of an
acid catalyst
• Esters contain the –COO- functional
group.