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topic 4-chemical changes (strong acids and weak acids (strong acids ionize…
topic 4-chemical changes
Acids and Bases
pH scale goes from 0 to 14
lower the pH solution the more acidic it is
the higher the pH of a solution the more alkaline it is
a neutral substance has pH 7
measure pH of solution
indicator
changes color depending on whether its above or below a certain pH (universal indicator)
pH probe
can be used to measure pH electronically when it is placed in a solution giving a numerical value - more accurate than indicator
acids and bases neutralize each other
an acid is a substance that forms aqueous solutions with a pH of less than 7. form
H+ ions in water
a base is a substance which has a pH greater than 7
an alkali is a
base that dissolves in water
. form OH- ions in water
acid + base---> salt + water
H+ + OH- ---> H2O
titrations
method of analyzing the concentration of solutions
If the sample is a solid, it is weighed using an accurate balance, and then dissolved to make up a known volume of solution (usually 100cm3).
A pipette is used to measure accurately a volume of this solution - for example, 10cm3. A safety pipette filler is used to draw solution into the pipette. This is emptied into a conical flask.
A few drops of an indicator may be added to the conical flask. This will show a change of colour when the titration is complete.
A second chemical is placed in a burette. This other solution is of a chemical that will react with the synthesised chemical sample in the conical flask. Often the solution in the burette is an acid or alkali, and it must be of a precise, known concentration.
The solution from the burette is run into the conical flask. The solution is added one drop at a time, with swirling to mix the solutions as the end-point is approached. Eventually, a colour change shows that the correct amount has been added to react completely with the synthesised chemical in the sample.
The volume of solution added from the burette is noted. The titration results can then be used to calculate the amount of the synthesised chemical in the sample, and therefore find its purity.
indicators: litmus or methyl orange
strong acids and weak acids
strong acids ionize completely in water, all particles dissociate to release H+ ions
weak acids do not fully ionize, only small proportion of acid particles dissociate to produce H+ ions
the ionization of a weak acid is a reversible reaction
If concentration of H+ ions is higher, rate of reaction will be faster = strong acids are more reactive than weak ones at same concentration
for every decrease of 1 on the pH scale the concentration of H+ increases by a factor of 10
pH of an acid or alkali is a measure of concentration of H+ ions in the solution
H+ ions change by = 10-x
acid strength tells you what proportion of the acid molecules ionize in water
concentration measures how much acid there is in a certain volume of water (eg squash-has water)
reactions of acids
metal oxides and metal hydroxides are bases
some dissolve (soluble) in water -
alkalis
all bases take part in neutralization of acids
Acid + metal oxide-----> salt + water
acid + metal hydroxide----> salt + water
acids and metal carbonates produce CO2
acid + metal carbonate ----> salt + water + carbon dioxide
metal carbonates are also bases
you can make soluble salts using an insoluble base
reactivity series
lists metals in the order of their reactivity
reactivity is determined for metals by how easily they lose electrons- forming positive ions
potassium, sodium, lithium, calcium, magnesium, carbon, zinc, iron, hydrogen, copper
how metals react with water tells u about reactivity
acid + metal------>salt + hydrogen
the more reactive the metal, faster hydrogen will be given off.
metals react with water
metal + water-----> metal hydroxide + hydrogen
more reactive metals will react, less reactive ones wont
separating metals from metal oxides
metals often have to be separated for their oxides
lots of metals (iron and aluminum) react with oxygen to form an oxide
an reaction that separates an oxide from its ore is
reduction
some metals can be extracted by carbon
extracted chemically by reduction using carbon
the ore is reduced as oxygen is removed from it and carbon gains oxygen so is oxidised
the position of the metal determines whether it can be reduced by carbon
metals higher than carbon use electrolysis
metals below carbon can be reduced by carbon
carbon can only take oxygen away from metals which are less reactive than itself
some metals are so unreactive they are found in the earth as the metal itself
redox reactions
if electrons are transferred its a redox reaction
a loss of electrons is called oxidation
a gain of electrons is called reduction
reduction and oxidation happen at the same time
displacement reaction are often redox
a more reactive metal will displace a less reactive metal
its always the metal ion that gains electrons and is reduced. The metal atom always loses electrons and is oxidized
ionic equations
only the particles that react and the products formed are shown
spectator ions= ions that are not oxidized or reduced
spectator ions are left out of an ionic equation
electrolysis
means splitting up with electricity
during electrolysis an electric current is passed through electrolyte. the ions move to electrodes where they react and the compound decomposes
electrolyte= molten or dissolved iconic compound
the
positive ions
move to the cathode (negative) and are reduced (gain electrons)
the
negative ions
move to the anode (positive) and are oxidized (lose electrons)
this creates a flow of charge through the electrolyte as the ions travel to electrodes
as ions gain or lose electrons they form an uncharged element and are discharged from the electrolyte
electrolysis of molten ionic solids form elements
an ionic solid cant be electrolyzed because ions cant move
molten ionic compounds can be electrolyzed because their ions move freely and conduct electricity
metals can be extracted
1) aluminum is extracted from ore bauxite using electrolysis
2) aluminum oxide has a very high melting temp so its mixed with cryolite to lower it
3) the molten mixture contains free ions so itll conduct electricity
4) the positive Al3+ ions are attracted to cathode where they pick up 3 electrons and turn into neutral aluminium atoms. then they sink to bottom of tank
5) negative O2- ions are attracted to anode where they lose 2 electrons. neutral molecules combine to form O2 molecules
electrolysis of aqueous solutions
there are ions from the compound, hydrogen and hydroxide ions from water
at the cathode hydrogen is produced if the metal ions form a elemental metal that is more reactive than hydrogen if less reactive a solid layer pure metal is produced
at the anode if halide ions are present a halide will be formed (chlorine, bromine etc). if none are present the oxygen will be formed
half equations
show reactions at each electrode