Chemical changes Flashcards
What is the pH scale
a measure of how acidic or alkaline a substance is
the lower the pH scale..
the more acidic it is
the higher the pH scale…
the more alkaline it is
What is the pH of a neutral substance
pH 7
pH of car battery acid and stomach acid
car battery acid = 1
stomach acid = 2
pH of vinegar , lemon juice
3
pH of acid rain
4
pH of normal rain
5/6
pH of pure water
7
pH of washing up liquid
9
pH of pancreatic juice
10
pH of soap powder
11
pH of bleach
12
pH of caustic soda(drain cleaner)
13/14
What is an indicator
a dye that changes colour depending on wether it is above or below a certain pH
What are wide range indicators
contain a mixture of dyes so gradually change colour over a broad range of pH
what are wide range indicators useful for
estimating the pH of a solution
An example of a wide range indicator
Universal indicator
What colours does a universal indicator have on its pH scale
-classic colours
-red for acid goes yellower then to green for neutral
-then darker to blues and purples
What is a more accurate way to measure pH than an indicator
pH probe attached to a pH meter
-as it gives it as a numerical number rather than a colour
What does a pH probe attached to a pH meter do
-be used to measure pH electronically
-the probe is placed in the solution u are measuring and the pH is given on a digital display as a numerical number
What is acid as a substance
-a substance that forms aqueous solutions with a pH of less than 7
What ions do acids form in water
H+ ions
what is a base
a substance with a pH greater than 7
What is an alkali
a base that dissolves in water to form a solution with a pH greater than 7
what ions do alkalis form in water
OH- ions
What is the reaction between acids and bases called
neutralisation
What is the word equation for a neutralisation reaction
acid+base —> salt + water
What is the symbol equation for a neutralisation reaction
H+ (aq) + OH- (aq) —> H2O (l)
what are the products of a neutralisation reaction
the products are neutral ,have a pH 7
how can we know if a neutralisation reaction is over
an indicator can be used
What do acids do in aqueous equations
-they ionise
-they produce hydrogen ions H+
What do strong acids do in water
-ionise completely
-all acid particles dissociate to release H+ ions
Examples of strong acids
sulfuric, hydrochloric and nitric acids
What do weak acids do in water/solution
-do not fully ionise
-only a small proportion of acid particles dissosiate to release H+ ions
Examples of weak acids
-ethanoic, citric and carbonic acids
What type of reaction is the ionisation of a weak acid
- a reversible reaction which sets up an equilibrium between the undissociated and dissociated acid
Where does the position of equilibrium lie of the ionisation of a weak acid
since only a few of the acid particles release H+ ions
-the position of the equilibrium lies well to the left
What do the reactions of acids involve
the H+ ions reacting with other substances
If the concentration of H+ ions is higher…
-the rate of reaction will be faster
-so strong acids will be more reactive than weak acids of the same concentration
What is the pH of an acid or alkali based off
the measure of the concentration of H+ ions in the solution
what happens for every decreased of 1 on the pH scale
the concentration of H+ ions increases by a factor of 10
Compare the pH of a strong and weak acid
the pH of a strong acid is always less than the weak acid if they have the same concentration
So what is acid strength
tells u what proportion of the acid molecules ionise in water
But what is the concentration of an acid
DIFFERENT to strength
-measures how much acid there is in a certain volume of water
the larger the amount of acid in a certain volume of water…
the more concentrated the acid is
So what combinations of strength and combination for acids can u have
can have a dilute but strong acid
-or a concentrated but weak acid
What will happen to the pH with increasing acid concentration
the pH will decreased
-regardless of wether it is a strong or weak acid
What are metal oxides and metal hydroxides
-soluble compounds which are alkalis
-dissolve in water
what do bases that don’t dissolve in water do with acids
can still take part in neutralisation reactions with acids
So what can all metal oxides and metal hydroxides react with
-acids
to form a salt and water
word equation for metal oxides or metal hydroxides reacting with acids
acid+metal oxide –> salt+ water
acid+ metal hydroxide–> salt+water
what salt is produced with the reaction of metal oxides or metal hydroxides reacting with acids
the salt produced depends upon the acid and the metal ion in the oxide or hydroxide
Example of the salt produced with sulfuric acid and potassium hydroxide
potassium sulfate is made
Example of the salt produced with hydrochloric acid and copper oxide
copper chloride
What are metal carbonates
are also bases
What do metal carbonates produce in a reaction with acid
-salt
-water
-carbon dioxide
word equation for metal carbonates reacting with acid
acid+ metal carbonate –> salt+ water+ carbon dioxide
How can u make soluble salts
using an insoluble base and acid
What is the reactivity series
-lists metals in order of their reactivity towards other substances
How are metals reactivity determined
is determined by how easily they lose electrons -forming positive ions
So the more easily a metal forms positive ions…
the higher a metal is on the reactivity series
What is the order further based off
-if u compare the relative reactivity of dif metals with either an acid or water
-then put them in order from the most reative to least
then u get the reactivity series
What do some metals react with acids to produce
a salt and hydrogen gas
equation for acid reacting with metal
acid+metal –> salt+hydrogen
What is the speed of reacton indicated by
-the rate at which the bubbles of hydrogen are given off
What does copper not react with
won’t react with cold dilute acids
examples of very reactive metals
-potassium,sodium,lithium, calcium all react expolsively
examples of less reactive metals
magnesium,xinc and iron react less violently
which non metals are included for reference in the reactivity series
-carbon
-hydrogen
How can u investigate the reactivity of metals
-by measuring the temperature change of the reaction with an acid or water over a set time period
-the more reactive the greater the temp change
What do the reactions of metals with water also show
the reactivity of metals
What metals will all react with water
potassium ,sodium, lithium and calcium
Which less reactive metals will not react with water
zinc,iron and copper
equation for metal reacting with water
metal+ water –> metal hydroxide+ hydrogen
How to get metals from the earth
have to extract them from a compound
What are the 2 different ways to extract a metal
-reduction
-electroylsis
What are lots of metals found as in ores
-oxides (process from oxidation, react with oxygen)
-seen with iron and aluminium
What is the reaction that seperates a metal from its oxide
called a reduction reaction
Formation of a metal ore
oxidation=gain of oxygen
equation of oxidation
2Mg + O2 –> 2MgO
Extraction of metal
Reduction= loss of oxygen
Equation for reduction
2CuO + C –> 2Cu + CO2
How can other metals be reducted
can be extracted from their ores chemically by reduction using carbon
What happens to the carbon in a reduction reaction
the carbon is oxidised
When can metals not be reducted with ion
-the metals higher than carbon in the reactivity series have to be extracted using electrolysis
which is expensive
When can metals be extracted by reduction
-the metals below carbon in the reactivity series can be extracted by redcution using carbon
What are the 3 metals that can be extracted with carbon
-Zinc
-Iron
-Copper
Why can carbon only be used for those metals
as carbon can only take oxygen away from metals which are less reactive than carbon itself
How are unreactive metals (eg.native metals) extracted from earth
-so unreactive that they are in the earth as the metal itself
-eg. gold is mined as its elemental form
What is oxidation in electrons
loss of electrons
What is reduction in electrons
gain of electrons
mnemonic for oxidation and reduction w/ electrons
OIL RIG
What are redox reactions
Reduction and Oxidation happen at the same time
Oxidation written example
Fe - 2e- –> Fe2+
Reduction written example
2H+ + 2e- –> H2
What are displacement reactions
a more reactive metal will displace a less reactive metal from its compound
What happens if u put a reactive metal into the solution of a dissolved metal compound
the reactive metal will replace the less reactive metal in the compound
In a displacement reaction what is oxidised and reduced
-the metal ion that gains electrons and is reduced
-the metal atom always loses electrons and is oxidised
What is shown in an ionic equation
-only the particles that react and the products they form are shown
What are spectator ions
-ones that don’t change in the reaction
-no interest here so can be crossed out
So what does the ionic equation for displacement concentrate on
the substances that are being oxidised or reduced
What does electrolysis use to cause a reaction
an electrical current
What happens during electrolysis
an electric current is passed through an electrolyte
-the ions move towards the electrodes,where they react, and the compound decomposes
What is an electrolyte
a molten or dissolved ionic compound
What happens to the positive ions in the electrolyte
they will move towards the cathode(- electrode) and gain electrons (they are reduced)
What happens to the negative ions in the electroylte
will move towards the anode ( + electrode) and lose electrons (they are oxidised)
so what is the charge through the electroylte
a flow of charge as ions travel to the electrodes
What happens as ions gain or lose electrons (electrolysis)
they form the uncharged element and are discharged from the electrolyte
Why can’t an ionic solid be electrolysed
because the ions are in fixed positions and can’t move
Why can molten ionic compounds be electrolysed
because the ions can move freely and conduct electricity
What are molten ionic liquids made of
are always broken up into their elements eg. lead bromide
Equation for a positive metal ions being reduced to the element at the cathode
Pb2+ + 2e- —> Pb
Equation for a negative non metal ions being oxidised to the element at the anode
2Br- —> Br2 + 2e-
Why is electrolysis an expensive method
as lots of energy is required to melt the ore and produce the required current
What is Aluminium extracted from
the ore bauxite by electroylsis
the ore contains aluminium oxide
What is aluminium’s melting temp
very high
How are high melting points reduced so there is a cheaper electrolysis
mixed with cryolite to lower the melting point
What are the positive Al3+ ions attracted to (electrolysis)
-attracted to the negative electrode
-where they each pick up 3 electrons and turn into neutral aluminium atoms
-sink to the bottom of tank
What are the negative O2- ions attracted to
-the positive electrode
-where they each lose 2 electrons
-the neutral oxygen atoms will then combine to form O2 molecules
Photo of electrolysis with aluminium oxide
What will there be in an aqueus solution (electrolysis)
-as well as the ions from the ionic compound there will be hydrogen ions (H+) and hydroxide ions (OH-) from the water
What will happen at the cathode if H+ ions and metal ions are present
hydrogen gas will be produced if the metal ions form an elemental metal that is more reactive than hydrogen
What will happen at the cathode if H+ ions and less reactive metal ions are present
-if the metal ions form an elemental metal that is less reactive than hydrogen a solid layer of the pure metal will be produced instead
What will happen at the anode if OH- and halide ions are present
-molecules of chlorine, bromine and iodine will be formed
-if no halide ions are present then the OH- ions are discharged and oxygen will be formed
Test for chlorine
Chlorine bleaches damp litmus paper turning it white
Test for hydrogen
hydrogen makes a squeaky pop with a lighted splint
Test for oxygen
oxygen will relight a glowing splint
What do half equations show
the reactions at the elctrodes
-the number of electrons need to be the same for each one
Negative electrode half equation for aqueaus sodium chloride
2H+ + 2e- —> H2
Positive electrode half equation for aqueaus sodium chloride
2Cl- - 2e- —> Cl2
Why are the electrons on either side of the half equation balanced
so they can be canclled out in the full ionic equation
When halide isn’t present in the aqueaus solution the half equation for the anode is
4OH- –> O2 + 2H20 + 4e-
