reactivity 3.2 Flashcards
how do metals (groups 1 & 2) change down a group (in relative ease of oxidation)?
the relative ease of oxidation increases
halogens are ______ agents. why?
oxidising agents, because they react with metals by accepting an e- from the metal atom
how does oxidising power of halogens change down a group? why?
decreases, halogens get less reactive
when does halogen displacement occur?
when a more reactive halogen displaces a less reactive halogen from an aqueous solution of its halide
what is the reactivity series (aka activity series)?
the ranking of metals in order of reactivity
what is a metal displacement reaction?
when metals higher in reactivity displace less reactive metals from their compounds in solutions or from their oxides
in a metal displacement reaction, which metal is the reducing agent? how does this determine ranking?
the more reactive metal, as this allows metals to be ranked from most reactive (strongest reducing agents) to least reactive
how can the reactions of acids and metals can be written as ionic equations?
showing only the species that has changed in the reaction
eg.
2HCl (aq) + Zn (s) → ZnCl2 (aq) + H2 (g)
turns into
2H+ (aq) + 2Cl– (aq) + Zn (s) → Zn2+ (aq) + 2Cl– (aq) + H2 (g)
turns into
2H+ (aq) + Zn (s) → Zn2+ (aq) + H2 (g)
define voltaic cells
a cell that generates a potential difference known as an electromotive force (EMF)
EMF is also known as ____. symbol representing this is __
cell potential, italicized E
voltaic cells are also known as _______ cells
galvanic
how can the absolute value of a cell’s potential be determined?
with the difference between one cell and another
think of it like arm-wrestling: you cannot determine the strength of an arm-wrestler unless you compare them to the other competitors
how do voltaic cells generate electricity?
from spontaneous redox reactions
Zn (s) + CuSO4 (aq) → Cu (s) + ZnSO4 (aq)
instead of electrons being transferred directly from the zinc to the copper ions, a cell is built which separates the two redox processes. what is each part called?
a half-cell
what is the purpose of dipping a rod of metal into a solution of its own ions?
to set up an equilbrium
look at sme notes primary cells for zinc example!! very important
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how is an EMF established?
if two different electrodes are connected, the potential difference between the two electrodes will cause a current to flow between them
how is a typical electrochemical cell made?
by combining a zinc electrode in a solution of zinc sulfate with a copper electrode in a solution of copper sulfate
(called a zinc-copper voltaic cell (AKA daniell cell)
(look at sme for this visual)
what allows ions to flow from one solution to another in a volatic cell? (what is it called & made of?)
a salt bridge, usually a piece of filter paper saturated with a solution of an inert electrolyte
what can EMF be measured with?
voltmeter
the two half cells are said to be __ _____ as the same current is flowing through both cells
in series
shorthand convention for electrochemical cell representation:
solid vertical (or slanted) line
shows a phase boundary (an interface between a solid and solution
shorthand convention for electrochemical cell representation:
double vertical line (sometimes seen as dashed vertical lines)
salt bridge
what are examples of chemical solutions used to make a salt bridge? x2, why? x1
potassium chloride
potassium nitrate
used because chlorides and nitrates are usually soluble
why is it necessary for the solution used to form a salt bridge to be soluble?
ensures that no precipitates form, as they could affect the equilibrium position of the half cells
which substance in each half cell is drawn next to the salt bridge?
highest oxidation state
how is cell potential difference shown?
with the polarity of the right-hand electrode
what can be deduced from this?
Zn (s)∣Zn^2+ (aq) ∥Cu^2+ (aq)∣Cu (s)
E cell = +1.10 V
that the copper half-cell is more positive than the zinc half-cell so that electrons would flow from the zinc to the copper
review fuel cells, esp h-o
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benefits of hydrogen-oxygen fuel cells x3
water is the only product (obvi more environmentally safe than other fuel cells, such as how other produce harmful nitrous oxides)
all the bond energy is converted into electrical energy instead of heat and light because it is takes place at room temp
have use on spacecrafts, supplies drinking water
risks & problems of hydrogen-oxygen fuel cells x4
hydrogen gas is highly flammable
expensive to store H gas safely & generally expensive
production of H is a by-product of crude oil industry (reliant on finite resource)
H has high energy density, but is a gas so E density per unit volume is low (needs large containers compared to liquid fuels)
how do electrolytic cells drive chemical reactions using electrical energy?
an electric current reverses the normal directions of chemical change and this is non-spontaneous
in electrochemical cells, where do oxidation and reduction take place?
oxidation takes place at the anode and reduction will occur at the cathode
in voltaic cells the anode is +/-?
in electrolytic cells the anode is +/-?
in voltaic cells the anode is negative
in electrolytic cells the anode is positive
how do secondary/rechargeable cells work?
they employ chemical reactions which can be reversed by applying a voltage greater than the cell voltage, causing electrons to push in the opposite direction
3 common examples of secondary cells
lead-acid batteries
nickel-cadmium / NiCad cells
lithium cells
composition of lead-acid batteries
consist of six cells joined together in series
how do lead-acid batteries work?
cells use lead metal as the negative electrode and lead(IV) oxide as the positive electrode
what electrolyte is used in lead-acid batteries?
sulfuric acid
lead acid batteries generate an EMF of about __ V, making the combined 6 cells give a combined voltage of about __ V
2, 12
what are lead-acid batteries used for? where are they often utilised?
designed to produce a high current for a short period of time
powering a starter motor in car engines
disadvantages of lead-acid batteries x3
very heavy
contain toxic materials: lead and lead(IV) oxide
sulfuric acid electrolyte is very corrosive
in NiCad cells, the ______ electrode consists of cadmium and the ______ electrode is made of a nickel(II) hydroxide-oxide system
negative, positive
disadvantages of NiCad cells x2
expensive
cadmium is a toxic metal
what are lithium-ion cells used for?
powering electronic devices
why is lithium used in lithium-ion cells?
it has a very low density and relatively high electrode potential
what makes up a lithium-ion cell? x3
a positive lithium cobalt oxide electrode
a negative carbon electrode
a porous polymer membrane electrolyte
what is the advantage to using a porous polymer membrane in lithium-ion cells?
the polymer electrolyte cannot leak since it is not a liquid or paste
EMF generated by lithium-ion cells, X to X
3.5 V and 4.0 V
what is the composition of a lithium-ion cell?
the cell consists of a sandwich of different layers of lithium cobalt oxide and carbon
how do lithium ions flow within the cell?
when the cell is charged and discharged the lithium ions flow between the negative and the positive through the solid electrolyte
what is the memory effect of NiCad cells?
they gradually begin to lose their charge after repeated charge cycles when the cell is not fully discharged
the cells appear to ‘remember’ their lower state of charge
problems with lithium-ion cells x2
current demand of lithium exceeds supply
lithium-ion cell fires, as it is a very reactive element in group 1 (high electrode potential)
example uses of electrolysis x4
purifying copper
plating metals with silver and gold
extracting reactive metals, such as aluminium
making chlorine, hydrogen and sodium hydroxide
how can electrolytic cells be constructed?
using a beaker or crucible as the cell depending on whether the ionic compound is in solution or molten
watch a video on electrolytic cells
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in electrolysis, the substance that the current passes through and splits up is called the _______
electrolyte
what does an electrolyte contain? x2
positive ions
negative ions
what happens to negative ions during electrolysis?
move to the anode & lose electrons (oxidation)
what happens to positive ions during electrolysis?
move to the cathode and gain electrons (reduction)
what happens to electrically neutral atoms/molecules during electrolysis?
they are released
since metals are always cations and non-metal anions, it is easy to predict the products of electrolysis of molten salts:
where will metals be formed? where will non-metals be formed?
metals will always be formed at the cathode and non-metals at the anode
define oxidising agent
a substance that oxidises another atom or ion by causing it to lose electrons. gets reduced
define reducing agent
a substance that reduces another atom or ion by causing it to gain electrons. gets oxidised
oxidation number decreasing = ?
is reduced, gaining electrons
what is needed for a reaction to be considered a redox reaction?
there must be both an oxidising and reducing agent
5 steps to balancing a redox equation
- identify the atoms which change in ox. no.
- deduce the oxidation number changes
- balance the oxidation number changes
- balance the charges
- balance the atoms
in redox titrations, the _______ agent is titrated against the ________ agent
oxidising, reducing
what are the 2 main redox titration examples that you need to know?
manganate(VII) acid against iron ions
iodine and thiosulfate titrations
8 rules for determining oxidation states
in ionic compounds, ox state of element = charge of the ion
in covalent compounds, assume ionic with more electronegative atom forming the negative ion
algebraic sum of all ox states in compound = 0
algebraic sum of all ox states in ionic compound = ion’s charge
pure elements have ox state = 0
oxygen ox state = -2, except for in peroxides (H2O2) where it is then = -1
hydrogen ox state = +1, except for some metal hydrides (eg. NaH) where it is then = -1
many elements show diff ox states in diff compounds. ox number (using roman numerals) is used when naming the element
look for how to make half equations (! spectator ions)
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