electrochemistry I Flashcards
what is a redox reaction?
donation (transfer) of electrons from one substance to another
key features of a redox reaction?
-involves electron transfer
-one substance must donate electrons and one substance must accept electrons
-the substance that donates electrons (loses) = oxidised
-the substance that accepts electrons (gains) = reduced
oxidation can be identified by:
-loss of electrons (on RHS of half equation)
-increase in oxidation number
-gaining of oxygen
-loss of hydrogen
reduction can be identified by:
-gain of electrons (found on LHS of half equation)
-decrease in oxidation number
-loss of oxygen
-gain of hydrogen
what must you ensure in a half equation?
that the charges are BALANCED
writing an overall redox equation?
-the number of electrons lost during oxidation MUST EQUAL the number of electrons gained during reduction
-treat like maths simultaneous equations, cancel out e-
define electronegativity:
a measure of the tendency of an atom (of that element) to attract a bonding pair of electrons
what are oxidants?
reactants that cause another substance to be OXIDISED. it pulls electrons away from something else, causing that substance to LOSE ELECTRONS
what are reductants?
reactants that cause another substance to be reduced
what side of the half equation is reduction?
LHS - gaining electrons (RHS NOT RED)
what side of the half equation is oxidation?
RHS - losing electrons
general oxidants and reductants
halogens (group 17) = oxidants
halogen ions = reductants
metals = reductants
metal ions = oxidants
define oxidation number
a measure of the electron density around an atom, compared to its elemental form
is 4+ or +4 an oxidation number (O.N)
+4
sign comes before number
trend of electronegativity and O.N
more electronegative elements = negative oxidation numbers
less electronegative elements = positive oxidation numbers
rule 1 for O.N
- the sum of all the oxidation numbers in any substance is equal to the total charge. this means that the elemental form will always have an oxidation number of ZERO
rules 2 and 3 of O.N
- fluorine has an oxidation number of -1 in compounds
- group 1 metals are +1 in compounds, group 2 metals are +2 in compounds
rules 4 and 5 for O.N
- when H is bonded to non-metals, it has O.N of +1, when H is bonded to metals, it has O.N of -1
- oxygen has oxidation number of -2 in compounds, but in peroxides, it has O.N of -1
rules 6 and 7 of O.N
- grp 7 elements are usually -1 in compounds. if two ore more different grp 7 elements are present, the more electronegative one if -1
- in absence of other info, grp 16 non-metals = usually -2, grp 15 non-metals = usually -3 and grp 3 metals are usually +3
outline the steps of KOHES
- identify Key element in half equation + balance
- balance number of Oxygen atoms (by adding water to other side)
- balance number of Hydrogen atoms (by adding H+ ions)
- balance charges on both sides by adding electrons to more pos side
- add state symbols
what are redox conjugate pairs?
a conjugate redox pair (one=reactant and one=product) share an atom that have gained or lost electrons
what pairs exist in redox conj. pairs?
reductant at start - conjugate oxidant formed at end
oxidant at start - conjugate reductant formed at end
what is the direction of electron transfer in a redox reaction?
FROM oxidised substance TO reduced substance
what is a galvanic cell?
a chemical system that produced an electric current (DC) from a spontaneous redox reaction
key features of a galvanic cell:
-contains two half cells, each containing a conjugate redox pair
-on electrode in each half cell - these conduct electricity
-contains a salt bridge
-contains anode and cathode
define anode
the site of oxidation
it is a NEGATIVE terminal (aNode = Neg)
define cathode
the site of reduction
it is a POSITIVE terminal (ca+hode = +)
what is the purpose of a salt bridge?
contains unreactive ions that maintain an even distribution of ions in each beaker + counteract any imbalance of charge and complete the electrical circuit
common = KNO3
movement of ions from salt bridge into half-cells?
anions (neg) go to anode (neg)
cations (pos) go to cathode (pos)
what does each half-cell contain?
each contains an electrode in contact with a solution. the species present in each half cell form a conjugate redox pair eg. Zn and Zn2+
direct contact in galvanic cells
when oxidant and reductant = in direct contact, electron transfer cannot be detected t/f energy is released in form of HEAT
galvanic cell electrical flow?
electrons are forced to travel along the external wiring to reach the substance that is being reduced (oxidant) t/f this is detected by a voltmeter or an ammeter = ELECTRICAL ENERGY
what is the mnemonic to remember??
AN OIL RIG CAT
what is the direction of electron transfer in a galvanic cell?
from anode to cathode
why must both oxidation and reduction occur simultaneously during any redox reaction?
because the loss of electrons by one substance must be accompanied by a gain of electrons in another substance
define displacement reaction
a type of redox reaction between a more reactive element and a compound containing a less reactive element, resulting in the less reactive element being displaced from the compound
what is a conjugate oxidant?
a substance produced when a reductant loses electrons, containing the element that has increased in oxidation number
what is a conjugate reductant?
a substance produced when an oxidant gains electrons, containing the element that has decreased in oxidation number
standard electrode potential E^o
the relative strength of the oxidant under standard conditions, compared to the H+/H2 half cells (0V). measured as a voltage produced
standard cell potential (E^0 cell)
the voltage/potential difference across two half-cells under standard conditions -> calculation = E cathode - E anode
electrochemical species and spontaneous reactions
an electrochemical species compares the relative strength of oxidants and reductants, so it can be used to predict whether a reaction will occur spontaneously (a redox reaction will occur)
will only occur if oxidant (left) = HIGHER than reductant (right side of half equation) NEED NEGATIVE GRADIENT
metal reactivity series acronym
police sergeant lieutenant commander MACZINTL has caught me stealing gold plates
what is the metal reactivity series?
potassium, sodium, lithium, calcium, magnesium, aluminium, CARBON, zinc, iron, nickel, tin, lead, HYDROGEN, copper, mercury, silver, gold, platinum
why can a metal displace another metal in a compound?
because it is a more reactive element that can displace a less reactive metal element from its compound and because it is a stronger reductant
