book 2!! Flashcards
what is an electrolyte
a substance that, when dissolved, gives a solution that conducts electricity
what type of substance are electrolytes
usually ionic substances
examples of electrolytes aka ionic substances
NaCl
KCl
Na2SO4
in an electrolyte solution,, how is current carried between electrodes
carried by the dissolved ions
what metal is used for electrodes
platinum
why is platinum used as the metal in a cell for electrodes
it’s inert
it’s unreactive
what is applied between the electrodes
voltage is applied
resistance is measured (R)
what is the resistance a measure of
it’s a measure of how difficult the motion of ions is between electrodes
what are the similarities and differences between the 2 electrodes
both platinum
same surface area
one positive (attracts negative ions)
one negative (attracts positive ions)
what does conductivity depend on
the conc
the higher the conc,, aka the higher the number of ions in solution,, aka more charge carriers available,, aka higher conductivity
strong electrolytes
fully ionise in solution
KCl
Na2SO4
when can ions behave independently in solution
when the solution is very dilute
when the ions are far apart
what’s an anion
negative ion
what’s a cation
positive ion
use of lithium instead of zinc does what to a battery
reduces its weight
gives a higher voltage (lithium is more reactive than zinc)
negative about alkaline and lithium batteries
they cannot be recharged.
their redox reactions aren’t reversible,, once the reaction has reached equilibrium,, it’s dead
how can batteries be recharged
by passing an electric current through them and so reversing the redox reaction
what does recharging a cell require
it requires the use of electricity to drive a chemical reaction in the non spontaneous direction
how is charge carried between the electrodes
by the dissolved ions from the electrolyte
what is an electrolyte
a substance that,, when dissolved,, gives a solution that conducts electricity
why is platinum used as an electrode
it’s inert
very unreactive
what is applied on the electrodes
voltage!!
what is measured when voltage is applied on the electrodes
the resistance is measured
R
what is resistance
a measure of how difficult the motion of ions is between electrodes
positive ions move towards what
they move towards the negative electrode
the anode
negative ions move towards the
the cathode
what happens at the anode
oxidation
the negative electrode loses electrons to give to the positive ions
what happens at the cathode
reduction
the cathode gains electrons from the negative ions
what does conductivity depend on
the number of ions in solution
a higher concentration of ions = more charge carriers available,, higher conductivity
strong electrolytes areeee
fully ionised in solution
strong electrolytes + conductivity
dilute = higher conductivity
due to interactions between ions affecting the movement of ions through the solution.
describe a daniel cell
Zinc in ZnSO4 LHS
Copper in CuSO4 RHS
1 moldm-3 each
what is current
the flow of electrons between the half cells
what is a salt bridge used for
charge is conducted by ions in solution. current is carried by ions
soaked in sodium sulfate
keeps electrical neutrality
flow of ions between cells
LHS is for the
anode
oxidation occurs here
Zn(s) -> Zn2+
RHS is for the
cathode
reduction occurs here
Cu2+ > Cu (s)
cell diagram steps
X (s) | X (aq) || Y (aq) | Y (s)
what is | in a cell diagram
phase boundary
what is || in a cell diagram
salt bridge
can we measure the potential of a single half cell
nope!!
the volt meter shows the potential difference between the 2 half cells
what can we use to measure the potential of a single half cell
use a cell to compare the rest to!!
use a standard hydrogen half cell
standard hydrogen half cell equation
2 H+ (aq) + 2e- —> H2 (g)
potential if a hydrogen half cell + what conditions it needs to be in
standard conditions: 1moldm-3, 1bar, 298k
0.00V
standard electrode potentials are actually
standard reduction potentials
E* values
the more positive the E* (reduction potential) the…
stronger the oxidising agent (it helps other lose e- by taking them)
the weaker the reducing agent (weaker bc it doesn’t help atoms gain electrons as it’s positive)
if the half cells E* value is more positive what does this mean
- oxidising agent
- accepts electrons
- reactions are written as reduction : the reaction is more likely to occur in the forward direction (the reduction direction)
what if the E* cell value is more negative
- reducing agent
- loses electrons
- more likely to occur in the reverse direction : bc reactions are written as reductions but it wants to lose electrons!!
the more positive reaction will do what do the more negative reaction
more positive,, wants to gain e-
so it oxidises the more negative reaction
Br2 added to NaCl and NaI (both 1moldm-3) what happens
- write the reduction equation (electrochem series)
- Br is added,, so we compare Br with I and Cl
- Br is more negative than Cl- (it can’t oxidise it - take it’s electrons away)
- Br is more positive than I- (it can oxidise it,, take its electrons away)
bc I is more negative,, its half reaction will be an oxidation,, we reverse the electrochem reaction) then we combine the 2!! 💗💗
equation for finding the E* of the cell!!
E* cell = E* cathode - E* anode
E* cell = E* reduction - E* oxidation
for the cell reaction to occur spontaneously,, what value must E*cell have
A POSITIVE VALUE!!!
what if the E* cell isn’t positive
the current reaction isn’t spontaneous
it would occur spontaneously in the opposite direction to the cell diagram
what does the E*cell value tell us
tendency of the overall cell reaction to happen
what is an example of the electrochemical cell doing work
work = the movement of electrons around the circuit
cell doing work!!
movement of charge (electrons) around a circuit against resistance.
required energy
work done by the cell.
what does the energy required to transfer electrons//charge represent
the electrical work the cell can perform
what is z
moles of electrons
what is Q
total charge
what’s is F
faraday constant
charge on 1 mole of electrons
96 485 coulombs
total charge produced equation
Q = zF
Q= total charge
z= moles
F= faraday constant
Electrical work equation
-Q x V
electrical work = -charge x potential difference
why is electrical work negative
bc it takes energy for it to occur
the reaction system loses energy
Welectrical can also be =
- zF x E*cell
MAX ELECTRICAL WORK THAT CAN BE TRANSFERRED
when does cell voltage = E*cell
resistance in circuit is large
zero current flows
what must occur for the cell to do work
resistance must be lowered so current can flow
change in gibbs energy is equal to whattttt
-zF x E*cell
equal to the max amount of work a cell can do
G = -zF x E*cell
considering gibbs and cell potential,, what values must they have and how are they linked
Wele = -zF x E*cell
E*cell = spontaneous if positive
G = -zF x E*cell
gibbs = max work a cell can do
gibbs: spontaneous if negative
what conditions is maximum work done in
done in reversible conditions
what are reversible conditions when it comes to cells
high resistance
little current
measured Voltage = E*cell
faraday constant units
C mol-1
E*cell units
V
z units
no units
so G = -zF x E*cell units
CVmol-1
CV = J
so G = Jmol-1
spontaneous Ecell must be
positive
spontaneous reaction must be
negative gibbs
gibbs and thermodynamic equilibrium are related in which equation,, and what does this tell us
G = -RT ln(K)
tells us we can use the E*cell to find equilibrium constant, K
how can we use E*cell to find equilibrium constant K
G = -zF x E*cell
G = -RT ln(k)
-zF x Ecell = - RT ln(k)
Ecell = RT/zF ln(k)
ln(K) = zF/RT E*cell
what happens when a cell reaction reaches equilibrium
it goes flat
it runs out
current is drawn from the cell, the half cell reactions occur, concentration of species changes,, then concentration of species stops changing + no current flows.
state where half cell reaction concentrations stop changing + no more current flows.
what does a large equ constant, K mean
it means that the reaction basically goes to completion,, all solid X forms Xions
all Y ions form Y solids.
there is complete conversion
what if the value of k is 1
reaction is at equilibrium
what if k is less than 1
the reaction is spontaneous in the reverse direction
E*cell dependence on concentration
E*cells are calculated under standard conditions,, they may change with different concentrations or gas pressures
how do we tackle the problem of E*cells being affected by conc and pressure
consider how Gibbs changes with rea and pro activity
G = G* + RTln(Q)
what is Q
reaction quotient
product of (activity x product ^stoich) // product of (activity of reactants ^ stoich) r
what does product of mean
it means u multiply the different things together.
like sum of but with multiplication instead of addition
what is the activity of a solid
1
Q = K when
at equilibrium!!
what is the activity of a gas, a liquid and a solid
gas : p/p*. pressure of gas / 1bar
liquid: conc/conc* conc/ 1moldm-3
solid = 1
finding the cell potential when conditions aren’t standard
Ecell = E*cell - (RT/zF) x ln(Q)
activity for stuff at standard conditions
1!!!
G = G* + RTln(Q) can be written as
-zF x Ecell = -zF x E*cell + RTln(Q)
nernst equation
Ecell = E*cell - RT/zF x ln(Q)
what does the nernst equation allow us to do
find the Ecell (cell potential) under non standard conditions
find cell potential when changing the concentration of the cell
can also find conc of species involved in an electrochemical cell.
what is a concentration cell
half cells made up of the same chemical reactions but at different concentrations
in electrochemical cells,, the chemical reactions are used to make what
used to produce energy
what is electrolysis
driving a chemical reaction by supplying it with electrical energy
input of energy allows what reactions to occur
non spontaneous reactions occur
what cells include reversible reactions that can be recharged
secondary cells
which battery is used in motor vehicles
lead - acid battery
electrolysis cell
looks like 1 half cell but is connected to an electrical power source (battery)
