ch9 - electrochemistry

galvanic cells

Explain electrochemistry

• Since REDOX reactions involve transfer of electrons, its possible to physically separate the oxidant (oxidising agent) and reductant (reducing agent) into different locations

• The process of electrons moving from reductant to oxidant is called ELECTRICITY

• Redox reactions can be used to generate electricity if the two-half reactions are PHYSICALLY SEPARATED

define electrochemical cell

a device that converts chemical energy into electrical energy (galvanic cells), or vice versa (electrolysis cells)

define a galvanic/voltaic cell

a type of electrochemical cell; a device that converts chemical energy into electrical energy

define battery

a combination of cells connected in series

identify all the parts in creating a battery / Daniell cell

• light globe or galvanometer (an instrument for detecting electric current)

• salt bridge

• 2 electrodes (usually solid & are labeled as cathode or anode) with corresponding (electrolyte) solutions

• 2 beakers

• wires to connect the electrodes to the light globe or galvanometer

define cathode & anode

cathode: an electrode at which a reduction reaction occurs - has a + charge

anode: an electrode at which an oxdiation reaction occurs - has a - charge

define salt bridge

an electrical connection between the 2 half cells in a galvanic cell; usually made from a material saturated in (relatively unreactive) electrolyte soln

define electrode

a solid conductor in a half cell at which oxidation or reduction reaction occur

define electrolyte

a chemical substance that conducts an electric current as a result of dissociation into positively & negatively charged ions

• in electrochemical cells, these ions migrate towards the negative & positive electrodes

ANY electrochemical cell consists of TWO half cells:

1. explain the setup of these half cells

2. state where oxidation occurs & why

3. state where reduction occurs & why

1. Each half cell consists of an electrode (metal or graphite) in contact with an electrolyte solution (sol. containing ions).

2. OXIDATION takes place at the ANODE (-ve electrode) as it generates electrons

• (An Ox)Oxidation = loss of electron

3. REDUCTION takes place at the CATHODE (+ve electrode)

• (Red Cat)Reduction = gain of electrons
  Electrons flow from the ANODE to CATHODE (-ve to +ve)

1. define external circuit

2. identify what part/s of an electrochemical cell is an external circuit

the section of an electrochemical cell in which electrons move

• this section of the circuit will include wires attached to the electrodes

1. define internal circuit

2. identify what part/s of an electrochemical cell is an internal circuit

1. the part of an electrochemical cell in which ions move

2. solutions & salt bridge

how is the reaction of chemical energy transforming into electrical energy found?

by adding the 2 related half equations together

in a half cell there is a conjugate redox pair - explain what this is

• an oxidising agent & the product that’s formed when the oxidising agent gains electrons

or

• a reducing agent & the product that’s formed when the reducing agent loses electrons

define inert

a substance that is unreactive

when labeling a diagram of a galvanic cell - what do you need to include?

• anode

• cathode

• electrode polarity

• direction of electron flow in external circuit

• direction of flow of ions in the solns & the salt bridge

explain/draw the setup of a galvanic cell that uses gas & a metal anode

how does a salt bridge allow a cell to produce electricity?

it prevents the accumulation of charge

• cations in the salt bridge move towards the cathode

• anions move to the anode

how can a galvanic cell reaction produce heat energy instead of electrical energy?

if the reactants are allowed to come into direct contacts, the chemical energy is converted into heat energy instead of electricity

How do you get the Eoox from reading the data sheet?

If you read the table backwards (right to left), the electrons are now gained and the Eo sign is flipped; E^o_ox

The numerical value of - E^o_red = E^o_ox

How do you know if a reaction is spontaneous or non-spontaneous?

The Eo cell determines whether a reaction will be spontaneous or not (i.e. whether electricity will be produced). When adding two half cell e naughts together;

a positive Eo = spontaneous
a negative Eo = nonspontaneous

Explain what it means when the Eored is positive or negative on the data sheet

• The more positive the value of Eored the stronger the oxidising agent (reduction).

• The more negative the value of Eored the stronger the reducing agent (oxidation).

what does E^o stand for & how are these values found?

standard electrode/reduction potential

• found by connecting the half-cell to a standard hydrogen half-cell and measuring the volage produced

what is E^o a numerical measure of? & what is the table showing these values called?

a measure of tendency of a half-cell reaction to occur as a reduction reaction

in other words: The voltage produced is a measure of the driving force for a redox reaction to occur.

• electrochemical series

what can comparing E^o values determine & what is the purpose of determining this?

we can determine relative strengths of oxidising & reducing agents which can be used to predict half-cell & overall cell reactions

Explain ‘E naught’ values in redox reactions

• This property is used to rank the strength of different oxidising agents.

• The table of standard reduction potentials is such a ranking.

  (These are found on your Data Sheet and are called ‘E naught’ values.)

for a spontaneous reaction to occur, can you use 2 oxidising agents or 2 reducing agents?

no - has to be 1 oxidising agent & a reducing agent

How do you find (cell) potential difference & what is this used for

difference = higher half-cell E^o - lower half-cell E^o

higher = oxidising agent

lower = reducing agent

• difference is used to see whether a reaction is spontaneous or not

the standard half-cell potentials in the electrochemical series are measured under standard conditions - why?

other conditions, the order of half-reactions may be different, therefore predictions based on the electrochemical series may not be reliable

what are the type of galvanic cells that convert chemical energy directly into electrical energy

primary, secondary & fuel cells

how can a cell be rechargeable?

the products of the discharge reaction have to remain in contact with electrodes

define primary cell

a galvanic cell that is non-rechargeable because the products of the reaction migrate away from the electrodes

define secondary cell

a rechargeable cell

how can a secondary cell be rechargeable?

by connecting them to an external source of electricity

during the recharging of a secondary cell, the equation for the reaction that occurs during recharging is the ___ of the equation for the cell discharging

during the recharging of a secondary cell, the equation for the reaction that occurs during recharging is the reverse of the equation for the cell discharging

A secondary cell can act as a galvanic cell & a electrolytic cell - explain how

• acts galvanic when it discharges = releasing electrical energy

• acts electrolytic when recharged = converting electrical energy into chemical energy

define fuel cell

a type of electrochemical cell in which the reactants are supplied continuously, allowing continuous production of electrical energy

explain 2 pros of using fuel cells than combusting fuel

• electricity generation using fuel cells is more efficient than if the electricity were generated by the combustion of the same fuel

• less emissions of greenhouse gases from using fuels cells than if the fuel were burnt in a power station or vehicle

some scientists predict that fuel cells will play a key role in the transition from a dependence on ___ ___ for energy to a ____ economy

some scientists predict that fuel cells will play a key role in the transition from a dependence on fossil fuels for energy to a hydrogen economy

define dry corrosion

the oxidation of a metal by oxygen gas

define wet corrosion

• involves oxygen gas & water

• can be considered as an electrochemical process

explain wet corrosion relating to iron

• during wet corrosion, the iron is oxidised to Fe²⁺ (aq) & oxygen gas is reduced to OH^- (aq)

• these form a precipitate of Fe(OH)₂ (s), which is converted to rust → Fe₂O₃·xH₂O

what are the protection measures against corrosion?

surface coating, alloying, cathodic protection & the use of a more reactive metal as a sacrificial anode

are galvanic cells spontaneous or non-spontaneous?

spontaneous