sac 1b: Redox chemistry & Galvanic cells (copy)

reducing agent

causes another reactant to gain electrons. the reactant itself is oxidised and loses electrons.

oxidising agent

causes another reactant to lose electrons. the reactant itself is reduced and gains electrons.

oxidation

where oxygen is a reactant, the process by which a metal atom or non-metal ion loses electrons.

reduction

process by which a chemical species gains electrons or oxidation number decreases

redox reaction

electron transfer occurs from the reducing agent to the oxidising agent, oxidation number of one element will increase and the oxidation number of another element will decrease

primary galvanic cells

a non-rechargeable cell, which facilitate spontaneous reactions where the products of the reaction migrate away from the electrode, converting chemical energy into electrical energy.

role of electrodes

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

electrolyte in fuel cell

an alkaline or acidic solution that aids in balancing the charge of the anodes and cathodes. they allow redox half reactions to occur on the surface of the porous electode. they appear in the half equations but not the overall

use and limitations of electrochemical series

uses

• used to predict reactions that are likely to occur when hald cells are

• conneted. allows you to predict maximum cell voltage underr standard conditions

limitations

• No indication of rate of reaction

• All under standard conditions (100kPa, 25 degrees celsius, 1M of all values)

fuel cells

type of electrochemical cell where reactants are continuously supplied from an external source. Using the chemical energy of Hydrogen and other fuels to cleanly and efficiently generate electricity with almost no pollution.

environmental benefits of fuel cells (4)

• reduce fossil fuel consumption, pollutant emissions and avoid energy losses, they also reduce the volume of greenhouse gases produced

• they have an efficiency of up to 85%

• zero emission device with water as their only product

• use of renewable reactants eg. green hydrogen.

potential difference

potential difference measures the tendency to push electrons into the external circuit. It is the electromotive force between two points in a circuit.

standard hydrogen electrode (SHE)

an electrode that scientists use for reference on all half-cell potential reactions. The value of the standard electrode potential is zero,

how E° values are determined

• The half-cell should be constructed at standard conditions (gas pressures of 100 kPa, concentrations of 1 mol L−1) and temperature of 25°C.

• The cell is connected to a standard hydrogen electrode to form a galvanic cell, and the potential

• difference of the cell is measured with a voltmeter. This value is the standard electrode potential (E°).

• If the electrode in the half-cell is negative, the E° is given a negative sign; if the electrode in the half-cell is positive the E° is given a positive sign.

hydrogen ON exceptions

in metal hydrides, H is -1. NaH, CaH2

oxygen ON exceptions

peroxides, O is -1. H2O2, BaO2. Compounds with fluorine, O is +2. OF2

green chemistry principles which fuel cells relate to

1) use of renewable feedstocks

2) maximum energy efficiency

Primary cell

• disposable and designed not to be recharged. They go flat when the cell reaches equilibrium when there is no tendency for the quantities of reactants and products to change

limitations of the galvanic cell

Small amount of reactants 

1. Cell must be discarded or recharged after short amount of time

secondary cell

• rechargeable cells that can be reused many times. recharging can occur because the products formed in the cell during discharge remain in contact with the electrodes in convertible form.

2 advantages and disadvantages of fuel cells

advantage

1) efficient

2) produce water and heat as by products when H is used, no greenhouse gases

3) generates electricity for as long as it is supplied

disadvantages

1) require constant energy supply

2) storage and safety issues with H fuel. explosive and flammable, hard to transport

3) expensive

steam reforming equations (grey hydrogen)

blue hydrogen

when steam reforming process is undertaken, but co2 produced is stored underground

green hydrogen

where green electricity (from solar or wind power) is used to break water up in electrolysis, into oxyfen and hydrofen as a non-spontaneous redox reaction that produces hydrogen in a completely renewable way.

microbial fuel cell

Use low grade waste materials to convert organic material to electrical energy by the action of microorganisms. Microorganisms form a film on the surface of the anode and oxidise organic material, producing co2. These microorganisms then transfer electrons to the anode. 21-35% efficiency.

organic material (s or aq) —> CO2(g) + H+(aq) + e-
O2(g) + 4H+(aq) + 4e- —> 2H2O(l)

Purpose of a salt bridge

balance charges formed in the two compartments. Cations move towards the cathode and anions move towards the anode. Without a salt bridge a negative charge would accumulate up in the cathode and a positive charge would accumulate up in anode, that would stop the reaction quite quickly. It is part of the internal unit. 

Alkaline Conditions half reactions

anode - H2(g) + OH- →2H2O(l) + 2e-

cathode - O2(g) + 2H2O(l) +4e- → 4OH-(aq)

Acidic Conditions half reactions

anode - 2H2(g) → 4H+(aq) + 4e-

cathode - O2(g) + 4H+(aq) + 4e- → 2H2O(l)

Overall Equation for both

2H2(g) + O2(g) → 2H2O(l)

Electrolyte

 chemical substance that conducts electric current as a result of dissociation into positively or negatively charged ions, migrate toward the negative and positive terminals of an electric circuit.

Renewable Feedstocks for fuel cells

renewable feedstocks for fuel cells include biofuels or any other energy-yielding substances that can be replenished in a short period of time

Maximising energy efficiency

use of catalyst speeds up reaction to decreasing the amount of time for energy to escape. electrode porosity an increase in pore numbers and decrease in size of pores will increase the surface area, increases the ability of the reactants to come into contact with the catalyst. using heat produced as a by-product to heat buildings.

Direct reaction

the electrons flow directly from the atoms of one metal to the cations of the other metal. A galvanic cell is designed so that the half-reactions occur in separate compartments of the cell, because oxidant and reductant do not come in direct contact with each other. Electrons are connected with an external unit therefore not direct reaction.

the voltmeter reads zero what is the reason

electrodes are no longer immersed, one of the ions in the salt bridge have been depleted, wires have been disconnected