electrochemical cells w the rechargable

What happens at the negative electrode (anode)?

Metal is oxidised (loses electrons) to form +ve ions.

Electrons make this electrode negative.

What happens at the positive electrode (cathode)?

Metal is reduced (gains electrons) to form metal element.

Electrons being used up make this electrode positive.

What does a salt bridge consist of?

Contains saturated solution of ionic compound.

Porous ends so ions can pass through.

What is the purpose of a salt bridge?

To allow ions to flow in the opposite direction to the electrons, completing the circuit.

Why is K⁺NO₃⁻ preferred as a salt bridge?

All nitrates are soluble and most potassium compounds are soluble.

How is cell e.m.f. calculated?

Cell e.m.f. = E°r-E°l

What is the standard hydrogen electrode (SHE) used for?

To compare standard electrode potentials for different cells.

Standard electrode potential of a half cell

Voltage measured under standard conditions when the half-cell is connected to a standard hydrogen electrode

Which side does the SHE go on?

Left.

What is the standard electrode potential of hydrogen?

0.00V

What happens in the SHE?

H₂ gas bubbled into a solution of H⁺ ions.

Redox equilibrium established at a Pt electrode.

Why is a platinum electrode used

.Unreactive

.Can conduct electricity

What reaction takes place in the SHE?

2H⁺(aq) + 2e⁻ ⇌ H₂(g)

What direction do electrons flow in cells?

From most negative electrode (anode) to least negative electrode (cathode).

What pattern of oxidation and reduction does standard cell notation follow?

R | O || O | R

In cell notation what does a single line (|) show?

Phase boundary.

In cell notation what does a double line (||) show?

Salt bridge.

Is a + e.m.f. thermodynamically feasible and why?

Yes because ΔG≤0

Is a - e.m.f. thermodynamically feasible and why?

No because ΔG>0

Why are lithium ion batteries useful?

Light - Li is least dense metal

Cannot leak - because electrolyte is solid polymer rather than liquid or paste

Charge can be topped up any time

Benefits of non-rechargable batteries

Cheaper

Benefits of rechargable batteries

Last longer than non-rechargable batteries

Cheaper in the long run

Less waste

Describe a lithium cell

Lithium cobalt oxide electrode ( LiCoO2) and Graphite electode.

Electrolyte is a lithium salt in an organic solvent

What happens during charging of a lithium ion battery?

Electrons flow from positive electrode to negative electrode.

Lithium ions flow from negative electrode to positive electrode.

Half equation at lithium cell positive electrode

Li+ + CoO2 + e- --> Li[CoO2]

Half equation at lithium cell negative electrode

Li --> Li+ + e-

What happens during discharging of a lithium ion battery?

Electrons flow from negative electrode to positive electrode.

Lithium ions flow from positive electrode to negative electrode.

Discharging

Spontaneous reaction that produces electricity (E°cell = +ve)

Recharging

Non-spontaneous reaction that requires electricity (E°cell = -ve)

Benefit of hydrogen fuel cell

More energy converted into kinetic energy.

No energy wasted as heat.

Only waste product is water

Dont need to be recharged

Disadvantages of hydrogen fuel cell

Requires energy to produce hydrogen and oxygen ( usually generated from burning fossil fuels)

Hydrogen is highly flammable

Describe hydrogen fuel cell

H2 fed in to negative electrode

O2 fed to positive electrode

Electrolyte KOH

Electrodes separated by an anion exchange membrane

Benefit of anion exchange membrane

Allows OH- and water to pass through but not hydrogen and oxygen gas.

Hydrogen fuel cell negative electrode equation

2H2 + 4OH- --> 4H2O + 4e-

Hydrogen fuel cell positive electrode equation

O2 + 2H2O + 4e- --> 4OH-

Overall hydrogen fuel cell equation

2H2 + O2 --> 2H2O

Strongest reducing agent

. Most easily oxidised

. Most negative value

. Lower oxidation number

Strongest oxidising agent

.Most easily reduced

. Most positive value

. Higher oxidation number

What are the standard conditions

Concentration - 1.00 mol dm⁻³

Pressure - 100 kPa

Temperature - 298 K.

Describe the differences between non-rechargeable, rechargeable, and fuel cells.

In many non-rechargeable batteries the zinc in the wall of the battery is oxidised and becomes Zn²⁺. The supply of Zn(s) is used up and so the walls become thin and the battery can leak. There is nothing to replace the solid metal and so the battery life has finished. Improvements have been made over time to give non-rechargeable batteries longer life spans, although they are more expensive.

Rechargeable batteries can be re-charged by reversing the chemical reactions in the cell when a high voltage is applied. One example of this type of battery is a car battery.

Why use a high resistance voltmeter

To stop the current from flowing in the circuit

What happens if a current is allowed to flow

The reactions will occur separately at each electrode. The voltage will fall to zero as the reactants are used up