An Electrochemical Cell That Derives Electrical Energy Biology Essay

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A Galvanic cell, or Voltaic cell, named after Luigi Galvani, or Alessandro Volta respectively, is an electrochemical cell that derives electrical energy from chemical reactions taking place within the cell. It generally consists of two different metals connected by a salt bridge, or individual half-cells separated by a porous membrane.

Volta was the inventor of the voltaic pile, the first electrical battery. In common usage, the word "battery" has come to include a single Galvanic cell, but a battery properly consists of multiple cells.

A Galvanic cell consists of two half-cells. In its simplest form, each half-cell consists of a metal and a solution of a salt of the metal. The salt solution contains a cation of the metal and an anion to balance the charge on the cation. In essence the half-cell contains the metal in two oxidation states and the chemical reaction in the half-cell is an oxidation-reduction (redox) reaction, written symbolically in reduction direction as

Mn+ (oxidized species) + n e− M (reduced species)

In a galvanic cell one metal is able to reduce the cation of the other and, conversely, the other cation can oxidize the first metal. The two half-cells must be physically separated so that the solutions do not mix together. A salt bridge or porous plate is used to separate the two solutions yet keep the respective charges of the solutions from separating, which would stop the chemical reactions.

The number of electrons transferred in both directions must be the same, so the two half-cells are combined to give the whole-cell electrochemical reaction. For two metals A and B:

An+ + n e− A

Bm+ + m e− B

m A + n Bm+ n B + m An+

This is not the full working, as anions must also be transferred from one half-cell to the other. When a metal in one half-cell is oxidized, anions must be transferred into that half-cell to balance the electrical charge of the cation produced. The anions are released from the other half-cell where a cation is reduced to the metallic state. Thus, the salt bridge or porous membrane serves both to keep the solutions apart and to allow the flow of anions in the direction opposite to the flow of electrons in the wire connecting the electrodes.

The voltage of the Galvanic cell is the sum of the voltages of the two half-cells. It is measured by connecting a voltmeter to the two electrodes. The voltmeter has very high resistance, so the current flow is effectively negligible. When a device such as an electric motor is attached to the electrodes, a current flows and redox reactions occur in both half-cells. This will continue until the concentration of the cations that are being reduced goes to zero.

For the Daniell cell, depicted in the figure, the two metals are zinc and copper and the two salts are sulfates of the respective metal. Zinc is the oxidized metal so when a device is connected to the electrodes, the electrochemical reaction is

Zn + Cu2+ → Zn2++ Cu

The zinc electrode is dissolved and copper is deposited on the copper electrode (as copper ions become reduced to copper metal). By definition, the cathode is the electrode where reduction (gain of electrons) takes place, so the copper electrode is the cathode. The cathode attracts cations, so has a negative charge when current is discharging. In this case, copper is the cathode and zinc the anode.

Galvanic cells are typically used as a source of electrical power. By their nature they produce direct current. For example, a lead-acid battery contains a number of galvanic cells. The two electrodes are effectively lead and lead oxide.

The Weston cell was adopted as an International Standard for voltage in 1911. The anode is a cadmium mercury amalgam, the cathode is made of pure mercury, the electrolyte is a (saturated) solution of cadmium sulfate and the depolarizer is a paste of mercurous sulfate. When the electrolyte solution is saturated the voltage of the cell is very reproducible, hence its use as a standard.

Galvanic corrosion is a process that degrades metals electrochemically. This corrosion occurs when two dissimilar metals are placed in contact with each other in the presence of an electrolyte, such as salt water, forming a galvanic cell. A cell can also be formed if the same metal is exposed to two different concentrations of electrolyte. The resulting electrochemical potential then develops an electric current that electrolytically dissolves the less noble material.

Strong electrolytes

A strong electrolyte is one which undergoes complete ionization when dissolved in water. The solution contains only the ions and not molecules.

Examples:

HCl, HNO3, H2SO4, NaOH, Ca(OH)2, NaCl, KCl, CH3COONa etc.

Weak electrolyte

A weak electrolyte is one which undergoes partial ionization or dissociation. Here, in solution the ions and the dissociated molecules will be in equilibrium with each other. When such a solution is diluted, the degree of ionization increases. It becomes complete at infinite dilution.

Examples:

HCOOH, CH3COOH, NH4OH, CH3NH2, CH3COONH4, H3PO4 etc.

Information directly from: http://www.tutorvista.com/content/chemistry/chemistry-iv/electrochemistry/electrolytes-types.php

http://www.saskschools.ca/curr_content/chem30_05/6_redox/redox2_2.htm

Experiment Plan

Aim: To determine how a change in the electrolyte of the Zinc anode of a galvanic cell affects the voltage, and whether or not having a common salt in one electrolyte and the salt bridge makes the voltage greater.

Hypothesis: The anode with the most soluble salt solution, which is Zinc Chloride, will create the circuit with the greatest voltage. This is because the more soluble the salt is, the easier the transfer of ions within the solution is. Reduction and oxidation can then occur more efficiently through easier ion transfer, leading to the greater voltage.

Variables:

Control

Dependent

Independent

Volume of electrolyte - Measure electrolyte using the same 250mL beaker. This is done in order to prevent any changes in the volume of the electrolyte which could affect the voltage.

Voltage of cell - The electrolyte will be changed in order to measure any changes in the voltage. This will be recorded and compared to the standard electrode potentials.

Electrolyte (anode) - Changed in order to measure the effect on voltage.

Concentration - Use electrolyte from the same batch (container) in order to not have any discrepancies in the tests. This makes for a more reliable experiment.

Electrode - Using the same electrode in each experiment will ensure that each reading is reliable, by preventing any changes in the properties of the metal.

Salt Bridge - Use filter paper from the same package to prevent any changes in the properties (materials used). Also, use the KNO3 from the same container to prevent any changes in concentration for each test.

Materials:

3 x 250 mL beakers. Two for electrolytes and one for salt bridge solution.

Electrodes (Zinc and Lead).

Electrolytes: 1.0M: Lead chloride, Zinc sulphate, Zinc chloride, Zinc nitrate

Voltmeter: 1.0MV-10.0MV: .0025 error

2 Wires (including alligator clips)

Filter Paper for salt bridge

Saturated KNO3 solution for salt bridge

100mL Measuring Cylinder - 1mL between each measurement, so error is +/- 0.5mL

SAME DECIMAL POINTS

Method:

Collect materials shown above, and set up in the following fashion with the Zinc electrode connected to the negative, and the Lead connected to the positive terminals.

Make Salt bridge by:

Folding filter paper into rectangle, then 'U' shape.

Pour KNO3 into 40mL beaker

Dipping into KNO3 solution.

Remove excess liquid by applying pressure, but ensure it stays damp.

Place between both beakers as shown above, ensuring that both ends are below the meniscus of the liquid.

Since Zinc Chloride was not available in the Science lab, it had to be made from solid Zinc chloride and Distilled Water.

150mL was needed, so 200mL in case more was required (for spills). This was measured in a measuring cylinder and transferred to a beaker.

Measured out 27.2g of solid zinc chloride. This was the amount required to gain 1.0M Zinc chloride for the 200mL solution.

100mL of distilled water was measured in the measuring cylinder and poured into the beaker. This was repeated one more time, and then the mixture was stirred until all signs of solid were dissolved.

150mL of the 200mL was poured into a clean beaker, ready for the experiment to begin

Lead sulphate will be used in all the experiments (cathode). Use the Zinc sulphate (150mL and 1.0M) in the anode first, by measuring out 100mL in a measuring cylinder twice, then puring into the beaker. The setup should will looks like the diagram below, and once the wires are connected

Repeat this process for these electrodes 3 times.

Once the experiment is over, clean the beakers and electrodes thoroughly in order to prepare them for the next Zinc salt.

Repeat steps 2, 4 and 5 for Zinc Chloride and Zinc Nitrate

Average the results for each salt and compare to standard electrode potentials.

Risk of Harm

Cause of Harm

Control measure to minimise risk

Eye injury.

Splashing of electrolyte.

Wear safety goggles.

Electrocution.

Mixing of liquids around live electric circuits.

Perform experiment at the furthest possible distance from any power points.

Burns/reactions.

Coming into contact with corrosive materials.

Handle each liquid with care and wear goggles and lab coats.

Chemical.

Type of Harm caused.

Control measure to minimise risk.

What to do if harm is caused.

Zinc Sulphate

- Harmful if swallowed.

- Can cause irritation to eyes if it comes in contact with them.

- Is irritating if it contacts skin.

- Toxic to certain marine life.

- Wear protective gloves, lab coat and safety glasses when working with Zinc Sulphate.

- Dispose of safely.

- If swallowed, seek medical help.

- If contact with eye, flush the eye for a minimum of 10 minutes.

- If contact with skin, wash off with water.

Zinc Chloride

- Irritation if in contact with skin.

- Harmful to respiratory if inhaled and can cause respiratory distress which can be fatal.

- Harmful to the digestive system if ingested, this is because of its corrosive nature.

- Highly toxic to marine life.

- Wear protective gloves, lab coat and safety glasses when working with Zinc Chloride.

- If exposure is high, breathing protection should be used, or the experiment be conducted in a fume hood.

- If inhaled or digested, immediately seek medical help.

- If contact with skin, wash off with water.

Zinc Nitrate

- Can cause burns or irritation if in contact with eyes

- Due to slightly corrosive nature, prolonged contact with skin can be harmful and if swallowed it can irritate the digestive system.

- Wear protective gloves, lab coat and safety glasses when working with Zinc Nitrate

- Do not digest the chemical

- If contact with eye, flush the eye, and seek help if irritation continues.

- If contact with skin, wash off with water.

- If swallowed, drink water and seek medical help.

Lead Chloride

- Harmful to respiratory if inhaled, causing irritation to bronchia or lungs. Chance of Lead poisoning.

- Lead is poisonous and can lead to many symptoms such as abdominal pain and spasms. In extreme cases muscle weakness, insomnia, coma and possible death occur.

- Contact with skin or eye may cause local irritation, pain and redness/swelling. It can be absorbed through the skin or eye, and the symptoms of ingestion may occur.

- Lead can also intensify any existing circulatory, nerve or kidney conditions.

- Wear protective gloves, lab coat and safety glasses when working with Lead Chloride.

- Conduct experiment under fume hood.

- Conduct experiment safely without inhaling or ingesting any of the chemicals.

- If inhaled, move to fresh air and if not breathing give respiration artificially.

- If ingested induce vomiting and seek medical attention.

- If the skin or eye has come in contact flush the site instantly for minimum of 10 minutes to ensure it is clear. Seek medical help.

http://cartwright.chem.ox.ac.uk/hsci/chemicals/zinc_sulfate.html

http://cartwright.chem.ox.ac.uk/hsci/chemicals/zinc_nitrate.html

http://www.ehow.com/facts_6052843_hazardous-effect-zinc-chloride_.html

http://www.jtbaker.com/msds/englishhtml/l2782.htm

Calculations

Solubility Table:

Anode

Solubility

Zinc Chloride

432 g/100mL

Zinc Sulphate

57.7 g/100mL

Zinc Nitrate

184.3 g/100mL

Equations:

The reduction of Lead is given by the following equation: Pb2+ + 2e- → Pb

The oxidation of Zinc is given by the following equation: Zn → Zn 2+ + 2 e-

The complete equation of the redox reaction of the galvanic cell used in the experiment is given by: Zn(s) + Pb2+(aq) → Zn2+(aq) + Pb(s

In order to calculate the voltage produced by this electrochemical cell, the sign of the potential for Zinc must be reversed as it is oxidised. The voltage can be calculated by:

Potential Difference of anode and cathode.

Zn2+ + 2 e− Zn(s)

 -0.7618 −0.7618

Pb2+ + 2 e− Pb(s)

 -0.13 −0.13

 

 

E°

 Zn(s)  → Zn2+(aq) + 2e-

 

0.7618 V

Pb+(aq) + 2e- → Pb (s) 

+

-0.13 V

 

Zn(s)+ Pb+(aq) → Zn2+(aq) + Pb (s)  

 

0. 6318V

Results

Results - Voltage (V)

Electrolyte (Lead Chloride not changing)

Trial 1

Trial 2

Trial 3

Average

Zinc Sulphate

0.15 ± 0.025

0.16 ± 0.025

0.155 ± 0.025

0.155 ± 0.025

Zinc Chloride

0.18 ± 0.025

0.2 ± 0.025

0.23 ± 0.025

0.203 ± 0.025

Zinc Nitrate

0.11 ± 0.025

0.1 ± 0.025

0.12 ± 0.025

0.110 ± 0.025

Trial 1

Trial 2

Trial 3

Average

Standard Deviation

Zinc Sulphate

0.15

0.16

0.155

0.155

0.004082483

Zinc Chloride

0.18

0.2

0.23

0.203

0.020548723

Zinc Nitrate

0.11

0.1

0.12

0.11

0.008164966

http://www.scribd.com/doc/14782080/Experiment-Seven-Electrochemical-Cells - LOOK HERE!!

Discussion

Summary

The results achieved in the experiment supported the hypothesis that stated Zinc Chloride would produce the most voltage because of its solubility. The voltage of the cell with Zinc Chloride as the Anode was, on average, double the voltage of the Zinc Nitrate which also reflected the solubility of it (being less than Zinc Chloride as shown on the solubility table, page 10). The experiment was conducted with the only deviation to the method being the mixing of Zinc Chloride from solid Zinc Chloride and distilled water. This had an effect on the accuracy because there were 4 measurements made, rather than the 1 measurement required if Zinc Chloride was pre-prepared. The risk assessment was conducted and the necessary precautions were undertaken.

Significance

The results achieved in the experiment were consistent but were far lower than expected. The voltage from the results was far lower than the calculated voltage of 0.6318V. This could be due to the conditions that the experiment was conducted in, such as:

Temperature: Research shows that with an increase in the temperature, the resistance of the wires increases, reducing the current that is able to flow in the wires, in turn decreasing the voltage.

Concentration: The concentration used may have been too low, therefore resulting in a lower voltage.

Research does not show any previous experiments that indicate solubility having an effect on the voltage of a galvanic cell. Although research does show that the following factors affect the voltage:

Electrode

Electrolyte (conductivity and type)

Concentration of electrolyte

Validity

The experiment was valid, because it reliably tested and proved the hypothesis. The experiment was directly aimed at testing the hypothesis, and the method used ensured this. Not all the variables were identified and controlled. The variables that were not controlled were: temperature,

Reliability

The results that were achieved were consistent and the 3 trials of each cell improved the reliability. The standard deviation of each galvanic cell was very low and could not be displayed on the standard graph. The results were repeatable over the 3 trials with a very slight change (maximum of 0.01mV difference) of each different anode which showed reliability. By comparing the results with the expected electrode voltages gained from the redox calculations

Accuracy

The accuracy of the experiment was not perfect. The measurement of each Zinc electrolyte was not the same for each separate electrolyte. Also, when measuring out the required 27.2g ± 0.01g of Zinc Chloride, the balance was not completely c lear of excess solid falling out of the spatula and from previous measurements (shared balance). This may have had an affect on the concentration of Zinc Chloride.

Evaluation

The experiment was conducted according to the method, but there are certain improvements that could be made for future investigation. They include:

Changing the Lead Chloride after each measurement. The same electrolyte was used for every measurement, and the reduction that occurred after each measurement could have changed the concentration of Lead in the liquid. This creates an inconsistency for the following measurements.

When mixing the Zinc Chloride with distilled water, the balance should be free of any particles that could affect the measurement, and if zinc chloride falls outside the beaker it should be cleaned and the measurement repeated. This was not done in the experiment which could have affected the concentration of the Zinc Chloride (1.0M was the aim)

Repeating the experiment 2 more times may have led to more reliable results, and less error in the measurement, although the error was still very low.

Improvements

Factors that affected the experiment

How it affected the experiment

Improvements

Not completely accurate measurements of electrolyte volume.

This could have created an error in the measurement of the voltage between the different electrolytes.

Accurately measure for every test using the 500mL measuring cylinder, rather than measuring with the 100mL cylinder twice.

Did not remove and replace the electrolyte between each repeat.

This would have created errors in the following measurement after each trial, because the electrolyte would not be pure.

Change each electrolyte after every measurement to ensure consistent measurements.

Conclusion

In conclusion, the experiment reliable and accurately tested the hypothesis. Although the results were different to the calculated and expected results, the experiment was still successful. It proved the hypothesis which stated that the electrolyte with the more soluble salt would have the greatest voltage. After comparing the results with the solubility of each salt it was clear that this relation was proved.

Rules for propagating errors

Presenting Errors

Adding/Subracting

(3.12g ±0.01) + (1.09g +/-0.01) = 4.21g +/-0.02

Multiplying

+ % errors -> convert back to absolute error

3.45g ± 0.01 in 100.0mL ± 0.02

% error of mass = 0.01/3.45 x 100 = 0.29%

Volume = 0.02/100 x 100 = 0.02%

Concentration = 3.45g/100.0mL = 0.0345g/mL (0.31% or ±0.0001)

Average

Range(max-min)/2 compare with error of each value use whichever is bigger

Balance = 3.12g +- 0.01g (only device not exactly half the limit of reading, always 0.01g)

All other measuring devices are half the limit of reading

Always 1 significant figure

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