Chemistry Module Five (Triple a Importance)

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Chemistry, Module FivexXTheDitzyBlondeXx

What are electrochemical cells? Made from two different metals dipped in salt solutions of their own ions and connected by a wire. The wire is the external circuit. Half-cells involving solutions of two aqueous ions of the same element use a platinum electrode. Conversion of ions happens on the surface of the electrode.

Half-cells are connected by a salt bridge made of filter paper soaked in KNO3(aq). Redox reaction occurs in an electrochemical cell.

K+ and NO3- ions flow through the salt bridge and balance out the charges in the two half-cells. Electrons flow through the wire from the most reactive metal to the least reactive metal.

Voltmeter in the external circuit shows the voltage between the two half-cells. This is called the cell potential or emf, Ecell. Ecell = ERHS - ELHS More negative standard electrode potential put on the left, so the cell potential is always positive. Swapping the sides the half-cells are on changes the sign of the cell potential.

What are electrode potentials? Standard electrode potential, E, of a half-cell is the voltage measured under standard conditions when the half-cell is connected to a standard hydrogen electrode. Standard conditions are solutions at 1M concentration, temperature of 298K and pressure of 101kPa. Ecell = ERHS - ELHS Electrode potential can be positive or negative, depending on which way the electrons flow.

Standard hydrogen electrode is hydrogen gas bubbled through a 1M solution of H+ ions with a platinized platinum electrode submerged in, at 101kPa pressure. Platinized platinum electrode means the surface of foil is finely powdered with platinum. The platinized surface absorbs hydrogen gas, causing an equilibrium to be set up. H2(g) H+(aq) + 2e Electrode potential of a standard hydrogen electrode is 0. Reference cell because it is used to measure the electrode potentials of other half-cells. Standard hydrogen electrode always put on the left.

What is the electrochemical series? The more reactive the metal, the more likely it is to lose electrons to form a positive ion. More reactive metals have more negative standard electrode potentials.

The more reactive a non-metal, the more likely it is to gain electrons to form a negative ion. More reactive non-metals have more positive standard electrode potentials.

Electrochemical series shows standard electrode potentials. More positive electrode potentials mean the left-hand substance is more easily reduced and the right-hand substance is more stable. More negative electrode potentials mean the right-hand substance is more easily oxidised and the left-hand substance is more stable.

What is the anticlockwise rule? Predicts whether a reaction is likely to happen. 1. Write half-equations out with the most negative standard electrode potential on top. 2. Draw on anticlockwise arrows to show the direction of the half-reaction. 3. Swap the top half-equation, and change the sign of the electrode potential. 4. Combine the half-equations and find the cell potential. Electrode potential chart is another way of showing the anticlockwise rule; upside down y-axis showing electrode potentials with the more negative electrode potential on top.

What problems are there with predicting electrode potential? If the electrode potential is positive then the reaction is feasible under standard conditions. Above 0.4V, the reaction goes to completion. Between 0 and 0.4V then the reaction will be reversible.

Changing concentration or temperature of the solution can cause the electrode potential to change. If you increase the concentration of the ions in the top half-equation, the equilibrium shifts left, making electron loss more difficult and the cell potential lower. If you increase the concentration of the ions in the bottom half-equation, the equilibrium shifts right, making electron gain easier and the cell potential higher.

The kinetics may not be favourable: Reactions occurs so slowly that it appears that nothing is happening. Reaction has a high activation energy, which stops it occurring.

Bigger cell potential = bigger total entropy change. ln

What are redox titrations? Titrations using transition element ions. Used to determine the amount of oxidising agent needed to exactly react with a quantity of reducing agent or vice versa. Measure out a volume of reducing agent into a conical flask. Add 20cm3 of dilute sulfuric acid to the conical flask. Add a solution of oxidising agent to the conical flask while swirling the conical flask. Record the volume of oxidation added when the end point is reached (rough titration). Repeat the titration, adding the oxidising agent dropwise when you are within 2cm3 of the end point, until you get two or more readings within 0.1cm3 of each other. Find average titre.

What can you calculate from redox titration? Concentration of reducing agent:1. 2. 3. 4. 5. 1. 2. 3. 4. 5. 6. Write the balanced equation for the reaction. Work out the number of mols of oxidising agent added to the conical flask. Use the stoichiometry of the equation to find the number of mols of the reducing agent that has reacted. Scale up if needed. Work out concentration of the reducing agent. Write the balanced equation for the reaction. Work out the number of mols of the oxidising agent added to the conical flask. Use the stoichiometry of the equation to find the number of mols of reducing agent that has reacted. Scale up if needed. Work out the mass of reducing agent. Work out the percentage purity.

Percentage purity:

What are iodine-sodium thiosulfate titrations? Used to find the concentration of oxidising agents. The more concentrated the oxidising agent is, the more ions will be oxidised.

1.2.

Potassium iodate(V), KIO3, (oxidising agent) is added to excess acidic potassium iodide solution, oxidising some of the iodide ions to iodine. IO3-(aq) + 5I-(aq) + 6H+(aq) 3I2(aq) + 3H2O(l) I2(aq) + 2S2O32-(aq) 2I-(aq) + S4O62-(aq) Colour change from blue/black to colourless.

Solution produced is titrated with sodium thiosulfate, Na2S2O3.

3. 4. 5.6. 7. 8.

Find average titre. Work out the number of mols of thiosulfate that reacted. Use the stoichiometry of the second equation to work out the number of mols of iodine that reacted. Use the stoichiometry of the first equation to work out the number of mols of iodate(V) ions that reacted. Scale up if needed. Calculate the concentration of potassium iodate(V).

How can iodine-sodium thiosulfate titrations be used to calculate percentage copper in a copper alloy?1.

2. 3.

Weighed amount of copper alloy dissolved in conc nitric acid and made up to 250cm3 with deionised water in a volumetric flask. 25cm3 portion added to conical flask Excess nitric acid is neutralised with sodium carbonate solution, until white ppate appears.White ppate removed by adding ethanoic acid.

4.

Excess potassium iodide added, which reacts with copper(II) ions.2Cu2+(aq) + 4I-(aq) 2CuI(s) + I2(aq) CuI is seen as a white ppate.

5.

Product mixture is titrated against sodium thiosulfate solution.I2(aq) + 2S2O32-(aq) 2I-(aq) + S4O62-(aq)

6. 7. 8. 9. 10. 11.

Find average titre. Work out the number of mols of sodium thiosulfate that reacted. Use the stoichiometry of the equations to find the number of mols of copper. Scale up. Work out the mass of copper. Work out percentage of copper in the copper alloy.

What errors can occur in iodinesodium thiosulfate titrations? Starch indicator must be added when most of the iodine has reacted, otherwise the blue colour will be very slow to disappear. Starch indicator must be freshly made, otherwise it wont behave as expected. Ppate of CuI makes seeing the colour of the solution quite difficult. Iodine produced in the reaction can evaporate from the solution, giving a false titration reading, leading to a percentage of copper lower than the actual value. Avoid this by keeping the solution cool.

What are fuel cells? Produces electricity by reacting a fuel such as hydrogen with an oxidant such as oxygen. 1. At the anode (fuel), the platinum catalyst splits H2 into protons and electrons. 2. Polymer electrolyte membrane (PEM) only allows H+ ions across, and forces electrons to travel around the circuit to get to the cathode. 3. This created an electric current in the circuit, with voltage about 0.6V, which is used to power things. 4. At the cathode, O2 combines with H+ from the anode, and electrons from the circuit to make water, which is the only waste product. Cell continues to produce a current as long as there is hydrogen and oxygen available. Hydrogen has to be made to use the fuel cell: Natural gas reacted with steam to produce hydrogen gas gives carbon dioxide as a waste product and needs fossil fuels to heat the process. Electrolysis of water to produce hydrogen requires electricity, which is generated using fossil fuels, but could be generated sustainably using renewable resources.

What fuels can be used in fuel cells other than hydrogen? Hydrogen-rich fuels are fuels with a high percentage of hydrogen in their molecules, e.g. ethanol or methanol. These are converted into hydrogen by a reformer.

Some fuel cells can use alcohols without have to reform them to produce hydrogen first. Alcohol oxidised at the anode in the presence of water.

H+ ions pass through the electrolyte and are oxidised to water.

CH3OH + H2O CO2 + 6e- + 6H+ 6H+ + 6e- + (3/2)O2 3H2O

Advantages of using alcohols: Higher hydrogen density than liquefied hydrogen. Methanol and ethanol can be made on a large scale using renewable resources. Alcohols are liquid at room temperature so dont need refrigerated stora