Kayla Price
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AS - Level Chemistry Flashcards on Redox II, created by Kayla Price on 03/06/2017.

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Kayla Price
Created by Kayla Price over 2 years ago
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Question Answer
Describe the redox titration used to determine the strength of a bleach 1) A sample of the bleach is added to acidic potassium iodide. Chlorate(I) ions (the active ingredient in bleach) oxidise the I- ions to produce iodine: ClO-(aq) + 2H+(aq) + 2I-(aq) --> I2(aq) + Cl- + H2O(l) 2) The quantity of I2 produced relates to the amount of ClO- ions present. So the solution produced is titrated against sodium thiosulfate to work out how much I2 was produced: 2S2O3^2- + I2 --> S4O6^2- + 2I- 3) The conc. of I2 is then calculated
What is an iodometric titration? Titrations involving iodine
How is the end point of a titration of iodine against sodium thiosulfate found? Starch is added when the pale yellow solution becomes very pale later on in the titration. The starch is added late because I2 bind to starch very strongly so it improves the accuracy of the titration.
What are errors? Differences between measured and true values
What are random errors? Uncertainties which arise when a measurement is repeated several times, giving values distributed equally on both sides of the true value.
What are systematic errors? An error that affects all the measurements taken in the same way, making all values have the same difference from the true value. This arises when a particular instrument with a degree of uncertainty is used to measure all the values.
What are zero errors? A form of systematic error caused by measuring instruments that have a false zero
How would you find the uncertainty of an instrument? 1/2 an increment
How are the uncertainties of different measurements combined when the measurements are added or subtracted? The are added together
How are the uncertainties of different measurements combined when the measurements are multiplied or divided? The percentage uncertainties are calculated: uncertainty of value/value x 100 and then added together
Why are salt bridges important in an electrochemical cell? It completes the circuit and allows the movement of ions across it, without it the electrons would not flow as there would be a build up of charge in each cell.
How would you measure the electrode potential of a half cell? The half cell is joined with a standard hydrogen electrode, which has an electrode potential of 0V. The EMF measured is therefore the electrode potential of the half cell. The standard hydrogen electrode always takes the left hand side.
What are the conditions required for a standard hydrogen electron? 198K temperature, 1atm pressure, all solutions 1M.
How would you determine whether a reaction is feasible using electrode potentials? Erhs - Elhs = Ecell If Ecell is positive the reaction is feasible and if it is negative the reaction is not feasible
How can you tell which electrode is more negative/positive without calculation? The sign of Ecell is always representative of the right hand electrode, so if it is the most negative the sign would be negative and is it is the most positive the sign would be positive.
In what direction would a reaction go if the electrode potential is negative? It would be more likely to oxidise - reactants favoured (its ions)
In what direction would a reaction go if the electrode potential is positive? It would be more likely to reduce - products favoured (the species in standard states)
How would a change in the standard conditions for an electrochemical cell affect the cell? It would change the position of equilibrium between the oxidised and reduced form of the species and hence affect the value of Ecell
How could you predict the extent at which a reaction would go? If the Ecell value is below +0.6V there would be an equilibrium with more products than reactants, above +0.6V the reaction would go to completion, more positive than -0.6V there would be and equilibrium with more reactants than products and more negative than -0.6V the reaction would not go.
How would you arrange an electrochemical cell to get the most feasible Ecell? Put the most negative electrode on the left hand side.
Why does the voltmeter of an electrochemical cell preferably have an infinitely high resistance? To avoid any flow of current through the circuit. If there was a low resistance in the circuit, electrons would flow from where there a lot of them to where there are less. If any current flows the voltage drops, making it lower than the actual emf.
Describe how an acidic hydrogen-oxygen fuel cell works At the anode a platinum catalyst splits the H2(g) into protons and electrons. The polymer electrolyte membrane allows H+ ions to pass through it but not electrons, which forces the electrons to pass through a circuit to get to the cathode. This creates an electric current. At the cathode O2 reacts with the H+ and electrons to form H2O, and to maintain an electrochemical gradient between the two electrodes
Describe how an alkaline hydrogen-oxygen fuel cell works. At the cathode the platinum catalyst catalyses the reaction between O2(g) and water to form OH- ions, which travel through an ion exchange membrane towards the anode. The H2(g) reacts with the OH- ions to produce electrons and water. The electrons travel from the anode to the cathode via an external circuit, creating a current.
What are the advantages and disadvantages of hydrogen-oxygen fuel cells? Advantages: More efficient Less pollution Easy to alter size and power of cell Disadvantages: Difficult to store Pollution from production of H2 fuel Platinum catalyst expensive Difficult to transport
Describe how a methanol fuel cell works At the anode the methanol reacts with water and oxidises to form carbon dioxide, H+ ions and electrons. The H+ ions travel across a PEM and the electrons travel via an external circuit to the cathode, where they react with O2(g) to form H2O.
What are the advantages and disadvantages of methanol fuel cells? Advantages: Easy to transport Readily available Non hazardous Biodegradable Doesn't require high temperatures or pressures to work Low cost Not at too high a temperature to convert N2(g) in the air to NOx Disadvantages: Produces CO2 - greenhouse gas Toxic and flammable Made from non renewable fossil fuels Low efficiency
Describe how disposable batteries work Zinc cathode and a cathode of carbon rod packed round with granules of manganese(IV) oxide. The electrolyte is a paste of ammonium chloride. The zinc oxidises to form zinc ions. At the cathode the manganese(IV) oxide is reduced to manganese(III). The electrons travel from the zinc to the cathode.
What are the advantages and disadvantages of lithium batteries? Lithium has a low density, so batteries are light and lithium is very reactive, producing a large emf. However, it readily combines with oxygen in the air, forming a non conductive later of lithium oxide. It also reacts rapidly with water.
How does a lithium cell work? At the anode, lithium oxidises to Li+ ions and produces electrons. The Li+ ions travel across a polymer electrolyte and electrons via an external circuit towards the cathode. At the cathode MnO2 reacts with the Li+ ions and reduces to form Mn2O3(s) and lithium oxide.
How would you identify the anode and cathode of an electrochemical cell? The anode is oxidised and the cathode is reduced
What are the limitations of predictions from Ecell values? They do not give an indication of the rate of reaction and only relate to standard conditions - changes in concentration, temperature and pressure affect electrode potentials, so predictions from cell emfs about feasibility of redox reactions may not occur in practice due to kinetic effects or non-standard conditions.