5E- Redox

which half equation will show electrons as reactants

reduction

which half equation will show electrons as products

oxidation

how do you balance a half equation in acidic conditions

add waters to balance oxygens, add H+ to balance hydrogens, add electrons to balance charge

how do you balance a half equation in alkaline conditions

add double OH- needed to balance oxygen, add H2O to balance hydrogen, add electrons to balance charge

how to form a redox equation from half equations

multiple them so they have the same number of electrons, add them

how can you balance a redox reaction

use oxidation numbers- total increase in oxidation no. = total decrease in oxidation no., once oxidation and reduction are balanced remaining elements can be balanced

how to find a half equation from the redox equation and other half equation

write the redox equation as an ionic equation, scale known half equation up so the key species are removed when subtracted, move electrons (add any other species) onto the correct side (so they are added not subtracted) and simplify half equation if needed

what titration is used to find the concentration of reducing agents

manganate (VII) titration

what is the manganate (VII) titration used for

used to find concentration of reducing agents

what titration is used to find the concentration of oxidising agents

iodine/thiosulfate titration

what is the iodine/thiosulfate titration used for

to find the concentration of oxidising agents

what happens during a manganate (VII) titration

manganate (VII) ions are reduced to manganese (II) ions

write the balanced half equation for the reduction of manganate (VII) ions to manganese (II) ions in the manganate (VII) titration

MnO₄^- + 8H⁺ + 5e^- → Mn²⁺ + 4H₂O

what is the set up for the Manganate (VII) titration

burette is filled with a solution of manganate (VII) ions, test solution is measured into a conical flash using a pipette, excess sulfuric acid added

why is excess of sulfuric acid added to either the manganate solution or test solution before titrating in the manganate (VII) titration

to provide H+ ions needed for MnO₄^- to be reduced

what’s the indication of the end point of the manganate (VII) titration

first permanent pale pink colour

what are manganate ions often used to find the concentration of

2 specific reducing agents: iron (II) ions and ethanedioic acid

why might it be unsuitable to use the manganate (VII) ions and iron (II) ions redox reaction in a redox titration at very high concentrations of iron (II)

Fe³⁺_(aq) is orange so it would make the pale pink end point hard to see

what is the set up for the iodine/thiosulfate titration

standard solution of thiosulfate ions is added to the burette, a sample solution of oxidising agent to be analysed is added to the conical flask with an excess of potassium iodide- iodide ions oxidised to form iodine

what happens to the iodide ions from the potassium iodine when they are added to the oxidising agent at the start of an iodine/thiosulfate titration

they are oxidised to form iodine

how can iodine/thiosulfate reaction be used to find the concentration of oxidising agent

iodine- formed by oxidising agent when excess potassium iodide is added- can be titrated against a standard solution of thiosulfate ions

what is added close to the end point of an iodine/thiosulfate titration and why

starch- at low concentrations the brown colour of iodine is hard to see, makes judging end point when iodine has been reacted away very challenging, starch provides a clearer colour change as it turns blue black in presence of iodine and is colourless in the absence

why is the starch not added before the iodine/thiosulfate titration begins

once it is added it’s impossible to judge how close the end point of the titration is, once iodine is straw coloured (fairly pale) end point is approaching, starch is added and from this point thiosulfate can be added dropwise

what is iodine/thiosulfate titration often used for

find concentration of oxidising agents: copper (II) ions and chlorate (I) ions

what is in a half cell

contains an element in two different oxidation states, together with a means of electrical contact e.g. piece of metal

what is the function of a salt bridge

allows ions to flow between the 2 half cells

why is a salt bridge necessary in half cells

completes the circuit, otherwise charge would build up in the half cells

what are the ways charge is carried in circuits using half cells

movement of electrons through wire joining the two half cells, movement of ions through salt bridge

what is generally used as a salt bridge

concentrated solution of an electrolyte that doesn’t react with the materials in either half cell e.g. piece of filter paper soaked in saturate KNO_{3 (aq)}

if there is no solid involved in the half reaction what material is conventional to use as the electrode

platinum- it’s inert

what is the diagram for a half cell that has a metal and one of its ions

metal electrode and solution with metal ions

what is the diagram for a half cell that has two aqueous substances

platinum electrode, solution with both metal ions

what is the diagram for a half cell that has a gas and solution of one of its ions

platinum electrode, solution with ion, gas chamber with gas

what needs to be included when drawing half cell diagrams

salt bridge, voltmeter, temperature, conc. of solutions, pressure if there are any gases

define standard electrode potential of a half cell

the e.m.f. of a half cell compared with a standard hydrogen half cell measured at 298K with solution concentrations of 1 mol dm^-3 and a gas pressure of 100 kPa

what is the standard against which the potential of a half cell electrode is measured

standard hydrogen half cell

labelled diagram of standard half cell

platinum electrode, H+ solution, H2 in gas chamber

when measuring standard electrode potential why is it necessary to arrange things so that no current is allowed to flow

once current starts to flow, solution concentrations will have changed, conditions will no longer be standard

what does it mean if an electrode potential is negative

the half cell is more likely to give up electrons than standard hydrogen half cell- acts as electron donor/reducing agent

what does it mean if the electrode potential is positive

half cell is less likely to give up electrons than hydrogen half cell- accepts electrons/oxidising agent

what type of half cell equation is most likely to donate electrons

the most negative one

how do you calculate the E⁰ for the cell

E⁰ (more positive electrode) - E⁰ (more negative electrode)

E⁰ should always be a positive number

why even when combinations of half cells are correctly connected the predicted reaction may not occur

• activation energy of the reaction may be very high so rate of reaction is too slow to be observed

• actual conditions used may not be standard so predictions are no longer valid (equilibria position and electrode potential will change)

what are the conditions E⁰ values are measured

1 mol dm^-3, 298K, 100kPa

what is needed for a redox to be feasible

half-reaction for the oxidising agent must have a more positive standard redox potential than the half-reaction for the reducing agent

how to work out feasibility of redox reactions

use E⁰ data for relevant half equations to predict the reaction that should occur, compare the prediction to the scenario present e.g. are the species being talked about both on the reactant side

what are storage cells

often known as batteries, consist of fixed amounts of chemicals, all materials remain in the cell, these materials are used up as the cell discharges and energy is used, when stock of reacting materials is exhausted cell is flat and no more energy can be drawn from it- storage cell has a specific amount of energy it can supply

depending on type of cell it may be possible to reverse the chemical reaction and recharge the cell by re-forming original materials

what are fuel cells

uses fuel and oxygen which undergo an exothermic chemical reaction, fuel and oxygen flow into cell and products flow out, electrolyte remains- can be acid or alkaline fuel cells depending on electrolyte used, reaction is arranged so the energy is produced in the form of electrical energy, as long as fuel and oxygen are available the fuel cell can continue to operate, fuel cell can in principle supply an infinite amount of energy

what are the types of cells used to deliver electrical power on the go

primary cells, secondary cells, fuel cells

what is the difference between a primary and secondary cell

primary cells are non-rechargeable and can only be used one, secondary cells are rechargeable and can be used many times. both storage cells

what are some uses of primary cells

torches, remote controllers, bike lights

what are the main advantages of secondary cells over primary cells

can be recharged and reused, reducing environmental impact- waste and raw materials- and overall long term costs

examples of uses of different types of secondary cells

• lead acid batteries in motor vehicles

• lithium ion batteries in phones

• NiCD and NiMH batteries in radios etc.

which reaction occurs at the positive elctrode

the more positive reaction (reduction)

which reaction occurs at the negative electrode

more negative reaction (oxidation)

what are the main advantages of fuel cells over conventional engines

more efficient so reduces greenhouse gas emissions

other than hydrogen state another fuel which is sometimes used in fuel cells

ethanol

CH₃CH₂OH + 3O₂ → 2CO₂ + 3H₂O