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14 & 15
Redox+Volumetric Analysis2
Question | Answer |
---|---|
Oxidation in terms of electron transfer: | takes place when a substance loses electrons and becomes more positive |
Reduction in terms of electron transfer: | takes place when a substance gains electrons and becomes more negative |
Oxidising Agent | is a substance that brings about oxidation in other substances and is itself reduced |
Reducing Agent | is a substance that brings about reduction in other substances and is itself oxidised |
OILRIG: | Oxidation Is Loss, Reduction Is Gain, of electrons |
Oxidation Number defintions | The oxidation number of an element is the charge which an atom of that element has or appears to have in a compound. |
Oxidation number of free elements | 0 |
Oxidation number of oxygen | -2, exceptions: Hydrogen Peroixide, (H2O2) Oxygen is -1, OF2 oxygen is +2 |
Oxidation number of Hydrogen | +1, except in metal hydrides -1, NaH |
Why is our main Oxidising Agent Potassium Permanganate not considered a primary standard? | It cannot be found in a pure enough state and decomposes in sunlight and heat |
Opposing reducing agent used for Potassium Permanganate? | Sulphuric Acid |
Why can we not use Hydrochloric Acid or Nitric Acid in our experiments with Potassium Permanganate? | If HCl were used the Cl- ions would be oxidised to Cl2 by KMnO4 If Nitric acid were used it would interfere with the reaction as it is a strong oxidising agent |
Indicator used with Potassium Permanganate? Colour change? | Potassium Permanganate is autoindicating. It goes from purple to colourless. |
Besides being our reducing agent there is a second reason for adding Sulphuric acid to this titration involving Potassium Permanganate. What is it? | To prevent formation of Mn4+ MnO2 ,Brown percipitate, as Potassium Permanganate will only oxidise Mn7+ to Mn2+ in an acidic solution, as it has enough H+ ions for complete reduction. |
Why is Iron Sulphate not considered a primary standard? | It is oxidised in air and reacts with water (hydrolysis) |
What compound provides us with a pure source of Iron Sulphate? | Ammonium Iron (II) Sulphate |
How do we prevent the Ammonium Iron (II) Sulphate from reacting with the water when dissolved? | Addition of Sulphuric Acid supplies H+ ions to the solution which prevents Hydrolysis. |
What is the ration between Fe+2 ions and Potassium Permanganate | 1:5 |
What illness is caused by lack or Iron in our diet? | Anaemia |
What is the function of iron in our diet? | It is an essential part of haemoglobin, the molecule that carries oxygen in our blood. |
Symptoms of anaemia? | Fatigue, weakness and pale skin |
One use of sodium thiosulphate in industry? | Used as a reducing agent in chemistry, water analysis, extracting gold, as an antidote to cyanide poisoning and in the development of photographs. |
Why is Sodium Thiosulphate not considered a primary standard? | It is not found in a 100% pure state & the crystals are efflorescent |
Why is Iodine not considered a primary standard? How do we supply pure Iodine to our titrations? | Iodine sublimes at room temperature. Excess Iodine is supplied by the addition of Potassium Iodide. |
What indicator is used when titrating Potassium Permanganate and Sodium Thiosulphate? When is the indicator added? | Starch indicator added. It is added when a straw yellow colour appears. We continue the titration until the colour changes from blue/black to colourless. |
What colour appears when the Potassium Permanganate and Potassium Iodide are mixed together. What causes this colour? | Purple to red-brown as pure Iodine is released from the Potassium Iodide |
Why is the bleach solution diluted with water? | The bleach is a concentrated solution which would require too much reagents for the titration |
Oxidation | addition of oxygen |
Reduction | removal of oxygen/addition of hydrogen |
Oxidation | Increase in Oxidation number |
Reduction | decrease in oxudation number |
Oxidation number of an ion | is the same as its charge |
Oxidation number of a compound | the sum of the oxidation numbers of all the elements in a compound must add up to zero |
Oxidation number of Halogens | Always have an oxidation number of -1, unless bonded to a more electronegative atomeg Cl20 Cl =+1 |
Oxidation number of a complex ion | the sum of the oxidation numbers of all the elements in a complex ion must add up to the charge on the ion. |
What is the thiosulphate ion oxidised to | the tetrathionate ion S4O62- |
Why wait until it's straw yellow, before adding the starch? | If starch were added at an early stage in the titration, the iodine present would become strongly absorbed on to the starch. |
What are the requirements of a primary standard - | High purity -Stable in air and in solution -Readily soluble in a solvent -Reasonably large relative atomic mass |
what are the three main tritation types - | Acid-base titrations, neutralisation -Redox titrations -Complexometric titrations |