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Firstly note that in general aldehydes are more reactive than ketones. This is a result of two effects:
1. Ketones are more sterically hindered.
2. Alkyl groups are electron donating and so reduce the partial positive charge on the carbonyl carbon.
As a consequence of this difference in reactivity aldehydes are oxidised more easily than ketones and so, by selecting a sufficiently weak oxidising agent, we can distinguish the two functional groups by oxidising one but not the other.
The test begins as two separate solutions - Fehling's A and Fehling's B. The first is a light blue CuSO4 solution while the second is a solution of a chelate and sodium hydroxide.
Equal volumes of the two solutions are mixed and the sample is added. The resulting solution is heated.
Aldehyde - The aldehyde is oxidised and a brick red Cu(I) oxide precipitates out,
Ketone - No reaction occurs.
The Silver Mirror Test
This test makes use of Tollen's reagent which contains the complex [Ag(NH3)2]+. It is easily made by mixing aqueous ammonia with aqueous silver nitrate.
Aldehyde - Upon heating with Tollen's reagent solid silver metal is produced as Ag+ is reduced to Ag.
Ketone - No reaction occurs.see more
A mass spectrometer is a piece of lab equipment used to measure the relative atomic masses of atoms and molecules. It is widely used to identify substances in the lab.
The process is often automated and requires a number of sequential steps that take places in a specially built vacuum chamber.
1. The sample is injected either as a gas or vapour
2. An electron gun creates a beam of electrons which collide with the sample atoms and, by removing electrons, ionise them. This creates cations which generally have a 1+ charge; about 5% gain a 2+ charge. Molecules often fragment producing a range of molecules with different masses.
3. A series of negatively charged plates with a small gap between accelerate the cations. This creates a beam of cations in which the lighter cations move faster than the heavier ones.
4. A magnetic field applied perpendicular to the direction of travel causes the beam to be deflected. The amount of deflection is dependent on the charge-to-mass ratio (m/z); heavier ions are deflected less than lighter ones. In this way the sample beam is separated by mass.
5. The magnetic field is gradually adjusted so that ions of increasing mass strike a detector one after another. As the cations strike they remove electrons from the detector, this results in an electric current proportional to the abundance of the specific ion.
This data is fed into a computer which creates a mass spectrum; a graph of m/z against relative abundance. By comparing the peaks to known sample graphs the identity of the original sample can be ascertained.see more
Speed and velocity are both measurements of the rate of travel but their is a subtle difference.
Speed tells us the rate at which an object is moving.
Velocity tells us the rate at which an object is moving in a specific direction.
Lets examine two simple scenarios:
1. A car moving forward at 5m/s
2. A car moving backward 5m/s
In scenario one the speed of the car is 5m/s and the velocity is also 5m/s.
In scenario two the speed of the car is 5m/s but the velocity is -5m/s.
We see that because the direction of the car is opposite in the second scenario we define its velocity as negative.
Speed and velocity are examples of a scalar and a vector respectively.
A scalar is any quantity which has only a magnitude and is not direction dependent.
A vector has both a magnitude and a direction.
Examples of Scalar Quantities:
distance, time, energy
Examples of Vector Quantities:
displacement, acceleration, forcesee more