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# Eqbm constant watch

1. Why is the following happening?:

In exothermic reactions, why does the value of Kc increase when we decrease the temperature and why does the value of Kc decrease when we increase the temperature?
2. (Original post by laurawoods)
Why is the following happening?:

In exothermic reactions, why does the value of Kc increase when we decrease the temperature and why does the value of Kc decrease when we increase the temperature?
At equilibrium the rate of the forward reaction equals the rate of the reverse reaction.

An exothermic process has a lower activation energy than the corresponding endothermic reverse process (draw it out if you can't see why)

Increasing the temperature of an equilibrium makes more energy available and shifts the distribution of energies more towards the higher end in the Maxwell Boltzmann distribution.

This has the effect of providing a greater proportion of the particles with the required activation energy. This is true for both exo and endothermic processes, BUT as the endothermic process has a higher activation energy it lies more to the RHS of the Maxwell Boltmann curve and is affected more then the exothermic process in terms of overall proportions of particles.

Hence the rate of the endothermic process increases more than that of the exothermic process and the equilibrium moves towards the direction of endothermic change until a new equilibrium is reached.
3. CH3COOCH3 is compound E.

Suggest the structure of the fragment responsible for the major peak in the mass spectrum of compound E and state its m/z value. Write an equation showing the formation of this fragment from the molecular ion.

I thought that the m/z of the major peak was 57 but the amrk scheme is saying that this is wrong,...I hope you can see where i got the value from...pls can you explain to me why I am wrong...what does the major peak actually mean...isnt it the one with the greatest m/z value...how do you accurately determine what the major peak is...any tips.?
4. (Original post by charco)
At equilibrium the rate of the forward reaction equals the rate of the reverse reaction.

An exothermic process has a lower activation energy than the corresponding endothermic reverse process (draw it out if you can't see why)

Increasing the temperature of an equilibrium makes more energy available and shifts the distribution of energies more towards the higher end in the Maxwell Boltzmann distribution.

This has the effect of providing a greater proportion of the particles with the required activation energy. This is true for both exo and endothermic processes, BUT as the endothermic process has a higher activation energy it lies more to the RHS of the Maxwell Boltmann curve and is affected more then the exothermic process in terms of overall proportions of particles.

Hence the rate of the endothermic process increases more than that of the exothermic process and the equilibrium moves towards the direction of endothermic change until a new equilibrium is reached.
CH3COOCH3 is compound E.

Suggest the structure of the fragment responsible for the major peak in the mass spectrum of compound E and state its m/z value. Write an equation showing the formation of this fragment from the molecular ion.

I thought that the m/z of the major peak was 57 but the amrk scheme is saying that this is wrong,...I hope you can see where i got the value from...pls can you explain to me why I am wrong...what does the major peak actually mean...isnt it the one with the greatest m/z value...how do you accurately determine what the major peak is...any tips.?
5. I'm assuming it means the molecular ion peak, which is often the highest?
In which case it'd be equal to the molecular mass, which is 74.

For any fragmentation equation, simply remember that the molecule is bombarded with an electron, e-, and the products are the ion and 2 electrons, 2e-.
6. (Original post by laurawoods)
CH3COOCH3 is compound E.

Suggest the structure of the fragment responsible for the major peak in the mass spectrum of compound E and state its m/z value. Write an equation showing the formation of this fragment from the molecular ion.

I thought that the m/z of the major peak was 57 but the amrk scheme is saying that this is wrong,...I hope you can see where i got the value from...pls can you explain to me why I am wrong...what does the major peak actually mean...isnt it the one with the greatest m/z value...how do you accurately determine what the major peak is...any tips.?
Yes, it's not an easy question. I can't actually see where you get m/e 57 from!

The major peak is the fragment that is most likely to be formed. This may be because it is particularly stable, or because there is more than one way of forming it (or both).

In the structure given there are two possible fragmentations that give rise to a [CH3]+ fragment, so I would judge it to be the most likely and hence to cause the most intense line in the MS at m/e=15.
7. (Original post by laurawoods)
Why is the following happening?:

In exothermic reactions, why does the value of Kc increase when we decrease the temperature and why does the value of Kc decrease when we increase the temperature?
The equilibrium constant, K... is also proportional to total entropy change, so when you increase decrease temperature of an exothermic reaction total entropy increases and the K constant will also increase
8. (Original post by laurawoods)
CH3COOCH3 is compound E.

Suggest the structure of the fragment responsible for the major peak in the mass spectrum of compound E and state its m/z value. Write an equation showing the formation of this fragment from the molecular ion.

I thought that the m/z of the major peak was 57 but the amrk scheme is saying that this is wrong,...I hope you can see where i got the value from...pls can you explain to me why I am wrong...what does the major peak actually mean...isnt it the one with the greatest m/z value...how do you accurately determine what the major peak is...any tips.?
I'd go with 43 being your major peak.
CH3COOCH3.+ --> CH3CO+ + .OCH3 Yeah, formatting isn't great here.

For A level the RCO+ ion is usually what they look for in a chunk of their questions as it's quite a stable ion, either that or the CH3+ that the above poster mentioned due to the amount of ways it can be formed.

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