A2 Chemistry questions- please help!

Hiya,
I’m currently doing some A2 Chemistry revision for my edexcel exam tomorrow and a few questions have come up that I was hoping someone on here could answer. I know there are quite a few questions, so even if you could answer a few that would be great :smile:

1) Why would adding a catalyst change a rate equation?

2) How do you calculate rate? Is it 1/time or 1/concentration?

3) An acid, HA has an acid dissociation constant pKa of 4.30. In a solution of HA of concentration 0.5dm3 what approximate % of HA molecules will have dissociated?

4) If you have a rate equation Rate=k[A]/[X] , what does this mean for the order with respect to [X]?

5) When is using colourimetry appropriate measure the rate of reaction? When the product/reactant is halogen ions, halogen ions in solution, halogen gas, halogens attached to organics substances?

6) If you say, double the concentration of a reactant and the rate of reaction doesn’t double but triples or increases by a multiple that is not a multiple of 2, is the order with respect to that reactant 0?

7) Is the enthalpy change of hydration basically the enthalpy change of solution for water, if that makes sense?

8) What is the significance of the half-equivalence point? I know what it is but what do you do with it?

8) Does anyone know of any list of commonly asked questions about experiments for organic questions such as “Why is ethanol used?”

Thanks soo much for your help! Good luck if you have a chem exam :smile:

(edited 12 years ago)

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Original post by ashleyb123

Hiya,
I’m currently doing some A2 Chemistry revision for my edexcel exam tomorrow and a few questions have come up that I was hoping someone on here could answer. I know there are quite a few questions, so even if you could answer a few that would be great :smile:

1) Why would adding a catalyst change a rate equation?

2) How do you calculate rate? Is it 1/time or 1/concentration?

3) An acid, HA has an acid dissociation constant pKa of 4.30. In a solution of HA of concentration 0.5dm3 what approximate % of HA molecules will have dissociated?

4) If you have a rate equation Rate=k[A]/[X] , what does this mean for the order with respect to [X]?

5) When is using colourimetry appropriate measure the rate of reaction? When the product/reactant is halogen ions, halogen ions in solution, halogen gas, halogens attached to organics substances?

6) If you say, double the concentration of a reactant and the rate of reaction doesn’t double but triples or increases by a multiple that is not a multiple of 2, is the order with respect to that reactant 0?

7) Is the enthalpy change of hydration basically the enthalpy change of solution for water, if that makes sense?

8) What is the significance of the half-equivalence point? I know what it is but what do you do with it?

8) Does anyone know of any list of commonly asked questions about experiments for organic questions such as “Why is ethanol used?”

Thanks soo much for your help! Good luck if you have a chem exam :smile:

1. A Catalyst reduces the activation energy of the reaction. This in turn means that due to boltzman distribution of energies amongst molecules, that more molecules have enough energy to react. When more molecules have enough energy to react, there is greater frequency of collisions resulting in reaction per unit time.

2. Rate= k[A][C] or more simply you can use Change in reactants/Change in time.

3. I can't remember atm.

4. That means that [x] is inversely proportional to rate. [X] doubled then rate halves.

5. Could depend on the halogen, but I would say either gas or attached to organic compound.

6. I've never heard that before, I thought that was impossible. Where are you getting these questions?

7. No, look in your text book for proper definitions.

8. Yes they frequently ask why is (an alcohol) used instead of Hydrogen in a fuel cell?