questions in chemistry

[batista_123]

1. in examkrackers, it says increasing temperature always increases rate, regardless of exothermic or endothermic. in TPR, it says only for endothermic reactions this is true, for exothermic, inreasing temperature decreases rate.
which is true?

2. what affects the rate constant? only temperature or other things also?

3. what is degree of branching? how do you tell which molecule has higher degree of branching?

4. finally, ochem. in every mcat prep book, they present all these reactions. for example, an alkene can be formed this way. but wait, there is also this other way that an alkene can be formed. this other way also exists, but the temperature has to be really high. oh by the way, if the alkene has this group, then this reaction doesnt work...i am just wondering if you have to know all these reactions? i used to flashcard them and memorize, but people on sdn said i am wasting my time. i want to know how to study these reactions?

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[Marjan Islam]

These are very good questions! I've also always had a bit of trouble with them, but I sat down and figured it out once and for all for myself!

1.) Rate will ALWAYS increase when temperature is increased.

Rate k = Ae^[-Ea/RT]

That's the arrhenius equation, you don't need to know it for the MCAT. BUT, by looking at it, you see that increasing temperature INCREASES the rate. In terms of the endothermic/exothermic stuff, that affects EQUILIBRIUM.

A + heat <=> B

Here, if heat is added, the reaction going to the right. If heat is removed, reaction moves to the left (Le Chat's principle). The RATE at which things are moving left and right increases with temperature, but the DIRECTION will depend on whether the reaction is endothermic (heat is a reactant) or exothermic (heat is a product).

The reason temperature INCREASES rate is because it gives the molecules more average kinetic energy (Avg. KE = 3/2RT) which allows them to overcome activation energy (Ea which is seen in the arrhenius equation).

2.) What affects rate? Look at the arrhenius equation: temperatue (which in turn lowers Ea) affects it.

CONCENTRATION of reactions also affects rate. Because although the rate constant k = k = Ae^[-Ea/RT], the RATE LAW is:

Rate Law = k [concentration of reactants A][concentration of reactants B]

Note, the concentration if reactants can be raised to an exponent. This is done looking at that chart, the results of which are attained experimentally (you know, the one where certain reactants in trials are held constant, while another is increased or decreased, and we see how it affects the rate of reaction... if you don't know what I'm talking about, review it in a text book!)

The only thing that affects EQUILIBRIUM is temperature! NOTHING ELSE!!

Another thing that affects rate are CATALYSTS. Why? Because they lower the Ea of the reaction. Again, it increases rate so increases both forward and reverse reactions. It doesn't affect equilibrium, just speeds up the time it takes to get to equilibrium.

3.) Degree of branching is just how much a molecule has BRANCHED:

The first figure is branched more, which increases the molecule's SURFACE AREA that is exposed to the environment. This lowers boiling point usually, while melting point is more affected with how symmetrical the molecule. For the above figures, I'd guess the first has lower boiling point, while the second molecule has higher melting point.

4.) As for the reactions, know the GENERAL IDEA. Like, a nucleophile will attack the positive cabonyl carbon, and the molecule will have an addition reaction if it's a ketone or aldehyde, and substitution if it's a carboxylic acid.

I wouldn't memorize the reactions, but it'd help to know the REACTANTS used. Like H20 will add a hydroxyl group to an alkene, or hot concentrated acid will protonate the -OH group, turning it into water, the leaving group, the H+ drops off, and the alkene is re-established.

Know reducing reagents, SN1, SN2, E1, E2 etc. etc.

Know Ch. 1, 2, and 4 from the EK organic very well. Chapter 3 is just, general ideas.

 

[Bernoull]

These are very good questions! I've also always had a bit of trouble with them, but I sat down and figured it out once and for all for myself!

1.) Rate will ALWAYS increase when temperature is increased.

Rate k = Ae^[-Ea/RT]

That's the arrhenius equation, you don't need to know it for the MCAT. BUT, by looking at it, you see that increasing temperature INCREASES the rate. In terms of the endothermic/exothermic stuff, that affects EQUILIBRIUM.

A + heat <=> B

Here, if heat is added, the reaction going to the right. If heat is removed, reaction moves to the left (Le Chat's principle). The RATE at which things are moving left and right increases with temperature, but the DIRECTION will depend on whether the reaction is endothermic (heat is a reactant) or exothermic (heat is a product).

The reason temperature INCREASES rate is because it gives the molecules more average kinetic energy (Avg. KE = 3/2RT) which allows them to overcome activation energy (Ea which is seen in the arrhenius equation).

2.) What affects rate? Look at the arrhenius equation: temperatue (which in turn lowers Ea) affects it.

CONCENTRATION of reactions also affects rate. Because although the rate constant k = k = Ae^[-Ea/RT], the RATE LAW is:

Rate Law = k [concentration of reactants A][concentration of reactants B]

Note, the concentration if reactants can be raised to an exponent. This is done looking at that chart, the results of which are attained experimentally (you know, the one where certain reactants in trials are held constant, while another is increased or decreased, and we see how it affects the rate of reaction... if you don't know what I'm talking about, review it in a text book!)

The only thing that affects EQUILIBRIUM is temperature! NOTHING ELSE!!

Another thing that affects rate are CATALYSTS. Why? Because they lower the Ea of the reaction. Again, it increases rate so increases both forward and reverse reactions. It doesn't affect equilibrium, just speeds up the time it takes to get to equilibrium.

3.) Degree of branching is just how much a molecule has BRANCHED:

The first figure is branched more, which increases the molecule's SURFACE AREA that is exposed to the environment. This lowers boiling point usually, while melting point is more affected with how symmetrical the molecule. For the above figures, I'd guess the first has lower boiling point, while the second molecule has higher melting point.

4.) As for the reactions, know the GENERAL IDEA. Like, a nucleophile will attack the positive cabonyl carbon, and the molecule will have an addition reaction if it's a ketone or aldehyde, and substitution if it's a carboxylic acid.

I wouldn't memorize the reactions, but it'd help to know the REACTANTS used. Like H20 will add a hydroxyl group to an alkene, or hot concentrated acid will protonate the -OH group, turning it into water, the leaving group, the H+ drops off, and the alkene is re-established.

Know reducing reagents, SN1, SN2, E1, E2 etc. etc.

Know Ch. 1, 2, and 4 from the EK organic very well. Chapter 3 is just, general ideas.

Marjan Kudos on the response!!!! I learnt a few things from it. Good Job