rate constant and temperature

[drdrdrdrdrdrdr]

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Hello everyone!
New to SDN...and slightly intimidated and impressed!

Why is temperature the only thing that affects rate constant? A conceptual explanation will be greatly appreciated.

Many thanks

 

[MT Headed]

Welcome!

Mathematically, the rate constant is derived from Arhenius equation which hides a lot of constants in the A, but clearly depends on T in the exponent. The higher the T the higher the rate.

Conceptually, a higher temperature means the molecules are moving faster, which means they bang into each other more frequently and when they do collide they have enough energy to break apart their old bonds. If you like those humpy graphs, temperature supplies the energy to get over the hump of the transition state and into the products.

Is there something else you think ought to affect the rate constant?

 

[dranet]

Spot on MT Headed.

 

[MedPR]

Hello everyone!
New to SDN...and slightly intimidated and impressed!

Why is temperature the only thing that affects rate constant? A conceptual explanation will be greatly appreciated.

Many thanks

Welcome!

Mathematically, the rate constant is derived from Arhenius equation which hides a lot of constants in the A, but clearly depends on T in the exponent. The higher the T the higher the rate.

Conceptually, a higher temperature means the molecules are moving faster, which means they bang into each other more frequently and when they do collide they have enough energy to break apart their old bonds. If you like those humpy graphs, temperature supplies the energy to get over the hump of the transition state and into the products.

Is there something else you think ought to affect the rate constant?

k=Ae^-Ea/RT

Other changes (pressure, volume, moles, etc) can affect the equilibrium of a reaction, but none of them will change the rate constant. The rate constant is dependent on Temperature.

 

[SaintJude]

Can I clarify something? The rate constant is only dependent on the temperature. Then Kaplan says "The addition of a catalyst, lowers the activation energy, thereby increasing the value of k."

So if there was an MCAT question that asked you to choose which factors can raise the values of k?

I. Temperature
II. Catalyst
III.. blah

Would you chose I &II only? or just I?

 

[MT Headed]

I and II only.

Other ways to affect the rate constant besides changing Ea or T would be to change e, R, or A. The first two are universal constants (well e is even a constant in other universes too I suppose), and if you change A you are changing the molecules that participate in the reaction itself.

 

[drdrdrdrdrdrdr]

Hi MT headed and everyone

Thank you so much for quick response!

I keep thinking back to PV/T, and therefore, I feel all three things are related. It was hard for me to understand... if change in T will change the k...how come change in P or V will not also change the k?

Equation makes it painfully clear, but since you asked what else I was thinking of, I was wondering why not P and V

 

[MT Headed]

Again, mathematically we can all see that the equation only relies on temperature. But what is temperature? It is a measurement of the kinetic energy contained in the molecules. You've probably seen

KEavg=(3/2)kT

by now, which tells us that literally the energy contained in the motion of molecules depends on temperature and nothing else. It is this kinetic energy of colliding molecules that supplies enough energy to break old chemical bonds and form new ones. More temp -> more kinetic energy -> more often the chemical reaction will occur.

There are several ways to change temperature and pressure. For instance if you applied work to the top of a piston that contained gases about to react, you would lower the V. You would raise the P. But most importantly you would also raise the temperature and that would cause the reaction to occur faster. If you surrounded this piston in a bath of water at a constant temperature, you could conceivably change the pressure and/or the volume of the piston without changing the temperature of the contents. And then the rate of the reaction would not change.

 

[milski]

You have to be careful and distinguish between rate constant and the rate of reaction. The rate constant depends on how hard molecules need to hit each other, if their orientation matter and other general properties of the reaction.

For most reactions the rate of reaction itself will depend not only on the rate constant but on the concentration/partial pressure of at least one of the reagents. Changing only V or P will change the rate of reaction but not the rate constant.

Reactions which depend only on the rate constant, also known as zero order reactions, usually are restricted by having to be in contact with some sort of solid catalyst or similar geometric considerations.


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[MedPR]

Hi MT headed and everyone

Thank you so much for quick response!

I keep thinking back to PV/T, and therefore, I feel all three things are related. It was hard for me to understand... if change in T will change the k...how come change in P or V will not also change the k?

Equation makes it painfully clear, but since you asked what else I was thinking of, I was wondering why not P and V

Because if you change P or V you don't necessarily change T. If you were to hold V and n constant, but change P, then that would necessitate a change in T and thus a change in the rate constant.

In other words, changing P or V may lead to a change in the rate constant, but only if the change first leads to a change in Temperature. If any change leads to a change in Temperature, the rate constant changes. Rate constants are specific for certain temperature.