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Offline Science geek

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Arrhenius equation
« on: August 07, 2015, 09:02:52 AM »
I have an equation in which i must find the time in which will a substance boil. So I have the given t1(time) also T2(temperature) T2 and p1(pressure) p2, I know the Arhenius equation
k = Ae exp.(-Ea/RT)
According to that
Δt1-1/Δt2-1 =exp ((Ea/R)*(T2-1-T1-1)
But what should I do with pressure i need to use it. And i need to find t2

Offline Science geek

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Re: Arrhenius equation
« Reply #1 on: August 07, 2015, 09:15:57 AM »
And i know the Ea

Offline Babcock_Hall

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Re: Arrhenius equation
« Reply #2 on: August 07, 2015, 09:17:37 AM »
The Arrhenius equation deals with rates of chemical reaction (Ea is the activation energy reaction).  I don't see what it might have to do with the time it takes something to boil.

Offline Science geek

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Re: Arrhenius equation
« Reply #3 on: August 07, 2015, 09:22:25 AM »
The problem is:
An egg boils at sea level at a temperature of 373K for 3 minutes (101,3 kPa). If the egg boils at 363 K and pressure of 70.10 kPa. How long would it take for a egg to boil at 70.10 kPa if Ea=417 kJ/mol

Offline mjc123

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Re: Arrhenius equation
« Reply #4 on: August 07, 2015, 11:20:01 AM »
You have the information to work out the ratio of the rate constants at the two temperatures. Assuming the course of the reaction is the same, how is the time to reach a given extent of reaction related to the rate constant?

Offline Science geek

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Re: Arrhenius equation
« Reply #5 on: August 11, 2015, 05:11:12 PM »
I still don't get it. Can you just direct me what should I do. Which formulas to use

Offline Corribus

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Re: Arrhenius equation
« Reply #6 on: August 11, 2015, 05:15:51 PM »
Is this an actual homework question or something? If so, could you reproduce it in its entirety?

It is rather unconventional to use Arrhenius equation to predict a boiling time, which depends, among other things, on the amount of substance there is and the rate at which heat is applied. And, what will be your units of rate? What is the conversion quantity you are measuring? The rate is usually expressed as an amount of produced substance per unit time, not an "onset of reaction", which is what you seem to want from your original post. This just doesn't make a whole lot of sense to me.
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Offline Science geek

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Re: Arrhenius equation
« Reply #7 on: August 11, 2015, 05:36:06 PM »
That is a problem in my chemistry book. Which formulas should I use apart from the Arrhenius equation? For me this problem is so confusing. I got the initial solution 123mins, which i know is not reliable nor correct.

Offline Corribus

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Re: Arrhenius equation
« Reply #8 on: August 11, 2015, 05:37:12 PM »
Could you please reproduce the problem exactly as it is written?
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline Science geek

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Re: Arrhenius equation
« Reply #9 on: August 11, 2015, 05:42:52 PM »
An egg boils at the sea level at a temperature of 373K and pressure of 101,3 kPa .If the egg boils at 363 K and pressure of 70.10 kPa,how long would it take for a egg to boil at 70.10 kPa if Ea  is 418kJ/mol.
It is in my native language,so I translated it in english,but it is 99,5% reliable.

Offline Corribus

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Re: Arrhenius equation
« Reply #10 on: August 11, 2015, 06:15:01 PM »
This is a very strange question. I don't really see how the Arrhenius equation is directly relevant to this kind of process, but I can play along.

First, you need to identify what is actually being asked for. Rate is generally speaking a function of something per unit time. In chemistry this is usually a unit of mass or moles per unit time, or even something non-dimensional like "reaction turnovers", but we aren't given anything that obvious here. The question is asking to calculate time for a process. So I'm going to call the something a unit of 1 egg, so that the rate is "eggs per time". I'm guessing "an egg boils in t seconds" means "an egg goes from an initially uncooked state to a fully cooked state in t seconds". The reaction here is the "cooking of the egg". Cooking is obviously a series of chemical reactions, although the way it's presented here is rather abstract. I guess in that sense it could be modeled by Arrhenius kinetics, although how the activation energy is being determined isn't really clear. Anyway, my interpretation of the question is that it wants to you to use Arrhenius equation to determine the amount of time it takes to fully boil one egg when the temperature is 363 K, based on the amount of time it takes to boil one egg when the temperature is 373 K. The pressures given, I'm assuming, have no direct bearing on the solving of the problem. They are only there to specify the state of the system - i.e., to explain why the temperature is changing. The question could have just as easily, and less confusingly, left it as "at sea level" and "on a mountain". I make this conclusion because of the use of the word "boiling" - the temperature of which is determined by the pressure.

So, when it comes down to it, you basically have

Rate 1 = 1 egg per t1 seconds
Rate 2 = 1 egg per t2 seconds

Given t1 and the Arrhenius expression, with activation energy, calculate t2.

This is typically a fairly straightforward type of problem, if we assume activation energy and pre-exponential factors are temperature independent. I would approach it by leaving it in terms of rates at first and then calculating t2 afterwards. But the problem is, unless you left it out, you never specify the time for the reference condition (t1). This isn't listed anywhere in the problem? If not, I must be missing something else that's obvious, because it seems you have too many unknowns.

(My suspicion of a missing piece of information is brought about by the first sentence "An egg boils at the sea level at a temperature of 373K and pressure of 101,3 kPa". This doesn't say anything about how long the process takes, which would seem to be a crucial piece of information. It can't be determined independently from the temperature or pressure, because, as mentioned above, one egg isn't a real unit - eggs come in different sizes.)
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline Science geek

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Re: Arrhenius equation
« Reply #11 on: August 11, 2015, 06:20:28 PM »
At the sea level t  boils for 3 minutes, i wrote it ..the first time...

Offline Science geek

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Re: Arrhenius equation
« Reply #12 on: August 11, 2015, 06:36:38 PM »
CAn you do some calculation,becouse all these explanations that u have made are hard to figure out in my langugage.

Offline Corribus

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Re: Arrhenius equation
« Reply #13 on: August 11, 2015, 07:18:19 PM »
OK, look:

Call T1 = 373 K and T2 = 363 K
Represent rates by k.

kn = 1 egg per tn minutes

So

k1 = 1 egg per 3 minutes or 0.33 egg/min

Solve for k2 is the first goal.

Arrhenius expression is

[tex]k_n = A \exp{(- \frac{E_a}{RT_n})}[/tex]

Letting n = 1 and 2, you can divide the two Arrhenius expressions and get:

[tex]\ln {\frac {k_1}{k_2}} = \frac {E_a}{R} (\frac {1}{T_2} - \frac {1}{T_1})[/tex]

Using this, you can solve pretty easily for k2, and then in turn determine what t2 is (1 egg per t2 minutes). Just be careful about units between the activation energy and the gas constant. I did the calculation and got a value for t2 that made at least a general sort of sense, although I maintain that it's still a very bizarre question.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline orthoformate

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Re: Arrhenius equation
« Reply #14 on: August 12, 2015, 11:49:09 AM »
OK, look:

Call T1 = 373 K and T2 = 363 K
Represent rates by k.

kn = 1 egg per tn minutes

So

k1 = 1 egg per 3 minutes or 0.33 egg/min

Solve for k2 is the first goal.

Arrhenius expression is

[tex]k_n = A \exp{(- \frac{E_a}{RT_n})}[/tex]

Letting n = 1 and 2, you can divide the two Arrhenius expressions and get:

[tex]\ln {\frac {k_1}{k_2}} = \frac {E_a}{R} (\frac {1}{T_2} - \frac {1}{T_1})[/tex]

Using this, you can solve pretty easily for k2, and then in turn determine what t2 is (1 egg per t2 minutes). Just be careful about units between the activation energy and the gas constant. I did the calculation and got a value for t2 that made at least a general sort of sense, although I maintain that it's still a very bizarre question.

You dropped the A when taking the ln of both sides. Did you do this on purpose?

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