Question About Enthalpy

[Sonim]

This is not am homework problem, just an personal question. So, my book introduces Thermochemistry with the concept of heat and how a calorimeter works. After that, it explains the story behind Hess Law and says that one of the reasons it was created is because a calorimeter can't calculate fast chemical reactions, but it doesn't really say the problem with it. I am supposing the heat is not enough to evaporate the water in the calorimeter and the chamber where the reactions happens is centralized in the calorimeter. Maybe there is something to do with some heat leaving the calorimeter. Why can't it calculate fast chemical reactions? Won't the water in the calorimeter heat up or cool down after some time?

Edited August 28, 2019 by Sonim

[studiot]

24 minutes ago, Sonim said:

This is not am homework problem, just an personal question. So, my book introduces Thermochemistry with the concept of heat and how a calorimeter works. After that, it explains the story behind Hess Law and says that one of the reasons it was created is because a calorimeter can't calculate fast chemical reactions, but it doesn't really say the problem with it. I am supposing the heat is not enough to evaporate the water in the calorimeter and the chamber where the reactions happens is centralized in the calorimeter. Maybe there is something to do with some heat leaving the calorimeter. Why can't it calculate fast chemical reactions? Won't the water in the calorimeter heat up or cool down after some time?

 

I would be interested to learn more of this, particularly the context.

Can you post a scan or photo of the page ?

What is the textbook please?

[Sonim]

I am brazilian, so you might have a little trouble understanding. So the book is called Química: Cotidiano e Transformações Parte II( Chemistry: Day by Day and Transformations). The first chapters are about solutions( density, molar concentration, etc) and colligatives properties, then we get to Thermochemistry. It starts with the concept of heat and talking about energy, later we get to endothermic and exothermic reactions. After all that, we are introduced to what is a calorimeter and Hess Law. I am posting the pic that says about the Hess Law.

 

Edited August 28, 2019 by Sonim

[studiot]

Thank goodness your English is excellent.

I can see from your page where is says that Enthalpy depends on temperature and pressure.

And that some reactions may be 'incomplete' in that further reaction is possible such as the combustion of carbon to form carbon monoxide (partial or incomplete)  or carbon dioxide (complete).
of course the incomplete product is itself combustible.

But I cannot see where is suggests that Enthapy depends on the speed of the reaction.
Perhaps you can point this part out?

There are several types of calorimeter.
Perhaps the special type used to measure the Enthalpy of combustion was not invented in Hess's day.
This type of calorimeter is called a 'bomb' calorimeter.
These are fired by an electric element to complete the reaction quickly.

[Sonim]

Sure, also I am posting the standard calorimeter the book is considering here. This is the direct translation of the paragraph:

As we saw, the heat involved in a transformation can be determined using a calorimeter. However, for many reasons, the calorimeter can't be used to determine the heat for every chemical reaction. There are situations where the reaction can be too slow or fast, can be explosive, it can even occur simultaneously with others undesirable reaction, what makes it impossible to measure the enthalpy variation for a specific reaction. How can we determine the heat involved in these type of transformations?

Edited August 28, 2019 by Sonim

[studiot]

Thanks for the translation. It is very useful.

I think it would be good to bear in mind that this is an introduction to the subject and also intended to be a lead in to indirect methods e.g. using Hess's Law.

The calorimeter offered is very basic. As I said there are several types of calorimeter including fast calorimeters.

https://www.springer.com/gp/book/9783319313276

That said, your book makes good points that many chemical reactions are complicated by side reactions, or are multistep so that one step happens very fast and another relatively slowly.
If you want the individual heats for each step this can be tricky to measure.

Perhaps other members here can contribute soemthing as well ?

+1 for the effort you have put in to holding a good discussion.

 

Here are a couple of points about Chemical themodynamics teminology you might like to keep in mind for the future.

1) Be sure about the difference between forms of energy, heat and work. They are accounted separately in Thermo.

2) An endothermic reaction refers to heat absorbed not energy absorbed and an exothermic reaction to heat evolved, not energy evolved.

3) Be aware that there are different sign conventions for energy accounting. Chemists use the simplest and most logical. Engineers and some Physicists use an older convention from the days of steam engines which leads to different signs in some equations.
The Chemists' convention is simple: All forms of energy evolved (heat, work  etc) are counted as negative. All forms of energy absorbed (heat, work  etc) are counted as positive.

Using this convention the First Law says

ΔE = q + w

Where

E is internal energy of the system

q is heat absorbed by the system

w is work done on the system  = - (work done by the system) ie is negative if the sysem does work.

Hope this helps.