# Double Displacement Question

1. Aug 5, 2015

### in the rye

1. The problem statement, all variables and given/known data
"The following reaction is a metathesis reaction. Please predict the products from the given reactants and label the driving force."

2. Relevant equations
MgO + HI -> ?

[edited]
3. The attempt at a solution
MgO(aq) + HI(aq) -> MgI2(aq) + H2O(l)
MgO being soluble because of the hydroiodic acid.
This makes the driving force water.

Last edited: Aug 5, 2015
2. Aug 5, 2015

### Staff: Mentor

No such thing as OH(aq).

I guess "driving force" (it is not a standard term in chemistry, although it makes limited sense for some simple cases, as a variant of Le Chatelier's principle) is a product that is highly stable or in some way removed from the mixture - either as an evolving gas, or as a precipitate.

3. Aug 5, 2015

### in the rye

Think I got it, is it:

MgO(aq) + HI(aq) -> MgI2(aq) + H2O(l)
MgO being soluble because of the hydroiodic acid.
This makes the driving force water.

Last edited: Aug 5, 2015
4. Aug 6, 2015

### Staff: Mentor

Yes, this is correct. Not balanced though.

This is a dangerous way of understanding of what is going on. Either you are taught a broken concept, or you misunderstood what you are being taught.

Driving force is always a change in free energy of the system (ΔG). This in turn depends on the change in enthalpy and entropy - ΔG=ΔH-TΔS. Reactions are spontaneous whenever ΔG > 0.

Are you sure "driving force" (as defined by your teacher/textbook) refers to a particular substance, and not to production of that particular substance? That would be still wrong in general, but much better.

5. Aug 6, 2015

### in the rye

The Professor using driving force for the production of gas, precipitate, or the presence of water in the product. She briefly said that for our cases we will use water as a driving force, but most chemists don't consider it to be one. It's a Summer course, so the depth of it has not been the greatest. And the book at times is hard to follow. I've found myself learning a lot from the aids, rather than the class or book.

I have one more question I've come across since posting this. If you could take a look at it, I'd appreciate it.

1. The problem statement, all variables and given/known data
"Write out the following reaction: Carbon reacts with nitric acid to produce nitrogen dioxide, carbon dioxide, and water. Is this a redox reaction? Is a specific type of redox reaction, specifically, combination, displacement, or decomposition?"

2. Relevant equations
n/a

3. The attempt at a solution

C + 4H(NO3) -> 4NO2 + CO2 + 2H2O

This one I balanced, and found it is a redox reaction. However, it doesn't stand out to me by anything we've covered (we've gone over single displacement, double displacement, combination, and decomposition). After doing a little bit of (online) research, I think it's a combustion reaction because of the product containing two gases and water. Is this right? She's not expecting us to know that, but I was just wondering for myself.

6. Aug 6, 2015

### epenguin

in the rye said: https://www.physicsforums.com/threads/double-displacement-question.826380/goto/post?id=5189947#post-5189947 [Broken]
Think I got it, is it:

MgO(aq) + HI(aq) -> MgI2(aq) + H2O(l)
MgO being soluble because of the hydroiodic acid.
This makes the driving force water
.

For the first your formulation is dangerous as it will not help or will mislead you for anything else. They are many situations where removal of water is a 'driving force'. They are situations where water is removed - which it is not here, the reaction is creating it! So your explanation is not logical at that point.

MgO is rather insoluble - have you ever come across 'milk of magnesia'? It is true that acids will tend to dissolve it, you would better see this if you know about ionic reactions.

If not you will have to make do with the fact that MgI2 is soluble and MgO isn't.

Last edited by a moderator: May 7, 2017
7. Aug 6, 2015

### in the rye

She literally said that if our product contains water, that water will be the driving force. No explanation beyond that. But, I see what you're saying. The reason that I got MgO is soluble is because of a solubility chart that we were provided with saying it is slightly soluble in acids. If I take that MgO is insoluble, what, then, would be the driving force? I guess that doesn't make sense to be since it's in the reactants and if the driving force is the production of a precipitate or gas, there'd be none.

I'm still lost with this second example. =/

Last edited by a moderator: May 7, 2017
8. Aug 6, 2015

### epenguin

Not a combustion reaction, which just means burning - unless you saw that happen!

http://www.etymonline.com/index.php?term=combustion&allowed_in_frame=0. (I'm sure the word 'urerere' is related to English 'ardent' and 'arson'.)

It is a redox all right, a complex reaction, I would not and doubt most chemists would be, troubled to name and classify it further.

9. Aug 6, 2015

### in the rye

Okay, thank you. I was getting frustrated.

Would this, then, be a similar situation?

Cu + 4 HNO3 -> Cu(NO3)2 + 2 NO2 + 2 H2O

It is redox, as well. But beyond that we can't really classify it? They remind me a little bit of single displacements, but not quite.

Last edited: Aug 6, 2015
10. Aug 6, 2015

### epenguin

Last edited: Aug 7, 2015
11. Aug 6, 2015

### in the rye

Thanks. So as far as classification goes, it's just a redox reaction with the HNO3 being a really strong oxidizing agent. I'm confused why she would ask us to classify it as a composition, decomposition, displacement, or metathesis and not just a redox reaction. She's giving me a migraine with this homework.

12. Aug 7, 2015

### in the rye

She ended up posting an answer key for a practice test with similar equations. She worded them exactly the same, but just labeled them as "Redox". So, I guess that's all she's looking for if they're not easily identifiable as anything beyond that. Thanks.

13. Aug 7, 2015

### epenguin

I don't blame you. Nitrogen chemistry is the trickiest, and not all has elementary rationalisation. Keeping track of oxidation numbers is meant to help, not to torture.
Classification is not much fun but seeing some of the chemistry in the laboratory is much more. So if you are not given this, google for you tube demos of oxides of nitrogen etc.
And for something quite important that you could not have predicted, google "diazotization you tube". You'll find both lab demos and theory. Don't worry if you don't understand everything, look at a number because individually the ones I saw are a bit rough!

Last edited: Aug 8, 2015