Double Displacement Reaction: Predicting Products and Identifying Driving Force

[in the rye]

Homework Statement

"The following reaction is a metathesis reaction. Please predict the products from the given reactants and label the driving force."

Homework Equations

MgO + HI -> ?

[edited]

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.

 

[Borek]

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.

 

[in the rye]

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.

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.

 

[Borek]

MgO(aq) + HI(aq) -> MgI₂(aq) + H₂O(l)

Yes, this is correct. Not balanced though.

This makes the driving force water.

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.

 

[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.

Homework Statement

"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?"

Homework Equations

n/a

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.

 

[epenguin]

in the rye said: https://www.physicsforums.com/threads/double-displacement-question.826380/goto/post?id=5189947#post-5189947
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.

 

[in the rye]

in the rye said: https://www.physicsforums.com/threads/double-displacement-question.826380/goto/post?id=5189947#post-5189947
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.

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. =/

 

[epenguin]

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.

Homework Statement

"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?"

Homework Equations

n/a

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.

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.

 

[in the rye]

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.

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.

 

[epenguin]

Well similar only to a point isn't it? Nitric acid is a strong a oxidising agen, but we can see some variety in reactions - maybe this is helpful. http://mrged.flexinet.com.au/download/chem12/term1/nitricredoxANS.pdf

 

[in the rye]

Well similar only to a point isn't it? Nitric acid is a strong a oxidising agen, but we can see some variety in reactions - maybe this is Heflin. http://mrged.flexinet.com.au/download/chem12/term1/nitricredoxANS.pdf

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.

 

[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.

 

[epenguin]

. She's giving me a migraine with this homework.

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!