Some easy questions!

A no, MgCl₂ is not purely ionic. It has some covalent character (Mg is a small ion with a relatively high charge, therefore polarizing)

B What has dissolving got to do with anything? The reaction 2 is strongly exothermic as the CaO reacts with the water

C sulphur burns in excess oxygen to form SO₂. It can't be oxidised further without catalytic help

D burning is exothermic, yes. But that's not the question. They are asking why nitrogen and oxygen don't react at RT

Oh, okay!


I was taking the example of Ammonium Chloride with water - it is an endothermic reaction and has many commercial uses. So won't Calcium Oxide be dissolving, too?



Oh, I didn't notice that.

But even if it has to do with 'why they don't react', the statement should also be correct. If burning is always exothermic, then statement 1 is wrong.

Mind you for the last bit, i believe chemistry books don't define the reaction "burning"
and combustion is the formal word. and yes, this is a question of whether is it spontaneous process and if it is, is it fast?

But isn't reaction with oxygen aka burning?

But isn't reaction with oxygen aka burning?

it is, but would you have seen standard enthalpy of burning in standard texts?

yes, but that is irrelevant. They are asking why it doesn't happen at RT, not whether it's endo or exo thermic. Whether something is endo/exo thermic only tells you the delta H of the reaction. It says nothing about whether it will spontaneously occur.

It is the enthalpy change when one mole of an element OR compound burns completely in air to give the products in their standard stats under standard conditions. I just can't understand that even you're agreeing that burning is exothermic than why is STILL statement number one correct? Even if it has nothing to do with the reaction at RT. It's wrong at first place.


So do you mean that even if the first statement is literally wrong, we just have to answer the question in the sense in which it's being asked?

It's a multi-choice question ffs! Not all the answers are correct (that's the whole point!). You have to pick the right one!

Example:

What is a dog?

A a four legged mammal
B a glass measuring instrument
C a country in Asia

(clue : only one of those is correct)

It is the enthalpy change when one mole of an element OR compound burns completely in air to give the products in their standard stats under standard conditions. I just can't understand that even you're agreeing that burning is exothermic than why is STILL statement number one correct? Even if it has nothing to do with the reaction at RT. It's wrong at first place.


So do you mean that even if the first statement is literally wrong, we just have to answer the question in the sense in which it's being asked?

Sorry for not mentioning about it before but it's a three-statement multiple completion item.

Here's the heading for this question:

But isn't reaction with oxygen aka burning?

Zishi, not all reaction with oxygen is burning (combustion).

Take for instance the corrosion of metals in air. This is a slow process of oxidation, which certainly cannot be described as burning.

The reaction of nitrogen with oxygen could also included as simple oxidation.

Combustion requires that the reaction with oxygen be self-sustaining. very exothermic and rapid.

Materials that undergo combustion almost invariably produce a vapour which combines exothermically with oxygen via a free radical process.

I did not agree that burning MUST be exothermic, though i might have implied that it is generally so. you are breaking very strong nitrogen triple bonds here, so you'd need very much energy to do so relative to the energy released when forming the new bonds. if you have a look at the standard enthalpy of formation of NO, which is equivalent to your rxn here(construct equations to satisfy yourself); you'd find that it is endothermic.

the point is again like what plato said, you'd need to think outside the box a little bit. yes, generally combustion is exothermic, but have you thought of why that must be? how can you change that?

and even if a reaction is exothermic, and spontaneously feasible, you still might not observe it at RT because of kinetic barrier. a good example is burning of wood. why isn't your wooden chair(presumably you have one) not burning now, eventhough like you said, combustion is exothermic(it is in this case).

so in that case then, your answer is C (as statements 2 and 3 are correct). Though it's a poor question as it's only statement 2 that really answers the question. Statement 3 is true but irrelevant

Zishi, not all reaction with oxygen is burning (combustion).

Take for instance the corrosion of metals in air. This is a slow process of oxidation, which certainly cannot be described as burning.

The reaction of nitrogen with oxygen could also included as simple oxidation.

Combustion requires that the reaction with oxygen be self-sustaining. very exothermic and rapid.

Materials that undergo combustion almost invariably produce a vapour which combines exothermically with oxygen via a free radical process.

Now Zishi, here is your genuine expert...

shengoc and I will go back to our sandpit

sorry, quite right, the answer is A

All your talk of burning (which is always exothermic) confused me. The reaction

$\frac{1}{2}N_2 + \frac{1}{2}O_2 \rightarrow NO$

is indeed endothermic

Sorry for asking this again and again, but just last time - this reaction IS NOT 'burning', right?

correct.

Some reactions with oxygen are burning, yes. But many are not, charco gave the example of rusting. You wouldn't call that burning, would you?

It's a multi-choice question ffs! Not all the answers are correct (that's the whole point!). You have to pick the right one!

Example:

What is a dog?

A a four legged mammal
B a glass measuring instrument
C a country in Asia

(clue : only one of those is correct)