Chemistry - Diffusion

bonkysleuth SG

Hi this question is regarding diffusion of 2 gases in glass tube. 36.5g of hydrogen chloride and 3.4g of ammonia gas were introduced into both ends of the tube A and B respectively. The ends of the tube were then sealed using stopper.

Stopper at B was removed after reaction was completed. moist red and blue litmus papers were inserted into glass tube at B. state with any reasons, any changes that occur (if any) to the moist litmus papers.

My guess is that there is no change because reaction is already completed and there is no longer acidic or any alkaline gases left. Is this accurate?

Thanks all.

eagle SG

Pardon me hor, UltimaOnline... snatching your job...

I also not 100% confirm of my answer... Never touch chemistry since 1999...

Ammonia: NH3
Hydrogen Chloride: HCl

NH3 + 3HCl => N Cl3 + 3 H2

This means that 1 unit of ammonia will react fully with 3 units of HCl

Molecular mass of NH3 = 7 + 3 = 10
Molecular mass of HCl = 1 + 17 = 18

so 3.4 g of NH3 will fully react with 3.4/10*18 * 3 = 18.36 g of hydrogen chloride

However, there's 36.5g of hydrogen chloride => excess of HCl around

Hence, it will be acidic.

Since blue litmus paper turns red under acidic conditions, the moist blue litmus paper will turn red, but the moist red litmus paper will remain as red.

UltimaOnline SG

Note the molar masses of the 2 gases (one of which is acidic, one of which is basic). The 2 gases will combine (once they diffuse towards and reach each other) to form ammonium chloride solid (which will not be formed equidistant from the two ends; due to different molar masses and hence rates of diffusion).

When heated (usually the question includes heating; in this question, even if no heat is supplied, the solid would still decompose over time; this is an equilibrium reaction, position of equilibrium depends on both temperature and pressure; here removing the stopper reduces the pressure, supporting the decomposition), the solid decomposes into the 2 gases.

The lighter gas will diffuse faster than the heavier gas. Since the lighter has is basic, the moist red litmus paper will turn blue first. When the heavier gas which is acidic, reaches the litmus papers subsequently, the colour of the litmus paper will naturally turn red.

UltimaOnline SG

Hi Eagle :)

Yes, your point regarding stoichiometry is valid.

36.5 g of HCl indicates 1 mol of HCl.

3.4 g of NH3 indicates 0.2 mol of NH3.

Indeed HCl is in large excess. Which means that the litmus papers would remain red (acidic) throughout.

My previous post is really more for the typical question that involves equal moles of NH3 and HCl.

So we're both right here :)

eagle SG

i didn't even know this is called stoichiometry...

UltimaOnline SG

Pardon me hor, UltimaOnline... snatching your job...

I also not 100% confirm of my answer... Never touch chemistry since 1999...

Ammonia: NH3
Hydrogen Chloride: HCl

NH3 + 3HCl => N Cl3 + 3 H2

This means that 1 unit of ammonia will react fully with 3 units of HCl

Molecular mass of NH3 = 7 + 3 = 10
Molecular mass of HCl = 1 + 17 = 18

so 3.4 g of NH3 will fully react with 3.4/10*18 * 3 = 18.36 g of hydrogen chloride

However, there's 36.5g of hydrogen chloride => excess of HCl around

Hence, it will be acidic.

Since blue litmus paper turns red under acidic conditions, the moist blue litmus paper will turn red, but the moist red litmus paper will remain as red.

>>> i didn't even know this is called stoichiometry... <<<

It's ok, as long as you can do it, doesn't matter what you call it.

But hor... ummm... your equation is a little off, Eagle.

>>> NH3 + 3HCl => N Cl3 + 3 H2 <<<

Why would nitrogen be tri-chlorinated and molecular hydrogen released? There's no chlorine radical or free radical substitution involved.

What actually happens in fact, is a proton transfer from the acid to the base. Nitrogen in ammonia has a lone pair that abstracts the proton from hydrogen chloride (heterolytic cleavage of the covalent bond between H and Cl, forming Cl- anion and NH4+ cation), and the consequent electrostatic attraction between the NH4+ cation and Cl- anion results in an ionic solid, ammonium chloride.

But thumbs up for contribution to the stoichiometry aspect... even if you didn't know whatsiscalled.

eagle SG

hmmm

I relearned some chemistry :D

bonkysleuth SG

Thanks guys. To reinforce what you have just said, may I know does removing the stopper (if there's equal moles of NH3 and HCl) decompose the new solid that has been formed back to the original form?(ammonia and HCl)

UltimaOnline SG

Yes, because removing the stopper decreases pressure by increasing available volume, causing the position of equilibrium to shift to the right (ie. decomposition to occur).

NH4Cl (s) ---> NH3 (g) + HCl (g)

Decomposition is also endothermic, (which is why usually this question involves heating), and hence heating will also cause position of equilibrium to shift to the right (ie. decomposition to occur).