brightsky's Chem Thread

[brightsky]

Is the decomposition of HI (2HI --> H2 + I2) exothermic or endothermic?

[zvezda]

Is the decomposition of HI (2HI --> H2 + I2) exothermic or endothermic?

I wouldnt think thats something youd necessarily need to know. If a questions about the reaction were to pop up, theyd give you the info im pretty sure

[Alwin]

I wouldnt think thats something youd necessarily need to know. If a questions about the reaction were to pop up, theyd give you the info im pretty sure

Yup, or they'd have to provide bond energies and then have half the state complain at the "unfairness" of such a q.

But, if you were asking because you genuinely wanted to know,
½ H2(g)  +  ½ I2(g)   ------- >  HI(g)
Endothermic and the ∆H = +28 kJ

So, decomposition of HI (2HI --> H2 + I2) should be exothermic and ∆H = -56 kJ ...

[brightsky]

@stankovic: Yeah I think the question is bit a dodgy, because it didn't tell us whether the reaction was exothermic or endothermic.
@Alwin: I'm getting mixed messages from google. Some sites say that the decomposition of HI is endothermic, while the formation of HI is exothermic...:/

[SocialRhubarb]

Yeah, if the formation of HI is exothermic, then its decomposition, the back reaction for the formation of HI, will be endothermic.

[brightsky]

Two questions:

1. When drawing concentration-time graphs, is it valid to state, as a general rule, that the higher the concentration, the greater the instantaneous increase in concentration (e.g. if we decrease the volume of the vessel that contains the system A(g) + B(g) --> C(g), where [A]>[ B ]>[C] at t=e1, will it always be true that A will experience the greater concentration increase, followed by B, followed by C?)?

2. If a question told us that a particular reaction is conducted at 25 degrees Celsius, does that mean that we need to adjust the number of sig figs to which our final answer is expressed accordingly (e.g. express it to 2 sig figs, etc.)? Do such data influence the degree of significance? What if the question told us that the reaction is conducted in a 1 L vessel? Does that mean we will have to express our final answer to 1 sig fig?

Thanks!

[lzxnl]

Two questions:

1. When drawing concentration-time graphs, is it valid to state, as a general rule, that the higher the concentration, the greater the instantaneous increase in concentration (e.g. if we decrease the volume of the vessel that contains the system A(g) + B(g) --> C(g), where [A]>[ B ]>[C] at t=e1, will it always be true that A will experience the greater concentration increase, followed by B, followed by C?)?

2. If a question told us that a particular reaction is conducted at 25 degrees Celsius, does that mean that we need to adjust the number of sig figs to which our final answer is expressed accordingly (e.g. express it to 2 sig figs, etc.)? Do such data influence the degree of significance? What if the question told us that the reaction is conducted in a 1 L vessel? Does that mean we will have to express our final answer to 1 sig fig?

Thanks!

1. Percentage? Nah, they're all the same. If you mean magnitude-wise, then yes.

2. If it says 1L, then yes you'll have to express it to 1 sig fig, unless the volume wasn't needed in the calculation of the result. As for 25 degrees Celsius, I'm still inclined to say that it's three sig figs (298 K) but don't quote me on that.

[brightsky]

1. In a newer version of the steam methane reforming (SMR) process (CH4(g) + H2O(g) <--> CO(g) + 3H2(g), dH = +206 kJ/mol), the reforming reactions occur in a tube surrounding a palladium membrane. The membrane selectively separates hydrogen from the gas mixture. Identity one disadvantage of this method of steam methane reforming (SMR).

2. The properties of methane include: toxic to humans, burns with an invisible flame, can be absorbed into the body by inhaling, dissolves in water in all proportions, passes through the skin into the bloodstream. Describe a specific safety procedure which would be used to protect workers in the manufacture, storage, or distribution of methane.

[jgoudie]

Not sure what exactly number 1 is asking for but here is my educated guess:

1) It is endothermic so will require a large amount of energy on a commercial scale.  Palladium is an expensive metal and thus the cost would be higher.  Carbon Monoxide is produced which is a toxic gas (however this is still produced in normal steam reforming).

2) Well the most obvious, workers should wear breathing apparatus and areas should be well ventilated.  Containers should be labeled accordingly.  Another which could be interesting is what happens with methane in general, an extra strong smelling gas is added to allow people to detect leaks.

1. In a newer version of the steam methane reforming (SMR) process (CH4(g) + H2O(g) <--> CO(g) + 3H2(g), dH = +206 kJ/mol), the reforming reactions occur in a tube surrounding a palladium membrane. The membrane selectively separates hydrogen from the gas mixture. Identity one disadvantage of this method of steam methane reforming (SMR).

2. The properties of methane include: toxic to humans, burns with an invisible flame, can be absorbed into the body by inhaling, dissolves in water in all proportions, passes through the skin into the bloodstream. Describe a specific safety procedure which would be used to protect workers in the manufacture, storage, or distribution of methane.

[brightsky]

The mercury (II) oxide/zinc metal cell is used as a source of energy for certain scientific instruments. The E0 values for the two half reactions in this cell are:
Reduction half-equation: Zn(s) + 4OH-(aq) --> ZnO2 2-(aq) + 2H2O(l) + 2e-
Oxidation half-equation: HgO(s) + H2O(l) + 2e- --> Hg(l) + 2OH-(aq)
Main features of the cell:
- Zinc rod dipped in solution containg K+, OH-, ZnO2 2-
- Below that there is a layer of mercury (II) oxide
- Below that there is a layer of mercury
- Below that there is a layer of inert metal base

1) There is a physical gap between the Zn rod and the HgO(s) which ensure that the electrons will flow through the eternal circuit once connected. However, in reality, there would be a porous separator in addition to the gap between the Zn rod and the HgO. Explain why this separator is needed fo this cell and why it still must be porous.

2) The mercury II oxide/Zn cell is a primary cell and therefore cannot be recharged. Suggest two distinct reasons as to why this is the case.

[brightsky]

Can anyone explain to me why, in the VCAA 2012 Chemistry Exam 1 Report, they have NaOH(aq)+HCl(aq) <---> NaCl(aq) + H2O(l) instead of the single headed arrow?

[Patches]

It's still an equilibrium reaction, even if the k value is very high because they're strong acids and bases.

[brightsky]

Hmm ive always been under the impression that if the k value of a reaction is exceedingly large, a singleheaded arrow is employed in lieu of a reversible arrow...apparently students that year got marks taken off for putting singleheaded arrow...or maybe my teacher is just being overly pedantic...

[lzxnl]

You've got to be kidding. At 298 K, the value of K is 10^14. You need a limit somewhere to enable one way arrows.

[brightsky]

Okay so im presuming vcaa is just being dodgy....

Also with mass spec, when im asked to give the formula of the fragment that gives rise to a particular peak, im never sure whether to include the . Indicating presence of unpaired electron or not. Is there a quick and foolproof way of checking whether a fragment ion is a radical?