Question help

[Scienuk]

Hello there!

I'm currently an IB1 Chemistry student who has his finals fast-approaching, so I would appreciate it if you could answer the following question with explanation.

Answers:

3. B

9. C

15. B

Thanks,

Edited May 8, 2016 by Scienuk

[kw0573]

3. If you choose D you have to remember that D is not an ionic compound and you don't get chloride ions. If the question were short answer then you would have to convert to a proper unit. But on the unit test, you can just use mol•dm^-3cm³ to compare. A has 1, B has 1.2 * 2 = 2.4, C has 2.1. On a short answer you would have to convert 1cm³ = 0.001 dm³ and get 0.01, 0.024, 0.021 instead.
9. Dative bond means one of the "atom" supplies both electrons in a bond.A do not react. B forms 1,2-dichloroethane (but you have to have taken organic chemistry to know this), which does not have dative bonds. C is an acid base reaction, and we get NH₄F (again you have to know about acid/base reactions), and there is a dative bond between the N nucleus and the H that was previously from HF. D also do not react because chlorine gas is not strong enough to displace fluoride ions. 
15. You need the enthalpy of vaporization values for II. Bond enthalpy is only for gaseous species. 

Definitely revisit #9 when you learn about organic chemistry and acid/base reactions. But by year one you should know how to do #3 and #15.

[Scienuk]

30 minutes ago, kw0573 said:

3. If you choose D you have to remember that D is not an ionic compound and you don't get chloride ions. If the question were short answer then you would have to convert to a proper unit. But on the unit test, you can just use mol•dm^-3cm³ to compare. A has 1, B has 1.2 * 2 = 2.4, C has 2.1. On a short answer you would have to convert 1cm³ = 0.001 dm³ and get 0.01, 0.024, 0.021 instead.
9. Dative bond means one of the "atom" supplies both electrons in a bond.A do not react. B forms 1,2-dichloroethane (but you have to have taken organic chemistry to know this), which does not have dative bonds. C is an acid base reaction, and we get NH₄F (again you have to know about acid/base reactions), and there is a dative bond between the N nucleus and the H that was previously from HF. D also do not react because chlorine gas is not strong enough to displace fluoride ions. 
15. You need the enthalpy of vaporization values for II. Bond enthalpy is only for gaseous species. 

Definitely revisit #9 when you learn about organic chemistry and acid/base reactions. But by year one you should know how to do #3 and #15.

Thank you for the swift response! Just a couple of questions, if you may:

For question 15, i'm pretty sure you can use average bond enthalpies to measure (Delta) H, the inaccuracy is however the fact that water is in a liquid state and not in the gaseous state, and because of so we must add the value of enthalpy of vaporisation to the (Delta) H value obtained. I say this because some Paper II (E.g. 2015 and 2013) questions provide you with a combustion equation such as for the combustion of Methane and provide H2O in its liquid state and in the next part ask you to comment on the accuracy of the calculation, for which you answer that enthalpy of vaporisation must be considered to obtain accurate results. Additionally, the questions asks for which of the following equations can bond enthalpy data be used ONLY to measure the enthalpy change, so why can't reaction I & III's enthalpy change be measured using other methods such as the sum of the enthalpy of formation of the products minus the sum of the enthalpy of formation of the reactants and why not by calorimetry? 

[kw0573]

Second question first: #15 is strictly speaking not a terribly-phrased question. Following strictest grammar, "using only enthalpy data" means getting the answer if you just know bond enthalpy, can you do it? If you can, then the answer is yes, if insufficient information, then the answer is no. If the question means you cannot use additional methods, "only using enthalpy data" which the answer would be no in every case, and "none of the above" is not an option. This is a problem for students coming from a country not predominantly speaking English, but IB does ensure there are no "distractor choices" due to ambiguous wording of question.

Now the discrepancy between past papers and this one (which also looks to be from 2015): the answer depends on the way the question is phrased. If it is short answer and you are asked to calculate the bond enthalpy for a liquid compound, you can't just say no I can't do it. But in this multiple choice, you are given the option of applying the method on II or not, and the best answer is to select I and III only.