Summary
Summary Chemistry notes.
- Course
- Institution
- Book
Notes! Summary notes.
[Show more]
Connected book
Book Title:
Author(s):
- Edition:
- ISBN:
- Edition:
Seller
Follow
alizay
Content preview
Chem FactsheetJanuary 2002 Number 29
Answering Questions On Born-Haber Cycles
To succeed in this topic you need to thoroughly understand how to write Born-Haber cycles (covered in Factsheet No. 28)
After working through this Factsheet you will know and understand:
• how to convert Born-Haber cycles into numerical answers given the relevant data;
• how the data given is used to test the understanding of definitions by candidates;
• the relationship between the marks for a question and the balance between writing the cycle and calculating an answer;
• the use of enthalpy level diagrams to test the understanding of candidates of definitions and Born-Haber cycles.
Questions on Enthalpy Level Diagrams 2. Calculating a numerical answer given all the other values and the diagram
There are two types of questions on these diagrams:
1. Identifying enthalpy changes
2. Calculating a numerical answer from given values on a diagram Ca2+(g) + O2−(g)
1. Identifying enthalpy changes X
Ca2+(g) + O(g)
Zn2+(g) + 2Br(g)
+250 KJ mol−1
c Zn2+(g) + Br2(g) b 2+
Ca (g) + ½O2(g)
d Zn+(g) + Br2(g) Zn2+(g) + 2Br−(g)
+1150 KJ mol−1 -3512 KJ mol−1
e Zn(g) + Br2(g) a Ca+(g) + ½O2(g)
f Zn(s) + Br2(g)
g +590 KJ mol−1
ZnBr(s)
Ca(g) + ½O2(g)
Example 1
+192 KJ mol−1
Question: Identify the stages a – g
Ca(s) + ½O2(g)
Method
For each arrow look to see what has changed. It is then a matter of -635 KJ mol−1
deciding which definition it matches and how many moles. CaO(s)
Answer: Question What is the value of X?
a = gaseous ions to ionic solid = L.E.
b = 2Br(g) → 2Br−(g) = 2 × 1st E.A. Method: Either convert to an enthalpy cycle:
c = Br2(g) → 2Br(g) = 2 × !H°at [½Br2]
d = Zn+(g) → Zn2+(g) = 2nd I.E. Identify each of: elements, compound and gaseous ions, then draw the
e = Zn(g) → Zn+(g) = 1st I.E. diagram:
f = Zn(s) → Zn(g) = !Hat°[Zn(s)] ∆Hf°
Elements Compound
g = elements → compound = !Hf° [ZnBr2(s)]
∆H1 L.E.
Gaseous ions
Example 2
Question: How are a – g linked by Hess’s Law? !H1 + L.E. = !Hf°
Method: Identify on the diagram: elements, compound and gaseous ions Identify the components in the diagram and put in the values.
∆Hf°
Elements Compound Answer -635 = + 192 + 590 + 1150 + 250 + X - 3512
-635 = X – 1330
∆H1 L.E. X = + 1330 – 635 = +695 kJ mol-1
gaseous ions
Or follow the diagram round, starting and finishing with the elements:
So: !H1 + L.E. = !Hf° 192 + 590 + 1150 + 250 + X + (-3512) − (-635) = 0
Answer: f + e + d + c + b + a = g Note: The direction of the arrow tells us whether to add or subtract. We are
going in the direction of the arrow for the -3512, so we add (-3512).
Or alternatively, "follow the diagram round" - starting and finishing with
the elements. If you go in the same direction as an arrow, you add the X − 695 = 0 so X = +695 kJmol-1
corresponding value; if in the opposite direction, subtract the value.
Finally, equate to zero: f + e + d + c + a − g = 0 For practice on this type of calculation, go to question 1.
1
The benefits of buying summaries with Stuvia:
Guaranteed quality through customer reviews
Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.
Quick and easy check-out
You can quickly pay through credit card or Stuvia-credit for the summaries. There is no membership needed.
Focus on what matters
Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!
Frequently asked questions
What do I get when I buy this document?
You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.
Satisfaction guarantee: how does it work?
Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.
Who am I buying these notes from?
Stuvia is a marketplace, so you are not buying this document from us, but from seller alizay. Stuvia facilitates payment to the seller.
Will I be stuck with a subscription?
No, you only buy these notes for $9.05. You're not tied to anything after your purchase.
Can Stuvia be trusted?
4.6 stars on Google & Trustpilot (+1000 reviews)
52510 documents were sold in the last 30 days
Founded in 2010, the go-to place to buy study notes for 14 years now