Volume affects temperature change? No. of hydrogen ions?

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Discussion Overview

The discussion revolves around a homework problem involving the temperature change observed when mixing hydrochloric acid and sodium hydroxide, as well as a question regarding the concentration of hydrogen ions in various acids. The scope includes conceptual understanding of enthalpy changes and acid dissociation.

Discussion Character

  • Homework-related
  • Conceptual clarification
  • Mathematical reasoning

Main Points Raised

  • One participant initially calculated that the temperature rise would be 8 degrees due to doubling the volume and moles, but later acknowledged that the correct answer is 4 degrees, raising a question about the relationship between enthalpy change and temperature change.
  • Another participant confirmed that enthalpy is indeed related to temperature change and prompted further exploration of this relationship.
  • In the second part of the question, a participant calculated the concentration of hydrogen ions in phosphoric acid based on its molecular formula but was corrected that it does not fully dissociate in solution, leading to confusion about the correct answer.
  • One participant expressed uncertainty about their understanding of pKa and requested an introduction to the concept.

Areas of Agreement / Disagreement

Participants appear to have differing views on the implications of the temperature change and the behavior of acids in solution. There is no consensus on the correct interpretation of the results or the understanding of pKa.

Contextual Notes

Participants have not fully explored the assumptions regarding enthalpy changes and the dissociation of acids, which may affect their conclusions. The discussion lacks clarity on the specific conditions under which the temperature changes were measured.

Who May Find This Useful

Students studying thermodynamics, acid-base chemistry, and those interested in the relationships between concentration, temperature change, and enthalpy in chemical reactions.

Kyoma
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Homework Statement



a) 100 cm3 of 0.1 mol/dm3 of hydrochloric acid was added to 100 cm3 of 0.1 mol/dm3 of sodium hydroxide and a rise in temperature of 4 degrees was observed. If 200 cm3 of 0.1 mol/dm3 of hydrochloric acid and 200 cm3 of 0.1 mol/dm3 of sodium hydroxide were used, the rise in temperature would be about?

b) Which solution contains the greatest concentration of hydrogen ions?
1. 2 mol/dm3 of sulfuric acid
2. 5 mol/dm3 of ethanoic acid
3. 2 mol/dm3 of sulfurous acid
4. 2 mol/dm3 of phosphoric acid

2. The attempt at a solution

a) I put 8 degrees as the answer since there are twice as many moles for each solution, but I was wrong. The correct answer is 4 degrees. Does this mean that enthalpy change is unaffected by the number of moles of the reactants?

b) I put 4 as the answer since its molecular formula is H3PO4, thus there are 3 moles of hydrogen ions in one molecule of the acid and multiplying it by the concentration gives me 6 moles of hydrogen ions, the highest, but I was wrong again. The answer is 1. I have no idea.
 
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Kyoma said:
a) I put 8 degrees as the answer since there are twice as many moles for each solution, but I was wrong. The correct answer is 4 degrees. Does this mean that enthalpy change is unaffected by the number of moles of the reactants?
The enthalpy is doubled -- you're right about that. What's the relationship between enthalpy and temperature change?

b) I put 4 as the answer since its molecular formula is H3PO4, thus there are 3 moles of hydrogen ions in one molecule of the acid and multiplying it by the concentration gives me 6 moles of hydrogen ions, the highest, but I was wrong again. The answer is 1. I have no idea.
Phosphoric acid can give up as many three protons. But it is not very willing to do that -- in an acidic solution, it will hang on to some of the three. This can be calculated. Have you studied pKa yet?
 
So, my answer for question one is correct? But the correct answer is 4 degrees?

And, no, I have not learned pKa yet... Is it possible if you can give a brief introduction on pKa? -.-
 
So, you are saying that for question a), I'm correct?
 

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