Volume affects temperature change? No. of hydrogen ions?

[Kyoma]

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.

 

[pmsrw3]

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 pK_a yet?

 

[Kyoma]

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? -.-

 

[Kyoma]

So, you are saying that for question a), I'm correct?

 

[DeeNos]

a) The volume of the reactants does not affect the enthalpy change of a reaction. The rise in temperature observed is a result of the exothermic reaction between hydrochloric acid and sodium hydroxide, which releases energy in the form of heat. The amount of heat released is determined by the amount of reactants present, not their volume.

b) The concentration of a solution is determined by the number of moles of solute in a given volume of solution. In this case, the solution with the greatest concentration of hydrogen ions would be 2 mol/dm3 of sulfuric acid, as it has the highest molar concentration of hydrogen ions (2 moles of H+ per 1 liter of solution). The molecular formula and number of moles of hydrogen ions in a molecule do not necessarily determine the concentration of a solution.