21 THER 368 Hear of Post-Laboratory Questions (Use the spaces provided for the a

Question

21 THER 368 Hear of Post-Laboratory Questions (Use the spaces provided for the answer and additional paper necessary) 1. A student was given only one graduated cylinder 3. The accepted AHeurn of hydrobromic acid (HBr) to use for this experiment. After using it to measure reacting wrhNaoHsolution and of HNO3reacting with 50.0mLof the assigned acid, the student failedtorinse potassium hydroxide (KOH) solution are identical. ordrythe cylinder before measuring out the50,5mLof (1) Write net ionic equations to show what aque- the base. Would the calculated AHeuten be higher, ous HBr and HNO, have in common. lower, or the same as the literature AHheutan? Briefly explain this difference as a result of using only one graduated cylinder for the experiment. (2) State what aqueous NaOH and KOH have in 2. Explain how the following changes in the proce- dure for this experiment would affect the results. (1) A glass beaker was used insteadofapressed polystyrene cup. (3) Explain why you would expect that AHheutzn for HBr reacting with NaOH solution and AHeutzm for HNO3 reacting with KOH solution would be identical. Write ap- propriate equations to support this explanation. (2) A pressed polystyrene top was used to cover the polystyrene cup after the acid and base solutions had been mixed. section

Explanation / Answer

1. If only one cyllinder was used to measure both acid and the base solution. Some of the acid has already been neutralized in the cyllinder by the base. Thus the heat generated inside the calorimeter during the experiment would be lower than the actual heat and so dH neutralization would be lower than the actual value.

2. Effect of the given changes,

(2) A glass beaker is not a perfect insulator and some of the heat would be lost from it to the surrounding. Thus the dHneutralization would be lower than the actual value. therefore, a perfect insulator such as pressed polystyrene cup is used.

(2) No effect, it would give the same result with no loss of heat.

3. Reaction of HBr or HNO3 with NaOH

(1) Net ionic equation for both,

H+ + OH- <==> H2O

(2) Both aqueous bases KOH and NaOH has OH- as the common reacting species with the added acid H+.

(3) Both the acids would yield the same dHneutzn as both react in 1 : 1 molar ratio with the added base.

HBr + NaOH ----> NaBr + H2O

HNO3 + NaOH ---> NaNO3 + H2O

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