Questions Accepted values for the enthalpy changes of neutralisation of some aci

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Accepted values for the enthalpy changes of neutralisation of some acids and bases are shown below.

Reaction

H   kJmol-1

HCl + NaOH NaCl + H2O

-57.9

HNO3 + NaOH NaNO3 + H2O

-57.6

HBr + NaOH NaBr + H2O

-57.6

CH3COOH + NaOH CH3COONa + H2O

-56.1

1. Compare the experimental values to the values in the table for the first two reactions. Account for any errors in your results.

2.   Why is the enthalpy change for ethanoic acid slightly lower than the values for the first three reactions? (Hydrochloric, nitric and hydrobromic acids are all strong acids. Ka (ethanoic acid) =1.7x10-5.

3.   Compare the temperature rise when 4.0M sodium hydroxide is used instead of 2.0M. Explain the relative values.

Reaction

H   kJmol-1

HCl + NaOH NaCl + H2O

-57.9

HNO3 + NaOH NaNO3 + H2O

-57.6

HBr + NaOH NaBr + H2O

-57.6

CH3COOH + NaOH CH3COONa + H2O

-56.1

Explanation / Answer

Q1 -- N/A since no data is present

Q2.

the enthalpy is slightly lower since the moelcule is much larger, meaning the "heat" released by H+ OH- neutralisation is actually used to "heat up" the molecules inside the solution AND the surroundings

So some heat will remain inside the solution, i.e. not will be used in the usrrounding heating.

Also note that there will be some hydrolysis, which is endothermic, so it also requires energy to form the acid HA form H+ and A-

Q3.

Note that, there will be TWICE water, since the concentration is HALF, so the volume required is twice

So the Temperature MUST decrease, since the heat will be used to heat up MORE water, which requires MORE energy, which is not present, so ther is only one way to heat this: less TEMPERATURE

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