An experiment is to be performed to determine the standard molar enthalpy of neu

Question

An experiment is to be performed to determine the standard molar enthalpy of neutralization of a strong acid by a strong base. Standard laboratory equipment and a supply of standardized 1.00-molar HCl and standardized 1.00-molar NaOH are available (a) What equipment would be needed? Page reference (b) What measurements should be taken? Page reference c) Without performing calculations, describe how the resulting data should be used to obtain the standard molar enthalpy of neutralization. (d) When a class of students performed this experiment, the average of the results was -55.0 kilojoules per mole. The accepted value for the standard molar enthalpy of neutralization of a strong acid by a strong base -57.7 kilojoules per mole. Propose two likely sources of experimental error that could account for the result obtained by the class.

Explanation / Answer

Ans. #a. Required equipment = Calorimeter (Coffee-cup calorimeter or Bomb calorimeter as availability. Bond calorimeter is highly sophisticated).

#b. The measurements to be taken-

            I. Volume of samples: Volume used to calculate the moles of acid or base in their respective samples using given molarity of the solution.

            II. Mass of samples: The mass of samples are required to calculate total heat change during the neutralization process. Mass can also be calculated using the formula “mass = Volume x Density”.

            III. Temperature: Initial and final temperatures of both the samples and calorimeter is to be recorded. For best accuracy, the initial temperature of all the samples and calorimeter must be the same.

#c. Balanced reaction of acid-base neutralization:

            H+(acid) + OH-(base) ---> H2O

# The amount of heat released during neutralization process is given by-

            Amount of heat gained/lost by the samples is given by-

q = m s dT                            - equation 1

Where,

q = heat lost

m = Total mass of sample (acid + base)

s1 = specific heat of reaction mixture

dT = Final temperature – Initial temperature

# Standard molar enthalpy of the neutralization reaction is given by-

            dH0 = -q / moles of H2O formed during neutralization reaction.

#d. Sources of experimental error-

I. Loss of heat from Calorimeter: The coffee-cup calorimeter is NOT perfectly insulated. So, some heat might be lost during the neutralization process. So, the recorded final temperature, and in turn, calculated value of q is lower than its actual value. A lower value of q gives a lower value of dH0.

II. Temperature recoding: If the final temperature is recorded too early or too late the thermal equilibrium phase, it would be lower than the actual final temperature. A lower temperature gives a lower value of q and, in turn, a lower value of dH0.

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