The calorimeter above was used to determine the enthalpy of the reaction between
ID: 548296 • Letter: T
Question
The calorimeter above was used to determine the enthalpy of the reaction between hydrochloric acid and sodium hydroxide. The relevant temperatures at the time of reaction (assuming the reaction was instantaneous) were determined as above. Temperatures were recorded as a function of time and extrapolated to the time of mixing. The generic graph is shown below. The temperature axis is again intentionally not displayed. The relevant information is given in the table below.
The data for the reaction between HCl and NaOH were as follows:
If the temperatures of the acid and base at the time of mixing are different, use their average as the temperature of the mixture before reaction. Use the average also as the initial temperature of the calorimeter.
What is the weight of the reaction solution in grams?
107.1 Correct
the change in temperature experienced by the reaction mixture (and the calorimeter) extrapolated back to the time of mixing?
15.5 Correct
What is the molar enthalpy, Hneut, of the reaction in:
H+ (aq) + OH- (aq) H2O (l)
in kJ/mol?
-57.3
Question:
The above experiment is repeated using the weak monoprotic acid, crotonic acid, in place of hydrochloric acid. The molar enthalpy of the reaction between the crotonic acid and sodium hydroxide, Hweak is determined to be -51.3 kJ/mol. Using Hess' Law and your result for HCl, what is the enthalpy of dissociation of crotonic acid in kJ/mol?
Value Units Concentration of HCl 2.45 M Volume of HCl 50.0 mL Density of HCl 1.02 g/mL Concentration of NaOH 2.39 M Volume of NaOH 55.0 mL Density of NaOH 1.02 g/mL Specific Heat of HCl & & NaOH & Reaction Mixture 3.97 J/g-K Temperature of HCl at 3.0 min, Tacid (from graph) 25.4 oC Temperature of NaOH at 3.0 min, Tbase (from graph) 25 oC Temperature of reaction mixture at 3.0 min, Tmix (from graph) 40.7 oC Temperature vs Time Mixture BASE baoc acid Acid 0.0 0.5 10 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.07.5 8.0 8.59.0 9.5 10.0 Time (min)Explanation / Answer
HCl (aq) + NaOH (aq) ----> NaCl (aq) + H2O (l) , H = -57.1 kJ/mole
Enthalpy of neutralization of any strong acid like HCl, HNO3 with a strong base like NaOH, KOH is always the same, because strong acids and strong bases are all completely ionized in dilute aqueous solutions.
Complete ionic equation
H+ (aq) + Cl- (aq) + Na+ (aq) + OH- (aq) ---> Na(+) (aq) + Cl(-) (aq) + H2O (l)
The common ions will cancel out
Net ionic equation
H+ (aq) + OH- (aq) ----> H2O (l)
Since strong acid and strong base completely ionize in aqueous solution, so number of H+ and OH- produced by 1 gram equivalent of strong acid and strong base is always the same.
If above experiment is repeated using the weak monoprotic acid, crotonic acid (CH3CH=CHCO2H) in place of hydrochloric acid. The molar enthalpy of the reaction between the crotonic acid and sodium hydroxide is - 51.3 kJ/mol .
NaOH ionizes completely as : NaOH (aq) ----> Na+ (aq) + OH- (aq)
CH3CH=CHCO2H (aq)<--------> CH3CH=CHCO2(-) (aq) + H+(aq)
When H+ of acid combine with OH- of base the equilibrium shifts to right and more of crotonic acid dissociates. Some of heat produced during combination of H+ and OH- ions is used up for complete dissociation of crotonic acid. The heat, which is used up is called enthalpy of dissociation or enthalpy of ionization.
Enthalpy of dissociation of crotonic acid = (57.1kJ/mol) - (51.3kJ/mol) = 5.8 kJ/mol
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