Question

Four chemical reactions occurred. All of the reactants are solutions

HCl + NaOH (25 mL of both) 2 molar for both                  -56480 J/mole

HCl + NaOH (50 mL of both) 2 molar for both                 -292900 J/mole

CH₃COOH + NaOH (25 mL of both) 2 molar for both          -56480 J/mole

CH₃COOH + NH₄OH (25 mL of both) 2 molar for both -49380 J/mole

1.       Write the net ionic equation for each of the reactions studied.

2.       Use the table of heats of formation in your textbook to calculate the standard value for the first and second experiments

3.       For the first experiment, determine your experimental error and the percentage error for this experiment.

4.       Within experimental error, how did dilution in the second experiment effect the heat of reaction per mole of water produced? Should the two values be the same or different?

5.       Were the heats of reaction per mole or reactants different for the strong acid/strong base, weak acid/strong base, weak acid/weak base reactions? Which had the highest heat of reaction? Which had the lowest? What conclusions can you make from these values? Why might the values be different from each other?

Some of the standard enthalpy values do not appear in the textbook guide. How do I find the value for the compounds if they are not in the correct state in the book.

Answer 1

1)

NaOH --> Na+ + OH-

HCl --> H+ + Cl-

Or

HCl + NaOH --> NaCl + H2O;

2)

Below data is obtained from online sources (textbook data should be close this data).

https://en.wikipedia.org/wiki/Standard_enthalpy_of_formation

Δ_f⁰ HCl (aq) = -167.2 kJ/mol

Δ_f⁰ NaOH (aq) = -469.15 kJ/mol

Δ_f⁰ NaCl (aq) = -407.27 kJ/mol

Δ_f⁰ H2O (aq) = -285.8 kJ/mol

For the first and second reactions,

Δ_f⁰ Hr = Δ_f⁰ NaCl (aq) + Δ_f⁰ H2O (aq) - Δ_f⁰ HCl (aq) - Δ_f⁰ NaOH (aq)

= -407.27 -285.8 + 167.2 + 469.15 = - 56.72 kJ/mol

= - 56720 J/mol

3)

Experimental error = -56480 + 56720 J/mol

= 240 J/mol

% Experimental error = 240 * 100 / 56480 = 0.425 %

4)

In the second experiment it appears that the heat of reaction per mole of water produced has increased upon dilution. Ideally the two values should be same (Hess’s Law).

5)

Δ_f⁰ CH3COOH (aq) = -483.5 kJ/mol

The heats of reaction per mole or reactants are different for the strong acid/strong base, and weak acid/weak base reactions. However, the heats of reaction per mole or reactants are observed to be same for the strong acid/strong base, and weak acid/strong base reactions.

Strong acid/strong base combination has the highest heat of reaction.

Weak acid/weak base combination has the lowest heat of reaction.

Since weak acids/weak bases dissociate partially in solutions, the reaction conversion is low. Therefore, the corresponding heat of reactions are observed to be lower than strong acid/strong base combinations.