Thermodynamics- Enthalpy of Reaction and Hess’s Law
Name
AP Chem Lab #4
Date
Purpose: ( brief statement of what you are attempting to do. These are similar to the
objectives. )
Procedure: (A brief description of the method you are using. You may refer to the lab
document for specific instructions, but you should include a brief statement of the
method. DO not include lengthy, detailed directions. A person who understands
chemistry should be able to read this section and know what you are doing. Include all
chemicals used and the major equipment. )
Pre Lab:
1. Define Hrxn.
2. Define specific heat.
3. The specific heat of a solution is 4.18J/(gC) and its density is 1.02 g/ml.
The solution is formed by combining 25.0 ml of solution A with 25.0 ml of
solution B, with each solution initially at 21.4 C. The final temperature of
the combined solutions is 25.3 C. Calculate the heat of reaction, q,
assuming no heat loss to the calorimeter. Use correct significant figures.
4. In problem 3 above, the calorimeter has a heat capacity fo 8.20 J/C. If a
correction is included to account for the heat absorbed by the
calorimeter, what is the heat of reaction, q?
5. If the reaction in question 3 is A(aq) + B(aq) -> AB(aq) , and the
molarity of A in solution A is 0.60 M and the molarity of B in solution B is
0.60 M, calculate the enthalpy of reaction, Hrxn, for the formation of 1
mole of AB in solution.
Results:
Observations: (General descriptions of visible appearances or changes that occur
during the experiment, such as “table salt is a white, cube-shaped crystal which dissolves
in water. (Qualitative))
Data: (Neatly arranged measured values listed in tabular form. The units of
measurement MUST be included with the numerical values. The accuracy of the
measurement can also be included as a range (+/-). Calculated answers that are derived
by performing a simple mathematical operation can also be included in the data table. If
the graphs are included, make the graphs an appropriate size. Label all axes and give
each graph a title.)
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Part 1 Data Table. Determination of the Heat Capacity of the Calorimeter
Initial temperature (C)
50.0 ml H2O – room temperature _______
50.0 ml H2O – heated
_______
Mixing Data
Time (sec)
20
40
60
80
100
Time (sec)
120
140
160
180
200
Temperature (C)
Tmix, C_______
qcal J
Tave, C ______
Ccal J/C ______
Temperature (C)
______
Part 2 Data Table: Determination of Heats of Reaction
Reaction 1: HCl(aq) + NaOH(aq) -> NaCl(aq) + H2O(l)
Initial temperature (C)
50.0 ml 2.0 M HCl
_______
50.0 ml 2.0 M NaOH
_______
Mixing Data
Time (sec)
20
40
60
80
100
Tmix, C_______
Temperature (C)
 H, kJ/mol
Time (sec)
120
140
160
180
200
Temperature (C)
_______
qrxn, J _______
2
Reaction 2: NH4Cl(aq) + NaOH(aq) -> NH3(aq) + NaCl(aq) + H2O(l)
Initial temperature (C)
50.0 ml 2.0 M NH4Cl
_______
50.0 ml 2.0 M NaOH
_______
Mixing Data
Time (sec)
20
40
60
80
100
Temperature (C)
Tmix, C_______
 H, kJ/mol
Time (sec)
120
140
160
180
200
Temperature (C)
_______
qrxn, J _______
Reaction 3: NH3(aq) + HCl(aq) -> NH4Cl(aq)
Initial temperature (C)
50.0 ml 2.0 M NH3
_______
50.0 ml 2.0 M HCl
_______
Mixing Data
Time (sec)
20
40
60
80
100
Tmix, C_______
Temperature (C)
 H, kJ/mol
Time (sec)
120
140
160
180
200
Temperature (C)
_______
qrxn, J _______
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Calculations: (Show all calculations with formula and appropriate units on all numbers.
Neatly demonstrate the math set-ups, including units. Label what is being calculated. –
make it organized. Show error calculations where appropriate. If experiments are
qualitative, this section may be omitted.)
Conclusion: (Make a simple statement concerning what you can conclude form the
experiment. Refer back to the purpose of the lab to write this section. (i.e. How was the
purpose of the experiment fulfilled?))
Discussion of Theory: (In this section you should include such information as: What
theory was demonstrated in this experiment (Include concepts used in the experiment)?
What do the calculations show? Why does (or doesn’t) the experiment work? This
section shows me that you understand the concepts used in the lab. Be detailed and ask if
you need help! )
Experimental Sources of Error: (What are some specific sources of error, and how do
they influence the data? Do they make the values obtained larger or smaller than they
should be? Which measurement was the least precise? Instrumental error and human
error exist in all experiments, and should not be mentioned as a source of error unless
they caused a significant fault. Significant digits and mistakes in calculations are NOT a
valid source of error. In writing this section it is sometimes helpful to ask yourself what
you would do differently if you were to repeat the experiment and wanted to obtain better
precision. If you can calculate percent error, do so and include in this section. )
Post Lab Questions: (Answer any questions included in the lab. Answer in such a way
that the meaning of the question is obvious in your answer. )
1. What is meant by calorimetery?
2. How does graphical analysis improve the accuracy of the data?
3. The equation for calculating the heat evolved in each reaction is:
a. q= -[(grams of solution x specific heat of solution x Tsolution) + (Ccal
x Tsolution)] What is the meaning of the negative sign in front of the
brackets?
4. Do the lab results support Hess’s Law?
5. How could the procedure be modified to achieve greater accuracy?
6. Find a table in a reference that lists standard heats of formations for the
species included in your net ionic equations. Use them to calculate Hrxn
for each of the three net ionic equations. Do these values support Hess’s
Law?
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