SCC 201-Thermochemistry: Heat of Neutralization and Hess’s Law
Learning Objectives
Students will be able to:
Determine the heat of neutralization of three separate reactions and manipulate the chemical equations
to find the heat of neutralization of a fourth reaction
Use their experimental ∆H values and compare it to literature values
Identify, handle and react acids
ases in a constructed calorimeter apparatus.
Measure temperature changes over extended reaction times, generate mixing curves and extrapolate
temperature changes to calculate heats of neutralization
Communicate their change of enthalpy findings through the formal lab report format of: Introduction,
Materials, Procedure, Results, Sample Calculations, Discussion and References
Background:
A chemical reaction is accompanied by an energy change described as a change in heat content: energy
is abso
ed (endothermic reaction) or released (exothermic reaction). In general, the
eaking of bonds
in reactants requires the consumption of energy and the creation of new bonds in products involves the
elease of energy. The potential energy that is stored in chemical bonds can be thought of as the heat
content of a system, enthalpy and when these chemicals react, a change in energy (abso
or release of
energy) will result in a change in enthalpy, ∆H. The overall change in energy will depend on the unique
properties of the reactants and products.
Calorimetry is the study of heat transfe
ed in a chemical reaction. The amount of heat abso
ed or
eleased during this thermochemical process is measured by a change in temperature. The device used
to measure the change is called a calorimeter (two nested styrofoam cups), where ideally, the
calorimeter would not abso
any heat from its su
oundings and at the same time not allow any heat
from the reaction to escape. An equation for calculating the heat associated with a temperature change
is:
(1) XXXXXXXXXXq = msΔT
where q = heat , m = mass, s = specific heat (J g-1 ºC-1) , ΔT = Temperature change
The specific heat of a substance, s, is the amount of heat required to raise the temperature of one gram
of a substance by one degree Celsius (an intensive property).
Enthalpy is a property of a substance that can be applied to determine the heat abso
ed or released in
a chemical reaction. The relationship between enthalpy change and heat is:
(2) ∆H = qp where ∆H is the enthalpy change and q is the heat
The “p” in equation 2 denotes that the reaction occurs at constant pressure. This is convenient, as a
great deal of reactions are open to a constant atmospheric pressure.
Hess’s Law : Unlike mass or temperature, there is no instrument that can measure heat or enthalpy, H.
However , an enthalpy change ∆H can be calculated from equation 1. In addition, if a reaction is ca
ied
out in a series of steps, ΔH for the overall process is equal to the sum of enthalpy change for each
individual step. Let’s look at an example- You would like to know the enthalpy change to transform
graphite into diamonds. This is an extremely difficult task (aside- need a high activation energy) in the
lab but if you know the enthalpy changes when the different forms of ca
on reacts with oxygen to
produce CO2, the enthalpy change of graphite to diamond can be calculated:
XXXXXXXXXXC (s,graphite XXXXXXXXXX2O2 (g) → 2 CO2 (g) XXXXXXXXXXΔH1º= XXXXXXXXXXkJ
XXXXXXXXXXC (s,diamond XXXXXXXXXXO2 (g) → CO2 (g) XXXXXXXXXXΔH2º= XXXXXXXXXXkJ
___________________________________________________________
XXXXXXXXXXC (s,graphite) → C (s, diamond) XXXXXXXXXXΔHoverallº= ?? kJ
We simply cannot add equations (1) and (2) to get the ΔHoverallº equation (3) because there would be
three oxygen molecules in the reactants and three ca
on dioxide molecules in the products and our
overall desired reaction (3) does not contain any O2 and/or CO2 molecules. Two possible operations to
manipulate the chemical equations so that our overall reaction (3) is attained are to a) multiply/divide
the chemical equation by a coefficient and/or b) reverse a chemical equation. When an operation (a or
) is done, the co
esponding ΔHº must undergo the same operation. In our case, we can flip reaction
(2) so that C (s,diamond) appears in the products and we can divide equation (1) by a factor of two.
XXXXXXXXXXC (s,graphite XXXXXXXXXXO2 (g) → CO2 (g) XXXXXXXXXXΔH1º= XXXXXXXXXXkJ (Equation (1) divided by 2)
XXXXXXXXXXCO2 (g) → O2 (g XXXXXXXXXXC (s,diamond) ΔH2º= XXXXXXXXXXkJ ( Equation (2) flipped)
___________________________________________________________
XXXXXXXXXXC (s,graphite) → C (s, diamond XXXXXXXXXXΔHoverallº= XXXXXXXXXXkJ
Adding ΔH1º+ ΔH2º= ΔHoverallº = 3 kJ
By altering the chemical equations, multiplying the coefficients ( and the co
esponding ΔH1ºvalue) in
equation (1) by 0.5 and by reversing equation (2) (and changing the sign of ΔH2º ) into an endothermic
eaction, the oxygen and ca
on dioxide molecules cancel when (1) and (2) are added.
Prelab questions:
1. You would like to determine how many calories are in 10 grams of chicken that is served in the
LaGCC cafeteria. Using your understanding of a bomb calorimeter (Figure 5.18 in your textbook)
a) Devise a scheme to find how many calories are in the meat and
) Assuming the specific heat of chicken is 3.68 J/g⁰C and the change in temperature was
1.90⁰C, how many calories are in 10 grams of chicken?
2. For the following hypothetical reactions:
2A + 3B → C + D ∆H = -46 KJ
E XXXXXXXXXXA → C ∆H = 50 KJ
A XXXXXXXXXX5D → B ∆H = 23 KJ
_________________________________________________
Overall reaction: 3A + B → E ∆H = ?
Calculate ∆H for the overall reaction
Your task:
In this lab, you will measure the enthalpy change that occurs in three separate exothermic acid
ase
eactions involving
a) NaOH and HCl
) NaOH and CH3COOH
c) NH3 and HCl
and calculate the respective enthalpy change for each reaction. For these experiments, you will assume
the specific heat of the each reactions is 4.18 J/g ºC and the density of the solutions is 1.0 g/cm3 .
Materials
Equipment Consumables
One thermometer
Two styrofoam cups (calorimeter)
One cup lid
50 mL graduated cylinder
250 mL beakers
Sti
ing Rod
Timer
1.0 M hydrochloric acid
1.0 M ammonia
1.0 M acetic acid
1.0 M sodium hydroxide
Distilled wate
Procedure development
1. Write the objective of your experiment
2. State the three acid
ase exothermic chemical reactions in your experiment
3. State equation(s) you will use to find the change in enthalpy
IN LAB, BEFORE BEGINNING YOUR EXPERIMENT, YOU WILL DEVELOP AND HANDWRITE A PROTOCOL
4. Compose a step by step procedure to determine ΔH for the reaction of HCl and NaOH. In terms
of details, your procedure should be clear to the point where a fellow colleague could read your
methodology and repeat the experiment without having to consult with you. Ensure that all of
the equipment and consumables are included in the procedure. Numerate your steps.
Suggestions on writing procedures:
-Draw and label your apparatus set-up.
-Indicate how much of each reactant you will use? Should they be the same/different amounts?
Is there a limiting reagent?
-Decide how many trials you will conduct?
- The maximum amount of solution in the calorimeter should not exceed 50 mL at any time.
-Finish your procedure with,
“Repeat steps __ to __ for the reactions between NaOH and HAc “
“Repeat steps __ to ___for the reactions between NH3 and HCl”
5. Create labeled