Lesson 6.5 Measuring Heats of Reaction
Suggested Reading
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Zumdahl Chapter 6 Section 6.2
Essential Question
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How are heats of reaction measured?
Learning Objectives
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State that combustion and neutralization are exothermic processes.
Apply the relationship between temperature change and enthalpy
change to classify a reaction as exothermic or endothermic.
Describe heat capacity and specific heat.
Calculate the heat energy change when the temperature of a pure
substance changes using the equation q = mc∆T.
Calculate the enthalpy change for a reaction using experimental data
on temperatures changes, quantities of reactants and mass of water.
Evaluate experimental results to determine enthalpy changes.
Introduction
So far we have introduced the concept of heat of reaction and have related
it to the enthalpy change. We also showed how to write thermochemical
equations, and from this equation how to calculate the heat of reaction for
any given amount of substance. Now that understand what heats of
reaction are, how would you measure them?
First, we need to look at the heat required to raise the temperature of a
substance, because a thermochemical measurement is based on the
relationship between heat and temperature change. The heat required to
raise the temperature of a substance is called its heat capacity.
Heat Capacity and Specific Heat
It requires heat to raise the temperature of a given amount of substance,
and the quantity of heat depends on the temperature change. The heat
capacity (C) of a substance is the quantity of heat needed to raise the
temperature of a sample of the substance by one degree Celsius (or 1
degree Kelvin). Changing the temperature of the sample from the initial
temperature ti to the final temperature tf requires heat equal to
q = C∆t
where ∆t is the change of temperature and equals tf - ti.
Heat capacity depends on whether the reaction takes place at constant
pressure or constant volume. We assume constant pressure unless
otherwise stated.
Suppose a piece of iron requires 6.70 J of heat to raise the temperature by
one degree Celsius. Its heat capacity is therefore 6.70 J/°C. The quantity of
heat required to raise the temperature of the piece of iron from 25.0°C to
30.0°C is
q = C∆t = (6.70 J/°C) x (30.0°C - 25.0°C) = 67.0 J.
Sometime you are asked to determine the heat capacity of a substance, in
which case you can use the equation
Cp = ∆H/∆T
Heat capacity is directly proportional to the amount of substance. Often
heat capacities are listed for the molar amounts of substances. The molar
heat capacity of a substance is its heat capacity for 1 mole of substance.
Heat capacities are also compared for 1 gram amounts os substances. The
specific heat capacity (s) (or simply specific heat) is the quantity of heat
required to raise the temperature of one gram of a substance by one
degree celsius (or one Kelvin) at constant pressure. To find the heat q
required to raise the temperature of a sample, you multiply the specific heat
of the substance, s, by the mass in grams, m, and the temperature
change, ∆t.
q = s x m x ∆t
Example: Relating Heat and Specific Heat
Calculate the heat absorbed by 15.0 g of water to raise its temperature
from 20.0°C to 50.0°C (at constant pressure). The specific heat of water is
4.18 J/(g •°C).
Solution
You substitute into the equation q = s x m x ∆t.
The temperature change is ∆t = tf - ti = 50.0°C - 20.0°C = 30.0°C. Therefore,
q = (4.18 J/(g •°C)) x (15.0 g) x (30.0°C) = 1.88 x 103 J
Measurement of Heat of Reaction
You measure the heat of reaction in a calorimeter, a device used to
measure the heat absorbed or evolved during a physical or chemical
change. The student labs we usually make calorimeters out of coffee cups,
which is often referred to as "coffee-cup calorimetry". More elaborate
calorimeters are employed when precise measurements are needed for
research, although the basic idea remains the same- to measure
temperature changes under controlled circumstances and relate these
temperature changes to heat.
The coffee-cup calorimeter is a constant pressure calorimeter. The heat of
reaction is calculated from the temperature changes caused by the
reaction, and since this is a constant-pressure process, the heat can be
directly related to the enthalpy change, ∆H. In research, these types of
calorimeters are used when gasses are not involved.
For reactions involving gasses, a bomb calorimeter is generally used. Lets
use an example to illustrate the operation of a bomb calorimeter. Consider
the heat of combustion of graphite, the allotrope of carbon used to make
pencil "lead". To measure the heat released when when graphite burns in
oxygen, a sample of graphite is placed in a small container in the
calorimeter. The graphite is surrounded by oxygen and sealed in a steel
vessel, or bomb. An electric current is activated to start the burning of the
graphite sample. The bomb is surrounded by water in an insulated
container, and the heat of reaction is calculated from the temperature
change of the calorimeter caused by the reaction.
Bomb calorimetry differs from coffee cup calorimetry in one very important
respect. Because the reaction of a bomb calorimeter takes place in a
sealed vessel it does not take place at constant pressure (it is sealed off
from the constant pressure of the atmosphere). Rather, bomb calorimeters
are constant volume systems. We solve these problems differently then we
do constant-pressure calorimetry problems. See below.
Now, study example 6.5 on page 239 and and example 6.6 on page 242.
The assumptions given at the beginning of the video apply. A word of
advice, use the working method to solve these problems. Make sure you
units match and note weather the temperature increases or decreases in
the problem. If it increases, then your final answer will be negative
(exothermic reaction). If it decreases, then your final answer will be positive
(endothermic reaction).
Summary of Equations and Assumptions Used in Calorimetry
Problems
Constant-pressure (coffee-cup) calorimetry
q = s x m x ∆T, where
q is the energy absorbed by the solution, which is opposite in sign to the
reaction. Alternatively, you can use the equation q = - s x m x ∆T, then you
don't have to worry about changing the sign at the end of the problem like
they did in the video. Usually, this equation is written without the negative
sign though. Either way you make sure you understand the implications of
the sign.
s is the specific heat capacity of the solution, which is assumed to be that
of water (4.184 J/°C • g) because the solution is dilute.
m is the mass of solution. We assume the density of water is 1 g/mL, then
1 mL = 1 g, and the mass of solution = the volume of solution in mL. This is
a very nice simplifying assumption!
∆T is the change in temperature of the solution, which in a coffee cup
calorimeter is the change in temperature of the surroundings.
Constant-volume (bomb) calorimetry
∆Ecombustion = ∆T x Ccalorimeter, where
∆Ecombustion is the energy released by the combustion reaction. Since
combustion is an exothermic process the sign is always negative.
∆T is the temperature increase.
Ccalorimeter is the heat capacity of the calorimeter.