Honors Chemistry
Presentation 09-09
Enthalpy Changes – Part 1
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Objectives:

When you complete this presentation, you will be able to
o Define enthalpy
o Define calorimetry
o Describe how to use a “coffee cup” calorimeter to determine the
change in enthalpy of a reaction and the specific heat of a material.
o Describe the parts of a bomb calorimeter
Introduction

We have been introduced to heat producing (exothermic) reactions and heat
using (endothermic) reactions.

Heat is a measure of the transfer of energy from a system to the
surroundings and from the surroundings to a system.

The change in heat of a system is called the change in enthalpy (ΔH) when
the pressure of the system in kept constant.
Calorimetry

We measure the transfer of heat (at a constant pressure) by a technique
called calorimetry.

In calorimetry ...
o the heat released by the system is equal to the heat absorbed by its
surroundings.
o the heat absorbed by the system is equal to the heat released by its
surroundings.

The total heat of the system and the surroundings remains constant.

We use an insulated device called a calorimeter to measure this heat
transfer.

A typical device is a “coffee cup” calorimeter.
Honors Chemistry
•
Presentation 09-09
Enthalpy Changes – Part 1
Page 2 of 3
To measure ΔH for a reaction ...
1. dissolve the reacting chemicals in known volumes of water
2. measure the initial temperatures of the solutions
3. mix the solutions
4. measure the final temperature of the mixed solution

The heat generated by the reactants is absorbed by
the water.

We know the mass of the water, mwater.

We know the change in temperature, ∆Twater.

We also know that water has a specific heat of
Cwater = 4.18 J/°C-g.

We can calculate the heat of reaction by:
qsys = ∆H = −qsurr = −mwater × Cwater × ∆Twater
Example:
When 25.0 mL of water containing 0.025 mol of HCl at 25.0°C is added to 25.0 mL of
water containing 0.025 mol of NaOH at 25.0°C in a coffee cup calorimeter, a reaction
occurs. Calculate ∆H (in kJ) during this reaction if the highest temperature observed
is 32.0°C. Assume the densities of the solutions are ρ = 1.00 g/mL.
Vfinal = VHCl + VNaOH = (25.0 + 25.0) mL = 50.0 mL
ρwater = 1.00 g/mL
∆Twater = Tfinal − Tinitial = 32.0°C − 25.0°C = +7.0°C
Cwater = 4.18 J/°C-g
Honors Chemistry

Presentation 09-09
Enthalpy Changes – Part 1
We can also do calorimetry at a constant volume rather than at a constant
pressure.

This is called “bomb” calorimetry.

A sample is placed in the crucible.

Oxygen is introduced into the
chamber.

The lid is tightened and the chamber
is placed in a water bath.

The ignition coil ignites the sample.

The heat generated in the chamber is
transferred to the water.

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The change in temperature is then
measured on the thermometer.