Enthalpy ChangesMeasuring and Expressing ∆H

Calorimetry

Objectives

• When you complete this presentation, you will be able to• Define enthalpy• Define calorimetry• Describe how to use a “coffee cup” calorimeter to determine

the change in enthalpy of a reaction and the specific heat of a material.

• 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 ...• the heat released by the system is equal to the heat

absorbed by its surroundings.• 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.

Calorimetry

• 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

Calorimetry

• 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

Calorimetry

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

mwater = Vfinal × ρwater = (50.0 mL)(1.00 g/mL) = 50.0 g

∆H = −m × C × ∆T = −(50.0 g)(4.18 J/°C-g)(+7.0°C)

∆H = −1463 J = −1.5×103 J = −1.5 kJ

Calorimetry

• 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.• The change in temperature is then measured

on the thermometer.

Calorimetry

• 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.

• We measure the transfer of heat (at a constant pressure) by a technique called calorimetry.

• We use an insulated device called a calorimeter to measure this heat transfer.

Summary

• Two calorimeters used are ...• the coffee cup calorimeter (for constant pressure

measurements)• the bomb calorimeter (for constant volume measurements)

Summary