Enthalpy Changes

Enthalpy Changes Measuring and Expressing ∆H Calorimetry

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 Objectives

We have been introduced to heat producing (exothermic) reactions and heat using (endothermic) reactions. • Heat is a measure of the transfer of energyfrom 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. Introduction

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. Calorimetry

We use an insulated device called a calorimeterto measure this heat transfer. • A typical device is a “coffee cup”calorimeter. Calorimetry

To measure ΔH for a reaction ... • dissolve the reacting chemicals in known volumes of water • measure the initial temperatures of the solutions • mix the solutions • measure the final temperature of the mixed solution Calorimetry

The heat generated by the reactantsis 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 volumerather 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

Enthalpy Changes