ENTHALPY CHANGESMeasuring and Expressing ∆H (Calorimetry)

OLEH :PUTRI ROSIDA R

8136141008

Introduction

We have been introduced to heat producing (exothermic) reactions and heat using (endothermic) reactions.

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.

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.

Calorimetry

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

A typical device is a “coffee cup calorimeter.”

Calorimetry

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 solutions4.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.

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.

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:

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

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 1.00 g/mL.Knowns: Vfinal = VHCl + VNaOH = (25.0 + 25.0) mL = 50.0 mL Dwater = 1.00 g/mL ∆Twater = Tfinal − Tinitial = 32.0°C − 25.0°C = +7.0°C Cwater = 4.18 J/°C-gCalculation: mwater = 50.0 g

∆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.”

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.

Calorimetry

We can also do calorimetry at a constant volume rather than at a constant pressure.

This is called “bomb calorimetry.”• Oxygen is introduced into the chamber.

Calorimetry

We can also do calorimetry at a constant volume rather than at a constant pressure.

This is called “bomb calorimetry.”• The lid is tightened and the chamber is placed in a water bath.

Calorimetry

We can also do calorimetry at a constant volume rather than at a constant pressure.

This is called “bomb calorimetry.”• The ignition coil ignites the sample.

Calorimetry

We can also do calorimetry at a constant volume rather than at a constant pressure.

This is called “bomb calorimetry.”• The heat generated in the chamber is transferred to the water.

Calorimetry

We can also do calorimetry at a constant volume rather than at a constant pressure.

This is called “bomb calorimetry.”• The change in temperature is then measured on the thermometer.

Summary

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)