Chemistry 122Chapter 17: Measuring and Expressing

Enthalpy Changes

CalorimetryCalorimetry measures heat flow into or out

of a system for chemical or physical processes

The heat released by a system is equal to the heat absorbed by the surroundings

The heat absorbed by the system is equal to the heat released by the surroundings

The device itself is called a calorimeter

Constant-Pressure CalorimetersThe heat content of a system at constant

pressure is the same as a property called enthalpy (H)

The amount of heat released or absorbed from a chemical reaction at constant pressure is referred to as a change in enthalpy (ΔH) of the system

Heat and enthalpy change are the same thing

q = ΔH

Same formula from before…• When you measure the temperature of the

water in the calorimeter before and after the chemical reaction, enthalpy can be calculated as follows:

qsurr = m x C x ΔT

This translates to: 'heat absorbed by the surroundings'

Since the surroundings is water, the known values of C is used (4.18 J/g·°C) in calculations involving the heat transfer

The opposite for the system…Conversely, if you are measuring the heat lost

or gained by the system, the enthalpy change is written as follows:

qsys = ΔH = -qsurr = -m x C x ΔT

The sign for an exothermic reaction (heat released) is negative and positive for an endothermic reaction (heat absorbed)

Bomb CalorimetersA sample is burned in a constant-pressure

chamber in the presence of oxygen at high pressure

The heat released by the reaction warms the water

By measuring the increase in temperature, it is possible to calculate the amount of heat released during the combustion

ExampleRemember to write down all that is known in

the example.Add up any volumes (or masses). Look for

conversions.Rely on known constants.

Complete questions 12 – 13, pg. 513

Thermochemical EquationsIN A CHEMICAL EQUATION, THE

ENTHALPY CHANGE FOR THE REACTION CAN BE WRITTEN AS EITHER A REACTANT OR A PRODUCT – depending on whether it is an exothermic or endothermic reaction

Ex. CaO(s) + H2O(l) → Ca(OH)2(s) + 65.2kJ A chemical equation that includes the

enthalpy change is called a thermochemical equation

Heat of ReactionThe enthalpy change for the chemical

equation exactly as it is written

Exothermic:CaO(s) + H2O(l) → Ca(OH)2(s) ΔH = -

65.2kJ

Endothermic:2NaHCO3(s) → Na2CO3(s) + H2O(l) + CO2(g)

ΔH = 129kJ

Refer to page 17.7, p. 515

This relates to stoichiometry(remember stoichiometry?)

The amount of moles factors into the amount of enthalpy released or absorbed

The states of matter are important as well This is because different physical states of the

same substance can have different ΔH valuesEx. Water is 4.184J/g∙˚C yet ice is 2.01J/g∙˚C

Example of Heat of Reaction – page 516Read through the question to identify what is

knownThis is a simple conversion questionYou know how much heat is absorbed when 2

moles of sodium bicarbonate is heated. However, the amount you want to investigate is 2.24 mol

Questions 14 – 15, p. 516

Heat of CombustionRefers to the amount of heat released when

one mole of substance is burned.

Ex. CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)ΔH=-890kJ

Unless otherwise indicated, standard conditions are being used in these equations. This means the temperature is 25°C and pressure is 101.3kPa.

Review

1) Select a substance from 17.2. Write the thermochemical equation.

2) Why is the ΔH negative?

3. Draw an enthalpy diagram for this reaction.

4. Is heat flowing into or out of the system?

5. What must the total energy (potential and kinetic) equal in any chemical or physical process?

For the remainder of class:Work on guided reading – section 17.2Questions 16 – 20 need to be completed to

check for understandingSection review 17.2

Practice Problems ?