Measuring Energy Changes

Introducing Heat Capacity and Specific Heat

Before Today’s Discussion Begins…

Remember the differences between temperature, thermal energy, and heat

Temperature is the average kinetic energy of all particles in a material

It is an intensive property – the amount of material DOES NOT affect the magnitude of this property

Thermal energy is the TOTAL energy of motion of molecules, atoms, or ions

Thermal energy is directly proportional to the temperature within a given system

+ molecules = + particle movement = + thermal energy= + temperature

Everything in the universe will contain some thermal energy if at a temperature above 0 Kelvin (the absolute scale)

Heat is the thermal energy TRANSFER that results from a difference in temperature

Introducing Heat Capacity

So, how do we relate a temperature to the amount of thermal energy?

With heat capacity!

Defined as the heat required to raise the temperature of an object by 1°C

Heat Capacity =Heat supplied

Temperature=

J

℃

Measuring Energy Changes

Look back to the Mind Catalyst

What are the two factors that determined the amount of energy (heat) contained by a substance?

Amount of substance being heated (number of grams)

Temperature change (number of degrees)

However, identical masses of different substances may contain different amounts of thermal energy even if at the same temperature!

So, identity of the substance is also a factor!

Different substances respond differently to being heated

Thus, different heat capacities

Do metals have relatively high or low heat capacities? When in your life have you noticed this?

What about water?

Relating Heat Capacities to Amount of a Substance More heat is required to raise the temperature of a large

sample of a substance by 1°C than is needed for a smaller sample

Specific heat capacity takes into account amount of the substance present

Defined as the amount of energy (in calories or joules) required to change the temperature of ONE GRAM of substance by 1°C

Specific Heat Capacity C =Heat capacity

Mass=

J℃

g=

J

g ∙ ℃

Specific heat capacities vary depending on state of matter!

The larger the specific heat of a substance, the less its temperature will change when it absorbs a particular amount of heat energy

Thus, more heat must be added to increase the temperature of a substance a given number of degrees

Real-World Application of Specific Heat Capacity

Oceans of Climate Change

Calculating the Amount of Heat Absorbed or Released From a Substance

We can use specific heat capacity as a conversion factor to calculate how much heat is absorbed or lost from a substance as long as its mass and change in temperature are known!

To do so, use the equation below:

Heat = Mass × Specific Heat × Temperature Change

q = mC∆T

q is energy (heat)

m is mass of sample in grams

C is specific heat capacity

ΔT is change in temperature in °C

Practice! How much heat is absorbed when 500 g of water with a specific

heat of 4.184J℃

g goes from 25°C to 35.0°C?

How much heat is absorbed when 500 g of copper with a specific

heat of 0.385J℃

g goes from 25°C to 35°C?

A 50.0 g block of ice with specific heat of 2.087 J℃

g absorbed 333 J

of heat energy. How much does the temperature of ice rise?

Practice!

The temperature of a silver coin (C = 0.24 J℃

g) falls by

353°C as it releases 5,550 Joules of heat. What is the mass of the coin?

Measuring ∆H Using Calorimetry

Determining ΔH using Calorimetry

Remember, we can easily measure heat changes (∆q) of any process in the laboratory

Under the conditions of constant pressure - as it is in the laboratory - we equate ∆q with ∆H

The process of measuring the change in heat of a chemical or physical change is done with a technique called calorimetry

More specifically, calorimetry is the process of measuring heat based on observing the temperature change when a body absorbs or releases energy as heat energy

Calorimetry is based on First Law of Thermodynamics

The total heat of the system and the surroundings remains constant

Performing Calorimetry

To be able to observe energy changes, we must be able to isolate our system from the rest of the universe

We use an insulated device called a calorimeter to measure this energy (heat) change

A typical device is a “coffee cup calorimeter”

Reaction is open to the atmosphere

Therefore, constant pressure and ∆q = ∆H

How Does Calorimetry Work?

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

qsystem + qsurroundings = 0

Assume the calorimeter does not absorb or leak any heat

How Does Calorimetry Work?

In other words:

The heat generated by the reaction (system) is absorbed by the water (surroundings)

We know the mass of the water, mwater

We know the change in temperature of the water, ∆Twater

We also know that water has a specific heat of cwater = 4.184 J/°C·g.

So, we can calculate the enthalpy of reaction by using the relationship:

qsys = ∆H = −qsurr

qrxn = ∆Hrxn = −qwater

qrxn = ∆Hrxn = -(mwater × Cwater × ∆Twater)

Remember, enthalpies of reactions are often expressed in terms of energy per moles of reacting substances or moles of produced substances

Divide calculated energy by amount of substance

May need to use stoichiometry to find amount of substance

Quantifying Energy Exchanges using Constant-Pressure Calorimetry

Calorimetry Example

http://mutuslab.cs.uwindsor.ca/schurko/animations/heatcapacitymetals/heat_metal.htm

Determination of ∆H Using a Bomb Calorimeter

A bomb calorimeter is a device used to measure heat of combustion at constant volume

Therefore, ∆q ≠ ∆H

The steel jacket isolates the system so that the heat produced by the combustion is taken up by calorimeter

−qrxn = qcalorimeter

Bomb Calorimeter Animation