Lesson : Enthalpy and Calorimetry

IB Chemistry Power Points

Topic 05
Energetics

www.pedagogics.ca
Lesson
Exothermic and
Endothermic Reactions
and Calorimetry

Much taken from

ENTHALPY
CHANGES

Great thanks to
JONATHAN HOPTON & KNOCKHARDY PUBLISHING
www.knockhardy.org.uk/sci.htm

Background and Review
First Law of Thermodynamics (Law of Energy Conservation)
Energy can be neither created nor destroyed but it can be converted from one
form to another

Energy Changes in Chemical Reactions

All chemical reactions are accompanied by some form of energy change

Exothermic Energy is given out

Endothermic Energy is absorbed

Examples Exothermic combustion reactions
neutralization (acid + base)

Endothermic photosynthesis
thermal decomposition of calcium carbonate

Activity : observing exothermic and endothermic reactions

Key Concept

The heat content of a chemical system is called
enthalpy (represented by H).

We cannot measure enthalpy directly, only
the change in enthalpy ∆H i.e. the amount of
heat released or absorbed when a chemical
reaction occurs at constant pressure.

∆H = H(products) – H(reactants)

∆Ho (the STANDARD enthalpy of reaction) is the
value measured when temperature is 298 K and
pressure is 101.3 kPa.

Enthalpy Diagram -Exothermic Change

If ∆H is negative, H(products) < H(reactants)
There is an enthalpy decrease and heat is
released to the surroundings.

enthalpy

Enthalpy Diagram - Endothermic Change

If ∆H is positive, H(products) > H(reactants)
There is an enthalpy increase and heat is
absorbed from the surroundings.

enthalpy

Example: Enthalpy change in a chemical reaction

Exothermic reactions release heat

N2(g) + 3H2(g)  2 NH3(g)
∆H = -92.4 kJ/mol

The coefficients in the
balanced equation
represent the number of
moles of reactants and
products.

N2(g) + 3H2(g)  2 NH3(g)
∆H = -92.4 kJ/mol

State symbols are ESSENTIAL as changes of state
involve changes in thermal energy.

The enthalpy change is directly proportional to the
number of moles of substance involved in the reaction.

For the above equation, 92.4 kJ is released
- for each mole of N2 reacted
- for every 3 moles of H2 reacted
- for every 2 moles of NH3 produced.

The reverse reaction
2 NH3(g)  N2(g) + 3H2(g)
∆H = +92.4 kJ/mol

Note: the enthalpy
change can be read
directly from the
enthalpy profile diagram.

Thermochemical Standard Conditions
The ∆H value for a given reaction will depend
on reaction conditions.

Values for enthalpy changes are standardized :

for the standard enthalpy ∆Ho
-Temperature is 298 K
- Pressure is 1 atmosphere
- All solutions involved are 1 M concentration

Assigned : Review Exercise 1

The ∆H value for a given reaction will depend
on reaction conditions.

Values for enthalpy changes are standardized :

for the standard enthalpy ∆Ho
-Temperature is 298 K
- Pressure is 1 atmosphere
- All solutions involved are 1 M concentration

Calorimetry – Part 1

Specific Heat Capacity
The specific heat capacity of a substance is a physical
property. It is defined as the amount of heat (Joules)
required to change the temperature (oC or K) of a unit
mass (g or kg) of substance by ONE degree.

Specific heat capacity (SHC) is measured in
J g-1 K-1 or kJ kg-1 K-1 (chemistry)
J kg-1 K-1 (physics)

Calorimetry – continued

Heat and temperature change
Knowing the SHC is useful in thermal chemistry. Heat
added or lost can be determined by measuring
temperature change of a known substance (water).

Q = mc∆T

heat = mass x SHC x ∆Temp

Determining the Specific Heat Capacity of Water

Read the background information and lab
activity instructions carefully

A kettle (and other electrical heating devices) have
power ratings (given in Watts). This tells you the
amount of heat energy supplied by the device. The
watt is equivalent to 1 J per second.

(heat) Q = P x t

DON’T FORGET TO CONSIDER UNCERTAINTY


What you need to do (DCP)
Plot data and choose a suitable range for analysis
(look for constant increase in temperature)
Organize the data from the range in a suitable table.
You may choose to do further processing at this time.
Plot the selected range of raw/processed data. Use
the slope to determine the specific heat capacity of
water.
Present all calculations / data processing clearly

DON’T FORGET TO CONSIDER UNCERTAINTY


What you need to do (CE)
Write a conclusion paragraph. Don’t forget to
compare data to the accepted value. Mention any
indication of presence (or absence) of random or
systematic error. Is your result valid?

Brainstorm a list of procedural/measurement
weaknesses or limitations.
-do you have evidence (data) that any of these
significantly affected the result (as indicated by the
data)? How could you improve the investigation?

Be prepared to discuss in class

Determining Specific Heat Capacity of Water

Sample results
1300 W kettle
supplies 1300 J of
heat each second.

1300 J heat gives Slope tells us
∆T of 0.2215 oC temperature
increases
0.2215 oC each
A ∆T of 1oC would second.
therefore require
5870 J of heat.

Determining Specific Heat Capacity of Water

Sample results (continued)
Mass of water in kettle was 1405 g. To change the temperature
of this water by 1 degree, 5870 J of heat were required.

The amount of heat required to change 1 g of water is
therefore 4.18 J. This is the SHC of water (very close to the
literature value)

SHC of H2O is 4.18 J g-1 K-1

For example
When 3 g of sodium carbonate are added to 50 cm3 of
1.0 M HCl, the temperature rises from 22 °C to 28.5 °C.
Calculate the heat required for this temperature change.

Calorimetry – Part 2

Applications
A calorimeter is used to
measure the heat
absorbed or released in
a chemical (or other)
process by measuring
the temperature change
of an insulated mass of
water.

Sample Problem 1
When 3 g of sodium carbonate are added to 50 cm3 of
1.0 M HCl, the temperature rises from 22.0 C to 28.5 C.
Calculate the heat required for this temperature change.

3 -3
Sample problem 2: 50.0 cm of a 1.00 mol dm HCl solution is mixed with 25.0
3 -3
cm of 2.00 mol dm NaOH. A neutralization reaction occurs. The initial
o
temperature of both solutions was 26.7 C. After stirring and accounting for
o
heat loss, the highest temperature reached was 33.5 C. Calculate the enthalpy
change for this reaction.

NaOH HCl
both 26.7o
.

26.7o  33.5o

3 -3
3 -3
o
o

After writing a balanced equation, the molar quantities and
limiting reactant needs to be determined.

Note that in this example there is exactly the right amount of
each reactant. If one reactant is present in excess, the heat
evolved will associated with the mole amount of limiting reactant.

3 -3
3 -3
o
o

Next step – determine how much heat was released.

There are some assumptions in this calculation
- Density of reaction mixture (to determine mass)
- SHC of reaction mixture (to calculate Q)

3 -3
3 -3
o
o

Final step – calculate ΔH for the reaction

Sample problem 3: to determine the enthalpy of combustion for ethanol (see
reaction), a calorimeter setup (below) was used. The burner was lit and allowed
to heat the water for 60 s. The change in mass of the burner was 0.518 g and
o
the temperature increase was measured to be 9.90 C.

What is the big
assumption made
with this type of
experiment?

Sample problem 3: to determine the enthalpy of combustion for ethanol (see
reaction), a calorimeter setup (below) was used. The burner was lit and allowed
to heat the water for 60 s. The change in mass of the burner was 0.518 g and
o
the temperature increase was measured to be 9.90 C.

First step – calculate heat evolved using calorimetry
Last step – determine ΔH for the reaction

3 -3
Sample problem 4: 100.0 cm of 0.100 mol dm copper II sulphate solution is
placed in a styrofoam cup. 1.30 g of powdered zinc is added and a single
replacement reaction occurs. The temperature of the solution over time is
shown in the graph below. Determine the enthalpy value for this reaction.

First step
Make sure you understand
the graph.

Extrapolate to determine
the change in
temperature.

The extrapolation is necessary to compensate for heat loss while the reaction
is occurring. Why would powdered zinc be used?

3 -3
Sample problem 4: 100.0 cm of 0.100 mol dm copper II sulphate solution is
placed in a styrofoam cup. 1.30 g of powdered zinc is added and a single
replacement reaction occurs. The temperature of the solution over time is
shown in the graph below. Determine the enthalpy value for this reaction.

Determine the limiting reactant

Calculate Q

Calculate the enthalpy for the reaction.

Review Exercise 2

Lesson : Enthalpy and Calorimetry

Related slideshows

More Related Content

Lesson : Enthalpy and Calorimetry