Reactivity 1.1 Measuring the enthalpy change By Anoosha Qaisar

IB chemistry notes Reactivity 1.1 By Anoosha Qaisar

1. Lecture notes
By
Ms. Anoosha Qaisar

2. Reactivity 1


R 1.1: Measuring enthalpy change

3. Energy: a measure of the ability to do work.
⚫Work: to move an object against an
opposing force.

⚫Energy (J) = Force (N) x Distance (m)

⚫Forms of energy include: heat, light, sound,
electricity, and chemical energy (energy
released or absorbed during chemical
reaction)

4. Figure 2 illustrates some different energy forms categorised into energy from movement, stored
energy and energy we can experience.

5. The three main processes for energy transfer include:
1.Conduction: transfer of energy by direct contact
2.Convection: transfer of energy by the movement of fluids
3.Radiation: transfer of energy by electromagnetic waves.

6. - Energy can be transferred and change from one form to
another.

- The total amount of energy remains the same in a closed
system, as stated by the law of conservation of energy.

- When a light bulb is turned on, electrical energy is
transformed into light, thermal, and some sound energy.

- Society focuses on the energy converted to the desired
form (e.g., light) and uses this to rate material efficiency.

- It may seem like energy is lost due to inefficiency, but it is
actually transformed into less useful forms, such as heat.
- The overall amount of energy in the system remains
constant.

7. Physical changes and energy transfer
- During melting, energy transfers from your hand to an ice cube,
weakening particle attraction and turning solid water into liquid.

- Your hand feels cold because it loses energy to the ice.

- When liquid water freezes, energy is released from the water into the
surrounding environment, which makes the freezer warm to the touch.

- These energy transfers occur not only in physical processes like
melting and freezing but also in chemical reactions.

- Understanding energy transfer is crucial for exploring how energy
flows during chemical reactions.

8. Chemical Reactions
●Elements can undergo a chemical reaction with other elements to form
a compound.
● Compounds can also undergo chemical reactions with other elements
or compounds to form new compounds.
●A chemical reaction is a process where a rearrangement of atoms
occurs forming new substance(s) with different properties.
●The initial substances used in a chemical reaction are called reactants
and the new substances formed as a result of the chemical reaction are
called products.

9. Transfer of energy in chemical reactions

All chemical reactions involve a transfer of energy between the system and
the surroundings, while total energy is conserved. In a chemical reaction,
energy cannot be created or destroyed; instead, it is either converted from
one form to another or transferred between the system and its surroundings.
The system refers to the components involved in the chemical reaction and
the surroundings encompasses everything outside the system. Figure 5
shows the system and surroundings for the processes mentioned earlier.

10. In a reaction, such as the combustion reaction of methane
(sometimes referred to as natural gas), methane reacts with
oxygen to produce carbon dioxide and water via the following
chemical reaction.
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
The chemical potential energy stored in the bonds of the
reactants is transformed into thermal energy and light energy
during the combustion process. The total amount of energy is
conserved.
During a chemical reaction, energy may be released or absorbed
in various forms such as heat, light or electrical energy. You have
probably encountered each of these forms in different chemical
reactions from processes such as combustion of wood in a
campfire (heat), luciferin reacting in fireflies (light) and the
oxidation and reduction reactions in a battery (electrical).

11. Heat and Temperature
- In chemical reactions, energy is often released as heat,
causing an increase in the temperature of the surroundings
(e.g., a warm beaker).
- Heat is quantitatively measured using a thermometer, which
records temperature changes in °C or K.
- Temperature:measures the average kinetic energy of
particles.
- Heat is the energy transferred due to a temperature
difference, measured in joules (J).
- Temperature and heat are related but distinct concepts; heat
transfer depends on additional factors like mass and material
properties.
- Example: A thermometer detects an increase in body
temperature during a fever, reflecting the body's use of energy
to fight illness.

12. TEMPERATURE AND HEAT
⚫Heat: a measure of the total energy in a
given amount of a substance (and
therefore depends on the amount of
substance present).

13. TEMPERATURE AND HEAT
⚫Temperature: a measure of the “hotness”
of a substance. It represents the average
kinetic energy of the substance (but is
independent of the amount of substance
present).

14. TEMPERATURE AND HEAT
⚫Example: Two beakers of water. Both have same
temperature, but a beaker with 100 cm
3
of water
contains twice as much heat as a beaker containing
50 cm
3
.

Same temp, but MORE HEAT

15. TEMPERATURE AND HEAT

⚫The increase in temp. when an object is heated
depends on
●The mass of the object
●The heat added
●The nature of the substance (different
substances have different “specific heat” values)

16. Enthalpy (H): a measure of internal energy stored in a
substance. The amount of heat that a system has is known as its enthalpy
(H). The enthalpy of a system is not easily measured but the changes in
enthalpy (ΔH) is something we can measure as heat is transferred between
system and surroundings. These changes in enthalpy are positive or
negative depending whether a reaction absorbs or releases heat.



⚫Enthalpy change (ΔH) in a reaction can be
measured (difference between reactants and
products).

⚫Standard enthalpy change of a reaction
(ΔH
ϴ
): measured at pressure = 1 atm; temp =
298 K

17. EXOTHERMIC & ENDOTHERMIC
Reactions
⚫Exothermic reactions:
release energy in the
form of heat (because
bonds in the products are
stronger than the bonds
in the reactants);
decreasing enthalpy has
neg. sign (ΔH < 0)

Diagram:
Enthalpy, H
reactants
products
extent of rxn
ΔH = negative

18. EXOTHERMIC & ENDOTHERMIC
Reaction
⚫Endothermic
reaction: absorb energy
in the form of heat;
increasing enthalpy,
positive value (ΔH > 0)

Diagram:
Enthalpy, H
reactants
products
extent of rxn
ΔH = positive

22. Worked example 1
A 150.0 g block of pure copper is heated so that the temperature increases from 18.0°C to 75.0°C.
Calculate the heat energy change in kJ if the specific heat capacity of copper is 3.86 × 10−1 J g−1 K−1.

24. CALORIMETRY
⚫The enthalpy change for a reaction can be
measured experimentally by using a
calorimeter.

bomb calorimeter
simple calorimeter
(a.k.a. “coffee cup calorimeter”)
Calorimetry is a technique used to measure the heat
transfer during a physical or chemical process. A
calorimeter is the apparatus used to measure temperature
changes to calculate the enthalpy for a reaction.

25. CALORIMETRY

⚫In a simple “coffee cup” calorimeter, all heat
evolved by an exothermic rxn is used to
raise temp. of a known mass of H
2
O.

⚫For endothermic rxns, heat transferred
from the H
2
O to the rxn can be calculated
by measuring the lowering of the
temperature of a known mass of water.

26. We can use a calorimeter to measure temperature changes from chemical
reactions and calculate the thermal energy transferred in the reaction. To
make these quantities specific to a chemical reaction, we can use the
thermal energy transferred (Q) to calculate the change in enthalpy for a
reaction (ΔH).
You may have noticed that the units of ΔH are kJ mol−1; this means we
need to involve the number of moles involved in a chemical reaction to
calculate enthalpy. Additionally, calorimetry involves measuring the
temperature of the surroundings, this means that the thermal energy (Q)
and the change in enthalpy (ΔH) will be opposite in sign to one another.

27. A copper calorimeter containing 100.0 g of water is used to measure the enthalpy of combustion
of ethanol. Water in the copper calorimeter has an initial temperature of 21.0 °C and a final
temperature of 67.0 °C. The burner containing ethanol had an initial mass of 25.83 g and a final
mass of 21.75 g. Calculate the enthalpy of combustion of ethanol in kJ mol−1.

29. Accounting for heat loss during data processing

When you consider using a calorimeter to determine the change in
enthalpy for a reaction, what do you think the largest source of error
would be? The system might be closed but it is not isolated;
therefore, it is always ‘connected’ to the earth in some way, making
heat loss unavoidable. This heat loss results in smaller changes in
temperature than we would expect theoretically. This means that
experimentally, we would record a final temperature lower than
theoretically possible for exothermic reactions and a final
temperature higher than theoretically possible for endothermic
reactions.

Can you think of any ways we can reduce heat loss experimentally?
Insulation of calorimeters is one way to reduce heat loss. This can
be done by surrounding the calorimeter with insulating materials,
increasing the thickness of these materials and minimising contact
with the air.

30. Compensating for heat loss:
Graph temp. v. time


By extrapolating the graph, the temp rise that would
have taken place had the reaction been
instantaneous can be calculated.

ΔT
cooling trendline
T
1
T
2
T
3
T
1
= initial temp.

T
2
= max temp. measured

T
3
= max temp. if no heat loss

ΔT = T
3
- T
1

37. CALCULATION OF ENTHALPY
CHANGES (ΔH)
⚫The heat involved in changing the
temperature of any substance can be
calculated as follows:

◦Heat energy = mass (m) x specific heat capacity (c)
x temperature change (ΔT)

◦(Remember q = mcΔT from H Chem?)

38. CALCULATION OF ENTHALPY
CHANGES (ΔH)
⚫Specific heat capacity of H
2
O = 4.18 kJ kg
-1
K
-1


◦Thus, 4.18 kJ of energy are required to raise the
temp of 1 kg of water by one Kelvin.

◦Note that this is effectively the same as the
complex unit J/g⋅°C (from Honors Chem)

39. CALCULATION OF ENTHALPY
CHANGES (ΔH)
⚫Enthalpy changes are normally quoted in kJ
mol
-1
for either a reactant or product, so it is
also necessary to work out the number of
moles involved in the reaction which produces
the heat change in the water.

40. Example 1: 50.0 cm
3
of 1.00 mol dm
-3
hydrochloric acid solution was added to 50.0 cm
3
of 1.00
mol dm
-3
sodium hydroxide solution in a polystyrene beaker. The initial temperature of both
solutions was 16.7 °C. After stirring and accounting for heat loss the highest temperature
reached was 23.5 °C. Calculate the enthalpy change for this reaction.
⚫Step 1: Write the equation for reaction


HCl(aq) + NaOH(aq) → NaCl(aq) + H
2
O(l)

41. Example 1: 50.0 cm
3
of 1.00 mol dm
-3
hydrochloric acid solution was added to 50.0 cm
3
of 1.00
mol dm
-3
sodium hydroxide solution in a polystyrene beaker. The initial temperature of both
solutions was 16.7 °C. After stirring and accounting for heat loss the highest temperature
reached was 23.5 °C. Calculate the enthalpy change for this reaction.
⚫Step 2: Calculate molar quantities

∴heat evolved will be for 0.0500 mol

42. Example 1: 50.0 cm
3
of 1.00 mol dm
-3
hydrochloric acid solution was added to 50.0 cm
3
of 1.00
mol dm
-3
sodium hydroxide solution in a polystyrene beaker. The initial temperature of both
solutions was 16.7 °C. After stirring and accounting for heat loss the highest temperature
reached was 23.5 °C. Calculate the enthalpy change for this reaction.
⚫Step 3: Calculate heat evolved

Total vol. sol’n = 50.0 + 50.0 = 100.0 mL
Assume sol’n has same density and specific heat
capacity as water, then…
Mass of “water” = 100.0 g
ΔT = 23.5 – 16.7 = 6.8 °C
heat evolved = 100.0g x 4.18J/g°C x 6.8 °C
= 2.84 kJ/mol (for 0.0500 mol)
neg. value = exothermic

43. Example 2: A student uses a simple calorimeter to determine the enthalpy change for the
combustion of ethanol (C
2
H
5
OH). When 0.690 g of ethanol was burned it produced a
temperature rise of 13.2 K in 250 g of water.

a)Calculate ΔH for the reaction

C
2
H
5
OH(l) + 3O
2
(g) → 2CO
2
(g) + 3H
2
O(l)
Heat evolved = 250g x 4.18J/g°C x 13.2°C
Heat evolved = 13.79 kJ (for 0.015 mol ethanol)
neg. value = exothermic

44. Summary:
●Chemical reactions involve a transfer of energy between the system and the
surroundings, while total energy is conserved.
●
●Exothermic reactions:
○involve a transfer of energy from the system to the surroundings.
○result in a measured increased temperature change.
○have products with a greater relative stability (lower potential energy)
than reactants.

●Endothermic reactions:
○involve a transfer of energy from the surroundings to the system.
○result in a measured decreased temperature change.
○have reactants with a greater relative stability than the products.

●The standard enthalpy change for a chemical reaction refers to the heat
transferred at constant pressure under standard conditions and states.
●Monitoring temperature changes during chemical reactions allows for the
calculation of thermal energy transfer and enthalpy change.