This document discusses thermochemistry and calorimetry. It explains that calorimetry involves using a calorimeter to measure the heat absorbed or released by chemical reactions and physical changes. A bomb calorimeter is used to determine the Calorie information on food packaging by measuring the heat from combustion reactions. Thermochemistry studies the heat changes that occur in reactions and phase changes using concepts such as exothermic, endothermic, system, surroundings, and enthalpy change (ΔH). The enthalpy change can be determined using calorimetry experiments by measuring the heat (q) gained or lost.

1. Chapter 15 :
Energy and Chemical Change
section 2 :
Heat

2.  Some chemical reactions absorb heat whereas others
release heat.
 The Calorie information that appears on packaged food
is obtained from combustion reactions carried out
in calorimeters.
Section 2: Heat

3. Calorimetry
 Calorimeter is an insulated device used for measuring the
amount of heat absorbed or released during a chemical or
physical process.
 A known mass of water is placed in an insulated chamber
to absorb the energy released from the reacting system
or to provide the energy absorbed The data to be collected is
the change in temperature of this mass of water.
 by the system.
Calorimetry

4. Figure 5 shows the kind of calorimeter, called a bomb
calorimeter, that is used by food chemists.

5. Determining specific heat
Satisfactory results can be obtained in your calorimetry
experiments using the much simpler foam-cup calorimeter.

6.  Assuming no heat is lost to the surroundings, the heat gained
by the water is equal to the heat lost by the metal.
 This quantity of heat can be calculated
 q = c × m × ΔT
 First, calculate the heat gained by the water
 Specific heat of water, 4.184 J/(g·°C).
 q water = 4.184 J/(g·°C) × 125 g × (29.30°C - 25.60°C)
 q water = 4.184 J/(g·°C) × 125 g × 3.70°C
 q water = 1940 J
Heat

7.  The heat gained by the water, 1940 J, equals the heat
lost by the metal,
 q metal , so you can write this equation.
 q metal = q water
 q metal = −1940 J
 c metal × m × ΔT = -1940 J
Measuring Heat:

8. Measuring Heat:

11.  Every chemical reaction and change of physical state either
releases or absorbs heat.
 Thermochemistry
 Is the study of heat changes that accompany chemical
reactions and phase changes.
 In thermochemistry, the system is :
 The specific part of the universe that contains the reaction or
process you wish to study.
 Everything in the universe other than the system is considered
the surroundings.
Chemical Energy and the Universe

12.  Therefore, the universe is defined as:
 Universe = system + surroundings
 During exothermic reactions
 Heat produced by the reaction flows from the system
to the surroundings.
 In an endothermic reaction or process ,
 The flow of heat is reversed. Heat flows from the
surroundings to the system
The system plus the surroundings.

13. Enthalpy and enthalpy changes
 it is impossible to know the total energy content
of a substance.
 Chemists are usually more interested in changes in
energy during reactions than in the absolute
amounts of energy contained in the reactants and
products.
 For many reactions, the amount of energy lost or
gained can be measured conveniently in a
calorimeter at constant pressure

14. Enthalpy (H)
Is the heat content of a system at constant
pressure.
Although you cannot measure the actual energy
or enthalpy of a substance, you can measure the
change in enthalpy, which is the heat absorbed
or released in a chemical reaction.

15. Enthalpy (heat) of reaction (Δ H rxn ).
The change in enthalpy for a reaction is called
the enthalpy (heat) of reaction (Δ H rxn )
Δ H rxn = H final - H initial
Δ H rxn = H products - H reactants
 The sign of the enthalpy of reaction
 Exothermic reaction lose heat.
 Therefore,
 H products < H reactants .
 Enthalpy changes for exothermic reactions are always
negative.

16.  Example heat-pack:
 4Fe(s) + 3 O 2 (g) → 2F e 2 O 3 (s) Δ H rxn = -1625 kJ

17. For this endothermic process,
H products > H reactants
Therefore,
A positive value for Δ H rxn is obtained.
Example cold -pack :
 N H 4 NO 3 (s) → N H 4 + (aq) + N O 3 - (aq) Δ H rxn = 27 k

19. The enthalpy change, ΔH, is equal to q p , the heat
gained or lost in a reaction or process carried out at
constant pressure.
q = Δ H rxn .