Section 17.1 Lecture for Honors & Prep Chemistry

1. Thermochemistry is the study of heat
changes that accompany chemical
reactions and phase changes.

2. There are two types of energy
Kinetic Energy and Potential Energy
As temperature increases, the kinetic
energy (motion) of particles increases.
Potential energy is stored energy.
Potential energy is stored in chemical
bonds.

3. There are two types of energy
Kinetic Energy and Potential Energy
Energy can be converted between the
two types but it cannot be created or
destroyed
= The law of conservation of energy

4. Heat (q) is energy that flows from a
warmer object to a cooler object.
• When the warmer object loses heat, its
temperature decreases and molecules
move slower.
• When the cooler object absorbs heat, its
temperature rises and its molecules speed up.
• Heat flows until both objects have
the same temperature.

5. HEAT vs. TEMPERATURE
Energy measures average
kinetic energy
Depends on the Independent of
amount of substance sample size
A drop of boiling water on your hand doesn’t
hurt as much as a pot of boiling water.
Same temperature (100ºC), but different
amounts of heat.

6. Cold and hot
• Cold is the sensation of molecules
slowing down (decreasing temperature)
• Hot is the sensation of molecules
speeding up (increasing temperature)
Hot and cold are relative. A warm cup can feel
very hot if your hand has been in snow.

7. In thermochemistry, it is useful to
divide the universe into two parts
The system- The thing we are studying
The surroundings- Everything else
universe = system + surroundings

8. • Enthalpy (H) is the heat content of
a system at constant pressure.
• Most processes involve a change in enthalpy (ΔH)
which can be measured in a lab.
• ΔH is measured in J or kJ

9. The change in enthalpy for a reaction is called the
enthalpy (heat) of reaction (∆Hrxn).

10. ∆Hrxn is the difference between the enthalpy of
products and the enthalpy of the reactants.
The total enthalpy change is a sum of all bonds
that are broken or formed during a reaction.
Bond breaking = requires energy
Bond forming = releases energy

11. A process or reaction is either
EXOTHERMIC or ENDOTHERMIC
Heat (energy) goes Heat (energy) goes
out of the system into the system
ΔH is negative ΔH is positive
energy is given off energy is absorbed

12. EXOTHERMIC =
Products have
lower energy than
reactants
ENDOTHERMIC =
Products have higher
energy than reactants

13. TNT
Higher energy → lower energy
+
energy released!

14. You can tell if a reaction is
endothermic or exothermic by touch.
Remember- the system determines the
sign of ΔH and we are part of the
surroundings
YOU
ARE
HERE

15. A reaction that feels
cold has heat going
from your finger
INTO THE SYSTEM
= endothermic
A hot reaction has heat
coming OUT OF THE
SYSTEM = exothermic

16. Equations for endothermic
processes have energy as a reactant.
sunlight + 6CO2(g) + H2O(l) 
C6H12O6(aq) + 6O2(g)
Equations for exothermic processes
have energy as a product.
Propane(g) + O2(g) 
CO2(g) + H2O(g) + heat