1. Unit 10: Energy in Chemical Reactions
Cypress Creek High School
Chemistry 1L
Chapter 10

2. Energy In this unit, we will study chemical systems & their energy
Energy is the ability to do work
All of these systems contain kinetic and potential energy
Kinetic energy – all particles are in motion
Potential energy – stored in the bonds of compounds 2

3. Chemical Reactions
When a substance undergoes a chemical change, it takes part in a chemical reaction.
After it reacts, it no longer has the same chemical identity.
What are the evidences that a chemical change has occurred?

4. Evidence for Chemical Changes
Color Change
Gas produced without heating
Precipitate formed
New odor develops
Large amount of heat or light produced

5. Collision Theory
The following three statements summarize the collision theory.
Particles must collide in order to react.
The particles must collide with the correct orientation.
The particles must collide with enough energy to form an activated complex, which is an intermediate particle made up of the joined reactants.
An effective collision is one that results in a reaction; new products are formed. 5

6. Activated Complex
The activated complex is an intermediate particle formed when reactants collide and stick together.
Old bonds break while new bonds form.
The minimum amount of energy colliding particles must have to form an activated complex is called the activation energy.
Particles that collide with less than the activation energy cannot form an activated complex. 6

7. Exothermic Reactions
In exothermic reactions energy is released into the surroundings
Potential energy, which is stored in the bonds, converts to kinetic energy when it is released.
Exothermic reactions feel warm because thermal energy is entering the surroundings (i.e. your hands)
The exothermic reaction releases heat; energy is a product. 7

8. Endothermic Reactions
In endothermic reactions energy is absorbed from the surroundings
Kinetic energy converts to potential energy when it is absorbed and is stored in bonds
Endothermic reactions feel cold because thermal energy is leaving the surroundings (i.e. your hands)
The endothermic reaction absorbs heat; energy is a reactant. 8

9. ΔHrxn = ΣH(products) - ΣH(reactants)
Enthalpy
Enthalpy is a measure of heat energy of a system at constant pressure
It can’t be measured directly, we measure change in enthalpy.
Change in enthalpy is called heat of reaction or ΔHrxn
ΔHrxn = ΣH(products) - ΣH(reactants)
This information will come from a table
“Δ” is the letter delta – it means “change”
“Σ” is the letter sigma – it means “sum”
“ o ” refers to standard temperature and pressure (STP)
Units: kJ/mol

10. Change in Enthalpy Negative ΔH indicates an energy loss (exothermic)
Positive ΔH indicates an energy gain (endothermic)

11. Chemical Equations Energy
Photosynthesis: plants consume carbon dioxide, water, and light energy (from the sun) and convert it into sugar (glucose), oxygen, and water.
6 CO2(g) + 12 H2O(l) + energy → C6H12O6(aq) + 6 O2(g) + 6 H2O(l)
Cellular Respiration: animals consume sugar (glucose) and oxygen to produce carbon dioxide, water, and chemical energy (ATP)
C6H12O6(aq) + 6O2(g) → 6CO2(g) + 6H2O(l) + energy
These chemical equations are important principles of biology.
Which is endothermic and which is exothermic?

12. Change in Enthalpy
All elements in their standard states have an enthalpy of zero
Because there is no change involved in their formation
Endothermic
When N2 and O2 combine to form NO2, the ΔH = kJ/mol
Exothermic
When S and O2 combine to form SO3, the ΔH = -396 kJ/mol.

13. Check for Understanding!
Do elements in their standard states possess zero energy?
Why are elements in their standard states assigned enthalpies of zero?
What does the on the graph tell you?
What does the -396 on the graph tell you?