Notes - Energy & Chemical Change Chapter 8, Lesson 3
Notes - Energy & Chemical Change
Chapter 8, Lesson 3
Energy and Chemical Reactions
• In chemical reactions, energy is either absorbed or released.
• In chemical reactions, atoms rearrange.
• Rearrangement involves some form of energy, usually thermal (heat) energy.
lightandsoundenergy
thermalenergy
Energy and Chemical Reactions
• Some reactions give off light with almost no thermal energy.
• Cold light occurs at room temperature.
Energy and Chemical Reactions
• Energy is stored in reacting molecules.
• The law of conservation of energy states that energy cannot be created or destroyed in chemical reactions.
• Energy simply changes from one form to another.
Energy and Chemical Reactions
• Molecules have chemical energy stored in their bonds.
• Energy is stored in the food that you eat and transferred to your cells.
Net Release of Energy
• When atoms rearrange, bonds are broken and new bonds form.
• Breaking bonds requires energy.
• Forming bonds releases energy.
Net Release of Energy
• An exothermic process releases energy in the form of heat.
• ex- means “out”
• The products in an exothermic reaction have less energy than the reactants.
Net Release of Energy
Net Release of Energy• An endothermic process
absorbs energy and feels cooler.
• endo- means “in”
• The products in an endothermic reaction have a higher energy than the reactants.
Net Release of Energy
Net Release of Energy
• Chemical reactions need energy to get started
• Activation energy is the minimum amount of energy that has to be added to start a reaction
• (i.e.) 2H2 + O2 → 2H2O is an exothermic reaction that needs a spark to get started
Classifying Chemical Reactions
• Synthesis Reaction – two or more substances combine to make a more complex substance • A + B → AB
• 2 H2 + O2 → 2 H2O
Classifying Chemical Reactions
• Decomposition Reaction – a compound reactant breaks down into simpler products • AB → A + B
• 2 H2O2 →2 H2O + O2
Classifying Chemical Reactions
• Replacement Reaction – one element replaces another in a compound, or two elements in different compounds trade places. • AB + CD → AC + BD• (i.e.) Making pure copper
• 2 CuO + C → 2 Cu + CO2
Classifying Chemical ReactionsPractice Classify each of the following reactions as
synthesis, decomposition, or replacement.
• 2 NH4NO3 → 2 N2 + O2 + 4 H2O
• 2 Al + Fe2O3 → Al2O3 + 2 Fe
• MgCl2 + K2S → MgS + 2 KCl
• P4O10 + 6H2O → 4 H3PO4
Classifying Chemical ReactionsPractice Classify each of the following reactions as
synthesis, decomposition, or replacement.
• 2 NH4NO3 → 2 N2 + O2 + 4 H2O decomposition
• 2 Al + Fe2O3 → Al2O3 + 2 Fe
• MgCl2 + K2S → MgS + 2 KCl
• P4O10 + 6H2O → 4 H3PO4
Classifying Chemical ReactionsPractice Classify each of the following reactions as
synthesis, decomposition, or replacement.
• 2 NH4NO3 → 2 N2 + O2 + 4 H2O decomposition
• 2 Al + Fe2O3 → Al2O3 + 2 Fe replacement
• MgCl2 + K2S → MgS + 2 KCl
• P4O10 + 6H2O → 4 H3PO4
Classifying Chemical ReactionsPractice Classify each of the following reactions as
synthesis, decomposition, or replacement.
• 2 NH4NO3 → 2 N2 + O2 + 4 H2O decomposition
• 2 Al + Fe2O3 → Al2O3 + 2 Fe replacement
• MgCl2 + K2S → MgS + 2 KCl replacement
• P4O10 + 6H2O → 4 H3PO4
Classifying Chemical ReactionsPractice Classify each of the following reactions as
synthesis, decomposition, or replacement.
• 2 NH4NO3 → 2 N2 + O2 + 4 H2O decomposition
• 2 Al + Fe2O3 → Al2O3 + 2 Fe replacement
• MgCl2 + K2S → MgS + 2 KCl replacement
• P4O10 + 6H2O → 4 H3PO4 synthesis
Products have a higher energy level than the reactants in an endothermic reaction.
A False
B True
8.3 Energy and Chemical Change
____ bonds ____ energy.
A Breaking; releases
B Breaking; requires
C Forming; requires
D Ionic; release
8.3 Energy and Chemical Change
The law of ____ states that energy is neither created nor destroyed in a chemical reaction.
A conservation of mass
B definite proportions
C conservation of matter
D conservation of energy
8.3 Energy and Chemical Change
Which does NOT describe an exothermic reaction?
A There is a net release of energy.
B The reactants have a higher energy than the products .
C The reactants are lower in energy than the products.
D The products are less stable than the reactants.
Energy is ____ in exothermic reactions.
A absorbed
B created
C released
D stored
SCI 5.c
Vocab – Energy and Chemical ChangeText p.358-363. For #5-8, use your notes.
1. chemical bond2. law of conservation of energy3. exothermic process4. endothermic process5. activation energy6. synthesis reaction7. decomposition reaction8. replacement reaction