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