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. light and sound energy thermal energy

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

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

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.) 2H 2 + O 2 → 2H 2 O 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 H 2 + O 2 → 2 H 2 O

Classifying Chemical Reactions Decomposition Reaction – a compound reactant breaks down into simpler products AB → A + B 2 H 2 O 2 →2 H 2 O + O 2

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 + CO 2

Classifying Chemical Reactions Practice Classify each of the following reactions as synthesis, decomposition, or replacement. 2 NH 4 NO 3 → 2 N 2 + O H 2 O 2 Al + Fe 2 O 3 → Al 2 O Fe MgCl 2 + K 2 S → MgS + 2 KCl P 4 O H 2 O → 4 H 3 PO 4

Classifying Chemical Reactions Practice Classify each of the following reactions as synthesis, decomposition, or replacement. 2 NH 4 NO 3 → 2 N 2 + O H 2 Odecomposition 2 Al + Fe 2 O 3 → Al 2 O Fe MgCl 2 + K 2 S → MgS + 2 KCl P 4 O H 2 O → 4 H 3 PO 4

Classifying Chemical Reactions Practice Classify each of the following reactions as synthesis, decomposition, or replacement. 2 NH 4 NO 3 → 2 N 2 + O H 2 Odecomposition 2 Al + Fe 2 O 3 → Al 2 O Fereplacement MgCl 2 + K 2 S → MgS + 2 KCl P 4 O H 2 O → 4 H 3 PO 4

Classifying Chemical Reactions Practice Classify each of the following reactions as synthesis, decomposition, or replacement. 2 NH 4 NO 3 → 2 N 2 + O H 2 Odecomposition 2 Al + Fe 2 O 3 → Al 2 O Fereplacement MgCl 2 + K 2 S → MgS + 2 KClreplacement P 4 O H 2 O → 4 H 3 PO 4

Classifying Chemical Reactions Practice Classify each of the following reactions as synthesis, decomposition, or replacement. 2 NH 4 NO 3 → 2 N 2 + O H 2 Odecomposition 2 Al + Fe 2 O 3 → Al 2 O Fereplacement MgCl 2 + K 2 S → MgS + 2 KClreplacement P 4 O H 2 O → 4 H 3 PO 4 synthesis

Products have a higher energy level than the reactants in an endothermic reaction. AFalse BTrue 8.3 Energy and Chemical Change

____ bonds ____ energy. ABreaking; releases BBreaking; requires CForming; requires DIonic; release 8.3 Energy and Chemical Change

The law of ____ states that energy is neither created nor destroyed in a chemical reaction. Aconservation of mass Bdefinite proportions Cconservation of matter Dconservation of energy 8.3 Energy and Chemical Change

Which does NOT describe an exothermic reaction? AThere is a net release of energy. BThe reactants have a higher energy than the products. CThe reactants are lower in energy than the products. DThe products are less stable than the reactants.

Energy is ____ in exothermic reactions. Aabsorbed Bcreated Creleased Dstored SCI 5.c

Vocab – Energy and Chemical Change Text p For #5-8, use your notes. 1.chemical bond 2.law of conservation of energy 3.exothermic process 4.endothermic process 5.activation energy 6.synthesis reaction 7.decomposition reaction 8.replacement reaction