5.1 The nature of chemical reactions

Pages 148 - 153

Goal 1

Understand what occurs during a chemical reaction and

recognize the signs

• Each compound or element– Has its own physical properties

• Taste• Melting temp• Boiling temp• Color

• Chemical reactions rearrange atoms– Reactants at start– Compounds are changed– Products at end– New properties shown

2H2 + O2 2 H2O

• Signs of chemical reaction1. Gas formed

• Bubbles• Odor• Smoke

• Signs of chemical reaction2. New solid formed

Sodium sulfide + cadmium nitrate cadmium sulfide

(clear liquid) (clear liquid) (yellow solid)

• Signs of chemical reaction3. Color change

• Signs of chemical reaction4. Heat / light energy release

• Signs of chemical reaction5. Corrosion

Goal 2

Describe energy movement and the difference between exothermic and

endothermicreactions

• Break bonds– Reactant break-up– Energy added– Heat, light, electricity, sound

• Form bonds– Product formation – Energy released

2H2 + O2 2 H2O

• Energy is conserved in chemical reactions

• All accounted for (what goes in…)– Absorbed– Stored– Released

• Exothermic– Overall reaction releases energy– Reactants had more stored energy than

products

• Endothermic– Overall reaction absorbs energy– Reactants had less stored energy than

products