1. Chemical Reactions Notes
Evidence and Energy

2. Chemical vs. Physical Change
Physical change – anything that alters the form of a substance, but not the identity.
Ex:, molding clay, melting ice, mixing a milkshake, etc…
Chemical change – atoms rearrange and new substances are formed.
Ex:, cooking, digestion, rusting, burning candle

3. Signs of a Chemical Change
1. Color Change – leaves changing colors, bread browning in oven, copper tarnishing to green, eggs cooking and changing to white, etc…

4. Other Signs of Chemical Change
2. A precipitate forming – a solid that forms in a solution from 2 liquids.
3. Gas Production - Examples: Carbon dioxide produced from yeast in bread, baking soda and vinegar

5. Other Changes
4. Changes in temperature – Ex: cold packs, calcium chloride, sodium metal and water

6. 5. Changes in properties
Baking a cake, texture, form and color changes.
Digestion – food changes from solids and is broken down into simplest forms.

7. What’s Happening on the Atomic Scale:
Chemical reactions occur when chemical bonds are either formed or broken apart.
Chemical bonds are the glue which keep atoms together.
Some chemical bonds are easy to break, some are strong and difficult to change.

8. Chemical Equations 2 H2 + O2 2 H2O
Subscript
Chemical Equations
Coefficient
2 H2 + O H2O
Reactant – the materials you start with – located to the left of the arrow
Product – substances formed in a chem. Reaction
Subscripts - # of atoms of an element in a molecule.
Coefficients - # of molecules

9. Conservation of mass
During a chem. Rxn, matter is neither created or destroyed.
Mass of atoms of the reactants always equals the mass of Atoms of the products

10. NaCl = ionic bond, sodium gives valence e-, chlorine takes valence e-
Bonding
NaCl = ionic bond, sodium gives valence e-, chlorine takes valence e-
When elements form compounds, they either take, give, or share electrons
Ionic bonds = give or take electrons
Covalent bonds = share electrons
Water molecule – hydrogen atoms share its electrons with the 1 oxygen atom, and vice versa

11. Example of Ionic Bond

12. Example of Covalent Bond

13. Rate of Chemical Reactions
Every Chemical reaction involves a change in energy. Some release energy, while others absorb energy

14. Exothermic
A reaction that releases energy in the form of heat

15. Endothermic Reaction
A reaction that absorbs energy and therefore feels cool to the touch.

16. Rates of Reaction
Concentration – increase amount of reactant speeds up reaction
Surface area – increase surface area, increase speed of reaction
Temperature – increase temp, increase reaction

17. Rates of Reaction
Catalyst- speeds up reactions by lowering activation energy It’s the match.com of chemicals!
Inhibitors – slows reaction down by raising activation energy. Ex: food preservatives.
Activation energy – energy required to get a reaction started.