Chemical Reactions Notes Evidence and Energy

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Chemical Reactions Notes
Evidence and Energy
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
with new properties.
• Ex:, cooking, digestion,
rusting, burning candle
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…
Other Signs of Chemical Change
• 2. A precipitate forming –
a solid that forms in a
solution from 2 liquids. limewater + CO2
• 3. Gas Production Examples: Carbon dioxide
produced from yeast in
bread, baking soda and
vinegar
Other Changes
• 4. Changes in
temperature – Ex: hot
pads, calcium
chloride, sodium
metal and water
5. Changes in properties
• Baking a cake, texture, form and color
changes.
• Digestion – food changes from solids and is
broken down into simplest forms.
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.
• Ionic bonds are stronger than covalent
bonds
Subscript
Chemical Equations
Coefficient
• 2 H2 + O2
2 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
The Law of 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
Bonding
NaCl = ionic
bond, sodium
gives valence e-,
chlorine takes
valence e-
Water molecule –
hydrogen atoms share
its electrons with the 1
oxygen atom, and vice
versa
• When elements form
compounds, they
either take, give, or
share electrons
• Ionic bonds = give or
take electrons
• Covalent bonds =
share electrons
Example of Ionic Bond
Example of Covalent Bond
Energy and Chemical Reactions
• Every Chemical
reaction involves a
change in energy.
Some release energy,
while others absorb
energy
Exothermic
• A reaction that
releases energy in the
form of heat
Endothermic Reaction
• A reaction that
absorbs energy and
therefore feels cool to
the touch.
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
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.
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