Transcript Observing Chemical Change

Targets for this Unit
the properties of the products of a chemical
reaction are different from the properties of
the reactants.
that in a chemical reaction no atoms are lost
or created, they are simply rearranged.
how to balance a simple chemical reaction
equation.
chemical reactions that absorb heat are
called endothermic reactions.
chemical reactions that release heat are
called exothermic reactions.
Observing Chemical
Change
Chapter 6.1
Page 214
• What happens during a chemical
reaction?
• Suppose you fill a sealable bag
with ice cubes, and you allow
the ice to melt. How would the
mass of the bag and ice before
melting compare to the mass of
the water and the bag after
melting? Explain your answer.
Section 1:
Observing Chemical Change
How can changes in matter be
described?
How can you tell when a chemical
reaction occurs?
Review
• Matter --- anything that has mass and
takes up space
• Chemistry --- the study of matter and how
it changes
• Changes in matter come in two forms:
physical changes and chemical
changes
Properties and Changes in
Matter
• Matter can undergo both physical
change and chemical change.
Chemical change
involves both reactants and products
Marshmallow + oxygen
carbon (ash) + water (vapor) + carbon dioxide
C16H22O11 + 2 O2
reactants
7 C2 + 11 H2O + 2 CO2
products
Reactants are the compounds you begin with in
a chemical reaction.
Products are the compounds produced in a
chemical reaction.
Properties and Changes in
Matter
• Chemical changes occur when bonds
break and new bonds form.
Reactants
Products
Evidence for Chemical Reactions
Chemical reactions involve changes in
properties and changes in energy that you
can observe.
1. Changes in properties
2. Changes in energy
1. Changes in Properties
• Physical change
– Color
– Precipitate
- Gas
- Texture
• Chemical change
– When two or more reactants chemically
combine to form new products, the chemical
properties of the products are different from
the properties of the reactants.
2. Changes in Energy
Chemical reactions usually absorb heat or
release heat.
When a reaction absorbs heat it is called an
endothermic reaction.
In an endothermic reaction, the products are
cooler than the reactants, because the
reaction absorbs heat from the reactants.
2. Changes in Energy
Chemical reactions usually absorb heat or
release heat.
When a reaction releases heat it is called an
exothermic reaction.
In an exothermic reaction, the products are
warmer than the reactants, because the
reaction releases heat to the reactants.
Energy in Chemical Changes
•A student places two
substances in a flask
and measures the
temperature once per
minute while the
substances react. The
student plots the time
and temperature data
and creates the graph
at left.
Energy in Chemical Changes
• Reading Graphs:
– What was the temperature
in the flask at 3 minutes?
When was the first time the
temperature was at 6ºC?
–At 3 minutes the
temperature in the flask
was about 30ºC. The first
time the temperature was
6ºC was at about 7
minutes.
Energy in Chemical Changes
• Calculating:
– How many degrees did
the temperature drop
between
2 minutes and 5
minutes?
–About 20ºC
Energy in Chemical Changes
• Interpreting Data:
– Is the reaction
endothermic or
exothermic? Explain.
–The reaction was
endothermic, because it
absorbed thermal energy
from the reaction
mixture, causing the
temperature to drop.
Energy in Chemical Changes
• Inferring:
– At what temperature
did the reaction stop?
How can you tell?
–The reaction stopped at
about 2ºC. You can tell
because that is the
lowest temperature
reached.
Energy in Chemical Changes
• Drawing Conclusions:
Suppose the temperature in
the flask increased instead
of decreased as the
reaction occurred. In terms
of energy, what kind of
reaction would it be?
If the temperature
increased instead, the
reaction would be
exothermic, because an
exothermic reaction is one
in which energy is
released.
Links on Chemical Changes
• Click the SciLinks button for links on
chemical changes.
End of Section:
Observing Chemical
Change
Do Cornell Notes for
Chapter 6, Section 1