Transcript energy change

Lab Skills Focus:
Filtration
Chemical Equation
MgSO4 + Na2CO3 → MgCO3(s) + Na2SO4
magnesium + sodium
sulfate
carbonate
→ magnesium +
carbonate
sodium
sulfate
Clear liquid
Word Equation
When solutions of two soluble salts are mixed, a
solid may form. This solid is called a precipitate.
and the reaction is called a precipitation reaction.
Precipitation reactions are used to make insoluble
salts.
In this example the soluble salts are magnesium
sulfate and sodium carbonate, and the insoluble
salt formed is magnesium carbonate.
Law of Conservation of Mass
Instructions
1. Measure 5 g of baking soda and
put into a flask.
2. Measure 20 mL of vinegar and carefully
pour it into a balloon.
3. Carefully attach the balloon to the top of the flask
without spilling the vinegar into the flask.
4. Place the flask with balloon on a weighing scale and
record the mass of the system.
5. Tip the vinegar from the balloon into the flask and
record your observations.
6. Measure the mass of the system after the reaction.
7. Pop the balloon and measure the mass again.
Summary
1. Explain whether your results support
the Law of Conservation of Mass.
There are times when mass might
appear to change during a chemical
reaction.
If you experience this during an
experiment, be sure to remember the
Law of Conservation of Mass.
The change in mass must be
accounted for in a way other than
destroying or creating atoms.
Endothermic and
Exothermic Reactions
(Heat Change)
Exothermic and Endothermic Reactions
An energy change accompanies the forming or
breaking of a bond between atoms in a molecule.
O Energy is released (produces heat) when a
bond forms
Exothermic
o Energy is absorbed from the surroundings
to break the bonds in a molecule
Endothermic
In your notebook:
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•
•
•
Draw a diagram of the set-up
Make a data table
Make a graph
Calculate temperature change as a
function of time
Endothermic
Reaction
Explore Lab 1:
Energy change
1. Make a blank table for recording results.
2. Pour 25 mL of acetic acid into a 400 mL beaker.
3. Record the temperature of acetic acid as the starting
temperature.
4. Keep the thermometer in the acetic acid and add 5 g of
baking soda into the beaker.
5. Watch the thermometer carefully
and record the time for each degree
Celsius drop.
5. Graph the rate of temperature change.
6. Calculate the rate of temperature change.
Temperature/T degrees C
Starting temperature- ex.
Time/sec
28
27
26
25
24
23
Calculating Rate of temperature change
Maximum ⁰C – Minimum ⁰C = Rate of temperature change
Total Time (sec)
Endothermic
Reaction
Explore Lab:
Energy change
Summary:
 Explain how this lab shows a chemical
reaction.
 How does the graph show that the reaction is
endothermic? Explain.
Endothermic reaction results:
• Starts at around 25 degrees C and
ends at around 21 degrees C
• Happens within about 2 minutes
http://www.youtube.com/watch?v=MyAzjSdc3Fc
Endothermic Reaction
Exothermic Reaction: Oxidation of steel wool
1. Put 8 grams of wool into a 400 mL beaker.
Explore Lab 2:
Energy change
2. Add enough acetic acid to completely
cover the wool.
3. Swirl for 2 minutes to remove the protective coating.
4. Pour the acetic acid into a waste beaker.
5. Quickly cover the beaker with Parafilm.
6. Insert a thermometer through
the film and into the steel wool.
7. Record the time for each degree Celsius rise and
follow the temperature/time profile through a 5
degree change.
Exothermic reactions
•The reaction produces heat.
•In this reaction vinegar is used
to remove the protective coating
from steel wool, allowing it to
rust.
•When the iron combines with
oxygen, heat is released.
Exothermic Reaction: Oxidation of steel wool
Summary:
 Explain how this lab shows a
chemical reaction.
 On the same graph (endothermic), show a
graph of temperature change as a function of
time.
 How does the graph show that the
reaction is exothermic? Explain.
Exothermic Reaction: Oxidation of steel wool
Applying the Law of Conservation of Mass:
How many atoms of Oxygen is needed to react
with 4 atoms of Iron to form 2 units of Iron oxide?
4 Fe +
4Fe +
 2 Fe2O3
___ O2
3O2
4 Fe + 6 O

2Fe2O3
=
4 Fe + 6 O