Targets for this Unit

1
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.

2
Observing Chemical Change

• Chapter 6.1
• Page 214

3

• 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.

4
Section 1Observing Chemical Change

• How can changes in matter be described?
• How can you tell when a chemical reaction occurs?

5
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

6
Properties and Changes in Matter

• Matter can undergo both physical change and
  chemical change.

7
Chemical changeinvolves both reactants and
products
Marshmallow oxygen carbon (ash)
water (vapor) carbon dioxide

• C16H22O11 2 O2 7 C2 11 H2O 2 CO2
• reactants products

Reactants are the compounds you begin with in a
chemical reaction. Products are the compounds
produced in a chemical reaction.
8
Properties and Changes in Matter

• Chemical changes occur when bonds break and new
  bonds form.

Reactants Products
9
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

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1. Changes in Properties

• Physical change
• Color - Gas
• Precipitate - 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.

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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.

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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.

13
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.

14
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.

15
Energy in Chemical Changes

• Calculating
• How many degrees did the temperature drop
  between2 minutes and 5 minutes?

• About 20ºC

16
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.

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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.

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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.

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Links on Chemical Changes

• Click the SciLinks button for links on chemical
  changes.

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End of SectionObserving Chemical ChangeDo
Cornell Notes for Chapter 6, Section 1