Title: Chemical Reactions Section 1: Observing Chemical Changes How can changes in matter be described?
1Chemical ReactionsSection 1 Observing Chemical
ChangesHow can changes in matter be described?
- In terms of two kinds of properties- physical
properties and chemical properties - Changes in matter can be described in terms of
physical or chemical changes
2What is a physical property?
- A characteristic of a substance that can be
observed without changing the substance into
another substance - Examples ice melting, color, hardness, texture,
shine, etc.
3What is a chemical property?
- A characteristic of a substance that describes
its ability to change into other substances - Examples burning magnesium, rusting
- Substances that undergo chemical changes
reactants - The new substance formed - product
4What is the difference between a physical and a
chemical reaction?
- A change in matter that produces one or more new
substances is a chemical change, or chemical
reaction - Physical change examples braiding your hair,
squashing a marshmallow - Chemical change examples burning of gasoline,
burning a marshmallow
5How can you tell when a chemical reaction occurs?
- Chemical changes occur when bonds break and new
bonds form - Chemical reactions involve two main kinds of
changes that you can observe-formation of new
substances and changes in energy
6What are some changes in properties that indicate
a chemical change?
- Precipitate a solid that forms from a solution
during a chemical reaction - Color change may indicate a chemical reaction
- Gas production bubbles
7How do you categorize changes in energy?
- Endothermic A reaction in which energy is
absorbed - More energy is required to break the bond of the
reactants than is released by the formation of
the new products - Examples baking soda and vinegar gets cooler
when combined
- Exothermic A reaction that releases energy in
the form of heat - The energy released as the products form is
greater than the energy required to break the
bonds of the reactants - Examples burning of airplane fuel
8 9Section 2 Describing chemical reactionsWhat
information does a chemical equation contain?
- Chemical equations use chemical formulas and
other symbols instead of words to summarize a
reaction - Reactants substances you have at the beginning
- Products new substances produced when the
reaction is complete
10What does the principle of conservation of mass
state?
- That in a chemical reaction, the total mass of
the reactants must equal the total mass of the
products - Matter is neither created nor destroyed
11What does open or closed system mean?
- Open system matter can enter from or escape to
the surroundings - Closed system matter is not allowed to enter or
leave
12What must a balanced chemical equation show?
- The same number of each type of atom on both
sides of the equation - Represents the conservation of mass
- Coefficients a number placed in front of a
chemical formula in an equation that tells you
how many atoms or molecules of a reactant or
product take place in the reaction
13How do you balance chemical equations?
- Count the of atoms of each element in the
reactants products - ID element that is not equal on both sides
- Add coefficient to the front of the formula that
will make the on both sides for that element
14Sample Problems
Reactants Products
1 Zn 1 Zn
1 H 2 H
1 Br 2 Br
- Sample Problem
- Zn HBr ? H2 ZnBr2
-
- Multiply HBr x 2 to correct (balance)
- Zn 2HBr ? H2 ZnBr2
Reactants Products
1 Zn 1 Zn
2 H 2 H
2 Br 2 Br
15Try these!!
2
2
- Na Cl2 ? NaCl
- Ca Cl2 ? CaCl2
- H2O ? H2 O2
- N2 H2 ? NH3
- Al2O3 ? Al O2
- P4 O2 ? P4O6
- Fe H2O ? Fe3O4 H2
2
2
3
2
4
3
2
3
3
4
4
16What are the three categories of chemical
reactions?
- Synthesis
- Decomposition
- Replacement
17What is a synthesis reaction?
- When two or more substances combine to make a
more complex substance - A B ? AB
- 2H2 O2 ?2H2O
- Example hydrogen and oxygen to make water
18What is a Decomposition reaction?
- Breaking down compounds into simpler products
- AB ? A B
- 2H2O2 ? 2H2O O2
- Example Hydrogen peroxide decomposes into water
and oxygen gas
19What is a replacement reaction?
- When one element replaces another in a compound,
or when two elements in different compounds trade
places - Example copper metal obtained by heating copper
oxide with carbon - Single (one element replaces another)
- AB C ? AC B
- Zn 2HCl ? ZnCl2 H2
- or double (elements appear to trade places with
another compound) - AB CD ? AC BD
- NaCl AgF ? NaF AgCl
20Section 3 Controlling Chemical ReactionsHow is
activation energy related to chemical reactions?
- The minimum amount of energy needed to start a
chemical reaction - All chemical reactions need a certain amount of
activation energy to get started
21What factors affect the rate of a chemical
reaction?
- Surface area the greater the surface area that
faster the reaction (ex. Chewing) - Temperature raising the temperature causes
particles to move faster and therefore have more
energy they also come into contact more often
lowering temperature slows things down - Concentration amount of substance in a given
volume increased concentration-increased
reaction - Catalysts increases the rate of a reaction by
decreasing the energy needed to start - Enzymes biological catalysts
- Inhibitors material used to decrease the rate
of reaction (ex. preservatives in food)