Download classification of chemical reactions

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Chemical Reactions and Chemical Equations
Chemical reaction (a chemical change)
formation of a new substance when chemical bonds break or new bonds are
formed
Example: 2Na + Cl2  2NaCl
Chemical change
change in matter that produces new substances
Example: rusting of iron
burning of wood
Physical change
a change that does not produce a new substance
a change in appearance or state
Example: chopping wood
melting ice
Evidence of chemical reactions
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A new substance is formed Na + Cl  NaCl
Gas production (bubbles) 2H2O2  H2O + O2
Color change
Temperature change (endothermic vs. exothermic)
Precipitate forms – solid substance that forms when two
substances mix
Chemical Equations
 shows chemical reactions using symbols and numbers instead of words
 shows the formula which contains the reactants and products
Example: 2 H2 + O2
 2 H2O
[reactant] [reactant]
[products]
reactants are found to the left side of the arrow
products are found to the right side of the arrow
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coefficients the large number in front of each formula, tells
how many of each substance that reacts, or is produced
subscripts the small number written below the symbol, tells
the number of atoms of an element present
Example: 2 H2 +
O2  2 H2 O
coefficients
subscripts
Law of Conservation of Mass
 matter cannot be created nor destroyed
 mass does not change in a chemical reaction
 chemical equations are written to show that atoms are neither created
nor destroyed [ balanced on both sides]
 atoms are just rearranged
Balanced equations follow the Law of Conservation of Mass:
# of reactants = # of products
# of atoms on the right = # of atoms on the left
Mg + O2  MgO balanced or unbalanced?
Answer: unbalanced
To balance, add coefficients in front of symbols until the # of reactants equals
the # of products
 Do not change or add subscripts
 Do not breakup a compound to add a coefficient
2 Mg
+
H2O
2H2O
O2

2 MgO
H2 + O2
2H2 + O2
2
Examples of coefficients
1) 3H2O
The coefficient is _____
2) 2H2SO4
The coefficient is ______
3) 4Fe2 O3
The coefficient is ______
What is the total number of atoms in the above formulas?
1) 3H2O= ___ hydrogen (H), ___ oxygen (O) = total of ___ atoms
2) 2H2SO4 = ___ hydrogen (H), ___sulfur (S), ___ oxygen (O) = total of ___atoms
3) 4Fe2 O3 = ____ iron (Fe), ___ oxygen (O) =total of ____ atoms
NOTE: Balanced equations demonstrate the Law of Conservation of Mass
CLASSIFICATION OF CHEMICAL REACTIONS
Reactions can be classified by what happens to the reactants and products
TYPES OF CHEMICAL REACTIONS:
1) Synthesis reaction (to put together)—____________________________
____________________________________________________________________.
Example:
A
+
B

AB
element + element
compound
Na + Cl2
 NaCl
Does this equation demonstrate the Law of conservation
of Mass? ______Is it balanced or unbalanced? _____________________
____ Na + ___Cl2
 ____ NaCl
2) Decomposition reaction---_______________________________________
__________________________________________________________________
Example:
AB

A
+
B
Compound 
element + element
NaCl
 Na
+ Cl
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3) Neutralization reaction
: acids + base = salt + water
Controlling Chemical Reactions
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chemical reactions need a certain amount of energy to get started
during a chemical reaction energy is either absorbed or released
energy is needed to break chemical bonds
broken bonds can form new chemical bond= new products
Activation energy—________________________________________________
________________________________________________________________________
Example: H2 + O2 + energy
started)
 H2O2 (energy is required to get this reaction
Energy and Types of reactions
Exothermic reactions
 A reaction that ____________________________________________
 Most chemical reactions ___________________________________
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 Energy is shown ___________________________________________
 Reactant
 product + heat energy
Endothermic reactions
 ____________________________________________________________
 energy is needed to keep the reaction going
 energy is shown on________________________________________
 Reactants + heat energy  products
Factors that affect the rates of Reactions
 Temperature: an ___________________in temperature _____________________ the
rate of chemical reactions (particles move faster, so reaction rate increases)
 Concentration: an ___________________in concentration ______________________
the reaction rate (more particles=more collisions, so the reaction rate
___________________
 Surface area: an _________________in surface area ___________________ the
reaction rate
 Catalysts:
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increases the reaction rate by lowering
the activation energy
They are not used up or destroyed in the
reaction
They can be used over and over again
Example of organic catalysts : enzymes
Hydrogen peroxide
Water + Oxygen
Catalyst
Is this a physical reaction or a chemical reaction? ____________
In the above equation, hydrogen peroxide is the ______________
In the above equation water and oxygen are the ______________
Where is the catalyst shown in a chemical equation? ___________________
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Inhibitors: slow down a reaction
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