Grade Science Bowling Green Junior High What are chemical reactions Chemical Reaction a change that takes place when two or more substances reactants interact to form new substances products with new properties ID: 913965
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Slide1
Chemical Reactions
7
th
Grade Science
Bowling Green Junior High
Slide2What are chemical reactions?
Slide3Chemical Reaction
–
a change that takes place when two or more substances (reactants) interact to form new substances (products) with new properties.
Slide4Compounds
Matter made of two or more different elements chemically bonded.
Cannot be separated by physical means
Has properties that are different from the elements that make it up.
Slide5Slide6MORE COMMON THAN ELEMENTS DUE TO MANY ELEMENTS BEING REACTIVE WITH EACH OTHER
THE ELEMENTS THAT COMBINE MAKE A NEW SUBSTANCE WITH NEW PHYSICAL PROPERTIES
FOR
A COMPOUND TO FORM OR BE BROKEN DOWN,
A CHEMICAL REACTION MUST TAKE PLACE
+
=
NaCl
TABLE SALT
Slide7Everyday examples of chemical reactions
Respiration (breathing)
Photosynthesis
Grilling foodStarting a vehicle
DigestionRusting metal
Slide8How do you know when a chemical reaction has taken place?
A new substance with new properties is formed
Slide9Signs of a chemical reaction
Temperature Change (heat given off or required)
FIZZES
OR BUBBLES
COLOR CHANGEODOR
LIGHT
GIVEN OFF
NEW SUBSTANCE FORMEDPrecipitate (solid)Precipitate (gas bubbles)
Slide10Two parts of a chemical reaction
Reactants
– Substances that
start a
chemical reaction (EX: chemicals on match head)Products
– Substances produced in the reaction
(EX: black
material on match)
Slide11Slide12CHEMICAL EQUATIONS
Chemical equations are symbols used to describe the details of a chemical reaction.
Shows how the reactants changed into the product.
This involves indicating all the atoms involved in the reaction.
Fe + O
2
FeO
2
Reactants
:
Iron and oxygen
Product
:
Ferrous oxide
(rust)
Plus Sign
:
Shows substances combine
Arrow
:
Means “yields”
takes the place of an = sign
Reactants are ALWAYS to the left of the arrow
Products are ALWAYS to the right of the arrow
Slide13Types of chemical reactions
Combustion
Synthesis
DecompositionSingle replacement
Double replacementNeutralizationOxidation/ReductionHydrolysisEndothermic/Exothermic
Slide14What do you have to have to burn something?
Slide15Combustion reactions
When oxygen (O
2
) combines with another compound to form water and carbon dioxide. Needs a fuel source
Takes place at high temperaturesFast process that results in an increase of temperature and production of fire.
Slide16Chemical reactions can be classified
Combustion Reaction
– always involves oxygen (O
2
) as a reactant.
O
C
CH
4
O
O
O
+
+
2O
2
CO
2
+
2H
2
O
Methane
Oxygen
Carbon Dioxide
Water
+
H
H
H
H
Slide174 types of reactions
Slide18Synthesis reactions
Two or more substances react to form a new substance(s)
A + B
AB
S + O2 SO
2
Slide19Chemical reactions can be classified
Synthesis Reaction
– combines two or more simpler reactants to form new, more complex products
.
N
N
O
O
O
O
N
2
+
2O
2
2NO
2
+
Nitrogen
Oxygen
Nitrogen Dioxide
Simple to complex
Slide20Decomposition reaction
One substance breaks down into two or more simpler substances
AB
A + B
CaCO3
CaO
+ CO
2
Slide21Chemical reactions can be classified
Decomposition reaction
– breaks a reactant into two or more simpler products
2H
2
O
Water
2H
2
+
O
2
Hydrogen
Oxygen
O
H
H
O
H
H
+
Complex to simple
Slide22Single Replacement
One element replaces another element in a compound
AB + C
AC + B
Zn + 2HCl H2 + ZnCl
2
Slide23Chemical reactions can be classified
Replacement Reaction
– elements switch places to form new compounds.
1) Single Replacement
Zn
Zinc
H
2
+
ZnCl
2
Hydrogen
Zinc Chloride
2HCl
Hydrochloric Acid
+
H
Cl
+
H
Cl
H
Cl
H
Cl
Zn
+
Slide24Double replacement
Elements from two different compounds switch places
AB + CD
AC + BD
HCl + NaOH
NaCl
+ H2O
Slide25Cl
Chemical reactions can be classified
Replacement Reaction
– elements switch places to form new compounds.
Double Replacement
Cl
FeS
Iron Sulphide
H
2
S
+
FeCl
2
Hydrogen Sulfide
Iron Chloride
2HCl
Hydrochloric Acid
+
Fe
S
+
+
H
H
Slide26All chemical reactions are going to release (give off) energy or absorb (take in) energy.
Some will require energy to start the reaction (activation energy)
EX: before you use a new cell phone, what’s got to happen?
Activation energy=energy required to start a chemical reaction.
Slide27Endothermic vs. exothermic processes
Slide28Exothermic reactions or processes
Exothermic reactions are exactly the opposite. While they take some energy to get going, called the activation energy of reaction, these reactions give off heat during the
reaction
Good examples of exothermic reactions are explosions like fireworks or combustion in engines.
Forming a chemical bond releases energy and is exothermicUsually feel hot because it is giving heat to you
Slide29Endothermic reactions or processes
Endothermic reactions are those which absorb heat during the reaction. They take in more energy than they give off, which leaves the surroundings cooler than the starting point
Evaporation of water by sunlight is a great example. The sun and the liquid water combine and the water absorbs energy and eventually becomes as gas.
Breaking a chemical bond requires energy and is endothermicUsually feel cold because it is taking heat away from you
Slide30Slide31Catalyst
Substance which speeds up a chemical reaction but is chemically unchanged at the end of the reaction.
The catalytic converter in a car contains platinum, which serves as a catalyst to change carbon monoxide, which is toxic, into carbon dioxide.
If you light a match in a room with hydrogen gas and oxygen gas, there will be an explosion and most of the hydrogen and oxygen will combine to create water molecules.
Slide32A way of writing which type of atoms and how many of each there are in a compound.
Slide33Chemical Formulas
Written as: C
4
H
10
Butane
Written as: CH
4
Methane
Slide34Subscripts= how many atoms
= how many total molecules
Slide35Counting Atoms
FeO
2
H2OCO
2MgBr2C6H12O
6
3OH
2H2O
Slide36Counting atoms in chemical equations
2Na + MgF
2
2NaF + Mg
Slide37Counting atoms in chemical equations
2K + Cl
2
2KCl
Slide38Counting atoms in chemical equations
2Na
2
O 4Na + O2
Slide39Law of Conservation of Matter
Matter cannot be created or destroyed, it just changes forms.
*
The
total mass of the reactants
MUST EQUAL
the
total mass of the product
.
Slide40Slide41Law of Conservation of Mass
http://www.sky-web.net/science/balancing_chemical_equations_examples.htm
Alka-Seltzer and Water
Slide42Balancing Equations
The number of atoms of the reactants must equal the number of atoms in the product. (Law of Conservation of Matter)
Ex: 2Na + Cl
2
-> 2NaCl 4P + 5O2 ->
P
4O10
Slide43Balancing Equations
Rules
Make sure that all atoms are equal on both sides.
You can only add coefficients.
Changing the subscripts will change the identity of the compound. H2
O & H
2
O2EX: 2Na + Cl2 -> 2NaCl
H2 + O2 -> 2H2O
(Not balanced… So…)
2H
2
+ O
2
-> 2H
2
O
Slide44Balancing Chemical Equations
Hg + O
2
HgO
H
2
+ Cl HCl
Mg + O
2
MgO
O
2
+ H
2
H
2
O
CH
4
+ O
2
CO
2
+ H2OFe + Cl
2
FeCl
3
Slide45Hg + O
2
HgO
Slide46H
2
+ Cl
HCl
Slide47Mg + O
2
MgO
Slide48O
2
+ H
2
H
2
O
Slide49Fe + Cl
2
FeCl
3
Slide50CH
4
+ O
2
CO
2
+ H
2
O