chemical reactions 7 th grade science bowling green junior high
TRANSCRIPT
CHEMICAL REACTIONS7th 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.
COMPOUNDS 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.
MORE 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
EVERYDAY EXAMPLES OF CHEMICAL REACTIONS
Respiration (breathing) Photosynthesis Grilling food Starting a vehicle Digestion Rusting metal
HOW DO YOU KNOW WHEN A CHEMICAL REACTION HAS TAKEN PLACE?
A new substance with new properties is formed
SIGNS OF A CHEMICAL REACTION Temperature Change (heat given off or
required) FIZZES OR BUBBLES COLOR CHANGE ODOR LIGHT GIVEN OFF NEW SUBSTANCE FORMED
Precipitate (solid) Precipitate (gas bubbles)
TWO PARTS OF A CHEMICAL REACTIONReactants – Substances that start a chemical reaction (EX: chemicals on match head)Products – Substances produced in the reaction (EX: black material on match)
CHEMICAL 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 + O2 FeO2
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
TYPES OF CHEMICAL REACTIONS
Combustion Synthesis Decomposition Single replacement Double replacement Neutralization Oxidation/Reduction Hydrolysis Endothermic/Exothermic
WHAT DO YOU HAVE TO HAVE TO BURN SOMETHING?
COMBUSTION REACTIONS When oxygen (O2) combines with another
compound to form water and carbon dioxide. Needs a fuel source Takes place at high temperatures Fast process that results in an increase of
temperature and production of fire.
Chemical reactions can be classified
Combustion Reaction – always involves oxygen (O2) as a reactant.
OC
CH4
O
O O
+
+
2O2 CO2+ 2H2OMethane Oxygen Carbon Dioxide Water
+H
H H
H
4 TYPES OF REACTIONS
SYNTHESIS REACTIONS
Two or more substances react to form a new substance(s)
A + B AB
S + O2 SO2
CHEMICAL REACTIONS CAN BE CLASSIFIEDSynthesis Reaction – combines two or more
simpler reactants to form new, more complex products.
N NO O
O O
N2 + 2O2 2NO2
+
Nitrogen Oxygen Nitrogen Dioxide
Simple to complex
DECOMPOSITION REACTION
One substance breaks down into two or more simpler substances
AB A + B
CaCO3 CaO + CO2
Chemical reactions can be classifiedDecomposition reaction – breaks a reactant
into two or more simpler products
2H2OWater
2H2 + O2Hydrogen Oxygen
OHH
OHH
+
Complex to simple
SINGLE REPLACEMENT
One element replaces another element in a compound
AB + C AC + B
Zn + 2HCl H2 + ZnCl2
Chemical reactions can be classified
Replacement Reaction – elements switch places to form new compounds.
1) Single Replacement
ZnZinc
H2+ZnCl2HydrogenZinc Chloride
2HClHydrochloric Acid
+
HCl+
HCl
HCl
HCl
Zn
+
DOUBLE REPLACEMENT
Elements from two different compounds switch places
AB + CD AC + BD
HCl + NaOH NaCl + H2O
Cl
Chemical reactions can be classified
Replacement Reaction – elements switch places to form new compounds.
Double Replacement
Cl
FeSIron Sulphide
H2S+FeCl2Hydrogen SulfideIron Chloride
2HClHydrochloric Acid
+
Fe
S
+ +H
H
All 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.
ENDOTHERMIC VS. EXOTHERMIC PROCESSES
EXOTHERMIC 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
exothermic
Usually feel hot because it is giving heat to you
ENDOTHERMIC 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 endothermic
Usually feel cold because it is taking heat away from you
CATALYST 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.
A way of writing which type of atoms and how many of each there are in a compound.
CHEMICAL FORMULAS
Written as: C4H10
Butane
Written as: CH4
Methane
Subscripts= how many atoms
= how many total molecules
COUNTING ATOMS
FeO2
H2O
CO2
MgBr2
C6H12O6
3OH 2H2O
COUNTING ATOMS IN CHEMICAL EQUATIONS
2Na + MgF2 2NaF + Mg
COUNTING ATOMS IN CHEMICAL EQUATIONS
2K + Cl2 2KCl
COUNTING ATOMS IN CHEMICAL EQUATIONS
2Na2O 4Na + O2
Law 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.
Law of Conservation of Mass
http://www.sky-web.net/science/balancing_chemical_equations_examples.htm
Alka-Seltzer and Water
BALANCING EQUATIONS• The number of atoms of the
reactants must equal the number of atoms in the product. (Law of Conservation of Matter)
Ex: 2Na + Cl2 -> 2NaCl
4P + 5O2 -> P4O10
BALANCING 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. – H2O & H2O2
EX: 2Na + Cl2 -> 2NaCl
H2 + O2 -> 2H2O
(Not balanced… So…)
2H2 + O2 -> 2H2O
Balancing Chemical Equations
Hg + O2 HgO
H2 + Cl HCl
Mg + O2 MgO
O2 + H2 H2O
CH4 + O2 CO2 + H2O
Fe + Cl2 FeCl3
Hg + O2 HgO
H2 + Cl HCl
Mg + O2 MgO
O2 + H2 H2O
Fe + Cl2 FeCl3
CH4 + O2 CO2 + H2O