CHEMICAL REACTIONS 7 th 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 REACTION Reactants – 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 + 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

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 (O 2 ) 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 (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

4 TYPES OF REACTIONS

SYNTHESIS REACTIONS  Two or more substances react to form a new substance(s)  A + B  AB  S + O 2  SO 2

CHEMICAL REACTIONS CAN BE CLASSIFIED Synthesis Reaction – combines two or more simpler reactants to form new, more complex products. N N O O O O N2N2 + 2O 2 2NO 2 + Nitrogen Oxygen Nitrogen Dioxide Simple to complex

DECOMPOSITION REACTION  One substance breaks down into two or more simpler substances  AB  A + B  CaCO 3  CaO + CO 2

Chemical reactions can be classified Decomposition reaction – breaks a reactant into two or more simpler products 2H 2 O Water 2H 2 + O2O2 Hydrogen Oxygen O H H O H H + Complex to simple

SINGLE REPLACEMENT  One element replaces another element in a compound  AB + C  AC + B  Zn + 2HCl  H 2 + ZnCl 2

Chemical reactions can be classified Replacement Reaction – elements switch places to form new compounds. 1) Single Replacement Zn Zinc H2H2 + ZnCl 2 HydrogenZinc Chloride 2HCl Hydrochloric Acid + H Cl + H H H Zn +

DOUBLE REPLACEMENT  Elements from two different compounds switch places  AB + CD  AC + BD  HCl + NaOH  NaCl + H 2 O

Cl Chemical reactions can be classified Replacement Reaction – elements switch places to form new compounds. Double Replacement Cl FeS Iron Sulphide H2SH2S + FeCl 2 Hydrogen SulfideIron Chloride 2HCl Hydrochloric 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: C 4 H 10 Butane Written as: CH 4 Methane

Subscripts= how many atoms = how many total molecules

COUNTING ATOMS  FeO 2  H 2 O  CO 2  MgBr 2  C 6 H 12 O 6  3OH  2H 2 O

COUNTING ATOMS IN CHEMICAL EQUATIONS 2Na + MgF 2  2NaF + Mg

COUNTING ATOMS IN CHEMICAL EQUATIONS 2K + Cl 2  2KCl

COUNTING ATOMS IN CHEMICAL EQUATIONS 2Na 2 O  4Na + O 2

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 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 + Cl 2 -> 2NaCl 4P + 5O 2 -> P 4 O 10

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. – H 2 O & H 2 O 2 EX: 2Na + Cl 2 -> 2NaCl H 2 + O 2 -> 2H 2 O (Not balanced… So…) 2H 2 + O 2 -> 2H 2 O

Balancing 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 + H 2 O Fe + Cl 2 FeCl 3

Hg + O 2 HgO

H 2 + Cl HCl

Mg + O 2 MgO

O 2 + H 2 H 2 O

Fe + Cl 2 FeCl 3

CH 4 + O 2 CO 2 + H 2 O