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A CHEMICAL REACTION IS A REARRANGMENT OF ATOMS IN WHICH SOME OF THE ORIGINAL CHEMICAL BONDS ARE BROKEN AND NEW BONDS AFE FORMED TO GIVE DIFFERENT CHEMICAL STRUCTURES.

A + B C + D reactants products

REACTANTS ARE THE THINGS YOU START OUT WITH, AND PRODUCTS ARE THE THINGS YOU END UP WITH.

HOW DO YOU KNOW A CHEMICAL REACTIION OCCURRED?

1) THE REACTANTS DISAPPEAR OR ARE REDUCED IN QUANTITY.

2) THE NEW SUBSTANCES THAT APPEAR (PRODUCTS) HAVE DIFFERENT PROPERTIES THAN THE REACTANTS.

3) ENERGY (HEAT, LIGHT, SOUND, ELECTRICITY) IS RELEASED OR ABSORBED.

CHEMICAL REACTIONS CAN BE DIVIDED INTO SEVERAL DIFFERENT TYPES:

1) COMBINATION REACTION (SYNTHESIS). HERE, TWO REACTANTS COMBINE TO GIVE A PRODUCT.

A + B AB

AN EXAMPLE WOULD BE MAGNESIUM METAL REACTING WITH OXYGEN TO FORM MgO, MAGNESIUM OXIDE.

2Mg + O2 2 MgO

2) DECOMPOSITION REACTION – JUST THE REVERSE OF A COMBINATION REACTION. A SUBSTANCE IS BROKEN DOWN INTO TWO OR MORE SIMPLER SUBSTANCES.

AB A + B

AN EXAMPLE WOULD BE WATER DECOMPOSING INTO HYDROGEN AND OXYGEN.

2 H2O 2 H2 + O2

3) SINGLE REPLACEMENT REACTION – ONE ELEMENT REPLACES ANOTHER IN A COMPOUND.

A + BC B + AC

AN EXAMPLE WOULD BE ALUMINUM IN THE THERMITE REACTION:

Al + Fe2O3 2Fe + Al2O3 + lots of heat

4) DOUBLE REPLACEMENT REACTION – TWO SUBSTANCES CHANGE PLACES WITH EACH OTHER IN COMPOUNDS – TYPICAL IN PRECIPITATION REACTIONS.

AB + CD AD + BC

AN EXAMPLE WOULD BE POTASSIUM IODIDE REACTING WITH LEAD (II) NITRATE TO FORM LEAD (II) IODIDE AND POTASSIUM NITRATE:

2 KI + Pb(NO3)2 2 KNO3 + PbI2 (yellow)

5) COMBUSTION REACTIONS – COMMON WITH HYDROCARBONS.

A HYDROCARBON REACTS WITH OXYGEN TO FORM CARBON DIOXIDE AND WATER.

CH4 + O2 CO2 + 2 H2O

REACTIONS CAN EITHER GIVE OFF HEAT OR ABSORB HEAT DEPENDING ON WHETHER THE REACTANTS HAVE MORE ENERGY IN THEIR CHEMICAL BONDS THAN THE PRODUCTS.

EXOTHERMIC REACTION – HEAT RELEASED

A PRACTICAL APPLICATION OF EXOTHERMIC REACTIONS IS IN THE HEATER PACKS USED IN “MRE’S” – MEALS READY TO EAT. THESE USE THE OXIDATION OF MAGNESIUM:

Mg + 2H2O Mg(OH)2 + H2 + heat

AN ALLOY OF MAGNESIUM AND IRON IS USED, AND THESE CAN PRODUCE ENOUGH HEAT TO BOIL WATER.

ENDOTHERMIC REACTION – HEAT ABSORBED

A GOOD EXAMPLE OF AN ENDOTHERMIC REACTION IS THE REACTION OF BAKING SODA AND CITRIC ACID.

ANOTHER WOULD BE DISSOLVING AMMONIUM CHLORIDE IN WATER.

ALL CHEMICAL REACTIONS REQUIRE A CERTAIN AMOUNT OF ENERGY TO GET THEM STARTED.

THIS MINIMUM AMOUNT OF ENERGY IS CALLED THE ACTIVATION ENERGY.