8th Grade Science

Adapted by T. Trimpe http://sciencespot.net/

http://www.unit5.org/chemistry/Equations.html

Remember: Chemical Changes

• Chemical change: a change that occurs causing the identity of the substance to change– Burning paper– Digesting food – Electrolysis of water

• A chemical change is called a chemical reaction

Remember: Chemical Reaction• Chemical reaction: occurs when the

original substance changes form into a new substance. Some clues to a chemical reaction may include bubbles, smoke, a strong smell, color change, temperature change, or a sizzling sound

Chemical Changes Cont’d• Indicators of a chemical change:

• Evolution of light

• Evolution of heat

• Evolution of a gas

• Color change

• Formation of a precipitate

Chemical Equations

• Depict the kind of reactantsreactants and productsproducts

• and their relative amounts in a reaction.

• 4 Al(s) + 3 O2(g) 2 Al2O3(s)

• The letters (s), (g), and (l) are the

• physical states of compounds

Chemical Equations

• Reactants – the substances that exist before a chemical change (or reaction) takes place.

• Products – the new substance(s) that are formed during the chemical changes.

• CHEMICAL EQUATION indicates the reactants and products of a reaction.

REACTANTS PRODUCTS

Chemical Equations

The number in the front is called a coefficientoefficient. . It tells you how many molecules in a reaction.• coefficient

•4 Al(s) + 3 O2(g) 2 Al2O3(s)

• subscript

•The number to the right of the symbols is called a subscript. It tells you how many of the same atom.

Chemical Equations

• When a coefficient is in front of a molecule and a subscript for an atom, you will multiply both numbers in order to determine the number of atoms that will be involved in the chemical reaction.

2 Al2O3

Al = 4 O = 6

Let’s Practice

Let’s Practice Balance Equations

Type of Reaction

Definition Equation

Synthesis

Decomposition

Single Replacement

Double Replacement

Watch the movie and then complete the chart.

A = Red B = Blue C = Green D = Yellow

A + B → AB

AB → A + B

AB + C → AC + B

AB + CD → AC + BD

Two or more elements or compounds combine to make a more complex

substance

Compounds break down into simpler substances

Occurs when one element replaces another one in a

compound

Occurs when different atoms in two different

compounds trade places

Identifying Chemical Reactions

____ P + O2 → P4O10 ____ Mg + O2 → MgO

____ HgO → Hg + O2 ____ Al2O3 → Al + O2

____ Cl2 + NaBr → NaCl + Br2 ____ H2 + N2 → NH3

2. Use colored pencils to circle the common atoms or compounds in each equation to help you determine the type of reaction it illustrates. Use the code below to classify each reaction.

S = Synthesis D = Decomposition SR = Single Replacement DR = Double Replacement

 

____ Na + Br2 → NaBr ____ CuCl2 + H2S → CuS + HCl 

____ HgO + Cl2 → HgCl + O2 ____ C + H2 → CH4

 

____ KClO3  →   KCl  +  O2  ____ S8 + F2 → SF6

 

____ BaCl2   +   Na2 SO4 →     NaCl   +   BaSO4