Notes – Chemical Equations Chapter 8, Lesson 2

Conservation of Mass When the end of the last period bell rang, everyone moved to a new room, and ended up in different groups in different rooms. The number of students IN the whole school did not change, only their arrangement. The amount of matter in a chemical reaction does not change, so the total mass at the start MUST equal the total mass at the end.

Conservation of Mass Matter is ALWAYS conserved in chemical reactions. Chemical equations show that in chemical reactions, atoms rearrange, but no atoms are gained or lost.

Conservation of Mass Antoine Lavoisier invented a balance to make more precise measurements. He also determined that the mass before and after a chemical reaction was the same. Mass of elements BEFORE Mass of elements AFTER

Conservation of Mass The law of conservation of mass states that the total mass before a chemical reaction is equal to the total mass after the reaction.

Chemical Equations Reactants are the starting materials in a chemical reaction and are placed on the LEFT side. Products are the ending materials in a chemical reaction and are placed on the RIGHT side. Reactant + Reactant → Product H 2 + O 2 → H 2 O 2

Chemical Equations “Tin + oxygen → tin oxide” reads as “tin plus oxygen produces tin oxide.” Word equations can be long and do not show that mass is conserved.

Chemical Equations Instead of writing long word equations, scientists use symbols and formulas. –Symbols represent atoms. –Formulas represent molecules. –Molecules may be elements or compounds.

Chemical Equations A molecule is the combination of 2 or more atoms. Some molecules have atoms that are alike, called diatomic (O 2 ). Most molecules are made of more than one type of atom (H 2 O) – a compound.

Chemical Equations

Chemical equations use the symbols and formulas instead of words.

Balancing a Chemical Equation Atoms are neither gained nor lost in a reaction, so both sides of the equation must have the same number of atoms.

Balancing a Chemical Equation A subscript tells how many atoms of an element are in one molecule. A coefficient tells how many atoms, molecules, or formula units are in a reaction. 2H 2 O subscript coefficient

Balancing a Chemical Equation (i.e.) “Oxygen reacts with hydrogen to form water.” H 2 + O 2 → H 2 O ReactantsProducts HOHO 2221 This is unbalanced, and we CANNOT change the subscripts! We add coefficients, a number in FRONT of a chemical formula that tells you how many molecules or atoms of each reactant & product act in the reaction. We do not write “1.”

We add a coefficient of “2” in front of the product: H 2 + O 2 → 2 H 2 O ReactantsProducts HOHO 2242

The oxygen atoms are now balanced, but the hydrogens are not anymore. Add a coefficient of “2” in front of the H 2 : 2 H 2 + O 2 → 2 H 2 O ReactantsProducts HOHO 4242

Balancing a Chemical Equation

Writing Balanced Equations Step 1Determine the correct symbols and formulas for reactants and products. Step 2Write reactant symbols and formulas to the left of an arrow and product symbols and formulas to the right. Step 3Count the number of each kind of atom on both sides. Step 4Use coefficients to make the number of each kind of atom the same on both sides of the arrow. Step 5Check to see that each kind of atom balances.