17-2 Describing Chemical Reactions

a shorter, easier way to show chemical

reactions, using symbols, not words, for the reactants and the

productshydrogen molecules react with oxygen molecules to form water molecules

H2 + O2 H2O

reactant + reactant “YIELDS” products

CHEMICAL EQUATION

Law of Conservation of Masstotal mass of reactants must equal total mass of products

matter is NOT destroyed or created in a chemical reaction

=

ELEMENTS – represented by a one or two-letter symbol (letter)

C Na HCOMPOUNDS– represented by a chemical formula

(word) which uses subscripts to show the ratio of elements in the

compound H2O2 CO2 C3H8OHOW MANY?

coefficient in front of the chemical formula tells “how many” atoms or molecules

3H2O2 4CO2 2C3H8O

Chemical Formulas & Counting Atoms

Balancing Chemical EquationsH + O     H2O

• Continue adding coefficients to obtain the same number of atoms of each element on both sides of the equation(change coefficient, then change counts)

• Add the subscript “2” to all diatomics that are ALONE in the equation (N, O, F, Cl, Br, I, H)(note: only time a subscript can & must be changed)

• Count the number of atoms that are present in the basic equation and record

• Pick an element that is not balanced on both sides of the equation ( H & O last)

• Add a coefficient in front of the chemical formula to adjust the count and record the new values

Balancing Chemical EquationsMg + O     MgO

• Continue adding coefficients to obtain the same number of atoms of each element on both sides of the equation(change coefficient, then change counts)

• Add the subscript “2” to all diatomics that are ALONE in the equation (N, O, F, Cl, Br, I, H)(note: only time a subscript can & must be changed)

• Count the number of atoms that are present in the basic equation and record

• Pick an element that is not balanced on both sides of the equation ( H & O last)

• Add a coefficient in front of the chemical formula to adjust the count and record the new values

C + Cl CCl4

Building Equations (basic equation is given)

Mg + O2 ------> MgO

C + Cl2 ------> CCl4

Building w/Marshmallows

Note: add like elements on the same side of the equation

Al + O2 Al2O3

SO2 + O2 SO3

Balancing Chemical Equations

• Continue adding coefficients to obtain the same number of atoms of each element on both sides of the equation(change coefficient, then change counts)

• Add the subscript “2” to all diatomics that are ALONE in the equation (N, O, F, Cl, Br, I, H)(note: only time a subscript can & must be changed)

• Count the number of atoms that are present in the basic equation and record

• Pick an element that is not balanced on both sides of the equation ( H & O last)

• Add a coefficient in front of the chemical formula to adjust the count and record the new values

Al + O Al2O3

SO2 + O SO3

Classifying Chemical EquationsSYNTHESIS – two or more substances combine to make a more complex substance

AB + C ABC (synthesize means to put together)

DECOMPOSTION - a complex substance breaks down into simpler substances

ABC AB + C (decompose means to break down)

REPLACEMENT – two elements in two different compounds trade places

AB + CD AC + BD (replace means to find a new partner)

Synthesis of Acid Rain2 SO2 + O2 + 2 H2O 2 H2SO4

Decomposition of Hydrogen Peroxide

Isolating Copper by Replacement

2 H2O2 2 H2O + O2

2 CuO + C 2 Cu + CO2

Synthesis, Decomposition, or Replacement