Chemical Reactions

Nature of Reactions

• A chemical reaction is when you create a new substance with unique physical and chemical properties.

Reaction Representation

• Chemists use symbols to represent what happens in a chemical reaction.

• This representation is called a chemical equation.

Na + Cl2 NaCl

Parts of an Equation• Reactants

– What is going into the equation.

• Products– What is coming out of the equation.

Law of Conservation of MassAtoms cannot be created or destroyed. The mass of the reactants entering the equation must equal the mass of the products exiting the equation.

“What goes in, must come out!!”

Parts of an Equation• Arrow

– “yields” (“creates”, “produces”, “equals”)

• Subscript– The little number after the element that tells

how many of each element is present.• H2O ZnCl2 Al2O3

• Valence Charge (aka…oxidation number)– The charge represented in the chemical

equation to represent how the element will react with the other reactants.

Parts of an Equation• Coefficient

– The big number written before the elements when balancing the chemical equations.• 2HCl 3Al2S3

• Representation of Gases– (diatomic elements) Gases are always

written with a subscript of 2 when they are alone in the equation• H2 O2 N2 Cl2 F2

Parts of an Equation

• Example of the parts:

2Al+3 + 3S-2 Al2S3

Coefficient Reactants

Valence Charge

Products

Yields

Subscripts

Label the Parts

• 2Ag+1I-1 + Mg+2Cl2-1 2AgCl + MgI2

• Label Each of these in the above equation.

1. Coefficients = Square2. Valence Charge = Triangle3. Subscripts = Circle4. Reactants = One Underline5. Products = Two Underlines6. Yields = Sunshine

Balancing Equations• Due to the law of conservation of mass..

– “what goes in, must come out” of an equation.

• In order to have the same number of atoms of each element that you started with…you must add coefficients. (big #s)

• You will place the coefficient in front of the symbol and will multiply by the subscripts.

Remember

2Al2O3

Coefficient Subscripts

Coefficients will multiply by the subscripts….2x2….2x3

Let’s Practice Counting Atoms

Let’s Try Balancing Equations

Atoms are not created or destroyed during a chemical reaction. Scientists know that there must be the same number of atoms on each side of a the equation. To balance the chemical equation, you must add coefficients to the different parts of the equation.