ch. 2 chemical reactions

CH. 2

CHEMICAL R

EACTIONS

S E C. 2

– C

HEM

I CA L F

OR

MU

L A S &

E QU

AT I ON

S

CHEMICAL F

ORMULAS

CHEMICAL FORMULAS

• Shortened way to represent a substance

• Use chemical symbols & numbers

• Show the type & number of atoms in a substance

• Number written to lower right of element• Tell how many atoms of each element are in

the substance

SUBSCRIPTS

• CO

• CO2

• NH4

• C6H12O6

• 1 carbon, 1 oxygen

• 1 carbon, 2 oxygen

• 1 nitrogen, 4 hydrogen

• 6 carbon, 12 hydrogen, 6 oxygen

HOW MANY ATOMS OF EACH ELEMENT?

WRIT

ING F

ORMULAS F

OR

COVALE

NT C

OMPOUNDS

WRITING FORMULAS FOR COVALENT COMPOUNDS

• Made up of 2 nonmetals

• Common prefixes used in covalent compound names

•Mono – 1•Di – 2• Tri – 3• Tetra - 4• Penta – 5

Carbon dioxide

1. Write chemical symbol of 1st element. Add a subscript if there is a prefix.

2. Write chemical symbol of 2nd element. Add a subscript if there is a prefix.

CO2

1. Carbon tetrachloride

2. Dihydrogen monoxide

3. Dinitrogen tetroxide

1. CCl4

2. H2O

3. N2O4

NOW YOU TRY…WRITE A FORMULA FOR EACH COVALENT COMPOUND

WRIT

ING F

ORMULAS F

OR

IONIC

COMPO

UNDS

REMEMBER…

• Ionic compounds are made up of a metal & a nonmetal chemically bonded together.

• The METALLIC element is always named 1st in the compound name.

• Sodium chloride – sodium is the metal &

chlorine is the nonmetal

1. Write the chemical symbol for the metallic ion first.

2. Write the chemical symbol for the nonmetallic ion.

3. Make sure the ions’ charges equal zero. Metallic ions always have a positive charge. Nonmetallic ions always have a negative charge. An ion’s charge is the same as the number of

electrons its group will gain or lose to become stable.

4. Add subscripts to the chemical symbols to make the compound’s charge equal zero.

STEPS FOR WRITING IONIC COMPOUND FORMULAS

1. Sodium’s chemical symbol is Na.

2. Chlorine’s chemical symbol is Cl.

3. Sodium is in group 1. It’s charge is 1+.

4. Chlorine is in group 17. It’s charge is 1-.

5. (+1) + (-1) = 0. No subscripts are needed.

1. Na

2. Cl

3. Chemical formula: NaCl

SODIUM CHLORIDE

1. Find the LCM of the ions’ charges.

2. Add subscripts where needed to equal the LCM.

Aluminum oxide

IF THE IONS’ CHARGES DO NOT EQUAL ZERO…

1. Al – group 13 – charge 3+

O – group 16 – charge 2-

2. LCM of 3 & 2 is 6

3. Subscript for Al is 2 (2 x 3=6)

Subscript for O is 3 (3 x 2=6)

4. Formula: Al2O3

1. Potassium chloride

2. Magnesium chloride

3. Sodium fluoride

4. Aluminum sulfide

1. KCl

2. MgCl2

3. NaF

4. Al2S3

NOW YOU TRY…..

CHEMICAL E

QUATIO

NS

CHEMICAL EQUATIONS

• Shortcut way to describe a chemical reaction

• Use symbols to show the relationship between the reactants & products

• Can be understood by scientists worldwide

REACTANTS

• Substances or molecules that participate in a chemical reaction

• The starting substances in a chemical reaction

• Are ALWAYS written on the LEFT side of the arrow in a chemical equation

1. Na + Cl2 2 NaCl

2. 2 H2O 2 H2 + O2

3. Zn + 2 HCl ZnCl2 + H2

1. Na + Cl2

2. 2 H2O

3. Zn + 2 HCl

IDENTIFY THE REACTANTS IN THESE EQUATIONS

PRODUCTS

• The substance that forms in a chemical reaction

• The ending substance(s) in a chemical reaction

• Are ALWAYS written on the RIGHT side of the arrow in a chemical equation

1. Na + Cl2 2 NaCl

2. 2 H2O 2 H2 + O2

3. Zn + 2 HCl ZnCl2

+ H2

1. 2 NaCl

2. 2 H2 + O2

3. ZnCl2 + H2

IDENTIFY THE PRODUCTS IN THESE EQUATIONS

ACCURACY IS IMPORTANT IN CHEMICAL EQUATIONS

• Incorrect symbols or formulas do not correctly describe a chemical reaction.

• Small mistakes will make a HUGE difference!

BALANCIN

G CHEMIC

AL

EQUATIO

NS

LAW OF CONSERVATION OF MASS

• Mass is neither created nor destroyed in an ordinary chemical or physical change.

• Chemical equations must show the same numbers & kinds of atoms on each side of the arrow.

SUBSCRIPTS VS. COEFFICIENTS

S U BS C R I P T S

• Written to the lower right of a chemical symbol

• Written smaller than chemical symbol

• Only apply to single atoms

C O E F F I C I E N T S

• Written to the left of the chemical symbol

• Written the same size as the chemical symbol

• Apply to every atom in molecule, including subscripts

1. O2

2. 2 CO

3. H2O2

4. 2 CO2

1. Subscript

2. Coefficient

3. Subscript

4. Both!

SUBSCRIPT OR COEFFICIENT?

CALCULATING THE NUMBER OF ATOMS

• A coefficient applies to every atom & every subscript in a molecule.

• If there is a subscript, multiply the coefficient by the subscript to find the number of atoms.

• Coefficients in an equation are “stopped” by the plus sign or the arrow.

2 H2 + O2 2 H2O

• Reactant side has 4 hydrogen atoms & 2 oxygen atoms. The coefficient 2 applies only to the hydrogen.

• Product side also has 4 hydrogen atoms & 2 oxygen atoms. The coefficient 2 applies to everything here. There is no plus sign or arrow to stop the coefficient.

1. 2 HCO3

2. 3 C6H12O6

3. NaF

4. 4 CaO

1. 10 atoms

2. 72 atoms

3. 2 atoms

4. 8 atoms

CALCULATE THE TOTAL NUMBER OF ATOMS

STEPS FOR BALANCING CHEMICAL EQUATIONS

1. Set up a T-chart for the reactants & products.

reactant product

2. Write the chemical symbol & number of atoms for each side.

reactant product

Ca – 1 Ca – 1

O – 2 O – 1

3. Compare to see if the equation is balanced.

This equation is not balanced.

Ca + O2 CaO

4. Find the LCM for each element that is

unbalanced.

reactant product

Ca – 1 Ca – 1

O – 2 O – 1 LCM = 2

Ca + O2 CaO

5. Add coefficients to balance the elements.

NEVER CHANGE A SUBSCRIPT!!

6. Recount the number of atoms for each

element. Repeat steps 4 - 6 as needed.

reactant product

LCM = 2 Ca – 1 Ca – 1 2 O – 2 O – 1 2

Ca + O2 2 CaO

reactant product

LCM = 2 Ca – 1 2 Ca – 1 2

O – 2 O – 1 2 LCM = 2

The equation is now balanced.

2 Ca + O2 2 CaO