Chemical reactions and chemical equations

Writing & balancing chemical equations

Subscript: number that appears partially below the line and to the right of a symbol in a chemical formula.

C6H12O6 = 6 carbon, 12 hydrogen, 6 oxygen

Ca(NO3)2 = 1 calcium, 1 X 2 = 2 nitrogen, 3 X 2 = 6 oxygen

Chemical reaction: a chemical change; atoms, molecules, or ions rearrange to form new substances.

–Substances entering the reaction are called reactants.

–Substances formed in the reaction are called products.

Chemical formula: shorthand method for showing the composition of a compound using element symbols and subscripts.

H2O, CaCl2, C6H12O6

Law of Conservation of Matter: atoms (matter) cannot be created or destroyed.

◦ Therefore, all atoms present in the reactants must also be present in the products, although they may rearrange.

During a reaction:

1. Chemical bonds that hold atoms together in the reactants are broken.

2. Some (or all) of the atoms rearrange.

3. New bonds are formed as atoms come together to form products.

Reaction of hydrogen (H2) and oxygen (O2) to form water (H2O)

Chemical equation: shorthand expression showing the reactants and products of a chemical change.

H2 + O2 → H2O

Coefficient: number that appears before a chemical formula

•Shows how many molecules are present

•Used to balance equations

•When none is shown, is understood to be one.

2 H2O = 2 water molecules

2 X 2 = 4 H 2 X 1 =2 O

Includes:1. A summary of the reaction.2. Formulas and numbers of reactants and

products present.3. States of reactants and products: solids (s) liquids (l) gases (g) aqueous solution (aq): dissolved in

water

2 H2 (g) + 1 O2 (g) → 2 H2O (g)

2 X 2 = 4 H 2 X 2 = 4 H1 X 2 = 2 O 2 X 1 = 2 O

Balanced chemical equation: contains same number of each kind of atom on each side of the equation.

1. Write the unbalanced equation.

2. Inventory the atoms in the reactants and products.

3. Balance the equation.◦ Hint: balance elements left-to-right on the Periodic

Table (metals first, then nonmetals)◦ Leave H and O until last

1. Write the unbalanced equation.__Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __ H2 (g)

2. Inventory reactants and products.__Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __ H2 (g)

Zn

Cl

H

Zn

Cl

H

1

1

1

1

2

2

2

BALANCED !

BALANCED !

3. Balance the equation (by changing coefficients.)

__Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __ H2 (g)

Zn

Cl

H

Zn

Cl

H

1

1

1

1

2

2

2

2

2__Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __ H2 (g)1 11

Try to balance this reaction:

__CH4 (g) + __O2 (g) __CO2 (g) + __H2O (l)

C

H

O

C

H

O

1

4

2

1

2

3 4

4

4

11 2 2

Try to balance this reaction:

__KClO3 (s) __KCl (s) + __O2 (g)

K

Cl

O

K

Cl

O

1

1

3

1

1

2 6

2

6

22 3

2

2

2

Try to balance this reaction:

__Al(OH)3 (s) + __H2SO4 (aq)

__ Al2(SO4)3 (aq) + __H2O (l)

Al

S

H

O

Al

S

H

O

1

1

5

7

2

3

2

13

12

18

8

1

2 3

6

2

10

3

12

18

“1 H2O” means: 1 molecule of water = 2 H atoms and 1 O

atom 1 mole of water = 6.02 X 1023 molecules =

18g H2O

A balanced reaction means:2 H2 (g) + O2 (g) → 2 H2O (g)

2 molecules H2 1 molecule O2 2 molecules H2O

2 moles H2 1 mole O2 2 moles H2O