Atomic Theory

Atoms are building blocks of elements

Atoms of the same element are similar

Different from atoms of other elements

Two or more different atoms bond in

simple ratios to form compounds

Chemical Equations: An expression in which symbols and formulas are used to represent a chemical reaction.

sodium metal + chlorine gas table salt

(sodium chloride)

A chemical equation identifies the starting and ending chemicals as reactants and products: reactants productsFormation of water2H2 + O2 2H20

A chemical equation is balanced when it reflects the law of conservation of mass.

Graphic: http://www.chemistry.ohio-state.edu/betha/nealChemBal/

The Law of conservation of mass states that matter cannot be created or destroyed in any chemical reaction

The atoms in the reactants are rearranged to form new compounds, but none of the atoms disappear, and no new atoms are formed.

HH

HH O O

Reactants Products

Remember that atoms don’t change in a chemical

reaction.• The number and kinds of atoms

present in the reactants of a chemical reaction are the same as those present in the products. When stated this way, it becomes the law of conservation of atoms.

So: Chemical equations are balanced, when the numbers and kinds of atoms on each side of the reaction arrow are equal.

HH

HH O O

Reactants Products

HH

HH

OO

4 Hydrogen 4 Hydrogen

2 Oxygen 2 Oxygen

Coefficients, the numbers placed in front of formulas to balance equations. They indicate the number of particles present in the reaction. If a number is not present it is understood that 1 is the coefficient.

2 H2 + O2 2 H2O

2H2O

Coefficient

subscript

Coefficients are used to balance equations. NEVER change or add subscripts!

When balancing a chemical equation, first you must determine the number of each kind of atom on each side of the yield arrow.

• Ca + O2 CaO

1 Ca atom

2 O atoms

1 Ca atom

1 O atomNotice that there are the same number of calcium atoms on each side of the equation but the number of oxygen atoms are not the same.

This equation is not balanced.

Ca + O2 CaO

• Coefficients are needed to make the number of oxygen atoms equal. A coefficient 2 is placed in front of CaO.

1 Ca atom

2 O atoms

1 Ca atom

1 O atom

2

• This coefficient indicates 2 Ca atoms and 2 This coefficient indicates 2 Ca atoms and 2 oxygen atoms. Now calcium is not equal. A oxygen atoms. Now calcium is not equal. A coefficient of 2 in placed in front of Cacoefficient of 2 in placed in front of Ca• The number of each kind of atom are equal on The number of each kind of atom are equal on both sides of the yield arrow. This equation is now both sides of the yield arrow. This equation is now balanced and follows the law of conservation of balanced and follows the law of conservation of mass. mass.

22 1 Ca atom

2 1 O atom

2 1 Ca atom

2 O atoms

Ca + O2 CaO

• Coefficients are needed to make the number of oxygen atoms equal. A coefficient 2 is placed in front of CaO.

1 Ca atom

2 O atoms

1 Ca atom

1 O atom

2

• This coefficient indicates 2 Ca atoms and 2 This coefficient indicates 2 Ca atoms and 2 oxygen atoms. Now calcium is not equal. A oxygen atoms. Now calcium is not equal. A coefficient of 2 in placed in front of Cacoefficient of 2 in placed in front of Ca• The number of each kind of atom are equal on The number of each kind of atom are equal on both sides of the yield arrow. This equation is now both sides of the yield arrow. This equation is now balanced and follows the law of conservation of balanced and follows the law of conservation of mass. mass.

22 1 Ca atom

2 1 O atom

2 1 Ca atom

2 O atoms

2

Notice that the number of hydrogen and oxygen atoms are equal

2

The number of oxygen atoms are not the same so the equation is balanced with coefficients.

Balancing Chemical EquationsThe following five steps can be used as a

guide to balance chemical equations.Balance this chemical reaction.Sulfuric Acid reacts with sodium hydroxide

to yield sodium sulfate and water Step 1: Write an unbalanced equation,

using correct formulas for all reactants and products.

H2SO4 + NaOH Na2SO4 + H2O

Step 2: Inventory all atoms found in the equation

H2SO4 + NaOH Na2SO4 + H2O

2 H + 1 H = 3H 2 Na1 S 1 S4 O + 1 O = 5 O 4 O + 1 O= 5 O 1 Na 2 H

Step 3: Compare the number of each atom on each side of the equation. Add coefficients to balance the number of atoms. Remember that adding a coefficient affects all elements in the compound.

H2SO4 + NaOH Na2SO4 + H2O

2 H + 1 H = 3H 2 Na 1 S 1 S 4 O + 1 O = 5 O 4 O + 1 O=

5 O 1 Na 2 H

2 24H

6O

4H6 O

2 Na

Equal Equa

l

Step 4: Check the equation to make sure the numbers and kinds of atoms on both sides of the equation are same. H2SO4 + 2 NaOH Na2SO4 + 2 H2O

2 H + 2 H = 4H 2 Na 1 S 1 S 4 O + 2 O = 6 O 4 O + 2 O= 6 O 2 Na 4 H

Step 5: Make sure the coefficients are reduced to their lowest whole-number value (ok here).

H2SO4 + 2 NaOH Na2SO4 + 2 H2O

1 : 2: 1 : 2