Chemical Equations Basic Chemistry

Rev. 4/30/02

Chemical equations show …

The reactants and the products

Reactants Products

Chemical equations show …The physical states of the reactants

and products:

(s) = solid (l) = liquid (g) = gas

(aq) = aqueous solution

HCl(g) + NH3(g) NH4Cl(s)

Chemical equations show …

• The progress of a reaction …

The shows progress from left to right.

A double arrow shows an equilibrium between reactants and products.

Chemical equations show …• The presence of a catalyst

A catalyst is a substance that appears not to take part in a reaction, but causes the

reaction to go faster.

The catalyst is written above the arrow.

Pt

Chemical equations show …

• Heat is added to the reaction.

Added heat is indicated by the Greek letter delta ( )

The delta is placed over the arrow

Coefficients• The coefficients show the

number of molecules or atoms of each reactant and product.

• The coefficients are written in front of the symbols

• The coefficients allow us to balance the equation.

Why balance an equation?

A balanced equation adheres to the Law of conservation of mass

“Matter is neither created, nor destroyed, only changed in

form.”

We must account for every atom that undergoes a reaction.

Balancing Equations

Example: balance the reaction of hydrogen and oxygen to make water:

H2(g) + O2(g) H2O(l)

Balancing Equations

• Make a table under the equation to indicate the number of each kind of atom as you change coefficients.

H2(g) + O2(g) H2O(l)

H H

O O

2 2

2

2

12

2 44

2

The equation is balanced!

We will balance these together:

1. Mn(s) + Cl2(g) MnCl4(S)

2. Al2(SO4)3 Al2O3 + SO3(g)

3. S(s) + O2(g) SO3(g)

4. Na2CrO4(aq) + Fe(NO3)3 Fe2(CrO4)3(s) + NaNO3(aq)

Balance the Following Equations

1. N2(g) + H2(g) NH3(g)

2. Fe(s) + O2(g) Fe2O3(s)

3. NaHCO3(s) Na2O(s) + H2O(g) + CO2(g)

4. Ba(NO3)2(aq) + Na2SO4(aq) NaNO3(aq) + BaSO4(s)

Answers …1. N2(g) + 3 H2(g) 2 NH3(g)

2. 4 Fe(s) + 3 O2(g) 2 Fe2O3(s)

3. 2 NaHCO3(s) Na2O(s) + H2O(g) + 2 CO2(g)

4. Ba(NO3)2(aq) + Na2SO4(aq) 2 NaNO3(aq) + BaSO4(s)

“Word Equations”

A balanced equation can be written from an

expression that describes a chemical reaction in

words, rather than symbols.

To write a balanced equation…1. Determine the reactants and

products. Look for clues: “added”, “produces”, “heated”, “decomposes”

2. Write the correct formulas and put in the state symbols and any other symbols (I.e. catalyst or

3. Balance the equation.

Consider the following:

Propane gas (C3H8) burns in air to produce carbon dioxide gas and water vapor.

Identify the reactants and products. Look for the clue.

… to produce …

If something burns in air, what does it react with?

Oxygen

What are the reactants?

Propane and oxygen

What are the products?

Carbon dioxide and water

Write the formulas and symbols:

C3H8(g) + O2(g) CO2(g) + H2O(g)5 3 4

Now balance the equation:

The End