Reactions in ChemistryWriting Balanced Chemical Equations

Writing reaction equations•Before you can write a balanced chemical

equation for a reaction, you need to write an unbalanced equation. To do this you need to identify several things:▫What chemicals are reacting?▫What is the proper formula for each

chemical?▫What type of reaction is it?▫What are the products that will be

produced?▫What are their proper formulas?

Types of chemical reactions•Remember, the five basic categories of

reactions are:▫Synthesis (a.k.a. Direct Combination)

▫Decomposition

ABBA

BAAB

Types of chemical reactions▫Single Replacement

▫Double Replacement

BAXBXA

BXAYBYAX

Types of chemical reactions▫Combustion

An element or compound reacts with oxygen, often producing energy in the form of heat and light.

If a hydrocarbon undergoes combustion, the products are normally CO2 and water.

Things to remember•When writing the chemical formulas:

▫Hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine are diatomic molecules.

▫When deciding on formulas for binary ionic compounds, use the crisscross method and remember to reduce the ratio of atoms if possible.

•Be sure that all chemical formulas are correct and that all reactants and products have been included before trying to balance the equation.

Example 1•A strip of magnesium is placed in a

hydrochloric acid solution, and hydrogen gas is released. Write a balanced equation for the reaction that takes place.

What are the reactants?

What are their formulas?

Magnesium metal and hydrochloric acid.

Mg and HCl

Example 1 (cont)What type of reaction would this be?

What are the products? (Were all of them mentioned?)

What are their formulas?

Single replacement

MgCl2 and H2

Magnesium chloride and hydrogen gas.

Example 1 (cont)What would the unbalanced equation be?

Now balance it:

22 HMgClHClMg

22 HMgClHClMg Reactants ProductsMg – Mg – H – H – Cl – Cl –

Example 2•Potassium chlorate decomposes, to evolve

oxygen.

What are the reactants?

What are their formulas?

Only potassium chlorate

KClO3

Example 2 (cont)What type of reaction would this be?

What are the products? (Were all of them mentioned?)

What are their formulas?

Decomposition

O2 and KCl

Oxygen and potassium chloride

Example 2 (cont)What would the unbalanced equation be?

Now balance it:

KClOKClO 23

KClOKClO 23

Reactants ProductsK – K – Cl – Cl – O – O –

Example 3•Copper(II) sulfate reacts with a sodium

hydroxide solution.

What are the reactants?

What are their formulas?

Copper(II) sulfate and sodium hydroxide.

CuSO4 and NaOH

Example 3 (cont)What type of reaction would this be?

What would the products be?

What are their formulas?

Double replacement

Cu(OH)2 and Na2SO4

Copper(II) hydroxide and sodium sulfate

Example 3 (cont)What would the unbalanced equation be?

Now balance it:

4224 SONaOHCuNaOHCuSO

4224 SONaOHCuNaOHCuSO Reactants ProductsCu – Cu – SO4 – SO4 –

Na – Na – OH – OH –

Example 4•The reaction that takes place in a butane

lighter. (butane is C4H10)

What type of a reaction is this?

What would the reactants be?

Burning would indicate combustion

Butane and oxygen

Example 4 (cont)What are the formulas for the reactants?

What would the products be?

What are their formulas?

C4H10 and O2

CO2 and H2O

Combustion results in carbon dioxide and water

Example 4 (cont)What would the unbalanced equation be?

Now balance it:

OHCOOHC 222104

OHCOOHC 222104

Reactants ProductsC – C – H – H – O – O –

Example 5•Soda pop gets its acidic taste because as

carbon dioxide is forced to dissolve in the pop, it combines with water forming an acid.

What type of a reaction is this?

What would the reactants be?

Synthesis (or direct combination)

Carbon dioxide and water

Example 5 (cont)What are the formulas for the reactants?

What would the products be?

What are their formulas?

CO2 and H2O

Only H2CO3

Synthesis results in only one product.In this case it would be carbonic acid.

Example 5 (cont)What would the unbalanced equation be?

Now balance it:

3222 COHOHCO

3222 COHOHCO

Reactants ProductsC – C – O – O – H – H –

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