ChemistryCh 8 - Chemical Reactions

Reactions & Equations

When you take substances and rearrange their atoms to form new substances you have created a chemical reaction!

Chemical reactions are another name for chemical changes. These occur all around us

What are some examples of chemical reactions?

How can you tell a chemical reaction has occurred?

Chemical Equations

Chemical Equations are used by chemists to communicate to other chemists what is happening during a chemical reaction.

Chemical Equation Chemical equations are used to represent

chemical reactions. They show: Reactants

The starting substances Products

The new substances formed (s) (l) (g) (aq)

Tells you the state of matter or if a water solution Coefficients

Tell you the lowest whole-number ratio of amounts of substances in the equation.

Reactant 1 + Reactant 2 + etc. Product 1 + Product 2 + etc.

Plus sign separate each substance

The arrow is used to separate the reactants from the product.

Sometimes above the arrow there is information telling us what substances were used to help the chemical reaction run. These substances are called catalysts

Balancing Equations

BalancingChemical Equations

Balance the formula equation by the use of coefficients to balance the number of each type of atom on the reactant and product sides of the equation.

Balancing Tips:

1. Count the atoms of the elements in the reactants

2. Count the atoms of the elements in the products

3. Change the coefficients to make the number of atoms of each element equal on BOTH sides of the equation. (NEVER change the subscript in a chemical formula to balance an equation).

4. Make sure the coefficients are in their lowest possible ratio.

5. Check your work!

Example

Hydrogen gas reacts with oxygen gas to produce water.

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

2 H2 (g) + O2 (g) 2 H2O (l)

Example w/ Polyatomic Ions

Solutions of lithium phosphate and calcium chloride react to make a lithium chloride solution with solid calcium phosphate.

Li3PO4 (aq) + CaCl2 (aq) LiCl (aq) + Ca3(PO4)2 (s)

2 Li3PO4 (aq) + 3 CaCl2 (aq) 6 LiCl (aq) + Ca3(PO4)2 (s)

Example

Solid iron(III) oxide reacts with carbon monoxide gas to produce solid iron (I) oxide and carbon dioxide gas.

Fe2O3 (s) + CO (g) Fe2O (s) + CO2 (g)

Fe2O3 (s) + 2CO (g) Fe2O (s) + 2CO2 (g)

One Last Example

Iron metal reacts with chlorine gas to form a solid iron(III) chloride.

Fe(s) + Cl2(g) FeCl3(s)

2Fe (s) + 3Cl2(g) 2FeCl3 (s)

Reactions follow:THE LAW OF CONSERVATION OF

MATTER

The amount of atoms of an element MUST be equal on both sides of the equation.

That is why we balance equations, to account for all the matter used in the reaction

Types of Reactions

Combination (Synthesis) reaction Decomposition reaction Single-replacement reaction Double-replacement reaction Combustion reaction Neutralization reaction

involve acids & bases

You will be able to identify the different types of reactions.

You will recognize them by their reactants & products

For some you will be able to predict whether a reaction will happen at all.

Combination Reaction

What does combination mean?

In this reaction two or more simpler compounds or elements form one product.

Also called a synthesis reaction

General Equation: A + B AB

Decomposition Reaction

What does Decomposition mean?

In this reaction: One reactant would break down into two or more simpler substances.

General Equation: AB A + B

Single-replacement Reaction

An element and a compound react together and a new element and new compound are formed

Sometimes called Displacement Reaction

General Equation: General Equation: A + BX A + BX AX + B AX + B Examples:

Na + KCl K + NaCl

F2 + 2 LiCl 2 LiF + Cl2 Watch the next two slides

Na + KCl K + NaCl

Na K Cl

F2 + 2 LiCl 2 LiF + Cl2

F

Li Cl

FLi Cl

Li

Li

More Single ReplacementReactions

Metals replace metals (and hydrogen) Examples:

2Al + 3CuSO4 Al2(SO4)3 + 3Cu

Zn + H2SO4 ZnSO4 + H2

2 Na + 2 H2O 2 NaOH + H2

We can tell whether a reaction will happen using the Activity Series of Metals

Some metals are more reactive than others

The MORE active metals replace less active metals

There is a list on page 217 – Table 8.2

You will be given this list on quizzes/tests

Single Replacement

A metal higher on the list replaces one lower on the list.

So if the element by itself is higher, the reaction happens, if element by itself is lower, it doesn’t

If there is no reaction we write NR after the arrow

Single Replacement

Note the * H can be replaced in acids

by metals higher than it on the list

Only the first 4 (Li - Na) react with water.

Activity Series Examples:

Will a reaction occur for the following?

Pb + KCl

Al + H2O

What does it mean that Ag is on the bottom of the list?

Single Replacement Continued

Nonmetals can replace other nonmetals Limited to F2 , Cl2 , Br2 , I2 (the Halogens)

The order of activity follows the periodic table. Higher replaces lower.

Will a reaction occur for the following?

F2 + KCl

I2 + LiBr

Br2 + KCl

Double-replacement Reactions

Two compounds react together to form two new compounds.

Use the charges on the ions to help in determining the new substances formed.

These are sometimes called double- displacement reactions

General Equation: General Equation: AX + BY AX + BY AY + BX AY + BX

Double Replacement

Reactants must be two ionic compounds or acids.

Usually in aqueous solution

NaOH (aq) + FeCl3 (aq) The positive ions change place.

Na+OH- + Fe3+Cl- Fe3+OH- + Na+Cl-

3 NaOH + FeCl3 Fe(OH)33 + 3 NaCl

Watch the next slide

3NaOH + FeCl3 Fe(OH)3 + 3NaCl

Na+ O2- H+

Na+ O2- H+

Na+ O2- H+

Fe3+

Cl-

Cl-

Cl-

Double ReplacementTips

The reaction one of the products will do the following:

A. doesn’t dissolve in water and forms a solid called a precipitate

B. is a gas that bubbles out.C. is a covalent compound (usually water).

Polyatomic ions don’t change from side to side

Combustion Reactions:

A hydrocarbon compound is reacted with oxygen gas

A hydrocarbon? What’s a hydrocarbon? If the combustion is complete, the products will

be CO2 and H2O.

If the combustion is incomplete, the products will be CO and H2O

or just C and H2O.

O2 will ALWAYS be the second reactant

Examples

Complete combustion of C4H10

Examples

Complete combustion of C6H12O6

Question for you:

How do you identify acids & bases?

Common Acids

Acids that you must know (memorize): HCl = hydrochloric acid HNO3 = nitric acid

H2CO3 = carbonic acid

H2SO4 = sulfuric acid

H3PO4 = phosphoric acid

Common Bases

How to know the bases: Look for hydroxide polyatomic ion (OH-) Name the base from the name of the metal

and then hydroxide. EX: NaOH potassium hydroxide Mg(OH)2

Neutralization Reactions

A reaction in which an acid and a base react in aqueous solution to produce a salt and water. A salt is an ionic compound made up of a cation

from a base and an anion from an acid. These reactions are double-replacement

reactions. General Equation:General Equation:

Acid (aq) + Base (aq) Salt (aq) + Water (l) EX: Mg(OH)2 (aq) + 2 HCl (aq) MgCl2 (aq) + 2 H2O (l)

More Neutralization Examples

H2SO4 + 2 NaOH Na2SO4 + 2 H2O

Ba(OH)2 + 2 HCl BaCl2 + 2 H2O

Notice – Reactants are always an acid and a base and one of the products is always water