Unit 5: Chemical Reactions Chapter 11

Objectives

35 Identify the five types of chemical

reactions

36 Write word and chemical equations based

on chemical reactions

37 Balance chemical equations

38 Predict the products of chemical equations

Chemical Reactions

A chemical reaction is a chemical change in

which the bonds of one or more reactants

break and reform to create one or more

products.

The way a chemical reaction is written can

tell a scientist a lot about the reaction.

Over the course of this tutorial, we will look

into the different aspects that are shown in a

chemical reaction.

Basic information

CH4 + 2O2 2H2O + CO2

Reactants Products

Coefficients

35. Identify the five types of chemical reactions

There are five common types of chemical

reactions.

◦ Synthesis

◦ Decomposition

◦ Single Replacement

◦ Double Replacement

◦ Combustion

Click on each to get a description.

◦ The key identifying part will be highlighted in

blue.

Synthesis A + X AX

A synthesis reaction occurs when two or more reactants become one product.

There are four situations to watch for: ◦ Two single elements added together: i.e.: 2Mg + O2 2MgO

◦ An metal oxide added to water makes a metal hydroxide. i.e.: 2Na2O + H2O 2NaOH

◦ A metal chloride added to oxygen makes a metal chlorate. i.e.: 2NaCl + 3O2 2NaClO3

◦ A metal oxide added to carbon dioxide makes a metal carbonate. i.e.: Na2O + CO2 Na2CO3

Return

Decomposition AX A + X

A decomposition reaction occurs when one reactant becomes many products.

There are four situations to watch for: ◦ A compound breaking into single element: i.e.: 2MgO 2Mg + O2

◦ A metal hydroxide becoming a metal oxide and water. i.e.: 2NaOH 2Na2O + H2O

◦ A metal chlorate breaking into a metal chloride and oxygen. i.e.: 2NaClO3 2NaCl + 3O2

◦ A metal carbonate breaking into a metal oxide and carbon dioxide. i.e.: Na2CO3 Na2O + CO2 Return

36. Write word and chemical

equations based on chemical reactions

Word and chemical equations are the

most common ways to see a chemical

reaction.

It is important to be able to go from one

to the other.

It is also important to show as much

information as possible about the

reaction.

Basic symbols in a chemical reaction

Below is list a basic symbols that are used

when writing chemical equations.

Symbol Meaning

Yields, produces, or creates

+ And or with

(s) Solid

(l) Liquid

(g) Gas

(aq) Aqueous

In the presence of a catalyst

In the presence of heat

Writing word equations from

chemical equations Using a chemical equation, determine the

reactants and the products and write their

names as discussed in Units 5 and 6.

Insert words to describe each symbol.

NaCl (aq) + AgBr (aq) AgCl (s) + NaBr (aq)

Aqueous sodium chloride and aqueous silver bromide yield solid

silver chloride and aqueous sodium bromide.

Writing chemical equations from

word equations From the word equation, identify the

reactants and the products and write their

formulas as discussed in Units 5 and 6.

Add in the necessary symbols.

Liquid carbon tetrahydride reacts with gaseous oxgyen in the presence of

heat to produce gaseous water and carbon dioxide gas.

CH4 (l) +O2 (g) H2O (g) + CO2 (g)

Single Replacement

A single replacement reaction occurs when one element replaces part of a compound to produce a single element and a new compound.

There are two types of single replacement reactions

◦ Cationic Single Replacement

A + BX B + AX

i.e.: Na + AgCl NaCl + Ag

◦ Anionic Single Replacement

AX + Y AY + X

i.e.: 2NaCl + F2 2NaF + Cl2 Return

Double Replacement

AX + BY AY + BX

A double replacement reaction occurs

when two ionic compounds switch their

cations.

i.e.: NaCl + LiF LiCl + NaF

Return

Combustion

CxHy + O2 CO2 + H2O

Combustion reactions occur when a

hydrocarbon reacts with oxygen to form

carbon dioxide and water.

A hydrocarbon is a covalent molecule that

contains carbon, hydrogen, and occasionally

oxygen.

◦ i.e.: CH4, C2H6, or C3H7O

Return

37. Balance chemical equations

When writing chemical equations, it is important to correctly write each formula.

This will make it appear as though the law of conservation of mass is not followed.

To correct for this, coefficients are placed before each part of the reaction to show how many of each compound or element would be used or produced.

When counting atoms with a coefficient, make sure to multiply the coefficient by the subscripts ◦ i.e.: 2MgCl2 would have 2 magnesiums and 4 chlorine

atoms

Mg + AlCl3 MgCl2 + Al

Take a look at the equation above:

Notice how each compound is correctly written to balance their charges.

However, the chlorine is not balanced.

The formulas cannot be changed so coefficients will be added.

A coefficient is a number placed before the elements or compounds.

The next two slides will work through how to balance this equation.

Balancing the equation with pictures

___Mg + ___AlCl3 ___MgCl2 + ___Al

To be balanced, there must be the same number of each element on the

reactant side and the product side.

Since the chlorine is not balanced, we will add another MgCl2 and another Mg.

Chlorine is still not balanced so we will add a AlCl3 this time and another Al.

Since chlorine is still not balanced, we will add another MgCl2 and another Mg.

Now, there is the same number on each side.

We can put the number of each in the blank in front of each chemical.

3 2 3 2

Balancing equations: Just numbers

___Mg + ___AlCl3 ___MgCl2 + ___Al

Mg: Mg:

Al: Al:

Cl: Cl: Without pictures, counting the elements works just as well.

Start with the unbalanced equation.

Notice how the chlorine is not balanced. We could add one at a

time or look for the least common multiple.

In this case, the least common multiple of 2 and 3 is 6.

Therefore, if we put a 3 in for the coefficient for MgCl2 and a 2 in

for the coefficient for AlCl3, the chlorine will be balanced.

We can finish by balancing the Al and Mg.

3

6

3

2

6

2

3

2

1

1

3

1

1

2

3 2

38. Predicting products

In chemistry, we want to know what products

we will get if we mix two chemicals.

The first step in this process is identify the

chemical reaction from only the reactants.

◦ A flow chart has been created to help you work

through this process.

The next step is to correctly write the

products.

◦ Do not worry at this point if the equation is not

balanced.

Finally, go back and balance the equation.

How many reactants?

One

Two

Decomposition

Is one a hydrocarbon? Yes

No

Combustion

Is there a single element? No

Yes

Are both ionic?

Yes No

Double Replacement

Is it oxygen?

Yes

No

Synthesis

Single Replacement

(check the activity series)

Start

Return

Predicting products

Select each of the following for a description

of how to predict its products.

◦ Synthesis

◦ Decomposition

◦ Single Replacement

◦ Double Replacement

◦ Combustion

Predicting-Synthesis

Once a reaction is determined to be synthesis, recall the 4 types of synthesis reactions.

◦ Two single elements will combine to make one compound.

2Na + Cl2 2NaCl

◦ Metal chloride and oxygen make the metal chlorate.

2NaCl + 3O2 2NaClO3

◦ Metal oxide and water make the metal hydroxide.

Na2O + H2O 2NaOH

◦ Metal oxide and carbon dioxide make the metal carbonate.

Na2O + CO2 Na2CO3

Return

Predicting-Decomposition

Once a reaction is determined to be decomposition, recall the 4 types of decomposition reactions. ◦ One compound breaking into its individual

elements. 2NaCl 2Na + Cl2

◦ Metal chlorate breaking into metal chloride and oxygen. 2NaClO3 2NaCl + 3O2

◦ Metal hydroxide breaking into metal oxide and water. 2NaOH Na2O + H2O

◦ Metal carbonate breaking into metal oxide and carbon dioxide. Na2CO3 Na2O + CO2

Return

Predicting Single Replacement

A single replacement reaction is identified by

having a single element and a compound.

Determine the charges on all the elements.

The two with like charges will switch places.

With single replacement, there is another

step.

Na + K Cl NaCl + K + + -

The activity series

Not all single replacement reactions occur.

An element must have enough reactivity to

replace an element already in the compound.

The activity series lists the elements in

order of decreasing reactivity.

An element can replace any element below

it on the list.

Activity Series

Assume you have a reaction between silver and potassium chloride.

Because both silver and potassium are positive, silver will attempt to replace potassium.

Check the portion of the activity series to the right.

Potassium is higher on the list which means it can replace silver but silver cannot replace potassium in a reaction.

Therefore, the equation would be written as:

Ag + KCl No Reaction

Activity

Series

Lithium Li

Potassium K

Calcium Ca

Sodium Na

Magnesium Mg

Silver Ag

Return

Predicting-Double Replacement

A double replacement reaction occurs when

two ionic compounds react.

Determine the charges on all elements.

The products will switch partners.

Remember to correctly write each formula

by putting the positive ion first.

Na+OH- + H+Cl- H+OH- + Na+Cl-

Return

Predicting-Combustion

Combustion reactions always have a

hydrocarbon (CxHy) being added to O2.

Combustion reactions are probably the

easiest to predict.

The products are always carbon dioxide

and water.

CH4 + O2 CO2 + H2O

Return

This concludes the tutorial on

measurements.

To try some practice problems, click here.

To return to the objective page, click

here.

To exit the tutorial, hit escape.

Definitions-Select the word to return to the tutorial

Anion: the negative portion of an ionic compound

Aqueous: dissolved in water

Catalyst: a chemical added a reaction to increase the reaction rate without changing the products

Cation: the positive portion of an ionic compound

Law of conservation of mass: Mass can neither be created or destroyed; the masses of the reactants must equal the masses of the products