Objective 4 1

Objective 4: use models for describing compounds and chemical reactions

This objective is further broken down into 3 outcomes

Outcome 4A: Demonstrate models of ionic and molecular compounds

Outcome 4B: Describe reactions by using word equations, chemical formulas, and molecular models and identify types of reactions

Outcome 4C: Balancing simple chemical reactions

Outcome 4A: Demonstrate models of ionic and molecular compounds

Models Models are often used

to show which elements are

making up either ionic or

molecular compounds and can

be either drawn with the atoms

directly touching, or with a

line attaching the atoms which

represent the bond between

them. They also show the

numbers of atoms that are making up the compound. When drawing models (either ionic or molecular)

the size of the atoms will represent which has a larger

atomic mass (look to the periodic table to find out as it is

arranged in order of increasing atomic mass). In the H2O

example, oxygen has a higher atomic mass than hydrogen

and will therefore be drawn larger. If there is more than

one atom of an element, this is shown in the subscript of

the chemical formula. For example, H2O is the chemical

formula for water and since there are two Hydrogen atoms,

the number 2 is written as a subscript. Once you know the

elements and the number of atoms, you can draw the

model.

The number of atoms can be counted by looking at a model or by looking at the chemical

formula. For example, it you look at the model of NaCl above, you can count 2 atoms. This can also be

determined by simply looking at the chemical formula NaCl. If a compound has many more atoms, you

just simply count them up. For example, the chemical formula for sodium acetate is NaC2H3O2. You can

see there are 4 elements that make up sodium

acetate but if you count the total number of

atoms you get 8. Another example: A certain

compound’s formula is NaHCO3C. There are 4

elements and 6 atoms that make up this formula

(carbon is found in 2 different places in the

compound but you can’t count it twice because it’s still carbon).

Objective 4 2

A subscript can also be used to indicate whether an element or compound is currently in a solid,

liquid, or gas state. For example, H2O(s) indicates that you are looking at water’s solid form (ice). H20(l)

would indicate the compound is currently in its liquid state, and H20(g) would indicate that water has

been boiled and is in its gaseous form. There is one more subscript you need to know. The subscript (aq)

stands for “aqueous” and means the compound is dissolved in water. For example, if salt (NaCl) was

dissolved in water, its chemical formula would be written as NaCl(aq)

In the examples above, different elements are indicated by different sizes. But there are many

different ways that models can represent different elements. Another common way is simply to use

different colors. For example, in the diagram below, hydrogen is represented as a different color than

oxygen, even though in

reality hydrogen atoms

are much smaller.

In the two

examples on the

previous page, different sizes and colors were used to identify which element was which. Sometimes,

however, there is no way to tell. For example, sometimes models only

represent the number of total atoms present as shown in the diagram to the

right. There is no way to tell which atom to the right is the sulfur atom and

which 3 are the hydrogen atoms. These models still may give you some

information however. For example, if you asked if the model to the right were

SeO3 or C4H, you would pick SeO3 based on the fact that SeO3 has 4 total

atoms and C4H has 5 total atoms.

Objective 4 3

1. Complete the table below. Remember to draw the proper size of the elements in relation to each other. Also,

when writing in the chemical formula, make sure you put in the subscript to indicate its state of matter. Boxes

with an in them don’t have to be done Chemical

formula

Model model (use different sizes for

different atoms)

# of elements # of atoms State of matter chemical found in

BeO(l)

….

2 2 Liquid………….

PO2(g)……….

.

2 3 Gas

Mg3P2(s) …….……

2 5 Solid…………

C12H22O11(aq)

(common name

is sugar)………

3 45 Dissolved in water

Use the following information to answer the next question

CrC2H3O2H

2. How many elements are found in the above compound? How many atoms are there?

4 elements. 9 atoms.

3. Match each compound with its model by placing the number next to the chemical formula underneath the

appropriate model.

1. N2 2. CO 3. nitrogen dioxide 4. Beryllium phosphoride 5. MgF2

2 5 3 1 4

---

---

Objective 4 4

Use the following information to answer the next question

4. What is the chemical formula for the compound on the right above?

A. S2Cl5P3 B. S2Cl4P4 C. S3Cl4P3 D.S3Cl3P4 E.S4Cl3P3 F.S4Cl4P2

D

Use the following information to answer the next question

I.

II

III

IV

5. Which of the above models is modeling C3H?

III Use the following information to answer the next question

6. Which of the following formulas could the model above represent?

A. S2 B. F2Br C. MgC3 D. CFCO

B

Use the following information to answer the next question

7. Which of the following formulas could the marshmallow model above represent?

A. He3 B. B2P C. NC3 D. BrCl

C ……………..

Objective 4 5

Use the following information to answer the next question

8. Write the chemical formula for the compound on the above right (it doesn’t matter what order you put the elements in)

H5S2N4

9. Fill in the chart. Show your work for balancing ionic compounds

Elements to

combine chemically

into compounds:

Ionic or

Molecular

Chemical formula (you

may need to look at

outcome 3D again if you

forget how to do this)

Chemical name (you may

need to look at outcome 3B

again if you forget how to do

this)

Model (use different

sizes for atoms)

# of

atoms

Lithium and fluorine

I

LiF

lithium fluoride

2

Two sulfur and one

chlorine atom

M

S2Cl

disulfur monochloride

3

Beryllium and

fluorine

I

BeF2

beryllium fluoride

3

Iron (2+) and

phosphorus

I

Fe3P2

Iron (II) phosphoride

5

Two nitrogen atoms

M

N2

nitrogen gas

2

----

Objective 4 6

Outcome 4B: Describe reactions by using word equations, chemical formulas, and molecular models and identify types of reactions

Word equations In previous outcomes you have looked at how elements combine to form compounds. You have looked

at the difference between ionic and molecular compounds and how to name each of them. You have also

learned how to model these compounds with molecular models. But what happens when these ionic and

molecular compounds react with elements or other compounds? In this outcome you will learn about some

common chemical reactions and how to represent them with word equations, chemical formulas, and molecular

models

All chemical reactions, whether simple or complex, involve changes in substances. One or more starting

substances (the reactants) are changed into one or more new substances (the products). In a word equation, the

reactants are written on the left, and the products are written on the right. They are connected by an arrow ( )

that is read as "yields" or "reacts” to produce."

Reactants Products Word equations are simply a different way of writing the chemical formula for a chemical reaction. The

models simply represent the chemical formulas and word equations in a visual way. Below is an example of a

chemical word equation which represents a chemical reaction. When writing the word equations, common

names are used if a compound has a common name (like methane or water). If a compound doesn’t have a common name, it is simply named using the naming systems learned in previous outcomes (like tricarbon

dioxide or sodium sulfide).

Chemical Formula H2O + H H4O

Word Equation water + hydrogen tetrahydrogen monoxide

Model

(Do not worry if the atoms do not

balance. There are only 3 hydrogen

on the reactant side but 4 on the

product side. You will learn how to

balance these in the next outcome)

+

NOTE: In the reaction above, you may have noticed that an “extra” hydrogen seemed to appear in the

product that wasn’t there in either of the reactants (the reactants only had three hydrogen, but the product has four.) This is because in this topic you are not required to balance chemical reactions. For now, just be able

to name each individual element or compound and be able to recognize the 3 types of reactions found below

(combustion, corrosion, replacement reactions).

Chemical Reactions The word equation, chemical formula, and model above represent a chemical reaction. While there are

many types of chemical reactions, you are going to learn how to identify just three: corrosion reactions,

combustion reactions, and replacement reactions

Combustion Reaction A combustion reaction is a reaction involving a fuel (which almost always has the element carbon in it)

and oxygen. In a combustion reaction heat is produced. When combined, the fuel is used up and heat is

released. The burning of paper, wood, or gasoline are examples of combustion reactions. Most combustion

reactions involve the burning of carbon in a substance. The

combination of the carbon and oxygen is what produces the

heat and light energy. All fossil fuels (gasoline, oil, propane,

coal, natural gas, methane) contain the element carbon

somewhere in their chemical formula. So do paper and wood.

Burning carbon will produce carbon dioxide gas (CO2(g)), which

is the main cause of global warming. The most basic

combustion reaction, involving pure carbon, is shown below.

Objective 4 7

Notice that although heat had to be produced, it isn’t included in the model. This is because heat isn’t made up of atoms.

Chemical Formula C(s) + O2(g) CO2(g)

Word Equation carbon + oxygen gas carbon dioxide

Model

+

While fossil fuels have carbon in them, they are not made of pure carbon. Therefore, most combustion

reactions will have a reactant that is made of carbon as well as other elements. Below shows the burning

(combusting) of methane, which is the main component of natural gas.

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

Word Equation methane + oxygen gas carbon dioxide + water

Model

+ +

Corrosion reaction When some metals are exposed to oxygen they undergo

corrosion. You learned in previous outcomes that corrosion

is the decomposition of a metal. All corrosion reactions

involve a metal, oxygen and water. In fact, corrosion will not

occur if there is no moisture present. When iron corrodes, it

is called rusting and is shown in the diagram to the right and

in the example below. Corrosion occurs quicker if there is more moisture and if it is more heat. It is also

sped up if there is salt in the water. Note that water is not usually included as a reactant, but its presence is

needed to transfer the electrons needed for corrosion to occur. The reason it is usually not listed as a

reactant is because the water’s chemical makeup is not usually rearranged into a new chemical but is simply present as a way to move electrons around in order for the corrosion to occur. It’s the same reason that “heat” is not usually included as a reactant in combustion although a little bit of heat is needed in order for combustion to occur (also, heat is not made of atoms but is a type of energy).

Chemical Formula Fe(g) + O2(g) Fe2O3

Word Equation iron + oxygen gas iron (III) oxide

Model

+

Replacement reaction When a compound reacts with an element or another compound, one element may replace another. For

example, when pure zinc reacts with the compound copper (II) sulfide, zinc replaces the copper leaving

pure copper and zinc sulfide

(Zn + CuS Cu + ZnS). This

is called a replacement

reaction and is shown below.

Chemical Formula Zn + CuS Cu + ZnS

Word Equation zinc + copper (II) sulfide copper + zinc sulfide

Model

+

+

Objective 4 8

10. Write the word equation for this chemical reaction: Be + O BeO

beryllium + oxygen beryllium oxide

11. Write the word equation for this chemical reaction: O2 + S2O2 S2O4

oxygen gas + disulfur dioxide disulfur tetraoxide

12. Write the word equation for this chemical reaction: Na + Al2O3 Na2O + Al

sodium + aluminum oxide sodium oxide + aluminum

13. Write the word equation for this chemical reaction: H + P3H4 P3 H5

hydrogen + triphosphorus tetrahydride triphosphorus pentahydride 14. What two things are necessary for a combustion reaction to occur? What is ALWAYS produced in a

combustion reaction?

Necessary: Fuel, oxygen Always produced: Heat 15. What are 8 substances mentioned in the reading that contain the carbon needed for combustion to occur?

paper wood gasoline oil

natural gas coal methane propane 16. What three things are necessary for a corrosion reaction to occur?

A metal, oxygen and water (heat would speed up the reaction)

17. Identify the below reactions as a combustion, corrosion, or replacement reaction (water is not represented in

the corrosion reactions below, even though in reality it would need to be present).

I Cr + FeCl2 CrCl2 + Fe Replacement II Ti + O2 Ti2O3 Corrosion III Al + HCl AlCl + H Replacement IV C + O2 CO2 Combustion V CH3OH + O2 CO2 + H2O Combustion VI Mg + O MgO Corrosion

18. How many products are seen in IV above?

1

Objective 4 9

19. What 3 conditions will speed up a corrosion reaction?

More moisture, warmer, salt

20. In which example will corrosion currently be working on the metal the slowest?

A. A coin in the sand of the Sahara Desert on an extremely hot day

B. The metal bottom of a boat sailing near Hawaii in December

C. A nail in the roof of a house in Los Angeles on a rainy day in summer

A (corrosion needs moisture to occur)

21. Fill in the information for the following combustion reaction (don’t worry about balancing out number of atoms in the reactants and products)

Chemical Formula C2H6(g) ……. + O2(g) CO2(g)……... + H2O(l)……..

Word Equation ethanol (common name)

+ oxygen gas carbon dioxide + water

Model

+

+

………………

22. Fill in the information for the following corrosion reaction (don’t worry about balancing out number of atoms in the reactants and products)

Chemical Formula Mg +.. O2(g) . MgO…..………

Word Equation magnesium +.. oxygen gas.. . magnesium oxide

Model ...

+..

.

23. Fill in the information for the following replacement reaction (don’t worry about balancing out number of atoms in the reactants and products)

Chemical Formula Be….. + AlO……….. Al…… + BeO Word Equation beryllium + aluminum oxide aluminum + beryllium oxide..

Model

+

..

+

……

-------

Objective 4 10

Outcome 4C: Balancing simple chemical equations

Water (H2O) is a common name for one type of

compound that is very important to us (the chemical name

for water is dihydrogen monoxide). Water is necessary

for life as we know it on Earth and makes up 70% of a

humans body weight. There are a few more common

names that you must know. Hydrochloric acid is the

common name for HCl which is the acid found in our

stomachs but is also used in the production of many

foods, medicines and countless other products. Sodium

hydroxide has the chemical formula of NaOH and is

used in many strong household cleaners such as drain

cleaner and oven cleaner. You know that the symbol for

sodium is “Na” but you have not learned that when

oxygen and hydrogen combine they can form an ion

called a “hydroxide” ion (represented as: OH

- ). The Na

+

ion and the OH

- ion combine to form sodium hydroxide

(NaOH), which is a very powerful cleaning agent.

Ammonia is another chemical found in some cleaners

and has the chemical formula of NH3. The most common

cleaner it is found in is Windex. It is also used in many

areas of product production from fertilizers and food to

the production of rubber and metals. In fact, 80% of

ammonia is used in the production of fertilizers (nitrogen

is one of the main atoms needed for plants to grow).

Ammonia is also the chemical that gives cat urine that

strong potent smell. Another important chemical that is

given a common name is methane and has the chemical

formula of CH4. It is a fossil fuel similar to gasoline oil or

coal. All of these fossil fuels, including methane, have

carbon and you learned earlier in this unit that carbon is

almost always the fuel for combustion. Methane is also

the gas that cows burp out. Cows burping methane

doesn’t seem like a big deal, but this activity contributes largely to global warming. A cow will burp out 200

pounds of methane gas a year and there are 1.5 billion

cows on the planet raised for milk and meat. And

methane is a much more powerful greenhouse gas than

carbon dioxide. In fact, it traps heat 23 times more than

carbon dioxide.

Objective 4 11

You learned previously about modeling elements and compounds. In those models, each element was

represented as a circle (as seen in the diagram below on the left. However, it is also common for

compounds (along with their chemical structure) to be represented in just one large circle. For example,

water (H2O) can be modelled as seen below on the right:

The reason that molecules are commonly many of these water molecules joined together can get confusing

using the above method so it is simpler to draw them as seen below:

You see above that the number “4” is put in front of the H2O to indicate that 4 water molecules joined

together. It also indicates the number of total atoms. The 4 is multiplied by the subscript to give the total

number of atoms of each element in the larger molecule. Since 4 multiplied by 2 is equal to 8 we know

there are a total of 8 hydrogen atoms. And since 4 multiplied by 1 is 4, we know that there are 4 oxygen

atoms (remember that when there is no subscript written we know that is actually a 1). The number in front

is not only needed to represent the number of water molecules joined together to form the larger molecule

but is needed to balance chemical equations. For example, when oxygen gas (O2) and hydrogen gas (H2)

join they form water. But since oxygen and hydrogen are only found as diatomic molecules (O2 and H2

instead of just O or H as single atoms) it would seem that you can’t make H2O because two hydrogen and

two oxygen would make H2O2 (which is called hydrogen peroxide and is extremely poisonous). What it

means is that two hydrogen gas (H2) molecules will always combine with one oxygen gas (O2) molecule to

form two (H2O) molecules. This is seen in the reaction below:

Chemical

equation: 2H2 + O2 2H2O

Models:

+

2 hydrogen

gas

+ oxygen

gas

2 water

You can see that the number of hydrogen atoms in the reactants and the product is now equal. This is also

true of the oxygen atoms. The above chemical equation represents a reaction where compounds are joining

into larger compounds. Sometimes however, compounds are broken down. An example of this is seen

below as ammonia breaks down into nitrogen gas and hydrogen gas.

Chemical equation: 2NH3 N2 + 3H2

Models:

+

2 ammonia nitrogen gas + 3 hydrogen gas

You can see that throughout the breakdown of ammonia, the number of nitrogen and hydrogen atoms is equal

on each side of the chemical equation but that two ammonia molecules were originally needed for this reaction

to occur.

Objective 4 12

1. Give the common name for the following chemicals and give 1 fact about water and sodium

hydroxide and 2 facts about hydrochloric acid, ammonia and methane.

HCl NH3 H2O NaOH CH4

Common

name Hydrochloric

acid

Ammonia Water Sodium

hydroxide

methane

Fact Found in

our stomach.

Used in

making

many

products.

Used in

certain

cleaners such

as Windex.

Used in

production of

fertilizers and

other products

Makes

up 70%

of human

body

weight

Used in

strong

cleaning

agents

Is a fossil

fuel. Also

found in

cow burps;

leads to

global

warming

Use the following information to answer the next question

2. Answer the following questions based on the diagram of the structure above

How many types

of compounds

are seen above?

How many

elements are

found above?

How many

atoms of carbon

and hydrogen are

found above?

How many total

molecules are

found above?

How would you write the

chemical formula for the

molecules that are joined

above?

1 (methane) 2 3 carbon

12 hydrogen

3 3CH4

3. Fill in the chart below based on the title below in bold (it may be easier to start with writing the

chemical formula)

Four hydrochloric acid molecules are joined together

Write the chemical

formula

Draw a model How many atoms

are there?

How many

elements are there?

4HCl

8

2

Objective 4 13

Use the following information to answer the next question

4. How many types of compounds are seen to the right?

3 5. How many elements are seen to the right?

2 6. How many total molecules are seen to the right?

11 7. How many atoms are seen to the right?

29

8. Draw a model representing the breakdown of two water molecules into hydrogen gas and oxygen gas

Chemical equation: 2H2O. 2H2 +. O2

Models:

+.

9. Draw a model representing chlorine gas and hydrogen gas combining into joined hydrochloric acid

molecules

Chemical equation: Cl2 +.. H2 ...... 2HCl

Models:

+

...

10. Hydrogen peroxide is the common name for H2O2 and has many uses from disinfecting wounds to paper

production. Draw a model representing 3 hydrogen peroxide breaking down into hydrogen gas and

oxygen gas

Chemical equation: 3H2O2. . 3H2 +. 3O2

Models:

+.

Objective 4 14

Objective 4 15

3A

A sodium ion is positively charged while a chlorine ion is negatively charged. Since different charges are

attracted to each other (much like magnets), they will combine to form NaCl (which is table salt). This

combining due to opposite charges is an ionic bond resulting in an ionic compound.

Objective 4 16

Three Fluorine and

two Chlorine atoms M F3Cl2 trifluorine dichloride

5

3D

Chemical Formula Li + ScP Li3P + Sc

Word equation lithium scandium phosphoride lithium phosphoride scandium

Model

+ +

Chemical Formula H2 + O2 H2O2

Word equation Hydrogen gas + Oxygen gas dihydrogen dioxide

Objective 4 17

Model

+

Chemical Formula Fe + O2 Fe2O3

Word equation iron + oxygen gas iron (III) oxide

Model

+

1. W rite the word equation for this chemical reaction: Fe + F FeF2

Iron + Fluorine iron (II) fluoride

H20 NaCl O2 ClN O MgF2 C6H12O6