Unit 7 Chemical Reactions

Chapter 7

pp. 175 - 200

What is a chemical reaction?

It is the process by which atoms of one or more substances are rearranged to form NEW different substances.

Consider the following demonstration… What happens when an antacid tablet is

added to water? Let’s take a look…

How can you tell if a chemical reaction has occurred? Temperature change – Adding a strong acid

to water causes a dramatic increase in temp. Color change – A rusty nail changes from

silver to orange/brown Odor – The smell of rotten eggs when you

burn sulfur in the lab Gas bubbles – When baking bread, the bread

rises because of gas production Precipitate or formation of a solid – A solid

appears after adding two liquids together

Chemical Equations In every chemical equation there are three

parts.1. Reactants

2. “Yield” symbol or “”3. Products

Reactant 1 + Reactant 2 Product 1 + Product 2

Sometimes there are symbols that go above the “Yield” symbol. Often times these symbols represent catalysts or energy added or released from a reaction.

Symbols in chemical equations

Physical States (s) - solid (l) - liquid (g) - gas (aq) – aqueous or in

solution

Symbols above the “” Heat – heat energy Δ – energy (often

heat) - electricity “Sy” – Elemental

symbol acting as a catalyst

An important reminder… The following elements exist as diatomic

molecules in their natural state.

H2

F2

Cl2

Br2

I2

O2

N2

Representing Chemical Reactions

Word Equations Writing equations in the English language to

explain the process of a chemical reaction.

Example:

iron(s) + chlorine(g) iron(III)chloride(s)

“Solid Iron and gaseous chlorine react to produce solid iron(III)chloride.”

Writing Word Equations

Let’s look at another example.

Sodium(s) + Water(l) Sodium Hydroxide(aq) + Hydrogen(g)

Now you try to write it correctly in English.

Answer: Solid sodium and liquid water reacted to form aqueous sodium hydroxide and hydrogen gas.

Writing Skeleton Equations Word equations are useful for describing

chemical reactions, however, they are cumbersome and lack important information.

Let’s look at an example of the earlier iron reaction…

iron(s) + chlorine(g) iron(III)chloride(s)

Skeleton equations use symbols in place of the names of atoms and compounds

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

Writing Skeleton Equations

Let’s look at an example:

Carbon(s) + Sulfur(s) Carbon disulfide(l)

Now you try to write the skeleton equation

Answer:

C(s) + S(s) CS2 (l)

Practice Write the following as Word Equations

hydrogen(g) + bromine(g) hydrogen bromide(g)

carbon monoxide(g) + oxygen(g) carbon dioxide(g)

Write the following as Skeleton Equations

Solid barium and oxygen gas react to produce solid barium oxide.

Solid iron and aqueous hydrogen sulfate (sulfuric acid) react to produce aqueous iron(III)sulfate and gaseous hydrogen.

Answers Hydrogen gas reacted with gaseous bromine

producing gaseous hydrogen bromide. Gaseous carbon monoxide reacted with

oxygen gas to produce carbon dioxide gas.

Ba(s) + O2(g) BaO(s)

Fe(s) + H2SO4(aq) Fe2(SO4)3(aq) + H2(g)

Chemical Reactions

Both of these types of equations are useful, but still something is missing…

When a reaction occurs it is important to remember the “Law of Conservation of Mass”.

Mass (atoms) cannot be created nor destroyed. Therefore, the previous types of equations won’t do for chemical analysis.

Balanced Chemical Equations Let’s look at the iron equation one more

time.

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

Notice anything strange? Where did the extra chlorine atom come

from in the iron(III)chloride? To accurately represent this equation it is

important to show that the number of atoms in the reactants is equal to the atoms of product.

Balancing Chemical Equations Whenever the number of atoms of

reactant(s) are equal to the number of atoms of product(s) we say that the equation is a Balanced Chemical Equation.

How do we get the reactants and products to balance out?

Coefficients – (different than subscripts!) Whole numbers placed in front of an atom or compound to indicate more than one of each.

Balancing Chemical Equations Let’s look at the previously discussed iron

equation when it is balanced…

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

Notice that there are 2 atoms of iron and 6 atoms of chlorine. (or 3 molecules of chlorine gas)

The same can be said for the atoms of iron and chlorine in the iron(III)chloride compound.

Steps for balancing equations. Step 1: Write the skeleton equation for the

reaction. Step 2: Count the atoms of the elements in the

reactants. Step 3: Count the atoms of the elements in the

products. Step 4: Change the coefficients to make the

number of atoms of each element equal on both sides of the equation.

Step 5: Write the coefficients in their lowest possible ratio.

Step 6: Check your work.

Steps for balancing equations example. Hydrogen gas reacted with chlorine gas to

produce hydrogen chloride. Step 1: H2(g) + Cl2(g) HCl(g)

Step 2: H2 + Cl2

(2 atoms of H) (2 atoms of Cl)

Step 3: HCl ( 1 atom H + 1 atom Cl)

Step 4: H2(g) + Cl2(g) 2HCl(g)

(2 atoms H) (2 atoms Cl) (2 atoms H +2 atoms Cl)

Steps for balancing equations example.

Step 5: The ratio 1 hydrogen molecule to 1 chlorine molecule to 2 hydrogen chloride molecules (1:1:2) is the lowest possible ratio because the coefficients cannot be reduced and still remain whole numbers.

Step 6: Make sure the chemical formulas are written correctly. Then, check that the number of atoms of each element is equal on both sides of the equation.

That’s it!

OK, maybe that’s not all…

Sometimes equations are more complex than others, especially when polyatomic ions are involved. Here are a couple of extra tips.

1. Begin balancing with the most complex formula.

2. Balance polyatomic ions as a single unit.

Practice Time

Write the balanced chemical equations for the following:

Aqueous sodium hydroxide and aqueous calcium bromide react to produce solid calcium hydroxide and aqueous sodium bromide.

Liquid carbon disulfide reacts with oxygen gas, producing carbon dioxide gas and sulfur dioxide gas.

Answers

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

CS2(l) + 3O2(g) CO2(g) + 2SO2(g)

Classifying Chemical Reactions

Why differentiate? When you go to the library, you see many

different types of books that are organized in a way to help you find them…– Fiction, Nonfiction, Mysteries, Biographies,

History, Fantasy, etc.

Like books, there are also many different kinds of chemical reactions and scientists need a way to organize them.

Types of Chemical Reactions

Synthesis Combustion Decomposition Single-replacement Double-replacement

Synthesis Reactions A chemical reaction in which two or more

substances react to produce a single product.

A + B AB

Example 1 - 2 elements

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

Example 2 - 2 compounds

CaO(s) + H2O(l) Ca(OH)2(s)

Example 3 - 1 element + 1 compound

2SO2(g) + O2(g) 2SO3(g)

Combustion Reactions Oxygen combines with a substance and

releases energy in the form of heat and light.

Example 1

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

Example 2

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

Notice that both of these reactions are also Synthesis reactions as well...

Combustion cont’d. Not all combustion reactions are also synthesis

types. Sometimes oxygen will react with compounds

called “hydrocarbons” that contain carbon and hydrogen.

In these reactions, such as the one below, carbon dioxide and water are produced.

CH4(g) + 2O2(g) CO2(g) + 2H2O(g)

This is the same type of reaction that powers a car. In that instance octane (C8H18), another hydrocarbon, is a reactant, but the products are the same.

Decomposition Reactions

A reaction in which a single compound breaks down into two or more elements or new compounds.

AB A + BExample

NH4NO3(s) N2O(g) + 2H2O(g)

These types of reactions often require an energy source, such as heat, light, or electricity, to occur.

Decomposition cont’d.

One of the best examples of decomposition is in automobile airbags.

Airbags are inflated when an electrical signal starts a reaction with sodium azide (NaN3) pellets within the bag.

These pellets produce nitrogen gas which quickly inflates the bag.

2NaN3(s) 2Na(s) + 3N2(g)

Single Replacement Reactions

Atoms of one element replace atoms of another element in a compound.

When studying this type of reaction we must look closely at the element that we believe will replace another element

Some metals are more reactive than others. If a metal is more reactive than the element

it is replacing, it will take its place. IF a metal is less reactive than the element

it is replacing, ultimately, it will not replace it.

Single Replacement Cont’d.

How do we know if one element will replace another?

Activity Series of Metals – A table indicating the general level of reactivity of different elements

Single Replacement Example 1

Mg + Zn(NO3)2 Mg(NO3)2 + Zn

Example 2

Mg + 2AgNO3 Mg(NO3)2 + 2Ag

Example 3

Mg + LiNO3 No Reaction

Double Replacement

An exchange of positive ions between two compounds in a chemical reaction

Generally, these types of reactions occur in solution

Often times, in these types of reactions, one of the products will come out of solution as either, a gas, a pure liquid, or a solid.

Double Replacement Cont’d. Example 1 – One product is soluble, the other

is a precipitate.

Na2S(aq) + Cd(NO3)2(aq) CdS(s) + 2 NaNO3(aq)

Example 2 – One product is a gas that bubbles out of the mixture

2NaCN(aq) + H2SO4(aq) 2HCN(g) + Na2SO4(aq)

Example 3 – One product is a molecular compound, such as water, which separates from the compounds in solution.

Ca(OH)2(aq) + 2HCl(aq) CaCl2(aq) + 2H2O(l)