section 2.6—chemical reactions

Section 2.6—Chemical Reactions

If we’re going to do chemical reactions with antacids, we’d better know how to write them!

Diatomic Molecules

This is not in the notes so you should pay attention, and add it in!!!!!!

Some elements are so chemically reactive that, when they are not bonded to any other elements, they find another atom of their same kind to bond with.

Diatomic Molecules

To write a formula for a diatomic molecule you write the symbol of the element and use a two as a subscript.

Which elements form diatomic molecules?

Well, I am glad you asked!

Diatomic Molecules

There are seven diatomic molecules:

Element MoleculeHydrogen H2

Nitrogen N2

Oxygen O2

Fluorine F2Chlorine Cl2Bromine Br2Iodine I2

Diatomic Molecules

There are two good ways to remember the diatomic molecules

First – Remember a funny word!

HOFBrINCl

Diatomic Molecules

Second – Remember a funny phrase:

The six that make a seven plus hydrogen.

Now back to our regularly scheduled notes.

Watch as 2 H2 and 1 O2 undergo a chemical reaction

OO OO

Bonds and atoms are rearranged to form new compounds.

HH

HH HH HH

2 H2 + O2 2 H2O

Chemical Reactions

OO OO

Bonds and atoms are rearranged to form new compounds.

HH HH

OOHH

HHOOHH

HH HH HHThe compounds in the end are different from those in the beginning

Bonds are broken and formed between different atoms

2 H2 + O2 2 H2O

Chemical Reactions

Chemical Equations

The “sentence” of chemistry that shows how the starting materials and the final products of a chemical reaction

Examples of Chemical Equations

H Cl

HMg H

H Cl

Mg ClCl

H Cl

HMg H

H Cl

Mg ClCl

Word equation: magnesium metal is reacted with aqueous hydrochloric acid to produce aqueous

magnesium chloride and hydrogen gas 

Visualization

Formula equation: Mg (s) + 2 HCl (aq) MgCl2 (aq) + H2 (g)

Parts of a Chemical Equation

NaCl (aq) + AgNO3 (aq) AgCl(s) + NaNO3 (aq)

Reactants

States of matter Arrow

Products

The starting materials for the reaction—each compound is separated by a “+”

s = solidl = liquidg = gasaq = aqueous (dissolved in water)

Read as:YieldsProducesFormsMakesetc.

Formed in the reaction

You must write each chemical formula correctly first! (Section 2.2)

“and”, “is mixed with” or “reacts with” = +“yield”, “produces” and “forms” =

Writing Chemical Equations

We will learn to balance equations in the next section, so don’t worry about it now!

Let’s Practice #1

Example:Write the word equation into symbol form

Aluminum metal is reacted with hydrochloric acid to form aluminum chloride and hydrogen gas

Aluminum metal is reacted with hydrochloric acid to form aluminum chloride and hydrogen gas

Let’s Practice #1

Al + HCl AlCl3 + H2

Example:Write the word equation into symbol form

Let’s Practice #2

Example:Write the word equation into symbol form

Copper (II) nitrate and sodium hydroxide form copper (II) hydroxide and sodium nitrate

Copper (II) nitrate and sodium hydroxide form copper (II) hydroxide and sodium nitrate

Let’s Practice #2

Cu(NO3)2 + NaOH Cu(OH)2 + NaNO3

Example:Write the word equation into symbol form

NaCl + AgNO3 AgCl + NaNO3

Double Replacement Reactions

The cations from two compounds replace each other.

ClCl

NaNa

AgAg

OO OO

NN

OO

ClCl

AgAg

NaNa

OO OO

NN

OO

Two ionic compounds switch ions

Double Replacement Reactions

A X B Z A XBZAA XX BB ZZ AA XXBBZZ

General format of a double replacement reaction:

Combine the cation of the first reactant with the anion of the second reactant

CaCl2 + AgNO3

1

Products of a Double Replacement

Combine the cation of the second reactant with the anion of the first reactant

CaCl2 + AgNO3

2

Products of a Double Replacement

& balance charges with subscripts when writing formulasRemember to write cations first …

AgCl

CaCl2 + AgNO3

3

Ca(NO3)2 +CaCl2 AgNO3+

Products of a Double Replacement

Only leave subscripts that are in the original compound there if they are a part of a polyatomic ion!

Neutralization Reactions

General format of a neutralization reaction:

X B XBH HO HO

HXX BB XXBBH HOO HO

HH

OOH

Neutralization reactions are double replacement reactions where one cation is “H” and one anion is “OH” and water is formed

Combine the cation of the first reactant with the anion of the second reactant

H2 SO4 + NaOH

1

Products of a Neutralization Reaction

Combine the cation of the second reactant with the anion of the first reactant

H2 SO4 + NaOH

2

Products of a Neutralization Reaction

& balance charges with subscripts when writing formulasRemember to write cations first …

HOH

H2 SO4 + NaOH

3

Na2SO4 +H2 SO4 NaOH+

Products of a Neutralization Reaction

Only leave subscripts that are in the original compound there if they are a part of a polyatomic ion!

You can write the water as “HOH” when you combine the cation & anion or you can change it to “H2O”

Let’s Practice #3

Example:Write the

products for this reaction

Sr(OH)2 + HBr

Let’s Practice #3

SrBr2 + HOH

Example:Write the

products for this reaction

Sr(OH)2 + HBr

Let’s Practice #4

Example:Write the

products for this reaction

HCl + Ca(OH)2

Let’s Practice #4

CaCl2 + HOH

Example:Write the

products for this reaction

HCl + Ca(OH)2