Unit 5 Notes: Chemical Reactions and Equations

(1)

Unit 5 Notes: Chemical Reactions and Equations Name: Period:

Chemical Reaction –

Examples:



 Burning gasoline

 Rusting

 Making bread

 Metabolism

Substances undergo chemical reactions with other substances to gain stability.

 Remember: atoms bond to gain a full set of valence electrons!

During a chemical reaction,

 atoms can form compounds,

 compounds can break apart to form atoms,

 or compounds can react with other compounds.

 New substances are produced as existing bonds are broken, atoms are rearranged, and new bonds are formed.

CHEMICAL EQUATIONS

Chemical reactions are represented by sentences known as chemical equations (

similar to mathematical equations

).

Describing Chemical Reactions 1. Word Equations –

Methane gas reacts with oxygen gas to produce carbon dioxide gas, liquid water, and heat energy.

methane (g) + oxygen (g)  carbon dioxide (g) + water (l) + heat 2. Formula Equations –

CH

4

(g) + O

2

(g)  CO

2

(g) + H

2

O(l) + heat

PHASES OF MATTER

We frequently want to indicate the phase of each reactant and product in a reaction.

 ionic compounds tend to be solid at room temperature, but reactions involving ionic compounds normally occur only if the reactants are dissolved in water; you can always check the solubility of an ionic compound by finding it in Table E.

 covalent compounds involved in the reactions we will study are commonly gas or liquid at room temperature

Phase Symbol Common Sense Memorize

Gas

Liquid

Solid

(2)

NOTES on chemical reactions 1617 kl - 2 -

Aqueous (water solution)

BALANCING CHEMICAL EQUATIONS Law of Conservation of Matter –

The same number of atoms of each element must appear on both sides of the arrow in a chemical reaction.

Does the reaction below follow the Law of Conservation of Matter?

CH

4

(g) + O

2

(g)  CO

2

(g) + H

2

O(l) + heat

To balance the above equation, we must change the coefficients before the molecules. If we change the subscripts, we would change the identity of the substance – not desirable!

CO

2

= H

2

O =

CO = H

2

O

2

=

Steps for Writing Balanced Equations

1. You might be given a description of the reaction in words, you will need to be able to “translate” this into chemical formulas.

Example: Methane reacts with oxygen gas to create carbon dioxide and water vapor, while librating a large amount of heat.

CH

4

(g) + O

2

(g)  CO

2

(g) + H

2

O(g) + heat 2. Balancing begins once you have the formula equation.

3. To balance the equation, coefficients must be changed. It is usually best to begin with those elements that occur in only one substance on each side of the equation. Then check the remaining elements in the equation.

Example: ___ CH

4

(g) + ___ O

2

(g)  ___ CO

2

(g) + ___ H

2

O(g) + heat

4. Verify the equation is balanced.

___ CH

4

(g) + ___ O

2

(g)  ___ CO

2

(g) + ___ H

2

O(g) + heat

Practice

1. Balance the following chemical equations:

Cd + HCl  CdCl

2

+ H

2

CS

2

+ O

2

 CO

2

+ SO

2

2. How does balancing equations illustrate the Law of Conservation of Matter?

(3)

Additional Questions - Write formula equations for the following reactions and balance them.

a. nitrogen (g) + hydrogen (g)  ammonia (g) b. hydrogen (g) + oxygen (g)  water (l)

c. copper (II) oxide (s) + carbon (s)  copper (s) + carbon dioxide (g) d. zinc (s) + hydrogen sulfate (aq)  hydrogen (g) + zinc sulfate (aq) e. ammonia (g) + oxygen (g)  nitrogen monoxide (g) + water (l)

CATALYSTS:

 catalyst :

 Chemists sometimes do not know precisely what role a catalyst plays in a particular reaction, only that it works!

 The formula of a catalyst is usually written over the arrow in a formula equation.

 Sometimes heat is considered a catalyst (if it gets a reaction started, for example); heat as a catalyst is indicated as a small triangle over the arrow

CLASSIFYING CHEMICAL REACTIONS There are six main types of reactions:

1. Synthesis Reactions 4. Single Replacement Reactions 2. Combustion Reactions 5. Double Replacement Reactions 3. Decomposition Reactions 6. Acid/Base Reactions

Reaction Types: Notes

Chemical reactions are grouped into 6 categories (combustion is sometimes listed as a sub-set of synthesis).

As your classmates present, take notes on each type of reaction.

Reaction Type: Synthesis (also called _____________________) Explanation (words):

Generic equation:

Example reaction:

(4)

Memory cues (grammar? from the skit?):

Reaction Type: Combustion (sometimes categorized as a sub-set of ____________________) Explanation (words):

Generic equation:

Example reaction:

Memory cues (grammar? from the skit?):

Reaction Type: Decomposition (also called ____________________) Explanation (words):

Generic equation:

Example reaction:

Memory cues (grammar? from the skit?):

Reaction Type: Single Replacement Explanation (words):

Generic equation:

Example reaction:

Memory cues (grammar? from the skit?):

Reaction Type: Double Replacement Explanation (words):

Generic equation:

Example reaction:

Memory cues (grammar? from the skit?):

(5)

Reaction Type: Acid Base Neutralization Explanation (words):

Generic equation:

Example reaction:

Memory cues (grammar? from the skit?):

Identify the reaction type for the following equations, and explain how you decided. Then balance each equation.

Reaction Type How did you decide?

1. FeCl

2

+ Na

2

SO

4

 FeSO

4

+ NaCl ________

2. Al + HCl  AlCl

3

+ H

2

________

3. Fe + O

2

 Fe

2

O

3

________

4. H

2

O

2

 H

2

O + O

2

________

5. Ag + S  Ag

2

S ________

6. Mg + HCl  MgCl

2

+ H

2

________

7. BaCl

2

+ Na

2

SO

4

 BaSO

4

+ NaCl ________

PREDICTING PRODUCTS OF CHEMICAL REACTIONS Synthesis Reactions (Direct

Combination):

A + B  AB

2 Na (s) + Cl

2

(g)  2 NaCl (s)

If two individual elements react with each other, they will combine in the appropriate ratio to have all electrons accounted for.

Meaning, if you make a new compound it

has to be a real compound (neutral)!

(6)

Combustion Reactions:

A combustion reaction is a special type of synthesis reaction where oxygen combines with a flammable compound to make two new oxygen-containing compounds.

 this is what you usually think of as a “burning” reaction

 a compound containing primarily carbon and hydrogen (but also oxygen in some cases) reacts with elemental oxygen to form water, carbon dioxide, and a lot of heat

 reactants: free oxygen (O

2

) and a compound of carbon, hydrogen, and sometimes oxygen (examples: CH

4

, C

4

H

10

, C

2

H

4

, CH

3

OH, C

6

H

12

O

6

)

 products of complete combustion: CO

2

and H

2

O

 these equations can be difficult to balance –balance oxygen last (one strategy – if necessary – is to use a fraction as the oxygen coefficient and then multiply all coefficients by the appropriate factor)

Fuel [compound containing C, H, and maybe O] + O

2

(g)  CO

2

(g) + H

2

O (g) C

3

H

8

(l) + 5O

2

(g)  3CO

2

(g) + 4H

2

O (g)

Combustion Practice: complete and balance the following reactions.

1. __ C

6

H

12

O

6 (s)

+ __ O

2

(g)  __ CO

2

(g) + __ H

2

O(g)

2. __ C

2

H

4

(g) + __ O

2

(g)  __________________________________

3. __ C

4

H

10 (l)

+ __ O

2 (g)

 __ CO

2 (g)

+ __ H

2

O (g) 4. Complete combustion of methane.

Decomposition Reactions (Analysis):

AB  A + B

2H

2

O (l)  2H

2

(g) + O

2

(g)

If a decomposition reaction happens, the compound breaks into more stable elements or compounds.

This means the products you get at the end will be logical, familiar, real compounds.

Single Replacement Reactions:

There are three subgroups of single replacement reactions:

a. one metal replaces another metal in an ionic compound

A + BX  AX + B BX and AX are generally ionic compounds and A and B are metals Mg (s) + CuSO

4

(aq)  MgSO

4

(aq) + Cu (s)

Fe (s) + CuSO

4

(aq)  FeSO

4

(aq) + Cu (s) b. one halogen replaces another halogen in an ionic compound

Y

2

+ AX  AY + X

2

AX and AY are ionic compounds and X and Y are halogens F

2

(g) + 2NaCl (g)  2NaF (aq) + Cl

2

(g)

Cl

2

(g) + 2CaBr

2

(g)  2CaCl

2

(aq) + Br

2

(l)

(7)

c. one metal replaces hydrogen in an acid

A + HZ  H

2

+ AZ A is a metal, H is hydrogen, Z is any anion Zn (s) + 2HCl (aq)  H

2

(g) + ZnCl

2

(aq)

2Al (s) + 3H

2

SO

4

(aq)  3H

2

(g) + Al

2

(SO

4

)

3

(aq)

Whether one element will replace another depends on the tendency of each element to react. This is also known as their chemical activity. A more active element will replace a less active element that is already in a compound.

Table N of Standard Electrode Potentials can be used to determine if one element replaces another element in a reaction. Metals higher in the table will be replaced by metals lower in the table. Halogens higher in the group will replace halogens lower in the group (F

2

will replace all the other halogens). Metals lower on the table will replace hydrogen in an acid.

Additional Practice for Single Replacement Reactions:

Use table N to predict if the following reactions will occur. If the reaction will occur, predict the products and then balance the reaction. If not, write “no reaction” instead of writing the products. (Assume a 2+ charge on transition metals – Pb, Cu, Zn - in ionic compounds.)

Pb (s) + CuSO

4

(aq)  Mg (s) + HCl (aq)  Al(s) + ZnCl

2

(aq)  NaCl (aq) + H

2

(g)  F

2

(g) + 2KI (g)  I

2

(aq) + 2CaBr

2

(g) 

Double Replacement Reactions:

AX + BY  AY + BX

CaCO

3

+ 2HCl  CaCl

2

+ H

2

CO

3

(When H

2

CO

3

is a predicted product, the actual products are water and CO

2

gas

[notice that the atoms in H

2

O and CO

2

add up to make H

2

CO

3

]).

(8)

For a Double Replacement reactions to occur, this first condition MUST be met…

…AND one of these statements is true.  1.

2.

3. If we observe any of these 3 things, a DOUBLE REPLACEMENT reaction has occurred.

Table E (which shows the solubilities of various ionic substances in water) can be used to help determine if a precipitate is formed or if a reactant is soluble.

Additional Practice for Double Replacement Reactions:

Use table E to predict if the following reactions will occur. If you predict the reaction will occur, explain how you

know. Occurs?

yes/no

Explain how you know.

AgNO

3

+ NaCl  AgCl + NaNO

3

3KOH + Al(NO

3

)

3

 3KNO

3

+ Al(OH)

3

NaOH + HCl  NaCl + H

2

O KBr + NaNO

3

 KNO

3

+ NaBr

Acid/Base Neutralization Reactions:

There are several definitions of acids and bases. For this unit we will use the simplest definitions.

 An acid is a compound that has Hydrogen as its cation (for example HCl or H

2

SO

4

). Table L in your data book has a list of acids and their names.

 A base is a compound that has Hydroxide as its anion (for example, NaOH or NH

4

OH).

When an acid and base react, it is a double replacement reaction in which they form water (HOH or H

2

O) and a salt (remember what a salt is?).

HA (aq) + BOH (aq)  HOH (l) + BA (aq) HCl (aq) + NaOH (aq)  H

2

O (l) + NaCl (aq)

(see Table E)

(9)

Acid/Base Reaction practice: predict products for the following reactions and balance (remember the formula for your salt must be neutral!).

1. ___ H

2

SO

4

(aq) + _ NaOH (aq)  _ H

2

O (l) + ___ Na

2

SO

4

(aq)

2. ___ HNO

3

(aq) + ___ Ca(OH)

2

(aq)  _______________________________

3. Aqueous nitric acid is combined with a solution of barium hydroxide.

COMPLETE IONIC AND NET IONIC EQUATIONS

Double replacement reactions can be written three different ways:

1. complete, balanced molecular equation – as shown in the examples above 2. complete ionic equation – with the ionic compounds written in the form of ions 3. net ionic equation – with only the ions involved in the reaction written