Chemical Reactions Reactions

Or How I Learned To Love Or How I Learned To Love Blowing Things UpBlowing Things Up

Using Everyday Equations

Every minute of the day chemical reactions are taking place both in and around you.

What makes something a chemical reaction?

A chemical reaction involves one or more substances, the reactants, changing into one or more substances, the products.

Reactants Products

“ ” means “yields” or “reacts to produce.”

Chemical reactions can be described many ways. One is by using a word equation.

A word equation is an equation where reactants and products are represented by words.

Example:

methane + oxygen carbon dioxide + water

Word equations, while useful, are cumbersome. To better describe a reaction, writing the formulas is used.

A chemical equation uses formulas of reactants (on the left) separated by an arrow from products (on the right).

Example:

Fe + O2 Fe2O3

(Skeleton equation)

- does not indicate the relative amounts of reactants and products

Most often symbols are used to describe the state (i.e., gas, solid, etc.) of each substance.

p. 206

Balancing EquationsBalancing EquationsWhat is wrong with this chemical equation?

Fe(s) + O2(g) Fe2O3(s)

You should notice that the number of iron and oxygen atoms don’t match on both sides.

For a chemical equation to represent a chemical reaction correctly, it must be balanced.

A balanced equation is one in which each side of the equation has the same number of atoms of each element.

Sometimes, in writing an equation, it is already balanced.

One example of this is the equation for the burning of carbon in the presence of oxygen to produce carbon dioxide

C(s) + O2(g) CO2(g)1 “C” atom 2 “O” atoms 1 “C” atom

2 “O” atoms

Sometimes, most times actually, equations will not be balanced. To solve them, it takes trial and error, but some rules can be followed.

p.208

Example:

H2(g) + O2(g) Pt H2O(l)2 “H” atoms 2 “O” atoms 2 “H” atoms

1 “O” atom

Solution –

1. H2(g) + O2(g)Pt 2H2O(l)

2 “H” atoms 2 “O” atoms 4 “H” atoms

2 “O” atoms

Notice the “2”. Everything behind it is doubled. Now there are 4 “H” on the right and 2 “H” on the left.

2. 2H2(g) + O2(g) Pt 2H2O(l)4 “H” atoms 2 “O” atoms 4 “H” atoms

2 “O” atoms

By placing a coefficient of 2 in front of the H2, there is now the same number of atoms on both sides and the equation is balanced.

REACTION TYPESREACTION TYPES

In learning how to balance equations you have come across most of the different types of reaction without realizing it.

There are five (5) types of reactions. Most often the only way to learn what the products of a reaction are is to carry out the experiment. (Which we will!) But you can learn to recognize patterns of chemical behavior based on the type of reaction.

The most simple of reactions is the combination/synthesis reaction.

A combination reaction is one in which two or more substances react to form a single product.

Examples:

When two nonmetals react, or when a transition metal (Group B) reacts with a nonmetal, more than one product is often possible.

S(s) + O2(g) -> SO2(g); S(s) + O2(g) -> SO3(g)

Fe(s) + S(s) -> FeS(s); Fe(s) + S(s) -> Fe2S3(s)

iron(II)sulfide iron(III) sulfide

Complete and balance:

a. Al(s) + O2(g) ->

b. Cu(s) + S(s) ->

NOTE: Cu is one of the “CMILT” elements. Therefore, since I didn’t say which state Cu is in, there are two possible answers for “b”.

Al2O3(s)4 3 2

Cu2S(s)copper(I) sulfide2

copper(II) sulfideCuS(s)

The next type of reaction is known The next type of reaction is known as a decomposition reactionas a decomposition reaction– In a decomposition reaction, In a decomposition reaction, a single a single

substancesubstance undergoes a reaction that undergoes a reaction that produces produces two or more simple two or more simple substancessubstances

The simplest kind of The simplest kind of decomposition reaction is the decomposition reaction is the decomposition of an ordinary decomposition of an ordinary compound into its elementscompound into its elements

AX A + X

2 HgO (s) 2 HgO (s) 2 Hg (2 Hg (ll ) + O ) + O22 (g) (g)

A compound made of two elements is known as a “binary” compound.

If an electric current is used to break down the substance it is called electrolysis

Decomposition of metal carbonates

•breaks down to metal oxide and carbon dioxide gas

CaCO3 (s) CaO (s) + CO2 (g)Calcium carbonate

Li2CO3 (s) Li2O (s) + CO2 (g)Lithium carbonate

Decomposition of metal hydroxides

Decomposition of metallic chlorates

•breaks down to metal oxide and water

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

2Li OH (s) Li2O (s) + H2O (g)

•breaks down to metal chloride and oxygen

2KClO3 (s) 2KCl (s) + 3O2 (g)

2LiClO3 (s) 2LiCl (s) + 3O2 (g)

Complete & balance the Complete & balance the following decomposition following decomposition

reactionsreactions::1.1. H H22O (O (ll))

elec.

2. Mercury (II) oxide2. Mercury (II) oxide Δ

3. Ag3. Ag22O (s)O (s) Δ

4. nickel (II) carbonate4. nickel (II) carbonate Δ

H2 (g) + O2 (g)2 2

Hg (l) + O2 (g)2 2

1. 2H2O (l) 2H2 (g) + O2 (g)

2. 2HgO (s) 2Hg (l) + O2 (g)

3. 2Ag2O 4Ag (s) + O2 (g)

4. NiCO3 NiO (s) + CO2 (g)

5. NH4NO3 N2O (g) + 2H2O (g)

elec.

REVIEWREVIEW

Al + F2

Complete and balance:

2Al + 3F2 2AlF3

MgO

2MgO 2Mg + O2

The next reaction is known as single-replacement.

In a single-replacement reaction, atoms on one element replace the atoms of a second element in a compound.

A + BX AX + B

Not all elements replace other elements in a single-replacement reaction.

What determines whether something is replaced by another element is the reactivity of both elements.

The activity series of metals is a list of metals in order of

decreasing reactivity. (Those on the bottom are less reactive than those on the top.)

Example: (A + BX AX + B)

Mg(s) + Zn(NO3)2 (aq) Mg(NO3)2 (aq) + Zn(s)

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

In the above examples, “Mg” can replace “Zn” and “Ag”.

Mg(s) + LiNO3 (aq) no reaction

In the above example, “Mg” can not replace “Li”.

If two ionic compounds (i.e., If two ionic compounds (i.e., potassium chloride and barium potassium chloride and barium carbonate) are mixed you get what carbonate) are mixed you get what is known as a double-replacement is known as a double-replacement reactionreaction– A A double replacement double replacement

reactionreaction involves an exchange involves an exchange of of positive ionspositive ions between two between two compoundscompounds

The general equation is:The general equation is:

AX + BYAX + BY AY + BXAY + BX

There are three generalizations that can beThere are three generalizations that can bemade about double replacement reactionsmade about double replacement reactions

1. Formation of a precipitate - after reaction, a 1. Formation of a precipitate - after reaction, a visible solid formsvisible solid forms

Pb(NOPb(NO33))22(aq) + 2KI(aq)(aq) + 2KI(aq) PbI PbI22(s) + 2KNO(s) + 2KNO33(aq)(aq)

2.2. Formation of a gas - insoluble gas bubbles out Formation of a gas - insoluble gas bubbles out of the mixtureof the mixture

FeS(s) + 2 HCl(aq)FeS(s) + 2 HCl(aq) H H22S(g) + FeClS(g) + FeCl22(aq)(aq)

3. 3. Formation of waterFormation of water

HCl (aq) + NaOH(aq) HCl (aq) + NaOH(aq) NaCl(aq) + H NaCl(aq) + H22O(O(ll))

P. 233 #56, a,b

(sulfuric acid=H2SO4)

(nitric acid=HNO3)

COMBUSTIONCOMBUSTION

The final type of reaction is known as combustion.

Combustion reactions are reactions in which an element or compound reacts with oxygen, often producing energy in the form of light and heat.

Most often hydrocarbons (compounds made of hydrogen and carbon. DUH! (scientists are clever sometimes, huh?!)) are reacted producing carbon dioxide and water.

CxHy + O2 CO2 + H2O + heat/light

If combustion is incomplete, “CO” and “C” may also be produced.

CxHy + O2 C + CO + CO2 + H2O

Example: (assume complete combustion)

1. C6H6 + O2

2. CH3OH + O2

"x" and "y" can be any set of numbers

CO2 + H2O2 15 12 6

CO2 + H2O2 3 2 4