Chapter 11 - Reaction Types

Chemical Reactions Or How I Learned To Love Blowing 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 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). carbon dioxide + water

Example: Fe + O 2 (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 Fe 2 O 3

Balancing Equations What is wrong with this chemical equation? Fe(s) + O 2 (g) 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 . Fe 2 O 3 (s)

One example of this is the equation for the burning of carbon in the presence of oxygen to produce carbon dioxide C(s) + O 2 (g) 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: H 2 (g) + O 2 (g) CO 2 (g) 1 “C” atom 2 “O” atoms 1 “C” atom 2 “O” atoms Pt H 2 O(l) 2 “H” atoms 2 “O” atoms 2 “H” atoms 1 “O” atom

Solution – 1. H 2 (g) + O 2 (g) Notice the “ 2 ”. Everything behind it is doubled. Now there are 4 “H” on the right and 2 “H” on the left. 2. 2 H 2 (g) + O 2 (g) By placing a coefficient of 2 in front of the H 2 , there is now the same number of atoms on both sides and the equation is balanced. Pt 2 H 2 O(l) 2 “H” atoms 2 “O” atoms 4 “H” atoms 2 “O” atoms Pt 2 H 2 O(l) 4 “H” atoms 2 “O” atoms 4 “H” atoms 2 “O” atoms

REACTION 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) + O 2 (g) -> SO 2 (g); S(s) + O 2 (g) -> SO 3 (g) Fe(s) + S(s) -> FeS(s); Fe(s) + S(s) -> Fe 2 S 3 (s) iron(II)sulfide iron(III) sulfide

Complete and balance: a. Al(s) + O 2 (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 ”. Al 2 O 3 (s) Cu 2 S(s) copper(I) sulfide 2 4 3 2 copper(II) sulfide CuS(s)

The next type of reaction is known as a decomposition reaction

In a decomposition reaction, a single substance undergoes a reaction that produces two or more simple substances

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

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

2 HgO (s) 2 Hg ( l ) + O 2 (g)

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

CaCO 3 (s) CaO (s) + CO 2 (g)

Calcium carbonate

Li 2 CO 3 (s) Li 2 O (s) + CO 2 (g)

Lithium carbonate

Decomposition of metal hydroxides Decomposition of metallic chlorates

breaks down to metal oxide and water

Ca (OH) 2 (s) CaO (s) + H 2 O (g)

2Li OH (s) Li 2 O (s) + H 2 O (g)

breaks down to metal chloride and oxygen

2KClO 3 (s) 2KCl (s) + 3O 2 (g)

2LiClO 3 (s) 2LiCl (s) + 3O 2 (g)

Complete & balance the following decomposition reactions :

1. H 2 O ( l )

H 2 (g) + O 2 (g) Hg (l) + O 2 (g) elec. 2. Mercury (II) oxide Δ 3. Ag 2 O (s) Δ 4. nickel (II) carbonate Δ 2 2 2 2

1. 2H 2 O ( l ) 2H 2 (g) + O 2 (g) 2. 2HgO (s) 2Hg (l) + O 2 (g) 3. 2Ag 2 O 4Ag (s) + O 2 (g) 4. NiCO 3 NiO (s) + CO 2 (g) 5. NH 4 NO 3 N 2 O (g) + 2H 2 O (g) elec.

REVIEW Complete and balance: Al + F 2 2Al + 3F 2 2AlF 3 MgO 2MgO 2Mg + O 2

The next reaction is known as single-replacement. 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. In a single-replacement reaction, atoms on one element replace the atoms of a second element in a compound. A + BX AX + B

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.)

In the above examples, “Mg” can replace “Zn” and “Ag”. In the above example, “Mg” can not replace “Li”. Example: (A + BX AX + B) Mg(s) + Zn(NO 3 ) 2 (aq) Mg(NO 3 ) 2 (aq) + Zn(s) Mg(s) + 2AgNO 3 (aq) Mg(NO 3 ) 2 (aq) + 2Ag(s) Mg(s) + LiNO 3 (aq) no reaction

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

A double replacement reaction involves an exchange of positive ions between two compounds

The general equation is:

AX + BY AY + BX

There are three generalizations that can be

made about double replacement reactions

1. Formation of a precipitate - after reaction, a

visible solid forms

Pb(NO 3 ) 2 (aq) + 2KI(aq) PbI 2 (s) + 2KNO 3 (aq)

2. Formation of a gas - insoluble gas bubbles out

of the mixture

FeS(s) + 2 HCl(aq) H 2 S(g) + FeCl 2 (aq)

3. Formation of water

HCl (aq) + NaOH(aq) NaCl(aq) + H 2 O( l )

P. 233 #56, a,b (sulfuric acid=H 2 SO 4 ) (nitric acid=HNO 3 ) PROBLEMS

COMBUSTION 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.

If combustion is incomplete, “CO” and “C” may also be produced. "x" and "y" can be any set of numbers CO 2 + H 2 O CO 2 + H 2 O C x H y + O 2 CO 2 + H 2 O + heat/light C x H y + O 2 C + CO + CO 2 + H 2 O Example: (assume complete combustion) 1. C 6 H 6 + O 2 2. CH 3 OH + O 2 2 15 12 6 2 3 2 4