CHAPTER 8 Chemical Reactions

Where are we headed? Past - We have been studying how chemicals are put

together (bonding) Current goal – Learn about how chemicals react with

one another. Chapter 8 – Chemical Reactions Chapter 9 – Stoichiometry

Stoichiometry is the study of the quantity of materials consumed and produced in chemical reactions.

But first we need to learn how to balance chemical reactions to see how many reactants are needed to form a certain number of products.

Section 8.1 Describing Chemical Change

The normal way that a chemical reaction is written is reactants → products.

The arrow means “yields”, “gives” or “reacts to produce” As reactants are converted to products, the bonds

holding the atoms together are broken and new bonds are formed.

However, the actual atoms don’t change, so if you start with a certain number of oxygen and nitrogen atoms, you will need to end with the same number of oxygen and nitrogen atoms. The atoms don’t change identity during a reaction.

Section 8.1 Remember that atoms are neither created or

destroyed in a chemical reaction, they are only rearranged.

The reactants are always written on the left side of the arrow and the products are always written on the right side of the arrow.

For example: Methane + oxygen → carbon dioxide + water This form is called a word equation because words

are used instead of chemical formulas.

Section 8.1

There are different types of chemical reactions. For example, here are three different types – what do you see that is different about them?

Iron + oxygen → iron(III) oxide Two reactants make one product

Hydrogen peroxide → water + oxygen One reactant breaks up into two products

Methane + oxygen → carbon dioxide + water Two reactants rearrange themselves into two different

products

Chemical Equations This is how you write a chemical equation using

chemical formulas Fe + O2 → Fe2 O3

Notice that this equation is not yet balanced because there is only 1 Fe on the left side and 2 Fe atoms on the right side.

This is called a skeleton equation because it does not yet contain the quantities of each reactant or product that balance the equation.

Another way to write this would be Fe (s) + O2 (g) → Fe2 O3 (s) this shows the phases (but it is still unbalanced!)

Chemical Equations

In some chemical reactions, a catalyst is used to speed up the rate of reaction - but it does not change the overall reaction or get used up by the reaction. If a catalyst is used it looks like this:

MnO2

H2O2 (aq) → H2O (l) + O2 (g) Here the MnO2 is the catalyst. It is neither a

reactant or a product because it does not get consumed or changed during the reaction.

Symbols Used in Chemical EquationsSymbol Explanation

+ Used to separate two reactants or two products (or more)

→ “Yields”, separates reactants from products

Used in place of for reversible reactions

(s) Designates a reactant or product in the solid state, placed after the formula

(l) Designates a reactant or product in the liquid state, placed after the formula

(g) Designates a reactant or product in the gaseous state, placed after the formula

(aq) Designates an aqueous solution; the substance is dissolved in water; placed after the formula

Δ heat Indicates that heat is supplied to the reaction

MnO2 A formula written above or below the yield arrow indicates its use as a catalyst, in this example, it is MnO2

Writing Chemical EquationsSample problem 8-1

Try this one: Write a skeleton equation for the reaction where solid sodium hydrogen carbonate reacts with hydrochloric acid to produce aqueous sodium chloride, water and carbon dioxide gas. Include appropriate phase symbols (solid, liquid, etc)

NaHCO3 (s) + HCl (aq) → NaCl (aq) + H2O (l) + CO2 (g)

In case you were wondering, “aqueous” sodium chloride means table salt dissolved in water.

Balancing Chemical Equations Let’s say we wrote this equation – is it balanced? C + O2 → CO2 There’s one C on the left side and one C on the right. There are 2 O’s on the left side and 2 O’s on the right, so it’s balanced. Now try this one: H2 + O2 → H2O The H’s are balanced on both sides, but not the O’s. Can you add coefficients in front of each to balance it? 2 H2 + 1 O2 → 2 H2O Now is it balanced? Note that all coefficients have to be

whole numbers, no fractions or decimals allowed.

Rules for balancing equations Count the number of atoms of each element in the

reactants and the products. If a polyatomic ion appears unchanged on both sides of

the equation, you can just count that as one single unit. Balance the elements one at a time, using coefficients (the

numbers in front of each chemical formula). If no coefficient is written, it is assumed to be = 1.

It’s best to begin the balancing with the elements that only occur once on each side of the equation.

Remember, you are not allowed to change any subscripts (the quantities of each atom in a compound).

PhET simulation on balancing equations/reactions balancing-chemical-equations_en.jar

MINOH - Details of what to balance first

1. M is for metals – balance metals first2. I is for ions – Look for polyatomic ions that

cross from reactant to product unchanged and balance them as one group or unit.

3. N is for non-metals. Look for the common ones such as Cl or S.

4. O is for oxygen.5. H is for hydrogen.

Let’s practice…

Balance AgNO3 + H2S → Ag2S + HNO3

What coefficient do we need in front of each one? 2AgNO3 + H2S → Ag2S + HNO3 (metals first)

2AgNO3 + H2S → Ag2S + 2HNO3 (polyatomic ion NO3 balances

next)

2AgNO3 + H2S → Ag2S + 2HNO3 are we balanced now?

Let’s practice #2

Balance Zn(OH)2 + H3PO4 → Zn3(PO4)2 + H2O

Metals 3Zn(OH)2 + H3PO4 → Zn3(PO4)2 + H2O

Phosphate ion 3Zn(OH)2 + 2H3PO4 → Zn3(PO4)2 + H2O

Hydroxide ion 3Zn(OH)2 + 2H3PO4 → Zn3(PO4)2 + 6HOH

Hydrogen 3Zn(OH)2 + 2H3PO4 → Zn3(PO4)2 + 6HOH

You just had to realize that water is the same as HOH here.

8.2 Types of chemical reactions

There are five general types of reactions Combination Decomposition Single-replacement Double-replacement Combustion

Sometimes a reaction could fit equally well into two categories.

Combination Reactions The combination reaction can also be called a

synthesis reaction. The general form is

R + S → RS This is where two or more substances react to form

a single substance (like what would happen if you were synthesizing a chemical).

The reactants can be two elements or two compounds. The product is always a compound. Reaction of elements with oxygen and sulfur Reactions of metals with Halogens Synthesis Reactions with Oxides There are others not covered here!

Combination Reactions Ionic examples

2 K(s) + Cl2 → 2 KCl(s) Fe(s) + S(s) → FeS iron(II) sulfide 2 Fe(s) + 3 S(s) → Fe2 S3 iron(III) sulfide

(Recall Dalton’s law of multiple proportions) Here’s a covalent example of multiple proportions

S(s) + O2(s) →SO2(g) 2 S(s) + 3 O2(s) →2 SO3(g)

Some nonmetal oxides + water = acid (H+ ions) SO2(g) + H2O(l) → H2SO4

Some metallic oxides + water = base (OH- ions) CaO(s) + H2O(l) → Ca(OH)2 (aq)

Try some combination reactions…

Write the balanced equation for Be + O2 → ??? 2 Be + O2 → 2 BeO Write the balanced equation to make SrI2 Sr + I2 → SrI2

Write the balanced equation to make Mg3N2

3 Mg + N2 → Mg3N2

Write the balanced equation to make Al2O3

4 Al + 3 O2 → 2 Al2O3

Decomposition Reactions When one reactant breaks down into two or more

products, this is a decomposition reaction. It will have the form RS R + S heat

Ex: CaCO3 (s) → CaO(s) + CO2 (g) If energy is input, bonds can be broken and

compounds can decompose. It isn’t always easy to tell what a complex

compound will break into. But a simple binary (two component) compound

like the one above is straightforward.

Decomposition examples

Decomposition of: Binary compounds H2O(l ) 2H2(g) + O2(g) Metal carbonates CaCO3(s) CaO(s) + CO2(g) Metal hydroxides Ca(OH)2(s) CaO(s) + H2O(g) Metal chlorates 2KClO3(s) 2KCl(s) + 3O2(g) Oxyacids H2CO3(aq) CO2(g) + H2O(l ) Oxyacids include sulfuric H2SO4, nitric HNO3 and

phosphoric H3PO4 (all contain oxygen) Oxyacids are simply polyatomic ions with a positively

polarized hydrogen, which can be split off as a cation.

Decomposition Reactions

The decomposition of TNT (trinitrotoluene) looks like this:

2C7H5N3O6 (s) → 3N2 (g) + 7CO(g) + 5H2O(g) + 7C(s)

So for every 2 moles of TNT, you get 15 moles of hot expanding gas, hence the explosion.

decomposition.mov

Combination vs. Decomposition

How can you tell if a reaction is a combination or decomposition reaction? What do you notice?

What kind of reaction is this? 4 Al + 3 O2 → 2 Al2O3

Well then what kind of reaction is this? 2 Al2O3 → 4 Al + 3 O2

Wait, aren’t those two the same thing? Yes, if it runs one way, it often releases energy. To run it the other way, you often need to give it

energy.

Single-Replacement Reactions

These are reactions of the form T + RS TS + R This represents the replacement of Metals by another metal Hydrogen in water by a metal Hydrogen in an acid by a metal Halogens by more active halogens

Reaction of K and H2O

If you put solid potassium K into water, a violent single replacement reaction occurs. The reaction produces hydrogen gas and a large quantity of heat.

2K(s) + 2H2O→ 2KOH + H2

or you might see it better as 2K(s) + 2HOH→ 2KOH + H2

The activity series of the metals

Lithium Potassium Calcium Sodium Magnesium Aluminum Zinc Chromium Iron Nickel Lead Hydrogen Bismuth Copper Mercury Silver Platinum Gold

Metals can replace other metals provided that they are above the metal that they are trying to replace.

Metals above hydrogen can replace hydrogen in acids.

Metals from sodium upward can replace hydrogen in water (explosively).

Mg(s) + Zn(NO3)2 → what? Mg(NO3)2 + Zn(s)

The activity series of the halogens

Halogens can replace other halogens in compounds, provided that they are above the halogen that they are trying to replace. Fluorine Chlorine Bromine Iodine

Try it:

2NaCl(s) + F2(g) what? 2NaF(s) + Cl2(g)

MgCl2(s) + Br2(g) what? no reaction

Single Replacement Reactions

single replacement demo.mov

Zn and SnCl2 single repl.mov

Na and K in H2O single repl.mov

Double Replacement Reaction

Double Replacement Reactions

The positive ions of two compounds exchange places in an aqueous solution to form two new compounds.

R+S- + T+U- R+U- + T+S- There is typically one of three outcomes: One of the compounds formed is usually a precipitate, an insoluble gas that bubbles out of solution, or a molecular compound, usually water. Let’s look at those choices a little closer…

Double Replacement Reactions- Three alternatives

1. One product is only slightly or not soluble, resulting in the formation of a solid precipitate.

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

2. One product is gas that bubbles out of the mixture. 2 NaCN(aq) + H2SO4 (aq) → 2 HCN (g) + Na2SO4(aq) (you better hope this gas doesn’t bubble out of the container!)

3. One product is a molecular compound such as water. Ca(OH)2(aq) + 2HCl(aq) → CaCl2 (aq) + 2H2O(l)

Ok, try this… If these are the products of a double replacement

reaction, what are the reactants and the overall balanced equation? Remember, in a double replacement reaction, the cations switch places.

1. → AgBr (s) + KNO3 (aq)

AgNO3 (aq) + KBr (aq) → AgBr (s) + KNO3 (aq)

2. → BaSO4 (s) + 2NaCl (aq)

BaCl2(aq) + Na2SO4(aq) → BaSO4 (s) + 2NaCl (aq) double replacement.mov

Combustion Reactions A substance combines with oxygen, releasing a

large amount of energy in the form of light and heat.

Typically involves hydrocarbons (H,C compounds).

The complete combustion of hydrocarbons releases a lot of energy as heat. So hydrocarbons such as methane (CH4), propane (C3H8) and butane (C4H10) and octane (C8H18) are important fuels.

The burning of natural gas, wood, gasoline C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g)

Other combustion reactions

Other reactive elements also combine with oxygen P4(s) + 5O2(g) P4O10(s)

(This is also a synthesis or combination rxn.) 2 Mg (s) + O2 (g) → 2MgO (s) S (s) + O2 (g) → SO2(g) Let’s try balancing the equation for the combustion

of a benzene ring: C6H6(l) + O2(g) → CO2(g) + H2O(g) 2C6H6(l) + 15O2(g) → 12CO2(g) + 6H2O(g)

Anyone know what this is?

Combustion of Fe and O2

http://www.youtube.com/watch?v=5MDH92VxPEQ

The combustion reaction of iron and oxygen forms iron oxide as follows:

Fe(s) + O2 (g) → Fe2O3 (s)

If you were to weigh the steel wool before you burned it and after you burned it, what do you think you would find?

combustion rxn.mov

Summary/Review – Combination Reactions

Combination or synthesis reactions General equation: R + S → RS Reactants: Generally two elements or two

compounds, where at least one compound is a molecular/covalent compound.

Probable products: A single compound

Summary/Review – Decomposition Reactions

Decomposition reactions General equation RS → R + S Reactants: Generally a single binary (2 parts)

or ternary (3 parts). Probable products: Two elements for a

binary compound (or three total for a ternary compound, a mixture of elements or compounds).

Summary/Review – Single Replacement Reactions

Single replacement reactions General equation T + RS → TS + R Reactants: An element and a compound. In a single

replacement reaction, an element replaces another element from a compound in aqueous solution. For a single-replacement reaction to occur, the element that is displaced must be less active than the element that is doing the displacing (taking its place).

Probable products: A different element and a new compound.

Summary/Review – Double Replacement Reactions

Double replacement reactions General equation R+S- + T+U- R+U- + T+S- Reactants: Two ionic compounds. In a double

replacement reaction, two ionic compounds react by exchanging cations to form two different compounds.

Probable products: Two new compounds. Double-replacement reactions are driven by the formation of a precipitate, a gaseous product, or water.

Summary/Review – Combustion Reactions

Combustion reactions General equation: Cx Hy + (x + y/4) O2 → xCO2 + (y/2) H2O Reactants: Oxygen and a compound of C,H

and sometimes O. Probable products: CO2 and H2O. With

incomplete combustion, C and CO may also be products.

Section 8.3 – Reactions in aqueous solution Objective : to use solubility rules to predict the precipitate formed in

double replacement reactions. If you have two ionic compounds dissolved in water, you might write

the reaction like this: AgNO3 (aq) + NaCl (aq) → AgCl (s) + NaNO3 (aq) But in reality, since the ionic compounds dissociate in water, it’s more

like this: Ag+ (aq) + NO3

- (aq) + Na+ (aq) + Cl-(aq) →

AgCl (s) + Na+ (aq) + NO3- (aq)

But if we cancel out the spectator ions that appear on both sides: (the orange and yellow ones)

Ag+ (aq) + NO3- (aq) + Na+ (aq) + Cl-(aq)→

AgCl (s) + Na+ (aq) + NO3- (aq)

Then it just becomes Ag+ (aq) + Cl-(aq) → AgCl (s) This is the net ionic reaction. Note it must be balanced.

Example problem 8-10 Identify the spectator ions and write a balanced net ionic

equation for this reaction: HCl (aq) + ZnS (aq) → H2 S (g) + ZnCl2(aq) Write the complete ionic equation H+ (aq) + Cl- (aq) + Zn2+ (aq) + S2- (aq) → H2S (g) + Zn2+ (aq) + Cl- (aq) Note how the gas isn’t in aqueous solution. The zinc and chlorine are spectator ions The balanced net ionic equation is H+ (aq) + S2- (aq) → H2S (g)

Try this… Write a balanced net ionic equation for the following:

Pb(NO3)2(aq) + H2SO4(aq) → PbSO4(s) + 2HNO3(aq)

Pb2+ + 2 NO3- + 2H+ + SO4

2- → PbSO4(s) + 2H+ + 2 NO3-

Pb2+ + 2 NO3- + 2H+ + SO4

2- → PbSO4(s) + 2H+ + 2 NO3-

Pb2+ (aq) + SO42- (aq) → PbSO4(s)

(I left the (aq) designation off the two equations in the middle because they wouldn’t fit on one line if I had it in there, but for completeness, the (aq)’s should be in there.)

8.3 Reactions in aqueous solution How do you predict if a reaction will form a

precipitate? Mixing solutions of two ionic compounds can

sometimes form an insoluble salt called a precipitate (it will look like a solid in the aqueous solution).

We can predict this using the general rules for solubility on the next slide.

So if you put BaSO4 in an aqueous solution, will it dissolve?How about Ba(NO3)2 ?

Solubility Rules for Ionic CompoundsCompounds Solubility Exceptions

1. Salts of alkali metals and ammonia

Soluble Some lithium compounds

2. Nitrate and chlorate salts Soluble Few exceptions

3. Sulfate salts Soluble Compounds of Pb, Ag, Hg, Ba, Sr and Ca

4. Chloride salts Soluble Compounds of Ag, and some Hg and Pb compounds

5. Carbonates, phosphates, chromates, sulfides, hydroxides

Most are Insoluble

Compounds of the alkali metals and ammonia

Let’s try a few problems using the table on the last slide

H2SO4 + BaCl2 → 2HCl + BaSO4

Al2(SO4)3 + NH4OH → 3(NH4)2SO4 + Al(OH)3

AgNO3 + H2S → Ag2S + 2HNO3

CaCl2 + Pb(NO3)2 →Ca(NO3)2 + PbCl2

Ca(NO3)2 + Na2CO3 →2NaNO3 + CaCO3

Identify the precipitate formed when solutions of these ionic compounds are mixed:

BaSO4 (rule 3)

Al(OH)3 (rule 5)

Ag2S (rule 5)

PbCl2 (rule 4)

CaCO3 (rule 5)