Chapter 10

Chemical Reactions

Reactions and Equations 10.1 A chemical reaction is when the atoms of one or

more substances are rearranged to form different substances

How to tell if a chemical reaction has taken place– Is there evidence? (Temperature change, color change,

odor, gas bubbles, general appearance) Equations are used to represent chemical

reactions– Show reactants: the starting substances – Show Products: resulting or ending substances– Do not show numerical values so arrow is used instead

of an equals sign

Types of Equations

Three types of equations– Word– Skeleton– Chemical

Word equations– Used to indicate the reactants and products of

chemical reactions– Uses the actual names of the substances

involved– Setup: Reactant 1 + Reactant 2 Product 1 Ex: Iron(s) + Chlorine(g) Iron(III)

Chloride(s)

Skeleton Equations– Uses chemical formulas to represent the substances

unlike the word equation– Chemical formulas take the place of the words– Iron(s) + Chlorine(g) Iron(III) Chloride(s) would

become: Fe(s) + Cl2(g) Fe Cl3(S)

Chemical Equations– Uses chemical formulas like the skeleton equation– Shows matter is conserved during a reaction (Law of

Conservation of Mass)– Chemical eqation: Uses identities and relative amounts

of the substances involved in a chemical reaction– Everything is balanced in a chemical equation

Balancing Chemical Equations– Finding the correct coefficients for chemical formulas in

the skeleton equation is very important Coefficient is usually a whole number and 1 is understood

Steps to Balance a Chemical Equation

1. Write the skeleton equation2. Count the atoms of elements in the reactants3. Count the atoms of elements in the products4. Place or change the coefficients to make both

sides of the equation equal (DO NOT CHANGE SUBSCRIPTS)

5. Write the coefficient(s) in the lowest ratio possible

6. Check your work– Ex:

H2(g) + O2(g) H2O(g) Balanced: 2H2 + O2(g) 2H2O(g)

Practice website www.chem.vt.edu/RVGS/ACT/notes/scripts/bal_eq1.html

Examples Hydrogen gas and bromine gas form gaseous

hydrogen bromide– H2 + Br2 HBr– Balanced: H2 + Br2 2HBr

Balancing hints:– When odd numbers of oxygen needed use a fraction,

then multiply by two Al + O2 Al2O3

2Al + O2 Al2O3 need three oxygens (odd number) 2Al + 1.5O2 Al2O3 fractions/ decimals cannot be left as

coefficients 4Al + 3O2 2Al2O3 Whole equation multiplied by two to get

rid of decimals or fractions– Try the following:

C2H6 + O2 CO2 + H2O

Examples/ hints cont. When balancing with polyatomic ions found

both in the reactants and products, replace the ions by a variable.– Fe2O3 + H2SO4 Fe2 (SO4)3+ H2O– Fe2O3 + H2X Fe2 (X)3+ H2O– Fe2O3 + 3H2X Fe2 (X)3+ 3H2O– Fe2O3 + 3H2SO4 Fe2 (SO4)3+ 3H2O

Balancing with elements that appear more than twice in different formulas (leave for last!)– CO2 + H2 CH4 + H2O hydrogen appears more than

twice– CO2 + H2 CH4 + 2H2O balance C and O– CO2 + 4H2 CH4 + 2H2O

Classifying Chemical Reactions10.2

There are five types of chemical reactions.1. Synthesis2. Combustion3. Decomposition4. Single-Replacement5. Double-Replacement

1. Synthesis Reaction– Chemical reaction in which two or more

substances combine to form one single product.

– A + B AB Ex: 2Na(s) + Cl2(g) 2NaCl(s) 4Fe(s) + 3O2(g) 2Fe2O3 (s)

2. Combustion Reaction– Oxygen combined with a substance releasing energy in

the form of light and heat – Many combustion reactions are also synthesis reactions

Ex: 2H2(g) + O2 2H2O(g)3. Decomposition reactions

– Only one compound– Breaks down into 2 or more compounds or elements– Setup: AB A + B– Often requires an energy source (i.e. heat, light,

electricity) Ex: NH4NO3(s) N2O(g) + 2H2O(g) The above reaction occurs when the reactant is heated

4. Replacement Reactions– Unlike the other reactions, replacement reactions

involve the replacement of an element in a a compound– 2 kinds of replacement reactions:

Single Double

5. Single Replacement reactions – When atoms of one element replace the atoms

of another element in a compound– Setup: A+ BX AX + B– Whether or not a metal will replace another

metal or any other substance depends on the reactivity it has

6. Double Replacement reactions– Invovles the exchange of ions between 2

compounds– Setup: AX + BY AY + BX– If a solid is produced during the reaction it is

called a precipitate

Practice – Classify the following equations

1. Cl2 + 2KBr KCl + Br2 2. 2Ag2O 4Ag + O2

3. 2Na + Cl2 2NaCl4. PbCl2 + Li2SO4 PbSO4 + 2LiCl

Reactions in Aqueous Solutions 10.3Vocab

Aqueous solution- Solution in which the solvent is water

Solvent- The water in the solution Solute- The compound dissolved in the water Precipitate- Solid formed in a chemical reaction Spectator ion- Ions that do not participate Complete ionic equation- Shows all particles as

they really exist Net ionic equation- Only includes particles that

participate (No spectator ions)

Aqueous Solutions When two aqueous

solutions react it is always a double replacement reaction.

The water doesn’t react

There are three possible products when aqueous solutions react, they are; precipitates, water, or gases.

Spectator ions do not participate in the reaction and are usually not included in the ionic equation.

Writing Ionic Equations Ionic and chemical equations differ in that

substances that are ions in the solution are written as ions in the equation.

To write an ionic equation you must show the reactants and the product as ions.

A complete ionic equation shows all of the particles in the solution as they realistically exist.

A net ionic equation is a simplified version of the complete ionic equation. It only shows particles that participate in the reaction.

To write a net ionic equation from a complete ionic equation cross out all spectators.

Ionic equations AgNO3(aq) + NaCl(aq) AgCl(s) +

NaNO3(aq) What type of reaction is this? A complete ionic equation shows

dissolved ionic compounds as their free ions.

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

Ionic Equations Ag+(aq) + NO3-(aq) + Na+(aq)

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

Ions that are not directly involved in a reaction are called spectator ions.

Rewriting the equation, leaving out the spectator ions, gives us the net ionic equation…

Ag+(aq) + Cl-(aq) AgCl(s)

Ionic Equations You also have to make sure that the

net charge is balanced. For example…

Pb(s) + 2Cl-(aq) PbCl2(s)

Examples HCl(aq) + ZnS(aq) H2S(g) +

ZnCl2(aq) Cl2(g) + NaBr(aq) Br2(l) + NaCl(aq)