10/7/2011
Chapter 8
Chemical Reactions
Chemical Reactions
For every chemical change which occurs in
nature, it must be supported by a chemical
equation:
Reactants → Products
Let’s see the observations which could be in
support of a chemical change.
Chemical Change
Evidence
There are 5 common evidences which a chemical
change may occur in a chemical reaction:
1.
2.
3.
4.
5.
Color Change
Formation of a solid
Formation of a gas
Absorb or release of heat energy
Emission of light energy
1
10/7/2011
Color Change
Figure 8.1-Color change as evidence of a chemical reaction. When a solution containing
hydroxide ion is added to crystal violet dye, the intensity of the color decreases with
time until it disappears. The product of the reaction is colorless in water solution.
Precipitation Formation
Figure 8.2-Formation of a solid as evidence
of a chemical reaction. When two clear,
colorless solution are combined, one
containing barium ion and the other
containing sulfate ion, solid barium sulfate is
formed. The white solid will eventually
settle to the bottom of the test tube.
Formation of a gas
Figure 8.3-Formation of a gas as evidence
of a chemical reaction. Alka-Seltzer
tablets contain citric acid and sodium
hydrogen carbonate. When added to
water, these two reactants combine to
form carbon dioxide gas as one of the
products.
2
10/7/2011
Absorb/Release of Heat
Figure 8.4-Evolution of heat and light as
evidence of a chemical reaction. This
photograph shows aluminum reaction with
iron (III) oxide to produce molten iron and
aluminum oxide. The quantity of heat
evolved from this chemical change is large
enough to cause the iron produced to be in
the liquid (molten) state.
Emission of Light
Figure 8.5-Emission of light as evidence of a
chemical reaction. The light emitted from a
light stick is a form of energy released in a
multistep chemical change that involves
several reactants. One of the reactants is
stored in a fragile glass vial inside of the
larger plastic tube. When you bend the
plastic tube, you break the glass vial, mixing
the reactants.
Heat Transfer
Figure 8.6-Heat transfer as evidence of a
physical change. When heat is absorbed by
a system, as in this instant cold compress,
your skin feels cold because you are the
source of heat needed to drive the change.
3
10/7/2011
Evidence of a Chemical Reaction
When performing a chemical reaction, it is
important to know what took place and how
do we write the chemical equation for the
process.
Let’s look at an example!!!
Example
When solid sodium is added to liquid water, a
reaction occurs, producing hydrogen gas,
sodium hydroxide solution, and heat:
Chemical Equations
Symbols
Chemists have developed a standardized shorthand
method for describing chemical reactions, such as the
reaction of sodium and water, in writing.
State Symbols and Their Meanings
Symbol
Meaning
(s)
solid
(l)
liquid
(g)
gas
(aq)
aqueous (dissolved in
water)
4
10/7/2011
Chemical Equation
Final Result
Solid sodium
Na(s)
plus
+
liquid water
H2O(l)
yields
→
hydrogen gas
H2(g)
plus
+
sodium hydroxide solution
NaOH(aq)
Chemical Equations
Do’s and Don’ts
DO: Balance the equation entirely by using
coefficients placed before the different chemical
formulas.
DON’T: Change a correct chemical formula in
order to make an element balance.
DON’T: Add some real or imaginary chemical
species to either side of the equation just to
make an element balance.
Writing a Balancing Chemical Equation
1. Write a qualitative description of the
reaction. In this step, you write the formulas
of reactants and products.
2. Quantify the description by balancing the
equation. Do this by adding coefficients. Do
not change the qualitative description of the
reaction.
5
10/7/2011
Balancing Chemical Equations
PROCEDURE (Step):
1. Place a “1” in front of the formula with the
largest number of atoms. If two formulas
have the same number of atoms, select
the one with the greater number of
elements. We will call this formula the
starting formula in the discussion.
Na(s) + H2O(l) → H2(g) + 1 NaOH(aq)
Balancing Chemical Equations
2.
Insert coefficients that balance the elements that
appear in compounds. Use fractional coefficients, if
necessary. Do not balance element-only formulas,
such as Na or O2, at this time. We call these
uncombined elements. Choosing elements in the
following order is usually easiest:
a) Elements in the starting formula that are in only one
other compound.
b) All other elements from the starting formula.
c) All other elements in compounds.
Na(s) + 1 H2O(l) → H2(g) + 1 NaOH(aq)
Balancing Chemical Equations
3. Place coefficients in front of formulas of
uncombined elements that balance those
elements. Use fractional coefficients, if
necessary.
1 Na(s) + 1 H2O(l) → 1/2 H2(g) + 1 NaOH(aq)
6
10/7/2011
Balancing Chemical Equations
4. Clear fractions, multiply everything by a
common integer.
2 Na(s) + 2 H2O(l) → 1 H2(g) + 2 NaOH(aq)
Balancing Chemical Equations
5. Check your work.
2 Na(s) + 2 H2O(l) → 1 H2(g) + 2 NaOH(aq)
2 Na atoms
4 H atoms
2 O atoms
2 Na atoms
4 H atoms
2 O atoms
Interpreting Chemical Reactions
The particulate-level interpretation of a
chemical equation:
2 H2(g) + O2(g) → 2 H2O(g)
Two molecules of hydrogen react with one
oxygen molecule to form two water molecules.
7
10/7/2011
Illustration
The grouping-unit-level interpretation of a
chemical equation
(grouping unit = dozen, mole, etc.):
Writing and Balancing a
Chemical Reaction
1. Classify the reaction type.
2. Write a qualitative description of the reaction. In this
step you write the formulas of the given reactants to
the left of an arrow and the formulas of the given or
predicted products to the right.
3. Quantify the description by balancing the equation.
Do this by adding coefficients. Do not change the
qualitative description of the reaction by adding,
removing, or altering any formula.
Combination
It is nicknamed a synthesis reaction because it
takes two or more substances (elements
and/or compounds) to yield 1 overall product.
8
10/7/2011
Example
Combination Reactions
Solid calcium reacts with gaseous fluorine to net
a precipitate.
REACTION:
Ca (s) + F2 (g) → ???
The charge of Ca is 2+ and the charge of F is 1-. The final
product is CaF2.
Ca (s) + F2 (g) → CaF2 (s)
Example
Combination Reactions
Solid sodium reacting with oxygen gas to yield solid
sodium oxide.
REACTION:
Na (s) + O2 (g) → ???
The charge of Na is 1+ and the charge of O is 2-. The final
product is Na2O.
Na (s) + O2 (g) → Na2O (s)
BALANCED: 4Na (s) + O2 (g) → 2Na2O (s)
Practice Problems
Predict the expected product and balance the
chemical equation.
a) Al (s) + Cl2 (g) → ???
b) solid potassium mixes with sulfur (S)
c) solid barium mixes with solid phosphorus
(P)
9
10/7/2011
Summary
Combination Reaction
Reactants:
Any combination of elements
and/or compounds
Reaction type: Combination
Equation type: A + X → AX
Products:
One compound
Decomposition
One substance breaks down to yield two or
more elements and/or compounds.
Example
Decomposition Reactions
Solid calcium carbonate decomposes to yield a gas and a
solid metal oxide.
REACTION:
CaCO3 (s) → ???
Carbonates yield carbon dioxide and to determine the metal oxide,
we know Ca has a charge of 2+ as an ion and O has a charge of 2ion.
CaCO3 (s) → CaO (s) + CO2 (g)
10
10/7/2011
Example
Decomposition Reactions
Soild iron (III) chloride decomposes to a solid and a gas.
REACTION:
FeCl3 (s) → ???
The element is Fe (for the metal) and the gas formed is
molecular chlorine. In a chemical equation, chlorine is written
as Cl2.
FeCl3 (s) → Fe (s) + Cl2 (g)
BALANCED: 2FeCl3 (s) → 2Fe (s) + 3Cl2 (g)
Practice Problems
Predict the expected products and balance the
chemical equation.
∆
a) MgCO3 →
∆
b) AlN →
∆
c) NaCl →
Summary
Decomposition Reaction
Reactants:
One compound
Reaction type: Decomposition
Equation type: AX → A + X
Products:
Any combination of elements
and compounds
11
10/7/2011
Single Replacement Reactions
An ion exchange between compound with an
element to form another element and
compound.
Example
Single Replacement Reactions
Solid potassium dissolves in an aqueous
solution of copper (II) nitrate.
REACTION:
2K (s) + Cu(NO3)2 → Cu (s) + 2KNO3 (aq)
Example
Single Replacement Reactions
Chlorine gas mixes with solid sodium iodide.
REACTION:
Cl2 (g) + 2NaI (s) → I2 (s) + 2NaCl (s)
12
10/7/2011
Practice Problems
Predict the expected products and balance each
equation.
a) MgCl2 (aq) + Al (s) →
b) Solid calcium dissolves in aqueous
copper(II) chloride
c) Solid lithium dissolves in hydrochloric acid
ILLUSTRATION
Single Replacement Reactions
Figure 8.15-Reaction of sodium with water.
Practice Problems
Write the expected products for the following
reactants.
a) Solid magnesium dissolves in liquid water
b) Solid aluminum dissolves in nitric acid
13
10/7/2011
Summary
Single Replacement Reaction
Reactants:
Element (A) plus a solution of
either an acid or an ionic
compound (BX)
Reaction type:
Single-replacement
Equation type:
A + BX → AX + B
Products:
An ionic compound (usually in
solution) (AX) plus an element (B)
Double Replacement Reactions
Ions exchange between compounds to form
two new compounds.
“Precipitation”
Double Replacement Reactions
Figure 8.18
The precipitation of silver
chloride.
“Precipitation” involves the formation of a solid.
EXAMPLE:
NaCl (aq) + AgNO3 (aq) → NaNO3 (aq) + AgCl (s)
14
10/7/2011
“Neutralization”
Double Replacement Reactions
“Neutralization” involves the formation of water.
A neutralization reaction involves an acid (releases H+ ions)
and when reacted with a base (releases OH- ions) and
always produces water and a salt.
EXAMPLE:
General Equation:
HX (aq) + MOH (aq) → H2O (l) + MX (aq)
2HCl (aq) + Ca(OH)2 → 2H2O (l) + CaCl2 (aq)
Practice Problem
Write the equation for the reaction between nitric acid and
aqueous barium hydroxide.
STEP 1:
Write the initial reactants.
HNO3 (aq) + Ba(OH)2 (aq)
STEP 2:
Predict the expected products.
HOH (l) + Ba(NO3)2 (aq)
STEP 3:
Write the unbalanced chemical equation.
HNO3 (aq) + Ba(OH)2 (aq) → HOH (l) + Ba(NO3)2 (aq)
Practice Problem
Write the equation for the reaction between nitric acid and aqueous
barium hydroxide.
STEP 4:
Count the number of ions on both sides.
HNO3 (aq) + Ba(OH)2 (aq) → HOH (l) + Ba(NO3)2 (aq)
1 H+ ion
1 NO3- ion
STEP 5:
1 Ba2+ ion
2 OH- ions
1 H+ ion
1 Ba2+ ion
1 OH- ion 2 NO3- ions
Provide coefficients where applicable (ignore writing 1 if it’s
a coefficient) and show the final balanced, chemical
equation.
2HNO3 (aq) + Ba(OH)2 (aq) → 2HOH (l) + Ba(NO3)2 (aq)
2 H+ ions
2 NO3- ions
1 Ba2+ ion
2 OH- ions
2 H+ ions
1 Ba2+ ion
2 OH- ions 2 NO3- ions
All of the coefficients again MUST be in whole number form!!!
15
10/7/2011
Additional Problems
Predict the products formed from the
following initial reactants.
a) An aqueous solution of potassium
carbonate mixes with a solution of lead(II)
acetate
b) A solution of diluted ammonium hydroxide
mixes with sulfuric acid
Summary
Double Replacement Reaction
Reactants:
Solutions of two compounds,
each with positive and negative
ions (AX + BY)
Reaction type:
Double-replacement
Equation type:
AX + BY → AY + BX
Products:
Two new compounds (AY + BX),
which may be a solid, water, an
acid, or an aqueous ionic
compound
SUMMARY
16