Transcript chemical reaction

Chapter 11
What is a chemical reaction?
• A chemical reaction describes a change
in composition.
• In a chemical reaction, the original
substances are the reactants, and the
substances that are created are called the
products.
• Observations that suggests that a
chemical reaction is taking place are the
production of :
• Energy (heat, light, or sound)
• Gas
• Precipitate
• Change in color
• Not all reactions are chemical.
• A physical change can occur rather than a
chemical one.
• For example, water turning into ice.
Reactions involve rearrangements
of atoms.
• Reactants are converted into products.
• The law of conservation of mass states
that mass cannot be created nor
destroyed.
• This means that the number of atoms in
the product must be equal to the original
reactants.
• Atoms do not become other kinds of
atoms, nor do they appear or disappear.
Concept check
• List four observations that signify a
chemical change.
• What happens to atoms during a chemical
reaction?
Chemical reactions release or
absorb energy
• Endothermic reaction- reaction in which
energy is absorbed.
- Bond formation always requires energy.
• Exothermic reaction- reaction in which
energy is released.
– Bond breaking always releases energy.
• Spontaneous reaction- when a reaction is
said to occur naturally or unaided.
For example a forest fire.
Spontaneous reactions can be
endothermic or exothermic.
• Particles must collide for a chemical
reaction to occur.
• Example, when a safety match is lit, the
reaction begins when the two substances
are brought together by striking the match
head across the striking surface.
• If this collision happens with enough
energy, the bonds in the reactants are
broken, allowing new bonds to form
between the atoms.
Types of Reactions
Types of Chemical Reactions
1)
2)
3)
4)
Synthesis (composition reaction)
Decomposition
Single replacement reactions
Double replacement reactions
Synthesis Reactions
• Two or more substances combine to form
a new compound
A +
X  AX
Where A and X can be elements or
compounds. AX is a compound.
Ex. A thin strip of magnesium metal is
placed in an open flame, it burns with
bright white light. When the metal strip is
completely burned, only a fine white
powder of magnesium oxide is left.
2 Mg (s) + O2 (g)  2 MgO (s)
• Some metals, such as iron, combine with
oxygen to produce two different oxides
2 Fe (s) + O2 (g)  2 FeO (s)
4 Fe (s) + 3 O2 (g)  2 Fe2O3 (s)
The first reaction is Fe +2
The second reaction is Fe +3
Decomposition Reactions
• Decomposition is a single compound undergoes
a reaction that produces two or more simpler
substances
AX  A + X
AX is a compound. A and X can be elements or
compounds
Most take place only when heat/electricity is added
Decomposition of binary
compounds
• Simplest type of decomposition
• Binary compound is decomposed into its
elements
electricity
2 H2O (l)  2 H2 (g) + O2 (g)
Decomposition of a substance by an electric
current is called electrolysis
Decomposition of Metal
Carbonates
• EX. When a metal carbonate is heated, it
breaks down to produce a metal oxide and
carbon dioxide gas
Δ
CaCO3 (s)  CaO (s) + CO2
Δ means heated
Single Replacement Reactions
• Single replacement reactions – one
element replaces a similar element in a
compound
A + BX  AX + B
Or
Y + BX  BY + X
A, B, X, and Y are elements. AX, BX, and
BY are compounds
Replacement of a Metal in a
Compound by Another metal
• A more active metal will replace a least active
metal.
2 Al (s) + 3 Pb(NO3) (aq) 
Looking at the activity series
Al is more reactive than Pb so Al WILL replace Pb
to form:
3 Pb (s) + 2 Al(NO3)3 (aq)
2 Al (s)+ 3 Pb(NO3) (aq) 3 Pb (s) + 2 Al(NO3)3 (aq)
Double Replacement Reactions
• The ions of two compounds exchange
places in an aqueous solution to form two
new compounds.
• One of the compounds formed is usually a
precipitate, an insoluble gas that bubbles
out of solution or a molecular compounds,
usually water.
• The other compound is often soluble and
remains dissolved in solution
Double Replacement Reaction
AX + BY  AY + BX
A, X, B, and Y in the reactants represent
ions. AY and BX represent ionic or
molecular compounds
Formation of a Precipitate
• The formation of a precipitate occurs when
the cations of one reactant combine with the
anions of another reactant to form an
insoluble or slightly soluble compound.
2 KI (aq) + Pb(NO3)2 (aq) PbI2 (s) + 2 KNO3 (aq)
Verify the products with the solubility sheet
Formation of a gas
• In some double replacement reactions,
one of the products is an insoluble gas
that bubbles out of the mixture
FeS (s) + 2 HCl  H2S (g) + FeCl2 (aq)
Formation of Water
• In some double replacement reactions, a
very stable molecular compound, such as
water, is one of the products
HCl (aq) + NaOH (aq) NaCl (aq) + H2O(l)
Balancing Equations
Balancing requires patience
• To satisfy the law of conservation of mass,
you should insert coefficients into the
chemical equation.
• This ensures that there are equal
numbers of atoms for each element on
each side of the equation when an
equation is balanced.
Testing an equation for balance
Reactants
Products
Balance?
Unbalanced
Formula equation CH3CH2OH +O2
CO2 + H2O
Carbon atoms
2
1
No
Hydrogen atoms
6
2
No
Oxygen atoms
3
3
Yes
• To balance this equation, there are two
carbon atoms in the reactant column and
only one in the product column.
• To balance the number of carbon atoms
you need to double the number of carbon
dioxide molecules in the products.
CH3CH2OH + ? O2
2CO2
+
? H2O
The 2 is called a coefficient and indicates that
there are two carbon dioxide molecules.
Coefficients are normal whole numbers, when a
coefficient is equal to 1, the 1 is not written for
simplicity.
Coefficients are written in front of the formula
and multiply the entire formula.
Testing an equation for balance
Reactants
Products
Balance?
Unbalanced
Formula equation CH3CH2OH +O2
2CO2 + H2O
Carbon atoms
2
2
Yes
Hydrogen atoms
6
2
No
Oxygen atoms
3
5
No
• The carbon atoms are now balanced by
the hydrogen atoms are not. In addition
the oxygen atoms have been thrown out of
balance.
• Let’s start with hydrogen, there are six in
the reactant column, so we must add three
to the product column.
Testing an equation for balance
Reactants
Products
Balance?
Unbalanced
Formula equation CH3CH2OH +O2
2CO2 + 3H2O
Carbon atoms
2
2
Yes
Hydrogen atoms
6
6
Yes
Oxygen atoms
3
7
No
• The oxygen atoms are still unbalance.
• Multiplying the number of oxygen
molecules by three results in an equal
number of oxygen atoms.
Testing an equation for balance
Reactants
Products
Balance?
Unbalanced
Formula equation CH3CH2OH + 3O2
2CO2 + 3H2O
Carbon atoms
2
2
Yes
Hydrogen atoms
6
6
Yes
Oxygen atoms
7
7
Yes
• Now you have written a balanced chemical
equation.
• Replacing the question marks in the
formula equation with the right coefficients
now give a correct chemical equation.
Tips for balancing equations
• 1. Save the elements that appear in several
reactants or products. (usually oxygen and
hydrogen )
• 2. If the same polyatomic ion appears on
both sides, treat them as single units.
• 3. After considering the first two steps,
balance the equation from left to right.
• 4. For ionic equations, be sure that charges are
balanced.
Testing an equation for balance
Reactants
Products
Balance?
Unbalanced
Formula equation
NH3 + O2
NO + H2O
Nitrogen atoms
1
1
Yes
Oxygen atoms
2
2
Yes
Hydrogen atoms
3
2
No
Testing an equation for balance
Reactants
Products
Unbalanced
Formula equation
Nitrogen atoms
Oxygen atoms
Hydrogen atoms
NH3 + O2
NO + H2O
Balance?
Testing an equation for balance
Reactants
Products
Balance?
Unbalanced
Formula equation
4NH3 + 5O2
Nitrogen atoms
4
Oxygen atoms
Hydrogen atoms
4NO + 6H2O
4
Yes
10
10
Yes
12
12
Yes
9-2 cont..
How are chemical equations for
reactions written?
• A chemical equation-describes the type
and number of atoms that are rearranged
during a reaction.
• A word equation describes a reaction.
ethanol + oxygen
carbon dioxide + water
The arrow means “react to form” and the plus
sign means “and”
• A word equation can then be replaced with
formulas to make a formula equation.
• The word ethanol can be replaced with its
formula
CH3CH2OH (l)
• The other three words in the equation are
also replaced with the corresponding
formula and symbol.
Word equation:
ethanol + oxygen
carbon dioxide + water
Formula equation:
?CH3CH2OH (l) + ? O2 (g)
?CO2 (g) +? H2O(l)
Question marks are used to signify that the number
of molecules for each is not yet known.
Numbers in Formulas
Coefficients
show
Amounts of reactants
and products in an
Equation
Ex: 3NaOH
Subscripts
show
Number of
each type of
atom in a formula
Ex: H2O
Superscripts
show
Amount
and type
of charge
on an ion.
Ex: K+
Sample problem p. 315
Sometimes it is useful to write the
equation in the form of a sentence.
The first step in the commercial production
of nitric acid is the combustion of ammonia
in the presence of a catalyst. The
exothermic reaction produces nitrogen
monoxide and water vapor. Write and
balance the equation for this reaction.
• Step one: Write the word equation
ammonia + oxygen
nitrogen monoxide + water
• Step two : Write the unbalanced formula
equation.
? NH3 + ?O2
?NO + ?H2O
• Step 3: Count the number of atoms for
each element and Balance