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Chemical Reactions
Chapter 7
Chap 7 Pretest
• Which of the following is an example of a
physical change?
•
•
•
•
A.
B.
C.
D.
Wood burns and becomes ash.
A steel nail rusts over time.
Ice melts and becomes water.
Milk curdles when acid is added to it.
Chap 7 Pretest
• Which of the following characteristics can you
determine about a substance based on its
chemical formula?
• A. The number and types of atoms that make
up a substance.
• B. The mass of an unknown sample of the
substance.
• C. The melting point of the substance.
• D. The density and state of the substance at
room temperature.
Chap 7 Pretest
• How do you find the atomic mass of an
element?
• The atomic mass of each element is listed
on the periodic table.
Chap 7 Pretest
• Which conversion factor would you
multiply 0.020 m by in order to express the
quantity in centimeters?
•
•
•
•
A.
B.
C.
D.
1000 m/1 km
1 km/1000 m
1 m/ 100 cm
100 cm/1 m
Equation Analogy
• Imagine that you work at a skateboard shop and you are
in charge of assembling the skateboards. Every
skateboard requires one deck (the board), two trucks
(the mounted axels), and four wheels.
• Your boss asks you to make five skateboards. How
many trucks do you need?
• The following diagram shows the “recipe” for one
skateboard. What do you notice about the relative
amounts of each part on either side of the arrow?
+
+
1D
+ 2T
+
4W
D 1T 2W 4
One deck plus two trucks plus four wheels makes one skate board.
7.1 – Describing Reactions
• Chemical equations are used to describe
reactions.
– Reactants
Products
subscript
– C + O2
CO2
– The coefficients show the molar ratios of the reactants
and products, and they can be changed in order to
coefficient
balance the equation.
– 2Cu + O2
2CuO
• Chemical equations are balanced to show that
mass is conserved in the chemical reaction.
– Only the coefficients, not the subscripts, should
change when balancing chemical equations.
7.1 – Balancing Chemical Equations
• Step 1 – count the number of atoms of each
element on each side of the equation.
• Step 2 – change one or more coefficients until
the equation is balanced.
– Start by balancing an element that appears in only one
reactant and product.
– Once one element is balanced, proceed to balance
another, and another, until all elements are balanced.
– Balance chemical formulas by placing coefficients in front
of them. Do not add subscripts, because this will change
the formulas.
– Coefficients are multipliers, so if we write 2 H2O it denotes
2x2=4 hydrogen atoms and 2x1=2 oxygen atoms.
7.1 – Describing Reactions
• Chemists use the mole, which is an amount
containing 6.02 x 1023 particles, to count large
numbers of small particles.
– Also known as Avogadro’s number.
• The mass of one mole of a substance is called a
molar mass.
– The molar mass of an element is the same as its
atomic mass expressed in grams.
– A compound’s molar mass is the sum of the atomic
masses of its component elements expressed in
grams.
7.1 – Describing Reactions
• Molar mass is used to convert between
molar and mass amounts of one
substance.
• A mole ratio is used to convert between
molar amounts of two different substances
in a chemical reaction.
Models of Reactions
• The following drawings represent reactants and products
of three different chemical reactions.
Reaction A
reactant
products
reactants
product
reactants
products
Reaction B
Reaction C
Models of Reactions
• 1. Synthesis means “putting something
together”. Which drawing represents a
synthesis reaction?
– Reaction B
• 2. Decomposition means “taking something
apart”. Which drawing represents a
decomposition reaction?
– Reaction A
• 3. Replacement means “something taking the
place of the other”. Which drawing represents a
replacement reaction?
– Reaction C
7.2 – Types of Reactions
• Synthesis reaction – two or more substances
react to form a single substance.
– “putting something together”
– A+B
AB
– 2Na + Cl2
2NaCl
• Decomposition reaction – a single substance
reacts to form two or more substances.
–
–
–
–
“taking something apart”
AB
A+B
2H2O
2H2 + O2
reverse of synthesis reaction
7.2 – Types of Reactions
• Single replacement reaction – one element
takes the place of another element in a
compound.
– A + BC
B + AC
– Cu + 2AgNO3
2Ag + Cu(NO3)2
• Double replacement reaction – two different
compounds exchange positive ions and form
two new compounds.
– AB + CD
AD + CB
– CaCO3 + 2HCl
CaCl2 + H2CO3
7.2 – Types of Reactions
• Combustion reaction – a substance reacts
rapidly with oxygen, often generating light and
heat.
– CH4 + 2O2
– 2H2 + O2
CO2 + 2H2O
2H2O
• Oxidation-reduction (redox) reaction – any
reaction that involves a transfer of electrons.
– 2Ca + O2
• Ca
• O + 2e
2CaO
2+
Ca + 2e
O2
7.3 – Energy Changes in Reactions
• During a chemical reaction, chemical bonds in
the reactants are broken and chemical bonds in
the products are formed.
– Breaking bonds requires energy.
– Forming bonds releases energy.
• A chemical reaction can be classified as:
– Exothermic (releases energy to the surroundings)
– Endothermic (absorbs energy from the surroundings)
• Energy is conserved during chemical reactions.
– Total amount of energy before and after the reaction
is the same.
7.4 – Reaction Rates
• Reaction rate – the time-rate at which
reactants change into products.
• Reaction rates are generally increased by
the following:
– Rise in temperature
– Increased surface area
– Concentration of reactants, agitation, and
catalysts
7.5 – Equilibrium
• Equilibrium – when there is no change in the
concentrations of reactants and products, and
reactants and products are forming
simultaneously.
– Can only take place when the forward and reverse
reactions occur at the same rate.
– Products
Reactants
• Equilibrium can also by physical in nature.
– Example: liquid water and water vapor inside a
closed container.
• Factors affecting chemical equilibrium include:
– Shifts in temperature, pressure, and concentration.