Download File - LSAmockscience

Chemical Reactions and
Equations
Chapter 7
• http://www.youtube.com/watch?feature=pl
ayer_embedded&v=dUMmoPdwBy4
What is a Chemical Reaction?
• One or more substances are converted to
one or more new substances
Why do Chemical Reaction
Occur?
• Chemical stability
• Atoms attempt to obtain a full set of
valence electrons (octet rule)
Parts of a Chemical Reaction
• Reactant(s)
– Starting substance(s)
– Left side of equation
• Product(s)
– Substance(s) produced
– Right side of equation
– Example
2Mg +
Reactants
O2

2MgO
Yield
Product
What are Chemical Equations?
• Representations of chemical reactions as
words or formulas
• 2 Types of chemical equations
– Word Equations – use the chemical names in
the equation
– Formula Equations – use the formulas in the
equation
Law of Conservation of Matter
In a chemical reaction:
The mass of Reactants always =
the mass of the Products.
Law of Conservation of mass = mass is
neither created or destroyed in a chemical
reaction.
Why balance chemical
equations
• Law of Conservation of Mass
– Mass can not be created or destroyed
• In a balanced chemical equation
– The number of atoms of each element on the
reactant side must equal the number of atoms
of each element on the product side
Parts of a Chemical Equation
Yield
Coefficient
2H2(g) + O2(g)  2H2O(g)
Reactants
Subscript
Products
State of Matter
How are atoms symbolized?
Cl2
2Cl
2Cl2
• Molecules may also have brackets to
indicate numbers of atoms. Ex: Ca(OH)2
• Notice that the OH is a group
O Ca O
H
• The 2 refers to both H and O H
• How many of each atom are in the following?
Na = 1, O = 1, H = 1
a) NaOH
b) Ca(OH)2 Ca = 1, O = 2, H = 2
c) 3Ca(OH)2 Ca = 3, O = 6, H = 6
Balancing equations:
• The law of conservation of mass states that
mass can neither be created or destroyed
• So, atoms are neither created or destroyed,
only rearranged in a chemical reaction
• So, the number of a particular atom is the
same on both sides of a chemical equation
• Example: Magnesium + Oxygen
• Mg + O2  MgO Mg + O O  Mg O
• However, this is not balanced
• Left:
Mg = 1, O = 2
• Right: Mg = 1, O = 1
To Balance Equations:
1. Count the number of atoms of each
element on both sides of the equation.
2. Place coefficients in front of the proper
elements or compounds in order to get
the same number of atoms on both sides
of the equation.
3. Make sure the coefficients are in the
lowest whole-number ratio.
4. Check your work by counting the atoms
of each element.
Mg + O2  MgO becomes _____________________
Examples:
Mg + HCl  MgCl2 + H2
Mg = 1
H=1
Cl = 1
Mg = 1
H=2
Cl = 2
Examples
Mg + 2HCl  MgCl2 + H2
Mg = 1
H=1=2
Cl = 1 = 2
Mg = 1
H=2
Cl = 2
Balance these equations:
a)
b)
c)
d)
e)
f)
g)
h)
Ca + N2  Ca3N2
NH4NO3  N2O + H2O
BiCl3 + H2S  Bi2S3 + HCl
C4H10 + O2  CO2 + H2O
O2 + C6H12O6  CO2 + H2O
NO2 + H2O  HNO3 + NO
Cr2(SO4)3+ NaOH  Cr(OH)3+ Na2SO4
Al4C3 + H2O  CH4 + Al(OH)3
Balance these equations:
a)
b)
c)
d)
e)
f)
g)
h)
3Ca + N2  Ca3N2
NH4NO3  N2O + 2H2O
2BiCl3 + 3H2S  Bi2S3 + 6HCl
2C4H10 + 13O2  8CO2 + 10H2O
6O2 + C6H12O6  6CO2 + 6H2O
3NO2 + H2O  2HNO3 + NO
Cr2(SO4)3+ 6NaOH  2Cr(OH)3+ 3Na2SO4
Al4C3 + 12H2O  3CH4 + 4Al(OH)3
Writing Formula Equations from
Word Equations
Things to remember:
There are elements that naturally form
diatomic molecules and come in pairs:
Br2 I2 N2 Cl2 H2 O2 F2
You must write the correct formula for
the word formula!!!!!
Write the formula equation for the reaction in which
magnesium reacts with nitrogen to produce
magnesium nitride
1. Write the word equation:
magnesium + nitrogen  magnesium nitride
2. Determine if any of the elements are diatomic:
H2, O2, N2, Cl2, Br2, I2, F2
3. Determine the formulas:
Mg2+ and N3-
so magnesium nitride is Mg3N2
4. Write the formula equation:
Mg + N2  Mg3N2
5.
Balance the equation:
3Mg + N2  Mg3N2
Counting with Moles
• Chemical reactions involve large numbers
of very small particles – so chemist use a
measuring unit called the mole to measure
amounts.
• One mole contains 6.02 x 1023 particles of
a substance. (Avogadro’s number)
Avogadro’s Number
• http://www.youtube.com/watch?feature=pl
ayer_embedded&v=TEl4jeETVmg
Molar Mass
• The mass of one mole of a substance is its
molar mass.
• The molar mass of an element = its atomic
mass expressed in grams.
• Examples:
Ca =
Fe =
C=
Zn =
• The mass of one mole of a substance is its
molar mass.
• The molar mass of an element = its atomic
mass expressed in grams.
• Examples:
Ca = 41.08
Fe = 55.847
C = 12.001
Zn = 65.38
Molar Mass
• To find the molar mass of a compound or
molecule:
1. Multiply the atomic mass of each atom x its
subscript
2. Add all of these together for one compound
or molecule
• Example:
H2O =
H = 1.0x2 = 2.0
O = 15.9x1=15.9
The sum = 17.9g/mole
Classifying Chemical Reactions
• There are 5 general types of reactions:
– Synthesis
– Decomposition
– Single Replacement
– Double Replacement
– Combustion
Synthesis reaction
• When two or more substances combine to
form a single product
For example: A + B  AB
2H2 + O2  2H2O
2 or more simpler substances  one complex
substance
Decomposition - opposite of
synthesis
• When one substance breaks down into 2
or more other substances.
AB  A + B
2H2O  2H2 + O2
One compound 2 or more simpler substances
Single Replacement reaction
• When one element replaces another element in
a compound
A + BC  AC + B
Element + compound  new element + new compound
Zn + CuSO4  ZnSO4 + Cu
• Metals usually replace metals and non-metals
usually replace non-metals.
An element can replace any element that is below
it in the list in a single replacement reaction.
Double Replacement
• When atoms or ions of two different compounds
replace each other.
AB + CD  AD + CB
2 compounds 2 new compounds
• Most double displacement reactions will not
occur unless the reactants are dissolved in water
so that they can become ions.
• Not all combinations of reactants will undergo a
reaction.
Combustion Reaction
• A substance reacts rapidly with oxygen
CH4 + 2O2

CO2 + 2H2O
Energy Changes in Reactions
• The breaking of chemical bonds requires
energy
• The forming of chemical bonds releases
energy
• During a chemical reaction energy is either
released or absorbed.
Energy Changes in Reactions
• During a chemical reaction energy is either
released or absorbed.
• Exothermic Reactions – release energy
into the surroundings
• Endothermic Reactions – absorb energy
from the surroundings
Energy Changes in Reactions
• Exothermic Reactions – the energy
released as bonds form in the products is
greater than the energy required to break
the bonds in the reactants.
• Endothermic Reactions – more energy is
required to break the bonds in the
reactants than is released by bonds
forming in the products.
Reaction Rates
• http://www.youtube.com/watch?feature=pl
ayer_embedded&v=OttRV5ykP7A
• Temperature
• Surface Area
• Stirring
• Concentration
• Catalysts