Chemistry
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N'vida E. Houndonougbo
Chapter 5:Chemical accounting:
Learning Objectives
I. Identify balanced and unbalanced chemical equations, and
balance equations by inspection. (5.1)
II. Write balanced equations for chemical processes.
III. Calculate the mass or number of moles of a reactant or
product from the mass or number of moles of another reactant
or product. (5.4)
IV. Define complete and incomplete combustion reactions.
V. Discuss the by products of combustion reaction and learn
about how carbon monoxide produced by incomplete reaction
can affect human health.
N'vida E. Houndonougbo
Chapter 5: Chemical accounting:
Learning Objectives
VI. Discuss the reduction of carbon monoxide in the atmosphere
using catalytic converters in engines.
VII. Define catalysts and how they speed up a chemical reaction
by decreasing the energy activation.
VIII. Calculate the concentration (Molarity, percent by volume,
percent by mass) of a solute in a solution. (5.5)
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Chapter 5: Chemical accounting
Suggested Assignment: Chapter 5
• Page 159: Review questions : 4-6
• Page 159: problems: 11-14; 15a,b; 16
• Page 160: Problems: 35-37; 39-46
• Page 363: problems: 5-12
N'vida E. Houndonougbo
Chapter 5: Chemical Accounting
Outline I. Chemical reactions
II. Balancing chemical equations
III. Moles and Equation Coefficients
IV. Mole–Mole Relationships in chemical reaction
V. Stoichiometry
Mole and Mass Relationships in Chemical Equations
VI. Solutions, solution concentrations, units of concentration
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Chemical changes/Chemical reaction
In the course of a chemical change, the reacting substances are converted to new substances. Chemical equations communicate a chemical change using symbols and formulas to represent the elements and compounds involved in a chemical reaction.
.
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Chemical Change/Chemical reaction
The substances which are present before the change occurs are called the reactants.
The substances which are present after the change occurs are called products.
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Chemical reaction equation
2 H2(g) + O2 (g) → 2 H2O (l)
The numbers placed in front of the chemical formulas are
called coefficients.
The arrow (→) means “yield(s)” or “react(s) to produce.
The following are used to denote the state of a species in an
equation:
(s) = solid (l) =liquid (g)=gas (aq)=aqueous solution
.
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Chemical reactions
• Seven nonmetals occur naturally as diatomic molecules: 1. Hydrogen (H2)
2. Nitrogen (N2)
3. Oxygen (O2)
4. Halogen (F2)
5. Halogen (Cl2)
6. Halogen (Br2)
7. Halogen (I2)
• These elements are written as diatomic molecules when
they appear in chemical reactions.
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Chemical reactions
Combustion reaction:
Burning in air is usually referred to as a combustion reaction.
– Hydrocarbons (compounds containing only carbons and hydrogens) such as gasoline and natural gas are typical fuels which undergo a combustion reaction.
– In the following combustion reaction, propane (C3H8 ) reacts with oxygen (O2 ) to produce Carbon dioxide (CO2),
water (H2O) and heat.
C3H8 + _O2 _CO2 + _ H2O + heat
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Chemical reactions
Early 1700s Lavoisier:
Law of conservation of mass:
During a chemical change matter is
neither created nor destroyed.
atoms neither appear or disappear, only
the connections between them change.
The number of atoms of each type in the
products must equal the number of atoms
of each type in the reactants
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Chemical Equations
Law of conservation of mass:
During a chemical reaction, atoms neither appear or disappear,
only the connections between them change.
This means the number of atoms of each type in the products
must equal the number of atoms of each type in the
reactants.
Is the law of conservation of mass satisfied in the following
combustion reaction? Discuss in class
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Balancing Chemical Equations Practice: Balance the following reaction
Hint: A polyatomic ion such as NO3
- appearing on both sides of an equation
can be treated as a single unit.
Answer: Discuss in class
12
FeBr3(s) + AgNO3(aq) → AgBr(s) + Fe(NO3)3(aq)
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Guidelines for Balancing Equations
Never change the subscripts in a chemical formula to balance a chemical equation.
Balance each element in the equation starting with the most complex formula.
Balance polyatomic ions as a single unit if it appears on both sides of the equation.
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Stoichiometry
Stoichiometry: the quantitative relationship between
reactants and products in a balanced chemical equation.
The coefficients of a balanced chemical equation represent
moles
• Chemists and chemical engineers must perform
calculations based on balanced chemical reactions to
predict the cost of processes.
• The calculations used to avoid using large, excess
amounts of costly chemicals are called stoichiometry
calculations.
• Example in class
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Mole and Mass Relationships in
Chemical Equations
Steps in a Stoichiometric Calculation:
1. Write and balance the chemical equation for the
reaction.
2. Determine the molar masses of the substances
involved in the calculation.
3. Use the coefficients of the balanced equation to
convert moles of the given substance to moles of
the desired substance.
4. Use the molar mass to convert moles of the
desired substance to grams of the desired
substance.
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Mole and Mass Relationships in
Chemical Equations
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Stoichiometry
Practice in class
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Complete and Incomplete Combustion
Complete combustion produces CO2 whereas incomplete
combustion produces product other than CO2
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Complete and Incomplete Combustion
Both reactions proceed simultaneously, competing
for the same reactants, the minor product produced
is called a by-product of the main reaction.
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Complete and Incomplete Combustion
Soot (an impure form of carbon) is a common
by-product of combustion and produced by incomplete
Combustion.
2 C4H10 + 5 O2 8 C + 10 H2O
Black soot: is often found in chimneys and on other solid surfaces close to the region of a flame undergoing incomplete combustion.
It also accounts for the black smoke emitted by diesel-powered engines.
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Complete and Incomplete Combustion
Indoor and outdoor Air Pollution
CO can be lethal at a high
concentration.
Concentrations of gases such CO
can be expressed as parts per
million (ppm).
The scale parts per million (ppm)
is used when reporting very small
concentrations of substances.
Example: Two part per million (2
ppm) of CO in air means that there is
two CO molecule in every one
million molecules of air.
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Complete and Incomplete Combustion
CO can be lethal at a high
concentration because it
interferes with O2 transport
from your Lungs to the cells in
your body.
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Complete and Incomplete Combustion
Indoor and outdoor Air Pollution:
Carbon monoxide can be produced by:
-Malfunctioning gas or oil furnaces
-Automobile engines
-Fireplaces
-Charcoal grills
-Kerosene heaters
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Complete and Incomplete Combustion
In engines Carbon monoxide can be destroyed by passing it
along with extra O2 through a catalytic converter.
A catalytic converter operates at a high temperature and converts
CO to CO2.
pt
2 CO + O2 2 CO2
A substance that can speed up
a reaction without itself being
consumed is called a catalyst.
In a chemical reaction equation
a catalyst is written above the arrow.
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Chemical reactions
How does a catalyst speed up the rate of a chemical reaction?
In most chemical reactions, one or more covalent bonds
must be broken and energy is required for this to
happen.
The minimum energy required for a reaction to take
place is called Activation energy (Ea).
Presence of catalysts speed up the reactions by lowering
the activation energy of the reaction.
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Chemical reactions
One way a catalyst can affect the rate of a reaction is to provide a surface on which reactants can meet
therefore increases the number of effective collisions
.
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Chemistry in water
Solution: homogeneous mixture composed of a solute
dissolved in a solvent
• Solute: A substance dissolved in a liquid.
• Solvent: The liquid in which a substance is dissolved.
• The term aqueous solution describes a uniform
mixture of a substance (solute) in water (solvent)
Example: table salt (solute ) dissolves in water(solvent)
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Solution concentrations
The amount of solute in a given amount of solvent is defined as solution
concentration.
A dilute solution contains relatively small amounts of solute in a given
amount of solvent.
A concentrated solution contains relatively large amounts of solute in a
given amount of solvent.
These concentration roughly estimate the amount of solute in the
solution. They give a qualitative changes in concentration.
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Units of Concentration
Scientific work generally require more precise concentration units. Quantitative work require the mole unit because substances enter into chemical reactions according to molar ratios.
The molar concentration, or molarity (M) is the most commonly used unit of concentration.
Molarity (M), is the number of moles of solute per liter of solution, and is expressed as moles/liter.
Memorize!!!!!
Moles of solute (mol)
Molarity (M or mol/L) = -----------------------
Volume of solution (Liter)
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Units of Concentration
Practice: in class
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Units of Concentration Percent Concentration:
For many practical applications such medicine and pharmacy
Solution concentrations are often expressed in percentages.
Memorize!!!
Percent by volume = × 100
volume of solute
volume of solution
Example:
Rubbing alcohol is a
water-isopropyl alcohol
solution that is 70 %
isopropyl alcohol by
volume:
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Units of Concentration
Percent Concentration by mass: many commercial
solutions are labeled with the concentration in percent
by mass.
Percent by mass = × 100 mass of solute
mass of solution
Example: Reagent grade
hydrochloric acid is sold as a 38%
mass solution of HCl in water.