BIOO211
Biochemistry for Complementary Therapists
Session 2
Chemical Reactions
Department of Bioscience
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Chemistry and Biochemistry
– Chemical Reactions
This session includes:
1.
2.
3.
4.
What is a chemical reaction?
Law of conservation of mass
Moles and formula masses (weights)
Types of chemical reactions
Combination; decomposition, combustion; reversible;
“exchange reactions” (anabolism and catabolism);
Redox – oxidation and reduction defined
Role of oxidation and reduction
Introduction to redox in biochemistry (eg NAD+/NADH)
Energy transfer and redox
Equilibrium reactions,
6.
Le Châtelier's principle
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Physical and Chemical Change
In a physical change,
o the identity and
composition of the
substance do not change
In a chemical change,
o new substances form
with different
compositions and
properties.
o A chemical reaction takes
place.
A physical change
involves a change
in energy but no
new substance is
produced e.g.
melting and
evaporation of
water H2O
(Timberlake, 2010)
Figure 7.1 A chemical change
produces new substances such
as the black substance on silver,
Silver sulfide, Ag2S (Timberlake,
2016, p. 269)
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What is a Chemical Reaction?
o In a chemical reaction, a
chemical change produces
one or more new substances.
o During a reaction, old bonds
are broken and new bonds
are formed.
 In a chemical reaction,
atoms in the reactants are
rearranged to form one or
more different substances.
o Metabolism is all the chemical
reactions in the body
In a chemical change, the iron on the
surface of nails, reacts with oxygen to
form rust (Timberlake, 2016, p. 269)
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Chemical Equations
A chemical equation
 Gives the chemical formulas of the reactants on the
left of the arrow and the products on the right.
Reactants
C (s) + O2 (g) 
Product
CO2 (g)
O2 (g)
CO2 (g)
C(s)
Writing a Chemical
equation (Timberlake,
2016 p. 270)
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Symbols Used in Equations
Symbols are used
in chemical
equations to show
 The states of the
reactants, s, l or g
 The states of the
products, s, l or g
 The reaction
conditions, aq, Δ
(Timberlake, 2016, p. 270)
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Chemical Equations Are Balanced
o In a balanced
chemical reaction,
• Atoms are not gained
or lost;
• The number of atoms
in the reactants is
equal to the number
of atoms in the
products.
• Called Law of
Conservation of
Mass
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A Balanced Chemical Equation
In a balanced chemical equation,
 There must be the same number of each type of
atom on the reactant side and on the product side.
 Numbers called coefficients may be used in front
of one or more formulas to balance atoms
Al +
S
2Al + 3S
Al2S3
Not Balanced
Al2S3
Balanced
2Al
=
2Al
3S
=
3S
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Periodic Table and the Mole
Molar mass of an element is found by
looking at its atomic mass on the periodic
table (Timberlake, 2016, p. 287)
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Mole
A mole is a collection that contains
 the same number of particles as there are carbon
atoms in 12.0 g of carbon 126C isotope
 6.02 x 1023 atoms of any element (Avogadro’s number).
1 mole of element
Number of Atoms
1 mole C
= 6.02 x 1023 C atoms
1 mole Na
= 6.02 x 1023 Na atoms
1 mole Au
= 6.02 x 1023 Au atoms
 A mole of a compound has Avogadro’s number of
molecules.
1 mole CO2 = 6.02 x 1023 CO2 molecules
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Subscripts Give Number Of Atoms
And Moles
1 mole C9H8O4 = 9 moles C and 8 moles H and 4 moles O
1.5 mol C9H8O4 = 1.5x 9 mol C & 1.5 x8 mol H & 1.5x 4 mol O
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Subscripts And Moles
The subscripts in a formula give:
 the relationship of atoms in the formula;
 the moles of each element in 1 mole of compound.
Glucose
C6H12O6
In 1 molecule: 6 atoms C 12 atoms H 6 atoms O
In 1 mole:
6 moles C 12 moles H 6 moles O
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Molecules To Moles – Scaling Up
4 NH3 + 5 O2
4 NO + 6 H2O
Four molecules of NH3 react with five molecules of O2 to
produce four molecules of NO and six molecules of H2O.
or
Four moles of NH3 react with 5 moles of O2 to produce
four moles of NO and six moles of H2O.
o The numbers (atoms or moles) of substance reacting
together is known as the stoichiometry
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Molar Mass Of A Compound
o Molar mass also called the formula weight
o For a compound, the molar mass is the sum of the
molar masses of the elements in the formula. The
molar mass of CaCl2 is calculated as follows:
Elements in 1 mol
Number of
moles
Atomic mass (from
periodic table)
Total mass
1 mol Ca atoms
1
40.1 g/mol
40.1 g
2 mol Cl atoms
2
35.5 g/mol
71.0 g
CaCl2 (compound)
111.1 g
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Types of Chemical Reactions
Chemical reactions are classified into general
types:
1. Combination (synthesis)
2. Decomposition
3. Replacement – single and double
4. Combustion
5. Reversible (equilibrium)
6. Exchange reactions – anabolism and
catabolism
7. Redox
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Some Types of Chemical Reactions
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Combination
In a combination reaction,
 Two or more elements (or simple compounds)
combine to form one product
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2Mg(s) + O2(g)
2MgO(s)
2 mol Magnesium + oxygen gives 2 mol magnesium oxide
2Na(s) + Cl2(g)
2NaCl(s)
2 mol Sodium + chlorine
SO3(g) + H2O(l)
gives 2 mol sodium chloride
H2SO4(aq)
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Decomposition
In a decomposition reaction,
 One substance splits into two or more simpler
substances.
2HgO(s)
2Hg(l) + O2(g)
2KClO3(s)
2KCl(s) + 3O2(g)
2SO3 (g)
2SO2 (g)+ O2 (g)
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Exchange or “Replacement”
Reactions
o Substances exchange atoms or groups of atoms
• consist of both synthesis and decomposition reactions
o Example
• HCl + NaHCO3 → H2CO3 + NaCl
• ions have been exchanged between substances
• Double replacement since two ions have been
exchanged, Cl- and HCO3 -
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Single Replacement
In a single replacement reaction,
 One element takes the place of a different element in a
reacting compound.
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Zn(s) + 2HCl(aq)
ZnCl2(aq) + H2(g)
Fe(s) + CuSO4(aq)
FeSO4(aq) + Cu(s)
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Double Replacement
In a double replacement,
 Two elements in the reactants exchange places.
AgNO3(aq) + NaCl(aq)
AgCl(s) + NaNO3(aq)
ZnS(s)
ZnCl2(aq) + H2S(g)
+ 2HCl(aq)
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Combustion Reaction
In a combustion reaction, a carbon-containing
compound that is the fuel burns in oxygen from the air
to produce carbon dioxide (CO2), water (H2O), and
energy in the form of heat or a flame.
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Redox - Oxidation and Reduction
An oxidation-reduction reaction
 Provides us with energy from food.
 Provides electrical energy in batteries.
 Occurs when iron rusts to iron (III) oxide
4Fe(s) + 3O2(g)
2Fe2O3(s)
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REDOX - Electron Loss and Gain
An oxidation-reduction reaction (redox)
 transfers electrons from one reactant to another.
A Loss of Electrons is Oxidation
Zn(s)
Zn2+(aq) + 2e-
(LEO)
A Gain of Electrons is Reduction
Cu2+(aq) + 2eCu(s)
(GER)
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Oxidation and Reduction Characteristics
“LEHO the lion
says GEHR”
(loss of electrons or
hydrogens is
oxidation;
Gain of electrons or
hydrogens is
reduction)
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Oxidation–Reduction in Biological
Systems
o In biological
systems, oxidation
may involve
• the loss of H or
• the gain of O.
o In biological
systems, reduction
may involve
• the gain of H or
• the loss of O.
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Antioxidants and Disease
Role of Free radicals
o Free radical atom has an unpaired
electron in its outmost shell
o Unstable and highly reactive
o Can become stable
• by giving up electron
• taking one off another molecule
(breaking apart important body
molecules)
o Free radicals attack important
macromolecules (DNA, protein,
phospholipid) leading to cell damage and
homeostatic disruption
o Damage may be slowed by antioxidants
o Linked to many diseases -- cancer,
diabetes, Alzheimer’s, atherosclerosis and
arthritis
©John Wiley and Sons. Inc.
Fig 2.3 What substances in the body can
inactivate oxygen-derived free radicals?
(Tortora & Derrickson, 2014, p. 31
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Concept Map Chemical
Reactions & Quantities
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Forms of Energy and Chemical
Reactions
o Chemical reactions involve energy changes
o Two principal forms of energy
• potential energy = stored energy
• kinetic energy = energy of motion
o Chemical energy is potential energy stored in the bond
of molecules
• digestion of food releases that chemical energy
o Law of conservation of energy
• energy can neither be created nor destroyed, just
converted from one form to another
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Heat of Reaction
in Chemical Reactions
o Forming new bonds
releases energy &
breaking old bonds
requires energy
o Chemical reactions
usually involve both
• exergonic - release
energy
• endergonic - absorb
energy
o Human metabolism
couples exergonic and
endergonic reactions
(Timberlake,
2013, p. 243)
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Redox reaction and Metabolism
Metabolism involves:
o Catabolic reactions
that break down large,
complex molecules to
provide energy and
smaller molecules
(oxidation).
o Anabolic reactions
that use ATP energy
to build larger
molecules.
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Redox reactins in Biochemistry
o Biological oxidation involves the loss of (electrons) and
hydrogen atoms
• coenzymes transfer hydrogen atoms to another
compound
• common coenzymes of living cells that carry H atoms
– NAD (nicotinamide adenine dinucleotide )
– NADP (nicotinamide adenine dinucleotide
phosphate )
– FAD (flavin adenine dinucleotide )
o Biological reduction is the addition of electrons (&
hydrogen atoms) to a molecule
• increase in potential energy of the molecule
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Reversible Reactions
o Chemical reactions can be reversible.
• Reactants can become products or products can revert to the original
reactants
o Indicates that the reaction is able to reach an equilibrium
o Indicated by the 2 arrows pointing in opposite directions between the
reactants and the products
o Suppose SO2 and O2 are present initially. As they collide, the forward
reaction begins.
2SO2(g) + O2(g)
2SO3 (g)
o As SO3 molecules form, they also collide in the reverse reaction that reforms the reactants. The reversible reaction is written with a double arrow.
forward
2SO2(g) + O2 (g)
2SO3(g)
reverse
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Chemical Equilibrium
• Before equilibrium is
reached, concentration of
reactants, [reactants],
decrease.
• As products build up, they
begin to collide and react.
• When equilibrium is
reached
• There is no further change
in the amounts of reactants
and products.
After some time, the two reactions occur at equal but opposite
rates but there is no further change in the concentration of
reactants or products at this equilibrium point (Timberlake, 2013,
p. 347)
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Le Châtelier’s Principle
o For a system at equilibrium, a change in the amounts of
reactants or products or the temperature causes stress.
o Le Châtelier’s principle states that the equilibrium will
“shift to relieve the stress”.
o That means that the rate of the forward and reverse
reaction will change until they are in equilibrium again.
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Changing Equilibrium Conditions
According to Le Châtelier's principle, equilibrium will
change if:
o concentration of product or reactant changes (for all
reactions);
o volume (pressure) changes for some reactions;
o temperature changes .
The effect of a catalyst will speed up the forward and
reverse reactions EQUALLY but will not change the
position of equilibrium
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Stress = Adding Reactant or
Product on equilibrium
Consider the following reaction at equilibrium:
Add H2
H2(g) + I2(g)
2HI(g)
If more reactant (H2 or I2) is added,
• the rate of the forward reaction increases to form more product until
the system is again at equilibrium.
• the equilibrium shifts toward the products.
Consider the following reaction at equilibrium:
Add HI
H2(g) + I2(g)
2HI(g)
If more product (HI) is added,
• the rate of the reverse reaction increases to form more H2 and I2
reactants.
• the equilibrium shifts toward the reactants.
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Effect of removing reactant or
product on equilibrium
Consider the following reaction at equilibrium:
Remove H2
H2(g) + I2(g)
2HI(g)
If some of a reactant (H2 or I2) is removed,
• the rate of the reverse reaction increases to form more reactant until
the equilibrium is reached and the equilibrium shifts toward the
reactants.
Consider the following reaction at equilibrium:
Remove HI
H2(g) + I2(g)
2HI(g)
When some of the product (HI) is removed,
• there is an decrease in collisions of HI molecules.
• the rate of the forward reaction increases and forms more product(HI).
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Effect of Decreasing or Increasing
the Volume
When a reaction at equilibrium contains different numbers
of moles of reactants than products,
A decrease in volume
o increases the concentration (mol/L) thus upsetting the equilibrium;
o shifts the equilibrium towards the fewer number of moles.
N2(g) + 3H2(g)
2NH3(g)
More moles
Decrease volume
Fewer moles
An increase in volume:
o decreases the concentration (mol/L) thus upsetting the
equilibrium;shifts the equilibrium towards the greater number of
Increase volume
moles.
N2(g) + 3H2(g)
2NH3(g)
More moles
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Heat and Endothermic Reactions
For an endothermic reaction at equilibrium,
o A decrease in temperature removes heat and
the equilibrium shifts towards the reactants.
o An increase in temperature adds heat and the
equilibrium shifts towards the products.
Decrease T
CaCO3 (s) + 133 kcal
CaO(s) + CO2(g)
Increase T
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Heat and Exothermic Reactions
For an exothermic reaction at equilibrium,
o A decrease in temperature removes heat and the
equilibrium shifts towards the products.
o An increase in temperature adds heat and the
equilibrium shifts towards the reactants.
Decrease T
N2(g) + 3H2(g)
2NH3(g) + 22 kcal
Increase T
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Summary of Changes on
Equilibrium
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Post-Session 2 Summary
/ Revision Questions
•
•
•
•
•
•
•
•
•
•
Describe the difference between a chemical and a physical change.
Define a chemical reaction and explain the symbols used
Define the components of a chemical equation (reactants, products, catalyst,
energy change)
Define exergonic (& exothermic) and endergonic (& endothermic).
Describe and classify chemical equations using five types: combination
versus decomposition, single replacement, double replacement, combustion.
Given a chemical reaction, demonstrate that the chemical equation is
balanced.
Describe the significance of a balanced equation.
Explain the concept of a mole of substance.
Recognise oxidation and reduction reactions and classify chemical reactions
Explain the principles and significance of redox (oxidation and reduction
reactions).
State two definitions of the term oxidized. State two definitions of the term
reduced. Given chemical reaction(s), identify oxidised and reduced
substances in the chemical equation(s) e.g. NAD + and NADH
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References & Diagrams
o Timberlake, KC 2016, General, organic, and biological
chemistry, 5th edn, Pearson, Boston.
o Timberlake, KC 2013, General, organic, and biological
chemistry, 4th edn, Pearson Benjamin Cummings,
Boston.
o Timberlake, KC 2010, General, organic and biological
chemistry, 3rd edn, Pearson Benjamin Cummings
o Timberlake, KC 2007, General, organic and biological
chemistry, 2nd edn, Pearson Benjamin Cummings
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