session 2 chemical reactions department of bioscience · pdf filesession 2 chemical reactions...

BIOO211Biochemistry for Complementary Therapists

Session 2

Chemical Reactions

Department of Bioscience

endeavour.edu.au

© Endeavour College of Natural Health www.endeavour.edu.au 2

Chemistry and Biochemistry

– Chemical Reactions

This session includes:

1. What is a chemical reaction?

2. Law of conservation of mass

3. Moles and formula masses (weights)

4. 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


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.

Figure 7.1 A chemical change

produces new substances such

as the black substance on silver,

Silver sulfide, Ag2S (Timberlake,

2016, p. 269)

A physical change

involves a change

in energy but no

new substance is

produced e.g.

melting and

evaporation of

water H2O

(Timberlake, 2010)

Physical and Chemical Change


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)

What is a Chemical Reaction?

© Endeavour College of Natural Health endeavour.edu.au 5

Chemical EquationsA chemical equation

Gives the chemical formulas of the reactants on the

left of the arrow and the products on the right.

Reactants Product

C(s)

CO2 (g)

O2 (g)

C (s) + O2 (g) CO2 (g)

Writing a Chemical

equation (Timberlake,

2016 p. 270)


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

Symbols Used in Equations

(Timberlake, 2016, p. 270)


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


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 Al2S3 Not Balanced

2Al + 3S Al2S3 Balanced

2Al = 2Al

3S = 3S


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)


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

Mole


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


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


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


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


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


Some Types of Chemical Reactions

©2016 Pearson Education, Inc.


Combination

In a combination reaction,

Two or more elements (or simple compounds)

combine to form one product

© ©2013 Pearson Education, Inc

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 gives 2 mol sodium chloride

SO3(g) + H2O(l) H2SO4(aq)


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)


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-


Single Replacement

In a single replacement reaction,

One element takes the place of a different element in a reacting compound.

© 2013 Pearson Education, Inc.

Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g)

Fe(s) + CuSO4(aq) FeSO4(aq) + Cu(s)


Double Replacement

In a double replacement,

Two elements in the reactants exchange places.

AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq)

ZnS(s) + 2HCl(aq) ZnCl2(aq) + H2S(g)


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.


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)


An oxidation-reduction reaction (redox)

transfers electrons from one reactant to another.

A Loss of Electrons is Oxidation (LEO)

Zn(s) Zn2+(aq) + 2e-

A Gain of Electrons is Reduction (GER)

Cu2+(aq) + 2e- Cu(s)

REDOX - Electron Loss and Gain


Oxidation and Reduction -

Characteristics

“LEHO the lion

says GEHR”

(loss of electrons or

hydrogens is

oxidation;

Gain of electrons or

hydrogens is

reduction)


Oxidation–Reduction in Biological

Systemso 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.


Antioxidants and Disease

Role of Free radicalso 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


Concept Map Chemical

Reactions & Quantities

©2016 Pearson Education, Inc.


Forms of Energy and Chemical

Reactionso 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


Heat of Reaction

in Chemical Reactionso 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)


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.

Redox reaction and Metabolism


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


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 re-

forms the reactants. The reversible reaction is written with a double arrow. forward

2SO2(g) + O2 (g) 2SO3(g)reverse


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)


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.


Changing Equilibrium Conditions

According to Le Châtelier's principle, equilibrium willchange 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


Stress = Adding Reactant or

Product on equilibriumConsider the following reaction at equilibrium:

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:

H2(g) + I2(g) 2HI(g)

If more product (HI) is added,

• the rate of the reverse reaction increases to form more H2 and I2reactants.

• the equilibrium shifts toward the reactants.

Add HI

Add H2


Effect of removing reactant or

product on equilibriumConsider the following reaction at equilibrium:

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:

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).

Remove H2

Remove HI


Effect of Decreasing or Increasing

the VolumeWhen 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)

An increase in volume:

o decreases the concentration (mol/L) thus upsetting the

equilibrium;shifts the equilibrium towards the greater number of

moles.N2(g) + 3H2(g) 2NH3(g)

More molesFewer moles

Increase volume

More moles Fewer moles

Decrease volume


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.

CaCO3 (s) + 133 kcal CaO(s) + CO2(g)

Heat and Endothermic Reactions

Decrease T

Increase T


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.

N2(g) + 3H2(g) 2NH3(g) + 22 kcal

Decrease T

Increase T


Summary of Changes on

Equilibrium


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


References & Diagramso 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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