chemical reactions - … · chemical equations represent chemical reactions chemical ... on the...

Chemical Reactions

BIOB111

CHEMISTRY & BIOCHEMISTRY

Session 3

Key concepts: session 3From this session you are expected to develop an understanding of the following concepts:

Concept 1: Chemical reactions

Concept 2: Bond breakage/formation in chemical reactions

Concept 3: Interpreting chemical reactions: coefficients and subscripts

Concept 4: Redox reactions

Concept 5: Avogadro’s number

Concept 6: Usefulness of the mole

Concept 7: Using the mole unit

These concepts are covered in the Conceptual multiple choice questions of tutorial 3

Session OverviewPart 1: Chemical equations

• Chemical vs Physical change

• Chemical equations represent chemical reactions

• Balancing chemical equations

Part 2: Redox reactions

• Redox reactions

Part 3: The mole

• What is the mole?

• Using the mole

Part 1: Chemical equations

• Chemical vs Physical change

• Chemical equations represent chemical reactions


• Physical change:– Matter can be changed from one state to another e.g. from solid into liquid

– Hallmarks of physical change:

• No new substances are created

• The type of matter present before the physical change is the same as the matter present after physical change

Changing the state of water via physical

change

Chemical vs Physical change

• Chemical change:– Matter can be changed from one substance into another

substance via a chemical reaction

– Hallmarks of chemical change:• One or more new substances are created

– The molecular structure of the original substance(s) changes in the chemical reaction forming one or more new substances

• The type of matter present before the chemical change is different from the matter present after the chemical change

Changing original substance(s) (reactants) to

different substance(s) (products) via chemical

change

Chemical reaction:

Chemical vs Physical change

Chemical equations represent chemical reactions

• In a chemical reaction, 1 or more original substances (REACTANTS) are converted to 1 or more new substances (PRODUCTS)

• A chemical reaction is usually written in a chemical equation

Chemical reaction:

Reactant 1 Reactant 2 Product 1Product 2

Chemical equation: Is a diagram of a chemical reaction

– Reactants are shown on the left & products on the right

– The arrow shows the progress of the chemical reaction

Progression through a chemical reaction:

– Start- step 1: The chemical bonds holding the reactants together are broken• Chemical bond breakage requires an energy input

• Example below: Chemical bonds broken within methane and oxygen

– Middle- step 2: Once the chemical bonds have been broken, the atoms rearrange to adopt new positions

• Example below: Carbon, hydrogen and oxygen atoms rearrange

– End- step 3: After atom rearrangement, new chemical bonds form between the atoms creating the products

• Chemical bond formation releases energy

Example below: New chemical bonds are formed to produce carbon dioxide and water


Chemical reaction:

Reactant 1 Reactant 2 Product 1Product 2

+ +H

H

HH

H H

H H

C C

OO

O OO O

O

O

Reactant 1:Methane Reactant 2:

Oxygen Product 1:Carbon dioxide

Product 2: water


Start- step 1:

Chemical bond breakage in reactants

+H

H

HH C

OO

O O


Oxygen

Covalent bond broken• Bond

breakage requires energy


Middle- step 2:

Atom rearrangement

+H

H

HH C

OO

O O


End- step 3:

Chemical bond formation in products

C

O O

H H

O

Product 1:Carbon dioxide

Product 2: Water

H

O

H

Newly formed covalent bond• Bond formation

releases energy Product 2: Water


+ +H

H

HH

H H

H H

C C

O O

O

O

HC 1 41 22 O 21 OC 1 H 22 O 1


Oxygen Product 1: Carbon Dioxide

Product 2:Water

OO

O O


1(CH4) 2(O2)+ 1(CO2) 2(H2O)+CH4 2O2+ 2H2OCO2 + Simplified

representation


Law of conservation of matter:

Atoms can neither be created nor destroyed, only rearranged

• According to the law of conservation of matter

for chemical reactions:– The number and type of atoms present in the reactants (left of the arrow) must equal

the number and type of atoms in the products (right of the arrow)• The chemical reaction must be balanced

– The conversion of reactants to products during a chemical reaction relies on atom rearrangement

• No new atoms are created in a chemical reaction

• No atoms are destroyed in a chemical reaction

CH4 2O2+ 2H2OCO2 +


The chemical formula of a molecule shows the number of

each type of atom present in the molecule

In 1 molecule of H2O:

1(H2O)

×

×

• 1 × H2 = 2 hydrogen atoms (in 1 molecule of H2O)

In 3 molecules of H2O:

3(H2O)

×

×

• 3 × H2 = 6 hydrogen atoms (in 3 molecules of H2O)

H H

O

• 1 × O = 1 oxygen atoms (in 1 molecule of H2O)

• 3 × O = 3 oxygen atoms (in 3 molecules of H2O)


Number of hydrogen atoms in 1 molecule of H2O:

Number of oxygen atoms in 1 molecule of H2O:

Number of hydrogen atoms in 3 molecules of H2O:

Number of oxygen atoms in 3 molecules of H2O:

Subscript– The subscript identifies the number atoms of a specific element present in a molecule

• The subscript only affects the atom type that the subscript number comes after

– Example: the subscript of 4 within CH4 establishes that there are 4 hydrogen

atoms in the CH4 molecule

CH4 2O2+ 2H2OCO2 +Chemical equations represent chemical reactions

– Subscripts cannot be changed in a chemical reaction

• If the subscript was changed, it would no longer be the same molecule

– If the subscript is 1, it is not shown• In theory, CH4 could be written as C1H4 ,

as there is one carbon atom in the CH4 molecule

CH4 2O2+ 2H2OCO2 +Coefficient– The coefficient identifies the number molecules present in the chemical equation

– The coefficient is always written in front of the formula• The coefficient can be changed when attempting to balance the equation

– Example: The coefficient of 2 in front of H2O means there is two separate H2O molecules on the product side of the equation


– The coefficient is always a whole number

– A coefficient of 1 is not normally shown

• In theory, CH4 could be written as 1CH4 , as there is one molecule of CH4 in the chemical

equation

Activity: Identify the coefficients and subscripts

in the chemical equation below

Why is the chemical equation on the left (blue) correct,

whereas the chemical equation on the right (red) is incorrect?

H2 + O2 → H2O 2H2 + O2 → 2H2O

Chemical equation is unbalanced

There are different numbers of one or

more atom types on the reactant and

product side of the chemical equation

Chemical equation is balanced

There are the same number of each atom

type on the reactant and product side of

the chemical equation


Method for balancing a chemical equation

Step 1: Determine if the chemical equation is balanced

Step 2: Balance the chemical equation by changing one or

more coefficients in front of the molecules

Step 3: Check whether adjustments to the coefficients have

balanced the chemical equation

Balancing chemical equations

Balancing chemical equations• To work out the number of an elements’ atoms present in a molecule

multiply the coefficient by the subscript

– For 2Fe2O3

Coefficient

SubscriptThe number of Fe atoms:

Coefficient is 2 and subscript is 2:

2 × 2 = 4 Fe atoms

• The chemical equation must have the lowest possible

coefficients

– E.g. 4Fe + 3O2 → 2Fe2O3 is correct

8Fe + 6O2 → 4Fe2O3 is incorrect

The number of O atoms:

Coefficient is 2 and subscript is 3:

3 × 2 = 6 O atoms


Step 1: Determine if the equation is balanced

• List the element types present on the reactant side and the product side of the

equation

– Count the number of each type of element present on each side of the equation

Fe + O2 → Fe2O3


Reactant atomtypes

Number of reactant atoms

Product atom types

Number of product atoms

Different numbers of Fe and O atoms

on the reactant and product side of the

equation = Unbalanced

equation

Fe (iron)

O (oxygen)

1

2

2

3

Fe (iron)

O (oxygen)


Step 2: Balance the chemical equation by changing one or

more coefficients in front of the molecules


Fe + O2 → Fe2O3

• Try changing one or more of the coefficients to balance the chemical equation

– Often trail and error is needed to get the correct answer

• Once coefficients have been changed, move to step 3 to check if the equation is balanced

4Fe + 3O2 → 2Fe2O3


Step 3: Check whether adjustments to the coefficients have balanced

the chemical equation

• List the element types present on the reactant side and the product side of the equation– Count the number of each type of element present on each side of the equation

– Essentially, a repeat of step 1

• If the chemical equation is not balanced repeat step 2 (try a different combination of coefficients)


4Fe + 3O2 → 2Fe2O3

Reactant atomtypes


Product atom types


The numbers of Fe and O atoms on the

reactant and product side of the equation are equal

= Balanced equation

Fe (iron)

O (oxygen)

4

6

4

6

Fe (iron)

O (oxygen)

Balancing the chemical equation below:

Na + N2 → Na3N

NNa N Na Na

Na

N

More sodium atoms are needed on the reactant side




3Na + N2 → Na3N

N

Na

N Na Na

Na

N

More nitrogen atoms are needed on the product side

Na

Na



3Na + N2 → 2Na3N

N

Na

N Na Na

Na

N

More sodium atoms are needed on the reactant side

Na

Na

Na Na

Na

N


Balance the following chemical equation:

6Na + N2 → 2Na3N

N

Na

N Na Na

Na

NNa

Na

Na Na

Na

N

Na

Na

NaSame number of sodium and nitrogen atoms present on the reactant and product side of the equation = balanced equation


Balanced chemical equation:

6Na + N2 → 2Na3N

N

Na

N Na Na

Na

NNa

Na

Na Na

Na

N

Na

Na

NaReactant atom

typesNumber of

reactant atomsProduct atom

typesNumber of

product atoms

Na (sodium) 6 Na (sodium) 6

N (nitrogen) 2 N (nitrogen) 2

Is the chemical reaction

below balanced?

Reactant atomtypes


Product atom types


2Al + Fe2O3 → 2Fe + Al2O3

Balance the chemical equation below:

H2 + O2 → H2O

Reactant atomtypes


Product atom types


Balance the chemical equations by

choosing the correct coefficient sets

Which set of coefficients balances the

equation?

N2 + H2 NH3

a) 2, 1, 3

b) 1, 3, 2

c) 1, 2, 3

Which set of coefficients balances the

equation?

__Mg + _N2 → __Mg3N2

a) 1, 3, 2

b) 3, 1, 2

c) 3, 1, 1

Describe what happens during a chemical reaction

by referring to the reactants and products.

How do atom rearrangements help facilitate

chemical reactions?

Is it possible for the products of a chemical reaction

to be converted back into the original reactants?

Why/why not?

Key concept: chemical reactions

Attempt Socrative questions: 1 to 4

Google Socrative and go to the student login

Room name:

City name followed by 1 or 2 (e.g. PERTH1)

1 for 1st session of the week and 2 for 2nd session of the week

Summary Part 1: Chemical equations

• Chemical vs Physical change– Chemical change occurs during a chemical reaction

• The type of matter present before the chemical change is different from the matter present after the chemical change

– Physical change involves a substance changing its physical state,

such as a solid being converted to a liquid• No new substances are created as a result of physical change

• No chemical reactions occur during a physical change

• Chemical equations represent chemical reactions– In a chemical reaction, the reactants on the left hand side are converted

into the products on the right hand side of the equation, with the arrow representing the progression of the chemical reaction

• CH4 + 2O2 → CO2 + 2H2O

– No atoms are created or destroyed in a chemical reaction, only rearranged to form new substances (products)

Summary Part 1: Chemical equations


– The same number of each type of atom must be present on both the reactant and product side of the chemical equation

• No atoms are created or destroyed in a chemical reaction, only rearranged to form new substances (products)

– To assess whether a chemical equation is balanced count the number of each type of atom on the reactant and product side

• If the number of each type of atom are equal on both sides, the chemical equation is balanced

• If the number of one or more types of atom are different on the reactant and product sides of the chemical equation, the chemical equation is unbalanced

– The coefficients in front of the molecules (reactants and products) in the chemical equation can be changed to balance the equation


• Redox reactions

Redox reactions

Combustion reactions are the most common type of chemical reaction

– Combustion reactions always involve a substance reacting with oxygen

– When organic hydrocarbons like methane (CH4) combust:

• Water, carbon dioxide and energy (light and heat) are produced

– All combustion reactions are

redox reactionshttp://www.chem.ucla.edu/harding/IGOC/C/combustion.html

https://www.freeimages.com/photo/fire-1495233

CH4 + 2O2 → CO2 + 2H2O + energy

Electrons H atoms O atoms

OXIDATION Loss of electrons Loss of H atoms Gain of O atoms

REDUCTION Gain of electrons Gain of H atoms Loss of O atoms

OXIDATION: Loss of 1 or more hydrogen atoms

REDUCTION: Gain of 1 or more hydrogen atoms

H1p+

1e-

Redox reactions

• Oxidation and reduction are complementary processes– Oxidation and reduction occur simultaneously because the compound that

loses hydrogen (oxidation) donates it to another compound which accepts the hydrogen (reduction)

• Losing or gaining hydrogen atoms is equivalent to losing or gaining electrons – Each hydrogen atom contains 1 electron (and 1 proton)

• In a redox reaction, electrons and/or hydrogen atoms are transferred from one compound to another

OIL RIG useful to remember what happens in a redox reaction:

Oxidation Is Loss of electrons and/or hydrogen atoms (OIL)

Reduction Is Gain of electrons and/or hydrogen atoms (RIG)

Redox reactions

Redox reactions

• In a redox reaction, one reactant is oxidised and one is reduced

• To determine which reactant is oxidised and which is reduced:– Assess how each reactant changes during the chemical reaction by looking at

the products

• The reactant that has lost hydrogen to become the product has been oxidised

• The reactant that has gained hydrogen to become the product has been reduced

CH4 2O2+ 2H2OCO2 +CH4 loses hydrogen = oxidised

O2 gains hydrogen = reduced

Redox reactions

• In a redox reaction, one reactant is oxidised and one is

reduced

Biological molecule loses 2 hydrogens = oxidised

Coenzyme gains 2 hydrogens = reduced

Adapted from Timberlake 2013, p223-224

• Redox reaction are vital to many

metabolic pathways required to

generate cellular energy (ATP)

including:

– Glycolysis

– Citric acid cycle

– The electron transport chain

• The coenzymes that are commonly

used in redox reactions (within

metabolic pathways) can exist in either:– An electron/hydrogen rich form

– An electron/hydrogen poor form

Electron poor coenzymes

Electron rich coenzymes

NAD+ NADH

NADP NADPH

FAD FADH2

Adapted from Timberlake 2013, p223-224

Redox reactions

Identify which reactant is oxidised and which is

reduced in the chemical reaction below

2NH3 + 3O2 → N2 + 6H2O

• NH3 loses hydrogen to become N2, hence NH3 is oxidised– Oxidation is loss of hydrogen/electrons

• O2 gains hydrogen to become H2O, hence O2 is reduced– Reduction is gain of hydrogen/electrons

• Oxygen is always reduced in redox reactions

NH3 loses hydrogen = oxidised

O2 gains hydrogen = reduced

Malate-H + NAD+ → Oxaloacetate + NADH

• Malate-H loses hydrogen to become oxaloacetate, hence

Malate-H is oxidised

– Oxidation is loss of hydrogen/electrons

• NAD+ gains hydrogen to become NADH, hence NAD+ is reduced

– Reduction is gain of hydrogen/electrons

Identify which reactant is oxidised and which is

reduced in the chemical reaction below

Malate-H loses hydrogen = oxidised

NAD+ gains hydrogen = reduced

When a molecule loses hydrogen atoms in a chemical

reaction, is the molecule oxidised or reduced?

What about if the molecule gains a hydrogen atoms?

In the above reaction, which molecule is oxidised?

How does the oxidised molecule

change during the redox reaction?

In the above reaction, which molecule is reduced?

How does the reduced molecule

change during the redox reaction?

Key concept: redox reactions

CH4 + 2O2 CO2 + 2H2O

Attempt Socrative questions: 5 and 6


Room name:




• Redox reactions

– A redox reaction involves one reactant being oxidised and one

reactant being reduced

• Oxidation and reduction occur simultaneously in a redox reaction

– Oxidation is the loss of electrons/hydrogen atoms

– Reduction is the gain of electrons/hydrogen atoms

– OIL RIG: Oxidation Is Loss, Reduction Is Gain

– The reactant that has loses hydrogen to become the product

has been oxidised

– The reactant that has gains hydrogen to become the product

has been reduced

Part 3: The mole

• What is the mole?

• Using the mole

• Amount of a substance is specified by numbers or collection terms:

– 1 dozen eggs

= 12 eggs

– 1 slab of beer

= 24 cans

– 1 mole of hydrogen atoms

= 6.02 × 1023 hydrogen atoms

What is the mole?

https://www.freeimages.com/photo/eggs-1194166

How was the mole determined?

The mole unit is relative to the amount of atoms in 12 grams of carbon-12 atoms

– 1 gram of Carbon-12 atoms:• Contains 6.02 x 1023 carbon 12 atoms

• Weighs 12.01 grams

– 1 mole of any component

= 6.02 x 1023 components of that thing

• 1 mole of Na = 6.02 x 1023 Na atoms

• 1 mole of H2O = 6.02 x 1023 H2O molecules

• 1 mole of CO2 = 6.02 x 1023 CO2 compounds

• 1 mole = 6.02 x 1023 components of that substance– 6.02 x 1023 = Avogadro’s number

Putting the mole into perspective

• One mole of hydrogen atoms is:– 6,020,000,000,000,000,000,000,000 hydrogen atoms

– Equal to 6.02 × 1023 hydrogen atoms (scientific notation)

• One mole of red blood cells

= the number of red blood cells

in every human on earth

• In a 1 litre bottle of water there are about

55.5 mole of H2O molecules

https://www.freeimages.com/photo/the-element-of-life-1364414

https://www.freeimages.com/photo/flowing-blood-1197512

What is the mole?

MOLE

How many individual components are in

one mole?

6.02 × 1023

COMPONENTS

1 MOLE OF HYDROGEN ATOMS =

6.02 × 1023

INDIVIDUAL HYDROGEN ATOMS

COUNTING VERY LARGE NUMBERS OF COMPONENTS E.G. ATOMS MOLECULES OR COMPOUNDS

ON A SIMPLE SCALE

Example

Useful for

Example

2 MOLE OF HYDROGEN ATOMS = 2 TIMES (6.02 × 1023)

INDIVIDUAL HYDROGEN ATOMS

What is the mole?

Number of components in one mole

is called AVOGADRO’S NUMBER

Using the moleWhat makes the mole useful?

• The mole can be used to quantify the amount of atoms,

molecules, or compounds in a sample on a useable scale

0 mole 1 mole 2 mole 3 mole 4 mole

Quantifying the number of

H2O molecules

6.02 × 1023

molecules1.20 × 1024

molecules

1.81 × 1024

molecules2.41 × 1025

molecules0

molecules

Example:

How many H2O molecules are in a 37 mL of water?

– 2.0 mole of H2O molecules = 1.20 × 1024 individual H2O molecules

• Much easier to count the number of H2O molecules in mole rather than

counting each individual H2O molecule

The chemical formula of a molecule shows the number of

each type of atom present in the molecule

In 1 molecule of H2O:

1(H2O)

×

×

• 1 × H2 = 2 hydrogen atoms (in 1 molecule of H2O)

In 3 molecules of H2O:

3(H2O)

×

×

• 3 × H2 = 6 hydrogen atoms (in 3 molecules of H2O)

H H

O

• 1 × O = 1 oxygen atoms (in 1 molecule of H2O)

• 3 × O = 3 oxygen atoms (in 3 molecules of H2O)


Number of hydrogen atoms in 1 molecule of H2O:

Number of oxygen atoms in 1 molecule of H2O:

Number of hydrogen atoms in 3 molecules of H2O:

Number of oxygen atoms in 3 molecules of H2O:

Using the mole

1 mole of molecules of H2O = 6.02 × 1023

molecules of H2O

× 1 mole

In 1 mole of H2O, how many mole of hydrogen atoms and oxygen atoms are there?

The chemical formula of a molecule (e.g. H2O) specifies the number of moles of atoms

of each element present in one mole of that molecule (e.g. H2O)

In 1 mole of H2O molecules:

1 mole (H2O)×

×

• 1 mole × H2 = 2 mole of hydrogen atoms (in 1 mole of H2O)

In 3 moles of H2O molecules:

3 mole (H2O) • 3 mole × H2 = 6 mole of hydrogen atoms (in 3 moles of H2O)

×

×

3 mole of molecules of H2O = 3 lots of

6.02 × 1023

molecules of H2O

× 1 mole

× 1 mole

× 1 mole

• 1 mole × O = 1 mole of oxygen atoms (in 1 mole of H2O)

• 3 mole × O = 3 mole of oxygen atoms (in 3 moles of H2O)

Number of hydrogen atoms in 1 mole of H2O:

Number of oxygen atoms in 1 mole of H2O:

Number of hydrogen atoms in 3 mole of H2O:

Number of oxygen atoms in 3 mole of H2O:

Attempt Socrative questions: 7 to 10


Room name:



Part 3: The mole

• What is the mole?– The mole is a way of counting very large number of any component

e.g. the number of atoms, molecules or compounds in a sample

– The number of components in 1 mole of a substance is 6.02 × 1023

• 6.02 × 1023 is Avogadro’s number

• Using the mole– The mole can be used to quantify the amount of atoms, molecules, or

compounds in a sample on a useable scale• E.g. 1.20 × 1024 hydrogen atoms = 2 moles of hydrogen atoms

• The chemical formula of a molecule (e.g. H2O) tells you the number of moles of atoms of each element present in one mole of that molecule (e.g. H2O)

• E.g. 1 mole of H2O molecules contains:– 2 moles of hydrogen atoms (1 mole × H2)

– 1 mole of oxygen atoms (1 mole × O)

Readings & Resources• Stoker, HS 2014, General, Organic and Biological Chemistry, 7th edn,

Brooks/Cole, Cengage Learning, Belmont, CA.

• Stoker, HS 2004, General, Organic and Biological Chemistry, 3rd edn, Houghton Mifflin, Boston, MA.

• Timberlake, KC 2013, General, organic, and biological chemistry: structures of life, 4th edn, Pearson, Boston, MA.

• Alberts, B, Johnson, A, Lewis, J, Raff, M, Roberts, K & Walter P 2008, Molecular biology of the cell, 5th edn, Garland Science, New York.

• Berg, JM, Tymoczko, JL & Stryer, L 2012, Biochemistry, 7th edn, W.H. Freeman, New York.

• Dominiczak, MH 2007, Flesh and bones of metabolism, Elsevier Mosby, Edinburgh.

• Tortora, GJ & Derrickson, B 2014, Principles of Anatomy and Physiology, 14th edn, John Wiley & Sons, Hoboken, NJ.

• Tortora, GJ & Grabowski, SR 2003, Principles of Anatomy and Physiology, 10th edn, John Wiley & Sons, New York, NY.

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