Transcript Biochemistry I (CHE 418 / 5418)

Introduction to Organic and
Biochemistry
(CHE 124)
Reading Assignment
General, Organic, and Biological Chemistry: An Integrated Approach
4th Ed. Raymond
Chapter 5: Reactions
Answers to odd numbered problems in textbook are found in the book’s
index.
Chemical Reactions
• Chemical Reactions – a chemical change in
which the covalent or ionic bonds that hold
elements or compounds together are broken
and new bonds are formed.
• Chemical reactions are represented by
chemical equations.
• C6H1206
Glucose
+
O2
oxygen
→
CO2
+
H 2O
carbon dioxide
water
This reaction MUST be balanced!
Balancing Chemical Reactions
• Chemical reactions are balance by adding
coefficients- numbers placed in front of the
formula.
– Never change the formula
• C6H1206(aq)
Glucose
+
O2(g)
oxygen
→
CO2 (g) + H2O
carbon dioxide
water
Demonstrate on board.
• C6H1206(aq) + 6O2(g) → 6CO2 (g) + 6H2O
Show products and reactants.
Writing / Balancing Chemical Equations
• 1. Write the equation (from a word problem)
– Reactants on the left
– Products on the right
• 2. Indicate physical state of reactants / products
–
–
–
–
(g) = gas
(l) = pure liquid
(s) = solid
(aq) = ion or molecular compound in aqueous (water) solution.
• 3. Balance equation so that the same number of atoms of
each element are on the left as on the right.
– Add coefficients in FRONT of chemical formula; NEVER change
subscripts on formulas
– Start by balancing an element that appears in only one species on each
side.
– USE TRIAL AND ERROR!
Types of Reactions
• Types of Reactions
–
–
–
–
Synthesis
Decomposition
Single Replacement
Double Replacement
• Reactions involving water
– Hydrolysis
– Hydration
– Dehydration
• Oxidation / Reduction
– Combustion
– Hydrogenation
Types of Reactions (Cont’)
• Synthesis – two or more elements or compounds
combine to form one more complex compound.
A+B→C
Example:
2N2 + O2 → 2N2O
Nitrogen can react with oxygen to produce dinitrogen
monoxide (aka nitrous oxide – laughing gas)
Types of Reactions (Cont’)
• Decomposition (reverse of synthesis) break
down of one compound to form elements or
simpler compounds.
AB → A + B
Example:
2H2O →
2H2 + O2
Under the proper conditions, water can decompose into
hydrogen and oxygen.
Types of Reactions (Cont’)
• Single Replacement – one element trades
places with a different element in a
compound.
A + BC → AC + B
Example:
Fe + CuSO4 → FeSO4 + Cu
Iron reacts with a solution of copper sulfate producing iron
sulfate and elemental copper.
Types of Reactions (Cont’)
• Double Replacement – two elements trade
places with each other.
AB + CD → AD + CB
Example:
NaCl(aq) + AgNO3(ag) → NaNO3(aq) + AgCl (s)
When an aqueous solution of sodium chloride is mixed with
silver nitrate, silver chloride precipitates leaving sodium
nitrate in solution.
Reactions involving Water
• Hydrolysis – Water (hydro) splits (lyses) another
molecule.
H+
• ester + water → carboxylic acid + alcohol
– This reaction requires an acid catalyst.
– Draw on board.
• Some drugs are catabolized (broken down) by hydrolysis
– Procaine, chloroprocaine
• Catalyst – a substance that speeds up a reaction without
itself being consumed or destroyed by the reaction.
Reactions involving Water (Cont)
• Hydration – addition of water across a double
bond (alkene) in the presence of H+ (acid)
catalyst.
– Draw on board.
Reactions involving Water (Cont’)
• Dehydration – reverse of hydration
H+/heat
Alcohol → Alkene + water
draw on board.
Oxidation / Reduction Reactions
• Oxidation / Reduction reactions are often
called Redox reactions.
– Oxidation – loss of electrons (e-)
– Reduction – gain of electrons (e-)
2 Na (s) + Cl2 (g) → 2 Na Cl
(s)
• Types of oxidation / reduction Reactions
– Combustion
– Hydrogenation
Oxidation / Reduction Reactions (Cont’)
• Combustion – special type of oxidation /
reduction reaction involving oxygen.
CH4(g) + 2O2 → CO2(g) + 2H2O
• Organic Chemist identify oxidation / reduction
– An atom is oxidized if:
• Gains electrons OR
– Attached to more oxygen in product than reactant
• Losses hydrogen
– Is attached to fewer hydrogens in product than reactant
– An atom is reduced if:
• Loses oxygen
• Gains hydrogen
• In the equation above, C is oxidized and oxygen is reduced.
• Many antiseptics are oxidizing agents. See p. 152
Oxidation / Reduction Reactions (Cont’)
• Hydrogenation – addition of H2 across a
double bond. Often requires a platinum (Pt)
catalyst.
– Draw on board.
Stoichiometry
• Stoichiometry – the quantitative relationship between
reactants and products in a chemical reaction.
• Making cheese sandwiches.
– If you have 20 slices of bread, how many sandwiches can
you make? (2 slices of bread/ sandwich)
• 2 bread slices + 1 cheese slice = 1 sandwich
• Conversion factors = 1 cheese slice or
2 bread slices
Work problem on board
2 bread slices
1 chees slice.
Stoichiometry (Cont’)
• Chemical Reactions (rxn)
2CO (g) + O2 (g) → 2CO2(g)
carbon
monoxide
oxygen
carbon
dioxide
Is this reaction balanced?
Demonstrate with dozen, molecules, moles.
Write conversion factors.
Stoichiometry (Cont’)
• Typical Question: If 6.0 mol CO is present,
how many moles of O2 will be required to
convert all of the CO to CO2?
• Typical Question: If 6.0 mol CO is present,
how many moles of CO2 will be produced if
all of the CO is used up?
Real Life Example
• Firefighters us a device called a “rebreather”
to enter smoky buildings. Rebreathers use
KO2 to remove carbon dioxide from air in the
unit using the reaction below.
4 KO2(s) + 2 CO2(g) → 2 K2CO3(s) + 3 O2(g)
• How many grams of KO2 are required to
completely react with 0.400 mol of CO2?
• Work on board.
• ALWAYS GO THROUGH MOLES!
Yield of Reaction
• In the real world reactants are not always
present in the exact ratio stated in the
balanced chemical reaction.
• We need to discuss:
– Limiting reagent
– Theoretical Yield
– Percent Yield
Limiting Reactant
• Limiting reactant – is the reactant that
determines (limits) the amount of product
that can be formed in a rxn.
Balanced
N2 (g) + 3H2(g) → 2NH3(s)
If 2.10 mol N2 and 5.70 mol H2 react, what is
the limiting reagent. Ask the question, how
many mol of H2 is needed to react with .10
mol N2?
Work on board.
Theoretical Yield
• Theoretical yield – the maximum amount of product
that can be obtained (controlled by amount of
limiting reagent.)
• How to determine
– 1. Determine the limiting reagent.
– 2. Determine the theoretical yield.
• MUST Start with balance equation
C3H8(g) + 5 O2(g) → 3 CO2(g) + 4 H2O(g)
If 2.2 mol propone (C3H8(g) ) reacts with 14 mol oxygen,
what is the theoretical yield?
Two step problem
1. Calculate limiting reagent
2. Determine the theoretical yield of carbon dioxide (CO2)
Percent Yield
• Most real-world reactions do not produce the
total theoretical yield, therefor we calculate
percent yield to determine what the yield is
% yield =
actual yield
X 100
theoretical yield
What determines if a Reaction will take place?
• Chemist classify reactions as:
– Spontaneous – a reaction that continues by itself
once it has started
– Nonspontaneous – a reaction will not take place
unless something starts it and keeps it going.
• Gibbs Free energy (G) determines if a reaction
will take place (be spontaneous) or not take
place (nonspontaneous)
– Spontaneous reactions have a - ΔG
– Nonspontaneous reactions have a + ΔG
– Reactions in equilibrium have ΔG = 0
C6H12O6 + 6O2 → 6CO2 + 6H2O
ΔG = - 686 kcal /mol
Energy released. Energy is used to break
and reform bonds.
Change in Free Energy (ΔG)
→
A
B
(Reactant)
E
n
e
r
g
y

(Product)
A
B
Progress of Rxn.

E
n
e
r
g
y

B
A
Progress of Rxn.

Negative ΔG
→
A
(Reactant)
E
n
e
r
g
y

B
(Product)
•
•
•
A
B
Progress of Rxn.

- ΔG
spontaneous
exergonic -energy
released
Positive ΔG
→
A
(Reactant)
E
n
e
r
g
y

B
(Product)
B
A
Progress of Rxn.

•
•
•
+ ΔG
non-spontaneous
endergonic energy consumed
Reactions
• Reaction Rate – how quickly products form
• Energy of activation – Ea – the energy barrier
that must be crossed to go from reactants to
products.
– Determines rate of reaction
• The greater Ea the slower the rate.
• Other factors influence rate
– Temperature
– Concentration of reactants
– Presence of catalyst