Lecture 4
Chemical Reactions
Ozgur Unal
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What happens to a piece of iron after it is exposed to air
with high humidity for a long time?
Rust is an example of a chemical change.
A chemical reaction is the process by which atoms in
substances are reorganized into different substances
Not every change is a result of chemical reaction.
Physical changes occur when there is change in a
substance’s appearance but not its composition.
Observe the papers shown to you, and determine which
underwent a chemical reaction.
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When scientists write chemical reactions, they express
each substance in the reaction in a chemical equation.
In a chemical equation, the
starting substances are called reactants,
which are shown on the left.
The substances formed after the
reaction are called products,
which are shown on the right.
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A chemical equation must be balanced, because the
total mass is always conserved in a chemical equation.
The number of atoms of each element in the reactant
side must equal the number of atoms of the same element
in the product side.
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Energy must be provided to start a chemical reaction.
Example: Cookie..
Why the coal (carbon) in front of you does not burn by
itself, although there is oxygen?
Activation energy is the minimum amount of energy
needed for reactants to form products in a chemical
reaction.
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Compare the two figure below.
Both reactions require activation energy to get started .
What is the difference?
Exothermic vs endothermic reactions.
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The chemical reactions in living organisms proceed very
slowly in laboratories because the activation energy is too
high.
To make it faster in living organisms, additional
substances must be present to reduce the activation
energy.
A catalyst is a substance that lowers the activation
energy in a reaction.
Catalysts do not increase the amount of products
produced.
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In living organisms special proteins, called enzymes, are
used as biological catalysts.
Enzymes speed up the rate of chemical reactions.
Most enzymes are specific to one reaction.
An enzyme’s name describes what it does.
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The reactants that
bind to the enzyme are
called substrates.
The specific location
where a substrate binds
on an enzyme is called the active site.
The active site and the substrate have complementary
shapes.
Factors such as pH, temperature etc affect enzyme
avticity.
http://www.stolaf.edu/people/giannini/biological%20ana
mations.html
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Lecture 5
Water and Solutions
Ozgur Unal
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In molecules electrons are shared in covalent bonds.
Sometimes, shared electrons are unequally distributed
within a molecule ------ > Polar molecules
Water is a polar molecule with a negatively charged and
a positively charged ends.
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A hydrogen bond is a weak interaction
involving a hydrogen atom and a
fluorine, oxygen or a nitrogen atom.
It is a strong type of van der Waals
force.
Water is also called “the universal solvent”.
Water can dissolve other polar molecules, such as sugar.
Check this out!
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A mixture is a combination of two or more substances in
which each substance retains its individual characteristics
and properties -------> No chemical change!
Examples: Salty water, coffee etc.
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When a mixture has a uniform composition throughout,
it is called a homogeneous mixture or solution.
In a solution there are two components:
Solvent and solute.
A solvent is a substance in which another
substance is dissolved.
A solute is the substance that is dissolved
in the solvent.
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In heterogeneous mixtures, the
components remain distinct unlike
homogeneous mixtures.
Examples: Salad, soup.
Suspension is a type of heterogeneous
mixture in which the particles settle to
the bottom.
Example: Sand in water
A colloid is a heterogeneous
mixture in which the particles do not settle out.
Example: Milk, fog, smoke, blood etc.
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