1. Lecture 4 Chemical Reactions Ozgur Unal
NIS - BIOLOGY
Lecture 4
Chemical Reactions
Ozgur Unal

2. Chemical Reactions
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.

3. Chemical Equations
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.

4. Chemical Equations
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.

5. Energy of Reactions
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.

6. Energy Change in a Chemical Reaction
Compare the two figure below.
Both reactions require activation energy to get started .
What is the difference?
Exothermic vs endothermic reactions.

7. Catalyst
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.

8. Enzymes
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.

9. How Enzymes Work 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.

10. Lecture 5 Water and Solutions Ozgur Unal
NIS - BIOLOGY
Lecture 5
Water and Solutions
Ozgur Unal

11. Water’s Polarity 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.

12. Water’s Polarity 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!

13. Mixtures with Water
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.

14. Mixtures with Water
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.

15. Mixtures with Water 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.