natural approach to chemistry chapter 4 physical and chemical change

Natural Approach to Chemistry

Chapter 4 Physical and Chemical Change4.1 Understanding Chemical Changes Frame 2-414.2 Chemical Reactions Frames 42-824.3 Chemical Reactions in the lab Frames 83-109

CHAPTER 4

Physical and Chemical

Change4.1 Understanding Chemical Changes

4.1 Understanding Chemical Changes

H2O(s) H2O(l)

Is it still the same substance?

Have the physical properties changed?


?

Can water go back to being ice?

H2O(s) H2O(l)


2C20H42(s) + 61O2(g) 42H2O(g) + 40CO2(g)

In the presence of a flame:

Is it still the same substance?

Have the physical properties changed?


2C20H42(s) + 61O2(g) 42H2O(g) + 40CO2(g)

If the flame cools down:

Does the candle come back?

?


2C20H42(s) + 61O2(g) 42H2O(g) + 40CO2(g)

If the flame cools down:

The candle does not come back by itself.

X

In this chemical reaction, the atoms were rearranged.

This change is irreversible.


In a physical change: - the molecules are rearranged- intermolecular forces are broken


In a physical change: - the molecules are rearranged-intermolecular forces are broken-interatomic forces are not broken

In a chemical change: - the atoms are rearranged- interatomic forces are broken


Intermolecular forces are much weaker than interatomic forces.

Interatomic forces.

Intermolecular forces.


Physical or chemical change?


OH H

OH H

H

O

H H

HH

H2O (water)

H4O (?)H3O (?)

Can we obtain any arrangement of atoms?

H H

H2 (hydrogen)

OO

O2 (oxygen)


OH H

H2O (water)

Can we obtain any arrangement of atoms?

H H

H2 (hydrogen)

OO

O2 (oxygen)

NO: each type of atom allows only certain chemical bonds to be formed

This is due to the

structure of the atom


Structure of the Atom

.

99.8% of the mass of the atom is in the nucleus!

An atom is not a hard ball.


. . .

The number of electrons corresponds to the atomic number of the element, as shown in the periodic table.


.

Electrons have a negative electric charge.


.


If negative charges repel each other, how does an atom

stay together?


Proton: A tiny particle in the nucleus that has a positive charge.

Neutral: Having zero total electric charge.


Protons in the nucleus have a positive electric charge.


Why don’t the negative electrons “fall” into the positive nucleus?


Why don’t the negative electrons “fall” into the positive nucleus?

Because the electrons have energy and momentum

- The same reason why the Earth does not fall into the sun, but gravitates around it instead.


Electrons are responsible for bond formation.

Electrons can be:

+-


In ionic compounds, electrons are NOT SHARED but TRANSFERRED.

1 electron

Sodium ion Chloride ion

Sodium atom Chlorine atom

Ionic bond

Cl -1

Cl

+1Na

Na


In ionic compounds, electrons are NOT SHARED but TRANSFERRED.

Magnesium ion

Magnesium chloride (MgCl2) is also an ionic compound.

1 electron 1 electron

Chloride atomChloride atom

+2Mg

Mg

Magnesium atom

-1Chloride ion Cl Cl -1 Chloride ion

Cl Cl

Ionic bondIonic bond


Covalent bonds

A chemical bond is formed by sharing or transferring electrons.

Covalent bond: A chemical bond that consists of two shared electrons.

Molecule: A neutral group of atoms bonded together by covalent bonds.

There are two covalent bonds in a water molecule.


Single bonds

Double bond Triple bond

Some elements can share multiple electrons with

the same atom.

Multiple bonds


Chemical bonds form because there is an advantage in energy.

Lower energy =

more stable


Lower energy =

more stable

Enthalpy of formation∆Hf (kJ/mole)

Chemical bonds form because there is an advantage in energy.


ReactantsAll chemical reactions involve three key components:

1. Reactants


Reactants ProductsAll chemical reactions involve three key components:

1. Reactants

2. Products


Reactants Products

Ener

gy

All chemical reactions involve three key components:

1. Reactants

2. Products

3. Energy (in or out)


Electrons are responsible for bonding among atoms.

Some elements are never found in pure form in nature: Ex: Lithium (Li), sodium (Na)

He Some elements are always found in pure form in nature: Ex: Helium (He), argon (Ar)


Reactivity: The tendency of elements to form chemical bonds.

Some elements are more likely to react than others.They are more reactive.


Electrons make bonds.

Electrons can be:


Can we predict if a compound is ionic or covalent?



Yes!

In an ionic compound, one atom is a nonmetal, and one atom is a metal

NonmetalMetal



Yes!

In a covalent compound, both atoms are nonmetals.

Nonmetal Nonmetal


Use the periodic table to help determine whether a compound is ionic or molecular.


Is the compound CF4 ionic or molecular?


Is the compound CF4 ionic or molecular?Both nonmetals

CHAPTER 4


Change4.2 Chemical

Reactions

4.2 Chemical Reactions

Electrolysis

?

?

Experimental setup:An electric current transfers a large amount of energy to water.

Observations:The mass of water decreases over time.The volume of gas increases.


Electrolysis

?

?

Experimental setup:An electric current transfers a large amount of energy to water.

Observations:The mass of water decreases over time.The volume of gas increases.

Hypothesis:The gas produced is water vapor.


Electrolysis

?

?


Observations on the gases:1. When cooled, the gas does not condense into liquid water.

2. One of the gases burns.

3. One of the gases causes a flame to get brighter.


Electrolysis

?

?


Observations on the gases:1. When cooled, the gas does not condense into liquid water.

2. One of the gases burns.

3. One of the gases causes a flame to get brighter.

Conclusion:The gas produced is not water vapor.


?

?

ElectrolysisClues:One gas burns.One gas causes a flame to get brighter.The gases come from water.


ElectrolysisClues:One gas burns.One gas causes a flame to get brighter.The gases come from water.

+


The energy from the electric current was high enough to cause a chemical change through a chemical reaction.

Chemical reaction: A process that rearranges the atoms in any substance(s) to produce one or more different substances.

Chemical change: A result of chemical reaction.

+


+

A chemical equation


The Chemical Equation

oxygen

hydrogen

oxygen

hydrogen



1

2

oxygen

hydrogen

2

2

oxygen

hydrogen

!

How can we make both sides the same number of each atom?


oxygen

hydrogen

oxygen

hydrogen



2

4

oxygen

hydrogen

2

4

oxygen

hydrogen


coefficients


2H2O(l) 2H2(g) + O2(g)


Reactants Products

CoefficientsTell you how many of each molecule

participate in the reaction(No coefficient appears when it equals 1).

Reaction arrow


Rules of balancing a chemical equation:

1. Determine if the equation is balanced.Count the number of each kind of atom on either side of the arrow.

2. If it is not balanced, use coefficients. Ex: H2O becomes 2H2O

3. Do not change the subscripts inside the molecular formulas.Ex: H2O cannot become H3O

4. Check that the equation is balanced.Count the number of each kind of atom on either side of the arrow with the new coefficients.



Is it balanced?

Reactants Products

Iron (Fe)

Oxygen (O)


Is it balanced?

Reactants Products

Iron (Fe) 1 2Oxygen (O) 2 3


Reactants Products


Not balanced


Rules of balancing a chemical equation:

1. Determine if the equation is balanced.Count the number of each kind of atom on either side of the arrow.

2. If it is not balanced, use coefficients. Ex: H2O becomes 2H2O

3. Do not change the subscripts inside the molecular formulas.Ex: H2O cannot become H3O

4. Check that the equation is balanced.Count the number of each kind of atom on either side of the arrow with the new coefficients.



Reactants Products

Iron (Fe)

Oxygen (O)

Add coefficients

? ? ?


Reactants Products


4 3 2

Balanced!


Photosynthesis is the basis of most life on Earth.

gases

a sugar(glucose) a gas


Verify that the equation is balanced

Reactants Products

Carbon (C)

Oxygen (O)

Hydrogen (H)


Reactants Products

Carbon (C) 6 6Oxygen (O) 18 18

Hydrogen (H) 12 12

Verify that the equation is balanced

Number of atoms or moles


Use the formula mass to convert from moles to grams.


108 g+ 264 g

372 g

180 g+ 192 g

372 g

Total mass of reactants Total mass of products


108 g+ 264 g

372 g

180 g+ 192 g

372 g

Total mass of reactants Total mass of products

Conservation of mass


Conservation of mass: Law that states that, in any chemical reaction, the total mass remains the same.

Total mass of reactants = Total mass of products


Reactants Products

Ener

gy

All chemical reactions involve three key components:

1. Reactants

2. Products

3. Energy (in or out)


Energy “in”

Energy as a reactant.

reactants products

An endothermic reaction requires an input of energy.


Energy “out”Energy as a product.

An exothermic reaction releases energy.

reactants products


Energy is absorbed

Energy is released


Energy is absorbed

Energy is released

First law of thermodynamics:Energy can neither be created nor destroyed.

Are these violations of the first law?


First law of thermodynamics:Energy can neither be created nor destroyed.


Respiration

Photosynthesis

Mixing water and CO2 doesn’t make sugar and O2 because energy input is needed.

Energy input is not needed

but just mixing sugar and O2 doesn’t make

water and CO2.

What is missing?


A candle burning gives off heat. It is an exothermic reaction.

However, the candle does not spontaneously light itself up.

What is missing?


Activation energy

C6H12O6 + 6O2 6H2O + 6CO2 + 2,800,000J


Activation energy

C6H12O6 + 6O2 6H2O + 6CO2 + 2,800,000J

an energy barrier!

CHAPTER 4


Change4.3 Chemical Reactions in

the Lab

4.3 Chemical Reactions in the Lab

Why would the presence of water on Mars be a sign that life might exist there?


Many reactions, including those that sustain life, involve chemicals dissolved in water.

Why would the presence of water on Mars be a sign that life might exist there?


DRY

Light blue powder

No reaction


DRY

No reaction

IN SOLUTION

Light blue powder NaS(aq) CuSO4(aq)

Chemical reaction

A precipitate is an insoluble compound.


A solution with water as the solvent is called an aqueous solution.

“Aqua” means “water”

SYMBOLS

(s) indicates a solid(l) indicates a liquid(g) indicates a gas(aq) indicates a substance dissolved in water (an

aqueous solution)


S O

O

O

OCu

Cu

S O

O

O

O

2-

2+

OH

H

OH

H

OH

HO

H H

A salt is an ionic compound that forms ions when dissolved in water.


Can you translate what you see into a chemical equation?



Zn(s) + CuSO4(aq)Cu(s)

and Zn(s) is disappearing


Zn(s) + CuSO4(aq)Cu(s)



a salt in water!


Cu2+(aq)

SO42-(aq)


Zn(s) +Cu(s)



Cu2+(aq)

SO42-(aq)

Zn(s) +Cu(s)


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

The copper ion gains 2 electrons to form solid copper.

Reduction: A chemical reaction that decreases the charge of an atom or ion by accepting electrons.


Cu2+(aq)

SO42-(aq)

Zn(s) +Cu(s)



The copper ion gains 2 electrons to form solid copper.

Where could those electrons

come from?


Cu2+(aq)

SO42-(aq)

Zn(s) +Cu(s)


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

Zinc gives up 2 electrons to become a zinc ion, dissolved in water.




Zinc gives up 2 electrons to become a zinc ion, dissolved in water.

Oxidation: A chemical reaction that increases the charge of an atom or ion by giving up electrons.



Reactants Products

reduction

REACTION NAME




Reactants Products

reduction

REACTION NAME

oxidation




Reactants Products

reduction

REACTION NAME

oxidation

These 2 reactions are happening at the same time.

To obtain the overall reactionadd the reactants, then the products.




Reactants Products

Cu2+(aq) + Zn(s) Cu(s) + Zn2+(aq)

reduction

REACTION NAME

oxidation

redox


OH H

OH

H- +

dissociationH2O OH- + H+

The double arrow indicates that the reaction goes in both ways.

The heavier arrow means that the reverse reaction is favored.


Classify these items as acids or bases.

HCl(l) H+(aq) + Cl-(aq)


H2O(l)

Acid: A chemical that dissolves in water to create more H+ ions than there are in neutral water.

Acids give foods like lemons their sour taste.Acids react with metal to create hydrogen gas (H2).Acids can corrode metal and burn skin.

Example:

Hydrochloric acid

NaOH(l) Na+(aq) + OH-(aq)


H2O(l)

Bases create a bitter taste.Bases have a slippery feel, like soap.Bases can neutralize acids.

Example:

Sodium hydroxide



H2O(l)

Bases create a bitter taste.Bases have a slippery feel, like soap.Bases can neutralize acids.

Example:

Sodium hydroxide

OH-(aq) + H+(aq) H2O(l)

Result: fewer H+ ions



H2O(l)Example:

Sodium hydroxide

OH-(aq) + H+(aq) H2O(l)

Result: fewer H+ ions

Base: A chemical that dissolves in water to create fewer H+ ions (or more OH- ions)than there are in neutral water.


Higher H+ concentration Lower H+ concentration

The pH scale helps to determine whether a solution is acidic or basic.

natural approach to chemistry chapter 4 physical and chemical change

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