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?

4.1 Understanding Chemical Changes

?

Can water go back to being ice?

H2O(s) H2O(l)

4.1 Understanding Chemical Changes

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?

4.1 Understanding Chemical Changes

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

If the flame cools down:

Does the candle come back?

?

4.1 Understanding Chemical Changes

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.

4.1 Understanding Chemical Changes

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

4.1 Understanding Chemical Changes

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

4.1 Understanding Chemical Changes

Intermolecular forces are much weaker than interatomic forces.

Interatomic forces.

Intermolecular forces.

4.1 Understanding Chemical Changes

Physical or chemical change?

4.1 Understanding Chemical Changes

Physical or chemical change?

4.1 Understanding Chemical Changes

Physical or chemical change?

4.1 Understanding Chemical Changes

Physical or chemical change?

4.1 Understanding Chemical Changes

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)

4.1 Understanding Chemical Changes

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

4.1 Understanding Chemical Changes

Structure of the Atom

.

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

An atom is not a hard ball.

4.1 Understanding Chemical Changes

. . .

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

4.1 Understanding Chemical Changes

.

Electrons have a negative electric charge.

4.1 Understanding Chemical Changes

.

Electrons have a negative electric charge.

If negative charges repel each other, how does an atom

stay together?

4.1 Understanding Chemical Changes

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

Neutral: Having zero total electric charge.

Electrons have a negative electric charge.

Protons in the nucleus have a positive electric charge.

4.1 Understanding Chemical Changes

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

4.1 Understanding Chemical Changes

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.

4.1 Understanding Chemical Changes

Electrons are responsible for bond formation.

Electrons can be:

+-

4.1 Understanding Chemical Changes

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

4.1 Understanding Chemical Changes

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

4.1 Understanding Chemical Changes

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.

4.1 Understanding Chemical Changes

Single bonds

Double bond Triple bond

Some elements can share multiple electrons with

the same atom.

Multiple bonds

4.1 Understanding Chemical Changes

Chemical bonds form because there is an advantage in energy.

Lower energy =

more stable

4.1 Understanding Chemical Changes

Lower energy =

more stable

Enthalpy of formation∆Hf (kJ/mole)

Chemical bonds form because there is an advantage in energy.

4.1 Understanding Chemical Changes

ReactantsAll chemical reactions involve three key components:

1. Reactants

4.1 Understanding Chemical Changes

Reactants ProductsAll chemical reactions involve three key components:

1. Reactants

2. Products

4.1 Understanding Chemical Changes

Reactants Products

Ener

gy

All chemical reactions involve three key components:

1. Reactants

2. Products

3. Energy (in or out)

4.1 Understanding Chemical Changes

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)

4.1 Understanding Chemical Changes

Reactivity: The tendency of elements to form chemical bonds.

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

4.1 Understanding Chemical Changes

Electrons make bonds.

Electrons can be:

4.1 Understanding Chemical Changes

Can we predict if a compound is ionic or covalent?

4.1 Understanding Chemical Changes

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

4.1 Understanding Chemical Changes

Can we predict if a compound is ionic or covalent?

Yes!

In a covalent compound, both atoms are nonmetals.

Nonmetal Nonmetal

4.1 Understanding Chemical Changes

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

4.1 Understanding Chemical Changes

Is the compound CF4 ionic or molecular?

4.1 Understanding Chemical Changes

Is the compound CF4 ionic or molecular?Both nonmetals

CHAPTER 4

Physical and Chemical

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.

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.

Hypothesis:The gas produced is water vapor.

4.2 Chemical Reactions

Electrolysis

?

?

Hypothesis:The gas produced is water vapor.

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.

4.2 Chemical Reactions

Electrolysis

?

?

Hypothesis:The gas produced is water vapor.

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.

4.2 Chemical Reactions

?

?

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

4.2 Chemical Reactions

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

+

4.2 Chemical Reactions

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.

+

4.2 Chemical Reactions

+

A chemical equation

4.2 Chemical Reactions

The Chemical Equation

oxygen

hydrogen

oxygen

hydrogen

4.2 Chemical Reactions

The Chemical Equation

1

2

oxygen

hydrogen

2

2

oxygen

hydrogen

!

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

4.2 Chemical Reactions

oxygen

hydrogen

oxygen

hydrogen

The Chemical Equation

4.2 Chemical Reactions

2

4

oxygen

hydrogen

2

4

oxygen

hydrogen

The Chemical Equation

coefficients

4.2 Chemical Reactions

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

The Chemical Equation

Reactants Products

CoefficientsTell you how many of each molecule

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

Reaction arrow

4.2 Chemical Reactions

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.

The Chemical Equation

4.2 Chemical Reactions

Is it balanced?

Reactants Products

Iron (Fe)

Oxygen (O)

4.2 Chemical Reactions

Is it balanced?

Reactants Products

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

4.2 Chemical Reactions

Reactants Products

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

Not balanced

4.2 Chemical Reactions

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.

The Chemical Equation

4.2 Chemical Reactions

Reactants Products

Iron (Fe)

Oxygen (O)

Add coefficients

? ? ?

4.2 Chemical Reactions

Reactants Products

Iron (Fe) 4 4Oxygen (O) 6 6

4 3 2

Balanced!

4.2 Chemical Reactions

Photosynthesis is the basis of most life on Earth.

gases

a sugar(glucose) a gas

4.2 Chemical Reactions

Verify that the equation is balanced

Reactants Products

Carbon (C)

Oxygen (O)

Hydrogen (H)

4.2 Chemical Reactions

Reactants Products

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

Hydrogen (H) 12 12

Verify that the equation is balanced

Number of atoms or moles

4.2 Chemical Reactions

Use the formula mass to convert from moles to grams.

4.2 Chemical Reactions

108 g+ 264 g

372 g

180 g+ 192 g

372 g

Total mass of reactants Total mass of products

4.2 Chemical Reactions

108 g+ 264 g

372 g

180 g+ 192 g

372 g

Total mass of reactants Total mass of products

Conservation of mass

4.2 Chemical Reactions

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

4.2 Chemical Reactions

Reactants Products

Ener

gy

All chemical reactions involve three key components:

1. Reactants

2. Products

3. Energy (in or out)

4.2 Chemical Reactions

Energy “in”

Energy as a reactant.

reactants products

An endothermic reaction requires an input of energy.

4.2 Chemical Reactions

Energy “out”Energy as a product.

An exothermic reaction releases energy.

reactants products

4.2 Chemical Reactions

Energy is absorbed

Energy is released

4.2 Chemical Reactions

Energy is absorbed

Energy is released

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

Are these violations of the first law?

4.2 Chemical Reactions

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

4.2 Chemical Reactions

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?

4.2 Chemical Reactions

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

However, the candle does not spontaneously light itself up.

What is missing?

4.2 Chemical Reactions

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

However, the candle does not spontaneously light itself up.

What is missing?

4.2 Chemical Reactions

Activation energy

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

4.2 Chemical Reactions

Activation energy

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

4.2 Chemical Reactions

Activation energy

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

4.2 Chemical Reactions

Activation energy

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

an energy barrier!

CHAPTER 4

Physical and Chemical

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?

4.3 Chemical Reactions in the Lab

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?

4.3 Chemical Reactions in the Lab

DRY

Light blue powder

No reaction

4.3 Chemical Reactions in the Lab

DRY

No reaction

IN SOLUTION

Light blue powder NaS(aq) CuSO4(aq)

Chemical reaction

A precipitate is an insoluble compound.

4.3 Chemical Reactions in the Lab

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)

4.3 Chemical Reactions in the Lab

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.

4.3 Chemical Reactions in the Lab

Can you translate what you see into a chemical equation?

4.3 Chemical Reactions in the Lab

Can you translate what you see into a chemical equation?

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

and Zn(s) is disappearing

4.3 Chemical Reactions in the Lab

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

and Zn(s) is disappearing

Can you translate what you see into a chemical equation?

a salt in water!

4.3 Chemical Reactions in the Lab

Cu2+(aq)

SO42-(aq)

Can you translate what you see into a chemical equation?

Zn(s) +Cu(s)

and Zn(s) is disappearing

4.3 Chemical Reactions in the Lab

Cu2+(aq)

SO42-(aq)

Zn(s) +Cu(s)

and Zn(s) is disappearing

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.

4.3 Chemical Reactions in the Lab

Cu2+(aq)

SO42-(aq)

Zn(s) +Cu(s)

and Zn(s) is disappearing

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

The copper ion gains 2 electrons to form solid copper.

Where could those electrons

come from?

4.3 Chemical Reactions in the Lab

Cu2+(aq)

SO42-(aq)

Zn(s) +Cu(s)

and Zn(s) is disappearing

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

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

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

4.3 Chemical Reactions in the Lab

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

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.

4.3 Chemical Reactions in the Lab

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

Reactants Products

reduction

REACTION NAME

4.3 Chemical Reactions in the Lab

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

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

Reactants Products

reduction

REACTION NAME

oxidation

4.3 Chemical Reactions in the Lab

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

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

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.

4.3 Chemical Reactions in the Lab

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

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

Reactants Products

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

reduction

REACTION NAME

oxidation

redox

4.3 Chemical Reactions in the Lab

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.

4.3 Chemical Reactions in the Lab

Classify these items as acids or bases.

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

4.3 Chemical Reactions in the Lab

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)

4.3 Chemical Reactions in the Lab

H2O(l)

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

Example:

Sodium hydroxide

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

4.3 Chemical Reactions in the Lab

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

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

4.3 Chemical Reactions in the Lab

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.

4.3 Chemical Reactions in the Lab

4.3 Chemical Reactions in the Lab

Higher H+ concentration Lower H+ concentration

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