natural approach to chemistry chapter 4 physical and chemical change 4.1 understanding chemical...
TRANSCRIPT
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Natural Approach to Chemistry
Chapter 4 Physical and Chemical Change
4.1 Understanding Chemical Changes Frame 2-414.2 Chemical Reactions Frames 42-824.3 Chemical Reactions in the lab Frames 83-109
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CHAPTER 4
Physical and Chemical
Change4.1 Understanding Chemical Changes
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4.1 Understanding Chemical Changes
H2O(s) H2O(l)
Is it still the same substance?
Have the physical properties changed?
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4.1 Understanding Chemical Changes
?
Can water go back to being ice?
H2O(s) H2O(l)
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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?
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4.1 Understanding Chemical Changes
2C20H42(s) + 61O2(g) 42H2O(g) + 40CO2(g)
If the flame cools down:
Does the candle come back?
?
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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.
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4.1 Understanding Chemical Changes
In a physical change: - the molecules are rearranged- intermolecular forces are broken
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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
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4.1 Understanding Chemical Changes
Intermolecular forces are much weaker than interatomic forces.
Interatomic forces.
Intermolecular forces.
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4.1 Understanding Chemical Changes
Physical or chemical change?
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4.1 Understanding Chemical Changes
Physical or chemical change?
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4.1 Understanding Chemical Changes
Physical or chemical change?
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4.1 Understanding Chemical Changes
Physical or chemical change?
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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)
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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
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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.
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4.1 Understanding Chemical Changes
. . .
The number of electrons corresponds to the atomic number of the element, as shown in the periodic table.
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4.1 Understanding Chemical Changes
.
Electrons have a negative electric charge.
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4.1 Understanding Chemical Changes
.
Electrons have a negative electric charge.
If negative charges repel each other, how does an atom
stay together?
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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.
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4.1 Understanding Chemical Changes
Why don’t the negative electrons “fall” into the positive nucleus?
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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.
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4.1 Understanding Chemical Changes
Electrons are responsible for bond formation.
Electrons can be:
+-
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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
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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
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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.
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4.1 Understanding Chemical Changes
Single bonds
Double bond Triple bond
Some elements can share multiple electrons with
the same atom.
Multiple bonds
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4.1 Understanding Chemical Changes
Chemical bonds form because there is an advantage in energy.
Lower energy =
more stable
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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.
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4.1 Understanding Chemical Changes
ReactantsAll chemical reactions involve three key components:
1. Reactants
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4.1 Understanding Chemical Changes
Reactants ProductsAll chemical reactions involve three key components:
1. Reactants
2. Products
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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)
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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)
HeSome elements are always found in pure form in nature: Ex: Helium (He), argon (Ar)
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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.
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4.1 Understanding Chemical Changes
Electrons make bonds.
Electrons can be:
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4.1 Understanding Chemical Changes
Can we predict if a compound is ionic or covalent?
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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
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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
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4.1 Understanding Chemical Changes
Use the periodic table to help determine whether a compound is ionic or molecular.
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4.1 Understanding Chemical Changes
Is the compound CF4 ionic or molecular?
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4.1 Understanding Chemical Changes
Is the compound CF4 ionic or molecular?Both nonmetals
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CHAPTER 4
Physical and Chemical
Change4.2 Chemical
Reactions
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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.
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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.
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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.
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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.
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4.2 Chemical Reactions
?
?
ElectrolysisClues:One gas burns.One gas causes a flame to get brighter.The gases come from water.
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4.2 Chemical Reactions
ElectrolysisClues:One gas burns.One gas causes a flame to get brighter.The gases come from water.
+
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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.
+
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4.2 Chemical Reactions
+
A chemical equation
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4.2 Chemical Reactions
The Chemical Equation
oxygen
hydrogen
oxygen
hydrogen
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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?
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4.2 Chemical Reactions
oxygen
hydrogen
oxygen
hydrogen
The Chemical Equation
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4.2 Chemical Reactions
2
4
oxygen
hydrogen
2
4
oxygen
hydrogen
The Chemical Equation
coefficients
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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
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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
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4.2 Chemical Reactions
Is it balanced?
Reactants Products
Iron (Fe)
Oxygen (O)
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4.2 Chemical Reactions
Is it balanced?
Reactants Products
Iron (Fe) 1 2
Oxygen (O) 2 3
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4.2 Chemical Reactions
Reactants Products
Iron (Fe) 1 2
Oxygen (O) 2 3
Not balanced
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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
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4.2 Chemical Reactions
Reactants Products
Iron (Fe)
Oxygen (O)
Add coefficients
? ? ?
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4.2 Chemical Reactions
Reactants Products
Iron (Fe) 4 4
Oxygen (O) 6 6
4 3 2
Balanced!
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4.2 Chemical Reactions
Photosynthesis is the basis of most life on Earth.
gases
a sugar(glucose) a gas
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4.2 Chemical Reactions
Verify that the equation is balanced
Reactants Products
Carbon (C)
Oxygen (O)
Hydrogen (H)
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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
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4.2 Chemical Reactions
Use the formula mass to convert from moles to grams.
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4.2 Chemical Reactions
108 g+ 264 g
372 g
180 g+ 192 g
372 g
Total mass of reactants Total mass of products
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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
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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
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4.2 Chemical Reactions
Reactants Products
Ener
gy
All chemical reactions involve three key components:
1. Reactants
2. Products
3. Energy (in or out)
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4.2 Chemical Reactions
Energy “in”
Energy as a reactant.
reactants products
An endothermic reaction requires an input of energy.
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4.2 Chemical Reactions
Energy “out”Energy as a product.
An exothermic reaction releases energy.
reactants products
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4.2 Chemical Reactions
Energy is absorbed
Energy is released
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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?
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4.2 Chemical Reactions
First law of thermodynamics:Energy can neither be created nor destroyed.
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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?
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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?
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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?
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4.2 Chemical Reactions
Activation energy
C6H12O6 + 6O2 6H2O + 6CO2 + 2,800,000J
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4.2 Chemical Reactions
Activation energy
C6H12O6 + 6O2 6H2O + 6CO2 + 2,800,000J
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4.2 Chemical Reactions
Activation energy
C6H12O6 + 6O2 6H2O + 6CO2 + 2,800,000J
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4.2 Chemical Reactions
Activation energy
C6H12O6 + 6O2 6H2O + 6CO2 + 2,800,000J
an energy barrier!
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CHAPTER 4
Physical and Chemical
Change4.3 Chemical Reactions in
the Lab
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4.3 Chemical Reactions in the Lab
Why would the presence of water on Mars be a sign that life might exist there?
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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?
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4.3 Chemical Reactions in the Lab
DRY
Light blue powder
No reaction
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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.
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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)
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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.
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4.3 Chemical Reactions in the Lab
Can you translate what you see into a chemical equation?
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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
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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!
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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
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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.
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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?
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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)
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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.
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4.3 Chemical Reactions in the Lab
Cu2+(aq) + 2e- Cu(s)
Reactants Products
reduction
REACTION NAME
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4.3 Chemical Reactions in the Lab
Cu2+(aq) + 2e- Cu(s)
Zn(s) Zn2+(aq) + 2e-
Reactants Products
reduction
REACTION NAME
oxidation
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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.
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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
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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.
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4.3 Chemical Reactions in the Lab
Classify these items as acids or bases.
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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
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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
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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
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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.
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4.3 Chemical Reactions in the Lab
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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.