chapter 7 chemical reactions 2015 -...

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Chemical Reactions

Section 7.1: Nature of Reactions

• When do chemical reactions take place?

• What is the role of energy in chemical

reactions?

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Chemical Reactions

• It is a change in matter that produces one or more NEW SUBSTANCES with different properties

• A chemical reaction is another name for a chemical change

• New properties are formed• No new atoms are made, but atoms are rearranged

• New compounds can be made• Old bonds are broken and new bonds are formed

• Always involves changes in energy

Chemical Reactions

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• What forms of evidence show that a chemical reaction took place?

– Color Change.

– New phase is made

• Precipitation

• Gas or solid formation.

– Changes in Temperature.

– Changes in Properties.

– Energy change

Chemical Reactions

Energy

Parts To A Chemical Reaction

C O2 CO2

1.Reactants: starting materials of achemical reaction

2.Products: substances formed at the end

Chemical Reactions

Yields

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Chemical Reaction 2

2Na Cl2 2NaCl

What Do You Notice About All The Atoms?

Energy and Reactions• Chemical reaction occurs when chemical

bonds break and new bonds form.

– Always takes a little energy

• Chemical reactions usually absorb heat or liberate (release) heat.

• Different forms of energy can be used:

» Heat

» Electricity

» Light

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Every Reaction Involves Energy

2. New bonds form…energy

released

Energy and Reactions

1. During a reaction… bonds absorb energy and break

� Forming bonds releases energy

� Energy is conserved

�Chemical Energy is energy

stored in the bonds of the

chemicals


food

fuel

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• If breaking bonds takes less energy than making them- it releases energy (exothermic)

•Exo- outside•therm- heat

• Exothermic reactions release energy•Get hot/warm•Give off light•Or release electricity


Exothermic Reaction

Exothermic Reactions

2 Na + Cl2 �� 2 NaCl + Energy

Energy is in the products in an

Exothermic Reaction


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Heat Released

• A chemical reaction in which energy is taken in.

– “absorb” heat

– Feels cool!

– Require heat or energy or they stop


Endothermic Reaction

Endo-intoTherm-heat

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Endothermic Reactions

6 CO2 + 6 H2O + Energy �� C6H12O6 + 6 02

Energy is in the reactants in an

Endothermic Reaction


Example: Photosynthesis

Heat is absorbed

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Exothermic vs. Endothermic


Exo and Endo Clip

What is the law of the conservation of mass?

• During a chemical reaction, matter is neither created nor destroyed.

• Atoms in = atoms out

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Section7.2 Balancing Chemical

Equations

• What is a chemical equation?

• What can a balanced chemical

equation tell you?

Describing Reactions

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Chemical Equation

• A symbolic representation of a

chemical reaction

• Putting chemical changes into words

• The plus mean “and”

• The arrow means “yield” or “make”

2Na + Cl2 2 NaCl


Element Symbols

• All elements are represented by a 1 or 2 letter symbol

C = Carbon Ne = Neon O = Oxygen• The symbols are shown on the periodic table

The burning of methane gas in oxygen is:

CH4 + 2 O2 → CO2 + 2 H2O


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Chemical Formulas

• Shows the elements & number of

atoms of each element in a molecule

H2SO4Hydrogen: 2 atoms

Sulfur: 1 atom

Oxygen: 4 atoms

7 atoms total

Subscript


• A formula may begin with a number.

• This number is called the coefficient.

– Represents the number of molecules of that compound or atom needed in the reaction.

– For example:

• 2H2SO4 – 2 molecules of Sulfuric Acid

• Never put a coefficient in the middle of a formula (2 NaCl is okay, Na2Cl is not)

• If there is no number, then “1” is understood to be in front of the formula.

Coefficients


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Coefficients2H2SO4

2 molecules of Sulfuric Acid

–A coefficient is distributed to ALL

elements in a compound

2 – H2 (for a total of 4 H atoms)

2 – S (for a total of 2 S atoms)

2 – O4 (for a total of 8 O atoms)

• The two sides of the equation are separated by an arrow.

– REACTANTS: The combination of chemicals before the reaction are on the left side of the arrow

– PRODUCTS: The right side indicates the combination of chemicals after the reaction.

Reading Chemical Equations

CH4 + 2 O2 → CO2 + 2 H2O


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2Na + Cl2 2 NaCl

Parts of a chemical Equation

“yields”Products

Everything

to the right

Reactants

Everything to

the left

Coefficientthe number of

Molecules in an

equation

Subscriptthe number of atoms

in a formula

Balanced Chemical Equations

• A balanced chemical equation follows the law of conservation of mass.

• It can tell you the amount of reactants you will need, and the amount of products you will get from the reaction.

• For example, – Methane + oxygen �� carbon dioxide + water

– CH4 + O2 �� CO2 + H2O

• Does not tell you how much of each compound you will need.

– CH4 + 2 O2 �� CO2 + 2 H2O

• This balanced equation does.

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Balance equations by changing coefficients

… never by changing formula subscripts

Balancing Chemical Equations

2Na + Cl2 2 NaCl

• Remember the Law Conservation of Mass:

Matter cannot be created or destroyed. That

means we need to have the same amount of

chemicals on each side of the �.

• Example


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Rules for balancing

1 Write the correct formulas for all the

reactants and products

2 Count the number of atoms of each type

appearing on both sides

3 Balance the elements one at a time by

adding coefficients (the numbers in front)

4 Check to make sure it is balanced.

• Begin balancing chemical equations by

putting numbers (coefficients) in front of

them.

– Example H2O on one side could become 2 H2O

– Remember that each side needs to have same number of Hydrogen and Oxygen

• Note – Don’t change the subscript

• Example H2O becomes H3O


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• A way to start off the process is to create

an inventory of your chemicals.

• Now look at the inventory and begin the

process of balancing the equation.

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One Sodium

Add a 2 and check if it balance

2

Did it balance? Are we done? NO

2

2

Time for a new chart….. Let’s try again and add a 2 to the other side.

2

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Does it balance this time? This is a balanced equation.

Photosynthesis Reaction

• Carbon dioxide + water � Glucose (sugar) + oxygen

6CO2 + 6 H2O �� C6H12O6 + 6O2

• Count the atoms on each side of the equation.

C : 6O : 18H : 12

C : 6O : 18H : 12

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Balancing Equations Practice

1. AgNO3 + Cu → Cu(NO3)2 + Ag

2. Mg + N2 → Mg3N2

3. P + O2 → P4O10

4. Na + H2O → H2 + NaOH

5. CH4 + O2 → CO2 + H2O

Elements Before After

More Balancing Equations Practice

a) Pb(NO3)2 + K2CrO4 → PbCrO4 + KNO3

b) MnO2 +HCl → MnCl2 + H2O+ Cl2c) C3H6 + O2 →CO2 +H2O

d) Zn(OH)2 + H3PO4 → Zn3(PO4)2 + H20e) CO + Fe2O3 →Fe + CO2

f) CS2 + Cl2 →CCl4 +S2Cl2g) CH4 + Br2 → CH3Br + HBr

h) Ba(CN)2 + H2SO4 → BaSO4 + HCN

Homework Your Turn

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There are 7 elements that exist in nature

as diatomic molecules.What elements exist as diatomic molecules?

H2, N2, O2, F2, Cl2, Br2, I2

Diatomic Molecules

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A. Potassium iodide reacts with bromine

forming potassium bromide plus iodine.

More Balancing Equations Practice

B. Nickel (III) chloride reacts with beryllium

oxide forming nickel (III) oxide plus beryllium

chloride.

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C. Chromium (II) sulfide decomposes

producing chromium (II) plus sulfur.

D. Titanium (III) oxide + copper (I) phosphate ->

titanium (III) phosphate + copper (I) oxide

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Balanced Equations

and Mole Ratios

• A balanced equation tells you the mole ratio, or proportion of reactants and products, in a chemical reaction.

In photosynthesis, for every 6 moles of carbon dioxide used, 6 moles oxygen is produced.

This is a 1:1 molar ratio

6CO2 + 6 H2O �� C6H12O6 + 6O2

Mole Ratios Can Be

Converted To Masses

• Mole Ratios can be converted to mass by

taking the molar mass of the molecule

times the number of moles required.

– O2 has a molar mass of 32g/mol, mass of 32 g

– 3 O2 has a mass of 3 mol x 32 g/mol = 96 g

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Section 7.3 Chemical Reaction Types

• How does learning about reaction types

help in understanding chemical reactions?

• In which kind of chemical reactions do the

number of electrons in atoms change?

• All the reactants get turned into products

• No reactants left

• Some reactions go both directions

• They are called reversible reactions

• 2H2(g) + O2(g) → 2H2O(g) + energy

• 2H2O(g) + energy → 2H2(g) + O2(g)

• 2H2(g) + O2(g) 2H2O(g) + energy

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• Helps to identify the kinds of chemical reactions and to predict the products of the chemical reactions.

• Five general kinds of reactions:– Synthesis

– Decomposition

– Single Displacement

– Double Displacement

– Combustion Reaction

Synthesis A+ B � AB

Decomposition AB � A + B

Single displacement AC +B �BC + A

Double displacement AC + BD � AD + BC

Combustion Ex: CH4 + 2O2 � CO2 +2H2O

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• Two or more substances (reactants)• Combine to form only one substance

(product)

Mg + O2 �� MgO

K + Cl2 �� KCl

Fe + O2 �� Fe2O3

Synthesis Reaction

• One substance (reactant) combine forms 2 or more substances (products)

H2CO3→ H2O + CO2

MClO3 → MCl + O2

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• One element and one compound (in reactants)

• Produces one element and one compound (in Products)

Zn + CuSO4 --> Cu + ZnSO4

F2 + 2KI �� I2 + 2KF

AC + B � BC + A

• 2 compounds (in reactants) produce• 2 compounds (in products)

AgNO3 + NaCl → AgCl+ NaNO3

Na2CO3 + CaCl2 ------> CaCO3 + 2NaCl

Double displacement

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Combustion Reaction

• A reaction in which a compound and oxygen burn.

•Water is a common product

Type of

Reaction

Definition �� Equation

Synthesis

Decomposition

Single –

Displacement

Or Replacement

Double –

Displacement or

Replacement

A = Red B = Blue C = Green D = Yellow

A + B → AB

AB → A + B

AB + C → AC + B

AB + CD → AC + BD

Two or more elements or compounds combine to make a more complex

substance

Compounds break down into simpler substances

Occurs when one element replaces another one in a

compound

Occurs when different atoms in two different

compounds trade places

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Classifying Reaction Practice

A. S8 + 8O2 � 8SO2 + energy

B. 6CO2 + 6H20 � C6H12O6 + 6O2

C. 2NaHCO3 � Na2CO3 + H2O + CO2

D. Zn + 2HCl � ZnCl2 + H2

Single-displacement

Decomposition

Synthesis

Synthesis

Section 7.4 Reaction Rates & Equilibrium

• What kind of things speed up a reaction.

• What does a catalyst do?

• What happens when a reaction goes

backward as well as forward?

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Things that Affect Reaction Rate

Anything that increases contact between particles will

increase the rate of a reaction

•Particle size

•Surface area

•Temperature

•Concentration

•Pressure


Particle size

•Massive, bulky molecules react

more slowly, because they

move more slowly.

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Surface Area

A large surface area

speeds up reactions.


Temperature

Most reactions go

faster at higher

temperatures.

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Concentration

Higher concentrations of

reactants react faster.


Pressure

Reactions are faster at

higher pressure.

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Catalysts - substances that speedup a reaction without being used up.Heat destroys catalysts

Inhibitor – a substance that blocks a catalyst, slowing the reaction down


Equilibrium Systems

• Chemical equilibrium- state

of balance in which the rate

forward equals the rate of

the reverse reaction.

– Products and reactants

remain unchanged.

– The reactions are still

running.

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• Equilibrium position- how much product and

reactant there are at equilibrium.

• Shown with the double arrow.

• Reactants are favored

• Products are favored

• Catalysts speed up both the forward and

reverse reactions so don’t affect equilibrium

position.

Equilibrium Systems

Which is favored? Product

• Bill Nye Video

chapter 7 chemical reactions 2015 -...

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