Download Section 1 Forming New Substances Chapter 9

Chemical Reactions
Chapter 9
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Section 1 Forming New Substances
chemical reaction is a process in which one
or more substances change to make one or
more new substances.
NEW SUBSTANCE◦ chemical /physical properties that form in a
chemical reaction differ from those of the
original substances.
Chapter 9
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Section 1 Forming New Substances
Signs OF chemical reaction
◦ color change
◦ formation of a gas or a solid
◦ the release or absorption of energy.
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Precipitate- solid that is produced as a
result of a chemical reaction in solution
Chapter 9
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Chemical Reactions
Chapter 9
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Section 1 Forming New Substances
SIGNALS OF
◦ they do not always guarantee that a reaction is
happening.
◦ most important sign - formation of new
substances that have new properties.
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chemical bond -is a force that holds two
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For a chemical reaction to take place
atoms together in a molecule.
◦ Chemical bonds MUST BREAK
(in starting substance)
◦ Atoms rearrange
◦ new bonds form make new substances
Chapter 9
Chemical Reactions
Chapter 9
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Section 1 Forming New Substances
Energy
◦ breaks chemical bonds
◦ As new bonds form energy is released
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exothermic reaction- chemical reaction in
which energy is released
can give off energy in several forms
Chapter 9
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Section 1 Forming New Substances
endothermic reaction- chemical reaction in
which energy is taken in
energy taken in  is absorbed from the
surroundings.
Ex: Photosynthesis
Chapter 9
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Section 1 Forming New Substances
Neither mass nor energy can be
◦ created or destroyed in chemical reactions.
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law of conservation of energy states that
energy cannot be created or destroyed but
can change from one form to another
(TRANSFER)
Chapter 9
Chemical Reactions
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The formation of water from its elements may
be represented as follows:
2H2 + O2  2H2O
What elements are present in water?
◦ Hydrogen and Oxygen
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How many atoms of each element are shown
in the reactants?
◦ 4-H; 2-O
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How many atoms of each element are shown
in the products?
4-H; 2-O
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The representation 2H2 + O2  2H2O is called
a chemical equation. Why do you think that
the numbers of atoms do not change?
Chapter 9
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Section 2 Chemical Formulas
and Equations
Chemical formulas are a simple way to
describe which elements are in a chemical
substance.
Chemical equations are a concise way to write
how atoms are rearranged in a chemical
reaction.
A balanced chemical equation shows the law
of conservation of mass.
Chapter 9
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Section 2 Chemical Formulas
and Equations
chemical formula- is a combination of
chemical symbols and numbers to
represent a substance.
◦ shows how many atoms of each kind of element
are present in a molecule.
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Subscript- is a number written below and
to the right of a chemical symbol in a
formula.
Chapter 9
Chemical Reactions
Chapter 9
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Section 2 Chemical Formulas
and Equations
covalent compounds -written by using the
prefixes in the names of the compounds.
ionic compounds- written by using enough
of each ion so that the overall charge
is 0.
Chapter 9
Chemical Reactions
Chapter 9
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Section 2 Chemical Formulas
and Equations
chemical equation- uses chemical
formulas, chemical symbols, and
coefficients to describe a reaction.
Reactants-starting materials in a chemical
reaction
Products-substances formed from a
reaction are
Chapter 9
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Section 2 Chemical Formulas
and Equations
writing a chemical equation
◦ Must be balancing –
◦ total number of atoms of each element in the
reactants must equal the total number of atoms
of that element in the products
◦ #reactants=#products
Chapter 9
Chemical Reactions
Chapter 9
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Section 2 Chemical Formulas
and Equations
law of conservation of mass -states that
mass cannot be created or destroyed in
ordinary chemical and physical changes.
This law means that the total mass of the
reactants is the same as the total mass of
the products.
Chapter 9
Chemical Reactions
Chapter 9
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Section 2 Chemical Formulas
and Equations
A coefficient is a number that is placed in
front of a chemical symbol or formula.
Coefficients are used to balance an
equation.
For an equation to be balanced, all atoms
must be counted.
Chapter 9
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Section 2 Chemical Formulas
and Equations
To count all the atoms in an equation,
multiply the subscript of each element in the
formula by the formula’s coefficient.
To balance an equation, change the
coefficients—not the subscripts.
Changing the subscripts in the formula of a
compound changes the compound.
Chapter 9
Section 2 Chemical Formulas
and Equations
Look at the following formulas for chemical
compounds, and identify the elements in each
formula.
NaCl
KBr
C12H22O11
NH3
SiF4
Fe(NO3)3
H2 O 2
MgSO4
What is the name and symbol of each element?
How many atom of each element are present
in each compound?
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A chemical equation describes what happens
in a chemical reaction.
The equation identifies
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reactants (starting materials)
and products (resulting substance),
the formulas of the participants,
the phases of the participants (solid, liquid, gas),
and the amount of each substance.
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Balancing a chemical equation
◦ refers to establishing the mathematical
relationship between the quantity of reactants and
products.
◦ quantities are expressed as grams or moles.
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3 steps to the process:
Write the unbalanced equation.
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Balance the equation.
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Chemical formulas of reactants are listed on the lefthand side of the equation.
Products are listed on the righthand side of the equation.
Reactants and products are separated by putting an arrow between them to show the
direction of the reaction. Reactions at equilibrium will have arrows facing both directions.
Apply the Law of Conservation of Mass to get the same number of atoms of every element
on each side of the equation. Tip: Start by balancing an element that appears in only one
reactant and product.
Once one element is balanced, proceed to balance another, and another, until all elements
are balanced.
Balance chemical formulas by placing coefficients in front of them. Do not add subscripts,
because this will change the formulas.
Indicate the states of matter of the reactants and products.
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Use (g) for gaseous substances.
Use (s) for solids.
Use (l) for liquids.
Use (aq) for species in solution in water.
Write the state of matter immediately following the formula of the substance it describes.
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Tin oxide is heated with hydrogen gas to
form tin metal and water vapor. Write the
balanced equation that describes this
reaction.
Write the unbalanced equation.
SnO2 + H2 → Sn + H2O
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1.Count the atoms◦ Look at the equation and see which elements are
not balanced.
◦ there are two oxygen atoms on the lefthand side of
the equation and only one on the righthand side.
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2. Balance them out◦ Correct by putting a coefficient of 2 in front of
water:
◦ SnO2 + H2 → Sn + 2 H2O
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2. Balance other elements◦ This puts the hydrogen atoms out of balance.
◦ Now there are two hydrogen atoms on the left and
four hydrogen atoms on the right.
◦ To get four hydrogen atoms on the right, add a
coefficient of 2 for the hydrogen gas.
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Remember, coefficients are multipliers, so if
we write 2 H2O it denotes 2x2=4 hydrogen
atoms and 2x1=2 oxygen atoms.
SnO2 + 2 H2 → Sn + 2 H2O
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The equation is now balanced.
Be sure to double-check your math!
Each side of the equation has 1 atom of Sn, 2
atoms of O, and 4 atoms of H.
SnO2(s) + 2 H2(g) → Sn(s) + 2 H2O(g)
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Indicate the physical states of the reactants
and products. To do this, you need to be
familiar with the properties of various
compounds or you need to be told what the
phases are for the chemicals in the reaction.
Oxides are solids, hydrogen forms a diatomic
gas, tin is a solid, and the term 'water vapor'
indicates that water is in the gas phase:
SnO2(s) + 2 H2(g) → Sn(s) + 2 H2O(g)
This is the balanced equation for the reaction.
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Balance equations
How is the law of conservation of energy true
for both endothermic/exothermic reactions?
What happens to bonds in chemical reaction?
As a solution of water and sugar dries, a
crystal forms on the end of a string that is
suspended in the solution. Is this an example
of a chemical reaction? Explain.
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Energy is always 1)___________ in
2)___________..
Which are classified as 3)___________if energy
is 4)________________ and stored in
5)____________such as in photosynthesis.
Which are classified as 6)___________ if energy
is 7)____________ from 8)__________ such as in
burning wood.
 Absorbed
 Chemical
equations
 Conserved
 Endothermic
 Photosynthesis  Released
 Exothermic
 Products
 Five
 Reactants
 Chemical bonds  progress
 Balanced
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Precipitate
Chemical reaction
Chemical equation
Chemical formula
Chemical bonds
Coefficient
Subscript
Yield sign
Product
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Reactants
Exothermic reaction
Endothermic reaction
Law of conservation
of energy
Diatomic molecule
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Chemical reaction
◦ What is an example of one?
◦ What happens to the bonds
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Chemical equations
◦ How to balance
◦ Why must be balanced
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Diagram
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Activation energy
Energy absorbed
Energy released
Exothermic/endothermic reaction