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CHEMISTRY II
UNIVERSIDAD AUTÓNOMA DE NUEVO LEONCIDEB
STAGE I: CHEMICAL REACTIONS IN OUR SURROUNDINGS
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Stage 1: Chemical reactions in our surroundings
Formative Objective: The student solves stoichiometric problems from the information contained in chemical equations about daily life phenomena. Also thinks about the consumption of fossil fuels to raise awareness and propose possible solutions to environmental phenomena
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Changes in matterPhysical change: the
composition of a substance is not affected.
Chemical change: a change in the atomic makeup (composition) of a substance. One or more substances are used up as others are formed.
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PRACTICE! Is it a physical or a chemical change?A glass bowl falls to the floorA mixture of hydrogen and
oxygen gases explode with a bang when ignited, producing water
Sharpening a pencilDigestion of a candy barA fireworks aerial displayMelting butter
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• The process by which the atoms are rearranged to form different substances. Always involves a chemical change.
Chemical
reactions
• Are used to represent, symbolically, what is taking place during the reaction. Tells us the materials we need and the products that will form in a chemical reaction.
Chemical
equations
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Visible Evidence of a Chemical Reaction
Change in the colorFormation of a gasFormation of a solidTemperature change
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Chemical ReactionsReactants: the starting substances
◦ Shown on the left side of the equation◦ Separated by a plus sign (+)
Products: the substances produced during the reaction◦ Shown on the right side of the equation
Reactants and products are separated by an arrow which means “yields” or “produces”
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Symbols used in chemical equations Symbol Meaning
+ Plus, is used to separate the reactants or products
"yields" is used to indicate the result of the reaction. Separates reactants from products
the reaction is reversible
(s) the substance is in a solid state
(l) the substance is in a liquid state
(g) the substance is in a gaseous state
(aq) the substance has been dissolved in an aqueus solution
Δ (over the reaction arrow) indicates the application of heat to the reactants
↑ indicates a change of state (into a gas)
↓ indicates a precipitate formed by a reaction
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Representing chemical reactions
• Indicates the reactants and products of chemical reactions.
• Aluminum (s) + bromine (l) aluminum bromide (s)
Word equatio
ns• Uses chemical formulas to identify the
reactants and products• Al(s) + Br2(l) AlBr3
Skeleton
equations • Shows that matter is conserved during the
reaction. Uses chemical formulas to show the identities and relative amounts of the substances involved
• 2Al(s) + 3Br2(l) 2AlBr3
Chemical
equations
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Practice problems on page 284
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Balancing chemical equationsIn a balanced chemical equation
the number of atoms of each kind of element represented as reactants and products must be equal.Step 1
• Write the skeleton equation for the reaction
Step 2• Count the atoms of the elements in
reactants and products. Change the coefficients to make the number of atoms of each element equal on both sides. (Never change a subscript)Step 3
• The coefficients should be the smaller posible whole numbers. Check your work, verify that the number of atoms of each element si equal on both sides of the equation.
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Practice problems (page 287)
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Reaction RatesThe change in concentration of a
reactant or product per unit of time.
Collision theory: states that atoms, ions and molecules must collide in order to react.
Activation energy: the minimum amount of energy that reacting particles must have to react.
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Fact
ors
aff
ect
ing
react
ion
ra
tes
• Some substances are more reactive than othersNature of
reactants
• Increasing the concentration of reactants, increases the rate of reactions
• The more particles present, the more often they collide
Concentration
• The smaller the size of particle, the greater the surface area.
• Increasing the surface area speeds up the rate of reaction by increasing the collision rate.
Surface area
• Increasing the temperature, increases the rate of reaction
• The higher the temperature, the higher the kinetic energy of particles, the more they collide.
Temperature
• Catalyst: substance that increases the rate of reaction
• Inhibitor: substance that slows down or inhibits reaction rates.
Catalysts and
inhibitors
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Classifying Reactions
CHEMICAL REACTIONS
Synthesis A + B AB
Combustion A + O2 AO
Decomposition AB A + B
Single Replacemen
t
A + BC AC + B
Double Replacemen
t
AB + CD AD + CB
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Classifying Reactions• When two or more substances react to produce a single product.
• A + B AB
Synthesis (Combination)
• Oxygen combines with a substance to release energy in form of heat and light.Combustion
• A single compound is broken down into two or more simple substances. Often requires an energy source, like heat, light or electricity to occur.
• AB A + B
Decomposition
• A reaction in which the atoms of one element replace the atoms of another element in a compound.
• A + BC AC + BSingle Replacement
• Involves an exchange of ions between two compounds. All of them produce either water, a precipitate or a gas.
• AB + CD AD + CB
Double Replacement
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Predicting products of Chemical Reactions
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Let´s practice!Balance the following equations and classify
them
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Energy changes in chemical reactions Exothermic reaction: involves the release of heat
energy. Occurs when more energy is released during product bond formation than is required to break bonds in the reactants.
Endothermic reactions: absorb or take up heat energy. Occurs when more energy is released during product bond formation than is required to break bonds in the reactants.
ACTIVATION ENERGY: the minimum kinetic energy that colliding molecules must possess for a reaction to occur.
Classify each of the following reactions as either exothermic or endothermic.
a) 2H2O(l) + heat → 2H2(g) + O2(g) b) Mg(s) + Cl2(g) → MgCl2(s) + heat
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STOICHIOMETRYThe study of quantitative relationships between the amounts of reactants used and amounts of products formed by a chemical reaction.
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MOLE
SI base unit used to measure the amount of a substance .
Is defined as the number of carbon atoms in exactly 12 g of the carbon-12 isotope.
Avogadro´s Number:
6.022 x 1023 particles
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Molar massThe mass in grams of a mole of
any substance that is the sum of the atomic masses of all the atoms represented in the formula and expressed in grams.
H2SO4 Al2(CO3) 3
98 gr / mol 234 gr / mol
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Law of conservation of massMatter is neither created nor
destroyed in a chemical reaction.
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Mole RatiosIs a ratio between the numbers
of moles of any two of the substances in a balanced chemical equation.
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Practice problems page 372
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Grams A
1 m
ol
Mola
r mass
(g)
Moles A Moles BMoles UNKNOWN
Moles KNOWN
Mola
r m
ass
(g
)
1 m
ol
Grams B
Solving stoichiometry problems
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Stoichiometric calculationsMOLE TO MOLE CONVERSION
Moles of UNKNOWN chemicalGiven moles of x ____________________________KNOWN chemical
Moles of KNOWN chemical
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Stoichiometric calculationsMOLE TO MASS CONVERSION
MOLES A MOLES B GRAMS B
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Stoichiometric calculationsMASS TO MASS CONVERSION
GRAMS A MOLES A MOLES B GRAMS B