Unit 1 Chemical reactions in our surroundings

CHEMISTRY II

UNIVERSIDAD AUTÓNOMA DE NUEVO LEONCIDEB

STAGE I: CHEMICAL REACTIONS IN OUR SURROUNDINGS

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

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.

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

• 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

Visible Evidence of a Chemical Reaction

Change in the colorFormation of a gasFormation of a solidTemperature change

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”

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

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

Practice problems on page 284

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.

Practice problems (page 287)

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.

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

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

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

Predicting products of Chemical Reactions

Let´s practice!Balance the following equations and classify

them

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

STOICHIOMETRYThe study of quantitative relationships between the amounts of reactants used and amounts of products formed by a chemical reaction.

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

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

Law of conservation of massMatter is neither created nor

destroyed in a chemical reaction.

Mole RatiosIs a ratio between the numbers

of moles of any two of the substances in a balanced chemical equation.

Practice problems page 372

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

Stoichiometric calculationsMOLE TO MOLE CONVERSION

Moles of UNKNOWN chemicalGiven moles of x ____________________________KNOWN chemical

Moles of KNOWN chemical

Stoichiometric calculationsMOLE TO MASS CONVERSION

MOLES A MOLES B GRAMS B

Stoichiometric calculationsMASS TO MASS CONVERSION

GRAMS A MOLES A MOLES B GRAMS B