chemical reactions chapters 5 sections 1, 2 & 3 6 sections 2, 3
TRANSCRIPT
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Chemical Reactions
Chapters 5 sections 1, 2 & 3
6 sections 2, 3
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Valence Electrons
•Only electrons involved in bonding. The number of valence electrons affects whether atoms bond or not.
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Electron Dot Diagram or Lewis Dot Structures
•Just the symbol and the # of valence electrons.
Li Br
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Bonding•Chemical bonds form when a chemical reaction occurs. The bonds form when either valence electrons are transferred, taken away or shared between atoms.
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Reactivity
•When bonds form the atoms become more stable.
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Ionic Bonds
•Electron Transfer: Atoms with fewer than 4 valence electrons will transfer them to an atom with four or more, causing the atoms to become stable.
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Ions
•If an atoms loses an electron it becomes a positive Ion.
•If it gains an electron it becomes negative.
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Ions and Ionic Bonds• Ionic bonds form as a result of the
attraction between positive and negative ions.
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Covalent Bonds
•When compounds are held together by sharing electrons.
•Common covalent compounds are Fat, proteins, carbs, cotton and wool.
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Covalent Bonds cont…
•They have low melting points and boiling points.
•Poor conductors of electricity.
•Elements from same side of the table bonded together.
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How Covalent Bonds Form• The force that holds atoms together in a
covalent bond is the attraction of each atom’s nucleus for the shared pair of electrons.
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Nonpolar Bonds
•Bonds cancel each other out Typically covalent bonds.
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Polar Compounds
•When electrons are shared unequally, it causes one atom to be slightly positive and the other will be slightly negative.
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Electrons are pulled closer to Oxygen giving it a slight negative charge.
Hydrogen gets a slight positive charge because the electrons are pulled further away from hydrogen.
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Chemical Reactions
Production of new materials that are
chemically different from the
beginning materials.
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Properties and Changes in Matter• Matter can undergo both
physical change and chemical change.
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Evidence for Chemical Reactions
•Color change•Precipitate•Gas production•Changes in temperature•Changes in properties
A solid that forms after a rxn.
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Chemical Equations• A shorter, easier way of showing
chemical reactions using symbols instead of words.
• Chemical Formula – shows the ratio of elements in a compound
•H2O
•NaCl
•CO2
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Structure of an Equation
•Reactants – materials you start with
•Products – materials you end with
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Reactant + Reactant Product
H2 + O2 H2O2
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Conservation of Mass•The amount of matter in a chemical reaction does not change, so the total mass of the reactants must equal the total mass of the products.
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Conservation of Matter
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Classifying Chemical Reactions
•Synthesis •Decomposition•Single Replacement•Double Replacement
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Synthesis
•Two or more elements combine to form a more complex compound.
•A + B AB•Hand + kerchief
handkerchief
•Example: C + 2Cl2 CCl4
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Decomposition
•Breaks down compounds into simpler substances.
•AB A + B•Schoolbook school + book• Example: 2Al2O3 4Al + 3O2
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Replacement• A reaction in which one element
replaces another in a compound, or two elements in different compounds trade places
• AB + C AC + B (Single)• AB + CD AC + BD (Double)• T.V. screen + door knob T.V.
knob + screen door• HCl + NaOH H2O + NaCl
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•To describe a reaction accurately, a chemical equation must show the same number of each type of atom on both sides of the equation.
Writing and Balancing Chemical Equations
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Writing and Balancing Chemical Equations
•CO2
Subscript= IDENTIFIES THE NUMBER OF ATOMS IN A COMPOUND.
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Writing and Balancing Chemical Equations cont…
Mg + O2 MgO2 2
Coefficient is the number in front of the chemical formula that represents the # of molecules.
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H2 + N2 NH3 3 2
HgO + Cl2 HgCl + O2 2 2
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Balancing Chemical Equations
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Balancing Equations Activity
• Click the Active Art button to open a browser window and access Active
Art about balancing equations.
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Balancing Chemical Equations
•Magnesium (Mg) reacts with oxygen gas (O2), forming magnesium oxide (MgO). To write a balanced equation for this reaction, first write the equation using the formulas of the reactants and products, then count the number of atoms of each element.
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Balancing Chemical Equations
• Balancing Chemical Equations:
• Balance the equation for the reaction of sodium metal (Na) with oxygen gas (O2), forming sodium oxide (Na2O).
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• Balancing Chemical Equations:
• Balance the equation for the reaction of tin (Sn) with chlorine gas (Cl2), forming tin chloride (SnCl2).
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Controlling Rates of Reaction
•Concentration: the more particles to react, the faster the reaction.
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Controlling Rates of Reaction cont…
•Surface area: the more particles that are exposed in a reaction, the faster it is.
•Temperature: If temperature is increased, then the rate of reaction increases.
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Controlling Rates of Reaction cont…
• Catalyst: Helps reaction happen by lowering activation energy.
•Inhibitor: Decreases rate of reaction.
•Enzyme: Biological catalyst in our bodies that allow important bodily functions to occur.
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Controlling Chemical Reactions
• Every chemical reaction involves energy in one of two ways:
• Exothermic: Energy is released.• Endothermic: Energy is absorbed.
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Endothermic Reaction
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Activation Energy
•The minimum amount of energy needed to get a reaction started.
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Acids and Bases
• Acid: Tastes sour, react with metals and turns litmus paper red.
• Base: Bitter, feels slippery and turns litmus paper blue.
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pH
•A scale that measures whether a chemical solution is acidic or basic.
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pH Scale
• The pH scale is from 0-14, 14 being basic, 0 being acidic and 7 being neutral.
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