Chemical Reactions

Chapters 5 sections 1, 2 & 3

6 sections 2, 3

Valence Electrons

•Only electrons involved in bonding. The number of valence electrons affects whether atoms bond or not.

Electron Dot Diagram or Lewis Dot Structures

•Just the symbol and the # of valence electrons.

Li Br

Bonding•Chemical bonds form when a chemical reaction occurs. The bonds form when either valence electrons are transferred, taken away or shared between atoms.

Reactivity

•When bonds form the atoms become more stable.

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.

Ions

•If an atoms loses an electron it becomes a positive Ion.

•If it gains an electron it becomes negative.

Ions and Ionic Bonds• Ionic bonds form as a result of the

attraction between positive and negative ions.

Covalent Bonds

•When compounds are held together by sharing electrons.

•Common covalent compounds are Fat, proteins, carbs, cotton and wool.

Covalent Bonds cont…

•They have low melting points and boiling points.

•Poor conductors of electricity.

•Elements from same side of the table bonded together.

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.

Nonpolar Bonds

•Bonds cancel each other out Typically covalent bonds.

Polar Compounds

•When electrons are shared unequally, it causes one atom to be slightly positive and the other will be slightly negative.

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.

Chemical Reactions

Production of new materials that are

chemically different from the

beginning materials.

Properties and Changes in Matter• Matter can undergo both

physical change and chemical change.

Evidence for Chemical Reactions

•Color change•Precipitate•Gas production•Changes in temperature•Changes in properties

A solid that forms after a rxn.

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

Structure of an Equation

•Reactants – materials you start with

•Products – materials you end with

Reactant + Reactant Product

H2 + O2 H2O2

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.

Conservation of Matter

Classifying Chemical Reactions

•Synthesis •Decomposition•Single Replacement•Double Replacement

Synthesis

•Two or more elements combine to form a more complex compound.

•A + B AB•Hand + kerchief

handkerchief

•Example: C + 2Cl2 CCl4

Decomposition

•Breaks down compounds into simpler substances.

•AB A + B•Schoolbook school + book• Example: 2Al2O3 4Al + 3O2

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

•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

Writing and Balancing Chemical Equations

•CO2

Subscript= IDENTIFIES THE NUMBER OF ATOMS IN A COMPOUND.

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.

H2 + N2 NH3 3 2

HgO + Cl2 HgCl + O2 2 2

Balancing Chemical Equations

Balancing Equations Activity

• Click the Active Art button to open a browser window and access Active

Art about balancing equations.

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.

Balancing Chemical Equations

• Balancing Chemical Equations:

• Balance the equation for the reaction of sodium metal (Na) with oxygen gas (O2), forming sodium oxide (Na2O).

• Balancing Chemical Equations:

• Balance the equation for the reaction of tin (Sn) with chlorine gas (Cl2), forming tin chloride (SnCl2).

Controlling Rates of Reaction

•Concentration: the more particles to react, the faster the reaction.

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.

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.

Controlling Chemical Reactions

• Every chemical reaction involves energy in one of two ways:

• Exothermic: Energy is released.• Endothermic: Energy is absorbed.

Endothermic Reaction

Activation Energy

•The minimum amount of energy needed to get a reaction started.

Acids and Bases

• Acid: Tastes sour, react with metals and turns litmus paper red.

• Base: Bitter, feels slippery and turns litmus paper blue.

pH

•A scale that measures whether a chemical solution is acidic or basic.

pH Scale

• The pH scale is from 0-14, 14 being basic, 0 being acidic and 7 being neutral.