Chemical Reactions 2/2018

Atoms are the building blocks for everything Elements are made of atoms Compounds are made of elements

Chemical Reactions Let's start with the idea of a reaction. In chemistry, a reaction happens when two or more molecules interact and something happens.

There are a few key points you should know about chemical reactions. 1. A chemical change must occur. You start with one compound and turn it into another. 2. A reaction could include ions, molecules, or pure atoms. A reaction can happen with anything, just as long as a chemical change occurs (not a physical one).

Compounds A substance formed from elements, but it does not have the same characteristics as the individual elements Ex. Iron oxide (rust)

Review

Chemical Formulas Chemists us chemical formulas to represent the composition (make-up) of the elements in compounds A short-hand (text message) way of writing a chemical compound NaCl- salt or Sodium Cloride H20- water

Electron shells Atomic number = number of Electrons Electrons vary in the amount of energy they possess, and they occur at certain energy levels or electron shells. Electron shells determine how an atom behaves when it encounters other atoms

Electrons are placed in shells according to rules: The 1st shell can hold up to two electrons, and each shell thereafter can hold up to 8 electrons.

Valence An atom is stable when it has 8 electrons in its outer shell. This makes them “fat and happy”

Electron Dot Structures Symbols of atoms with dots to represent the valence-shell electrons 1 2 13 14 15 16 17 18 H He:            Li Be  B   C   N   O  : F  :Ne :                    Na Mg  Al  Si  P S :Cl  :Ar :        

Why are electrons important? Elements have different electron configurations different electron configurations mean different levels of bonding The configuration determines how it bonds with other elements

Chemical Bond When an element comes together or combines with another element to form a compound An attempt to fill outer shell (make it fat and happy) with 8 electrons

Types of Bonds Ionic- giving or receiving of electrons to another element Covalent- sharing of electrons in the outer shell (valence). Covalent bonds form molecules

IONIC BOND bond formed between two ions by the transfer of electrons- metals combine with nonmetals

Ions Formed when electrons (-) are given away from element resulting in a positive charge, because the number of protons (+) is greater than the number of electrons (-) in the atom Positive ions are called cations

Ions Formed when electrons (-) are received from an element resulting in a negative charge, because the number of protons (+) is less than the number of electrons (-) in the atom Negative ions are called anions.

Formation of Ions from Metals Ionic compounds result when metals react with nonmetals Metals lose electrons to match the number of valence electrons of their nearest noble gas Positive ions form when the number of electrons are less than the number of protons Group 1 metals  ion 1+ Group 2 metals  ion 2+ Group 3A (13) metals  ion 3+

Ions from Nonmetal Ions In ionic compounds, nonmetals in 15, 16, and 17 gain electrons from metals Nonmetals add electrons to achieve the octet (8) arrangement Negative ions form when the number of electrons are more than the number of protons Group 5(15) nonmetals  ion 1- Group 6 (16) nonmetals  ion 2- Group 7 (17) nonmetals  ion 3-

Learning Check Give the ionic charge for each of the following: A. 12 p+ and 10 e- 1) 0 2) 2+ 3) 2- B. 50p+ and 46 e- 1) 2+ 2) 4+ 3) 4- C. 15 p+ and 18e- 1) 3+ 2) 3- 3) 5-

COVALENT BOND bond formed by the sharing of electrons

Covalent Bond Between nonmetallic elements of similar electronegativity. Formed by sharing electron pairs Examples; O2, CO2, C2H6, H2O, SiC

Covalent Bonds

Chemical Change Change from one substance to another Burning paper

Physical Change Change is physical or outward appearance. Does not change chemical make-up Wrinkled paper

Reactants the elements or compounds that are put together to form a chemical change Na + Cl  NaCl reactants

Products- the end result of reactants coming together Na + Cl  NaCl

Chemical Reactions Types Endothermic-energy absorbed Exothermic- energy is released

Rates of Reactions Catalyst- speeds up a chemical reaction Inhibitor- slows down a chemical reaction (takes longer)

Law of Conservation of Mass “In an ordinary chemical reaction, matter is neither created nor destroyed.” This means that you have neither gained nor lost any atoms during the reaction. They may be combined differently, but they’re still there.

Law of Conservation of Mass The mass of the products must equal the mass of the reactants (what I put into a reaction, I must get out) Mass is neither created or destroyed.

If the product has a mass less than the reactants what happed If the product has a mass less than the reactants what happed? Was their a gas, smoke or other byproducts? Example Burning paper- does the burnt paper (ashes) weigh as much as the original piece? Why?

Mass of Smoke =7 g

How to Balance Equations Draw boxes around all the chemical formulas. Make a Chemical Inventory (Look at the subscripts next to each atom to find the number of atoms in the equation.)

Write numbers in front of each of the boxes (coefficient) until the inventory for each element is the same both before and after the reaction. Multiply the coefficient Never change subscripts Try to leave single elements for last Whenever you change a number, make sure to update the inventory - otherwise, you run the risk of balancing it incorrectly. When all the numbers in the inventory balance, then the equation can balance

Finished Since both sides of the inventory match, the equation is now balanced!  All other equations will balance in exactly the same way, though it might take a few more steps in some cases.

Some practice problems: __NaCl + __BeF2 --> __NaF + __BeCl2 2. __FeCl3 + _Be3(PO4)2 --> _BeCl2 + _FePO4 3. __AgNO3 + _LiOH --> _AgOH + __LiNO3 4. __CH4 + __O2 --> __CO2 + __H2O 5. __Mg + __Mn2O3 --> __MgO + __Mn

Solutions 2 NaCl + 1 BeF2 --> 2 NaF + 1 BeCl2 2 FeCl3 + 1 Be3(PO4)2 --> 3 BeCl2 + 2 FePO4 1 AgNO3 + 1 LiOH --> 1 AgOH + 1 LiNO3 1 CH4+ 2 O2 --> 1 CO2 + 2 H2O 3 Mg + 1 Mn2O3 --> 3 MgO + 2 Mn

Practice at home www.chemistry-drills.com/balance.html