Chemical Reactions & Equations REVIEW for TEST View the first few slides to review concepts in this unit. Then answer the questions on the review. (The answer key to the review is found at the end of this power point.)

Review: How can you tell if a chemical Reaction has taken place? By the 4 Signs of a Chemical Change ! ! ! Review: How can you tell if a chemical Reaction has taken place? 1. Formation of a precipitate 2. Formation of a gas 3. Creation of heat or light 4. Color change

All chemical reactions Have two parts Reactants Are the substances you start out with Products Are the substances you end up with

Reactants turn into the products Reactants ® Products Reactants 3H2 + N2 → 2NH3 Products

Energy Exchange Reactions may give off energy (Exothermic) and the temperature of the surrounding will go up ↑. Reactions may absorb energy (Endothermic) and the temperature of the surrounding will go down ↓.

Chemical Reactions are cool ! The Law of Conservation will have the same amount of reactants as products. Chemical Reactions change the way atoms are joined. Chemical Reactions are cool ! However Atoms will never be created nor destroyed in the process That is called… LAW OF CONSERVATION

Symbols used in chemical equations 1. An arrow is used to separate the reactants from the products * Read the arrow as “reacts to form” or “Yields” 2. The plus sign (+) reads “and” copper + chlorine ® copper (II) chloride

More symbols (s) after the formula- Solid (g) after the formula- Gas (l) after the formula- Liquid (aq) after the formula - an aqueous solution (dissolved in water) Cu (s) + 2AgNO3 2Ag(s)+Cu(NO3)2 (aq) shows that heat is supplied to the reaction

Diatomic Elements There are 7 diatomic elements: (Meaning when these elements are not in a compound but are by themselves, they always come 2 together.) H2, N2, O2, F2, Cl2, Br2, I2

Remember! Atoms can’t be created nor destroyed (Law of Conservation) Balancing Equations Remember! Atoms can’t be created nor destroyed (Law of Conservation) The atoms that we start with… we MUST end up with. A balanced equation has the same number of atoms of each element on both sides of the equation.

® C + O2 ® CO2 This equation is already balanced What if it isn’t already?

® C + O2 ® CO We need one more oxygen in the products. We can’t change the formula, because it describes what actually happens

® 2 C + O2 ® 2 CO Another Carbon must be used to make another CO So ® C C O C O C O + O Another Carbon must be used to make another CO But where did the other C come from? Must have started with two C 2 C + O2 ® 2 CO

Rules for balancing equations Write the correct formulas for all the reactants and products Count the number of atoms of each type appearing on both sides Balance the elements one at a time by adding coefficients (the numbers in front of formulas or elements) Check to make sure it is balanced. You can see that this is balanced ! 2 C + O2 ® 2 CO

Change a subscript to balance an equation Never, Never, Never… Change a subscript to balance an equation If you change the formula, you are describing a different reaction. H2O is a different compound than H2O2 Never put a coefficient in the middle of a formula 2 NaCl is okay, Na2Cl is not.

If an atom appears more than once on a side, balance it last. Techniques If an atom appears more than once on a side, balance it last. If you fix everything except one element, and it is even on one side and odd on the other, double the first number, then move on from there.. How would you balance this one? C4H10 + O2  CO2 + H2O

Then What? Then What? C4H10 + O2  CO2 + H2O Double it. 4

Types of Reactions: #1 = Synthesis Reactions Synthesis – means put together A + X  AX 2 elements, or compounds combine to make 1 compound. 2Ca +O2 ® 2CaO

#2 = Decomposition Reaction AX → A + X Decompose - means fall apart Just like a Decomposition Reaction! They were together in the beginning and the ended up breaking apart. Yields egg shells + baby turtle Egg Hatching

Decompose - means fall apart Decomposition Reactions AX → A + X Decompose - means fall apart 2 NaCl 2Na + Cl2 CaCO3 CaO + CO2

One element replaces another A + BX → AX + B Y + BX → BY + X #3 = Single Replacement One element replaces another A + BX → AX + B Y + BX → BY + X Reactants must be an element and a compound. Products will be a different element and a different compound. Na + KCl ® K + NaCl F2 + LiCl ® LiF + Cl2

Na + KCl ® K + NaCl Look at some examples A + BX → AX + B Y + BX → BY + X Na + KCl ® K + NaCl Cl K Na

More examples F2 + LiCl ® LiF + Cl2 2 2 Cl Li Li F F Cl Li Li

Na + HOH ® Single Replacement NaOH + H2 Example: Na + H2O® Metals replace metals (and hydrogen) Example: Na + H2O® Think of water as HOH Na + HOH ® NaOH + H2 Metals replace one of the H, combine with hydroxide.

2Na + 2H2O → 2NaOH + H2 Na + HOH → NaOH + H2 This equation is balanced… How would you balance it? Na + HOH → NaOH + H2 2Na + 2H2O → 2NaOH + H2

Single Replacement We can tell whether a reaction will happen Some elements are more active than other More active replaces less active

Single Replacement Higher on the list replaces lower. If the element by itself is higher; it happens. If the element by itself is lower; it doesn’t happen.

Single Replacement Note the * H can be replaced in acids by everything higher Only the first 4 (Li - Na) react with water.

Single Replacement Nonmetals can replace other nonmetals Limited to F2 , Cl2 , Br2 , I2 The order of activity is that on the periodic table. ????????? Higher replaces lower. F2 + HCl ® Br2 + KCl ®

#4 = Double Replacement Reaction Two things replace each other. AX + BY → AY + BX Reactants must be two ionic compounds or acids. Usually in aqueous solution NaOH + FeCl3 Just like a double displacement reaction! Let’s try an example!

The positive ions change place. NaOH + FeCl3 ® Fe3+OH- + Na+Cl- The final equation will be… NaOH + FeCl3 ® Fe(OH)3 + NaCl Is this balanced or not? NO! 3NaOH + FeCl3 ® Fe(OH)3 + 3NaCl

3NaOH + FeCl3 ® Fe(OH)3 + 3NaCl

Double Replacement Will only happen if one of the products doesn’t dissolve in water and forms a solid (precipitate) or is a gas that bubbles out. or is a covalent compound usually water (water forming) .

How to recognize which type Look at the reactants E for element C for compound E + E Synthesis C Decomposition E + C Single replacement C + C Double replacement

#5 = Combustion Reaction A compound composed of only C and H is reacted with oxygen If the combustion is complete, the products will be CO2 and H2O. If the combustion is incomplete, the products will be CO and H2O. O2 will always be the second reactant

Oxidation-Reduction Reactions Also be familiar with: “Redox Reactions” Oxidation-Reduction Reactions Oxidation-reduction or redox reactions involve the transfer of electrons from one atom to another. 2 Mg + O2  2 [Mg2+][O2-] In this reaction, each magnesium atom loses two electrons to form an Mg2+ ion. Mg  Mg 2+ + 2 e- This is OXIDATION (atoms lose) And, each O2 molecule gains four electrons to form a pair of O2- ions. O2 + 4 e-  2 O2- This is REDUCTION (atoms gain)

Answer key to review: