Equations & Reactions

8.1 Describing Chemical Reactions A. Chemical Changes and Reactions produced 1. New substances are produced. breaknew bonds 2. Chemical reaction – chemical bonds between atoms or ions break, and new bonds form between atoms or ions. B. Evidence of a Chemical Reaction color change 1. color change formation of a precipitate 2. formation of a precipitate temperature change 3. temperature change formation of a gas 4. formation of a gas

C. Mechanics of a Chemical Reaction reactants 1. Starting Materials – reactants products 2. Ending Materials - products reactantsproducts 3. reactants → products yieldsproduces Arrow = yields or produces a set of valence electrons 4. Many reactions occur to complete a set of valence electrons. conditions 5. Symbols above the yield sign represent conditions necessary for a reaction to proceed. Ex) heat = delta = heat electrolysis = electrolysis

spontaneously 6. Some reactions occur spontaneously. state 7. Symbols represent the state of the reactants and products. lgscr Liquid = l Gas = g Solid = s Crystal = cr aqwater solution Aqueous = aq (solids in water solution) DEMO Ex) Al (s) + CuCl 2(aq) → AlCl 3(aq) + Cu (s) Ex) 2Al (s) + 3CuCl 2(aq) → 2AlCl 3(aq) + 3Cu (s)

state 8. Complete chemical equations include the subscript to indicate the physical state of each substance. 9. Diatomic molecules – certain elements exist in nature as diatomic molecules (X 2 ) List them: N 2 O 2 F 2 Cl 2 Br 2 I 2 H 2

Natural States of the Elements Diatomic Molecules Nitrogen gas contains N 2 molecules. Oxygen gas contains O 2 molecules.

Natural States of the Diatomic Elements

8.2 Balancing Equations A. Equations in Chemistry 1. Chemical equation: an expression that uses symbolsformulas symbols and formulas to describe a chemical reaction means “reacts with” yield 3. → means produces (Called the yield sign)

B. Balancing Chemical Equations mass before & after the reaction 1. Conservation of mass leads to balancing equations – the number of atoms of each element must be the same before & after the reaction. masssame 2. The Law of Conservation of Mass also states that the total mass before and after the reaction must be the same. You cannot lose or gain mass. MASS OF REACTANTS 3. Therefore the MASS OF THE PRODUCTS = MASS OF REACTANTS atoms 4. Subscript – indicates number of atoms of an element present in a compound. atoms molecules 5. Coefficient – indicates the number of atoms or molecules involved in the reaction.

6. Steps to Balance Equations: A. Write equation with symbols. B. Count # of atoms on each side of the reaction. C. Balance atoms using coefficients. D. General Rule: Balance all elements first. Then, balance C, H, and O. E. NEVER CHANGE SUBSCRIPTS!!!!

3 H 2 + N 2 → 2 NH 3 4 P + 5 O 2 → 2 P 2 O 5 2 NaNO 3 → 2 NaNO 2 + O 2 2 C 8 H O 2 → 16 CO H 2 O

8.3 Classifying Chemical Reactions direct combination A. Synthesis Reactions (direct combination) elements or compounds 1. Two or more elements or compounds combine complex to form a more complex product. A + B → AB 2. Ex. Fe + S → FeS CaO + H 2 O → Ca(OH) 2 sodium + chlorine → sodium chloride 2 Na + Cl 2 → 2 NaCl

Synthesis Reaction

Sodium Metal plus Chlorine Gas VideoSodium Metal plus Chlorine Gas Video 2 Na + Cl 2  2 NaCl Synthesis Reaction

analysis B. Decomposition Reactions (analysis) singlesimpler substances 1. A single reactant breaks down into simpler substances. AB → A + B opposite 2. The opposite of a synthesis reaction. 3. Ex. 2 HgO → 2 Hg + O 2 CaCO 3 → CaO + CO 2

Decomposition Reaction

CuCO 3(s)  CO 2(g) + CuO (s)

What is one of the products?

C. Single Replacement Reactions replace 1. Atoms of an uncombined element replace atoms of another element in a compound. A + BX → AX + B more 2. A more active element will replace a less active element. (See activity series.)

3CuCl 2 + 2Al  2AlCl 3 + 3Cu

ranking predict 3. An Activity Series is a way of ranking elements (usually metals) in order from greatest to least reactivity. It can be used to predict whether a reaction will occur or not. Fe + CuSO 4 → FeSO 4 + Cu FeSO 4 + Cu → No Reaction Mg + CuSO 4 → MgSO 4 + Cu MgSO 4 + Cu → No Reaction

D. Double-Replacement Reactions 2 different Atoms or ions from 2 different compounds replace each other. AX + BY → AY + BX Ex. CaCO 3 + 2HCl → CaCl 2 + H 2 CO 3 Ex. NaOH + HCl → NaCl + HOH

E. Combustion Reactions oxygen, O 2 1. One substance reacts with oxygen, O 2 to produce oxide compounds. burning 2. Occurs during burning. synthesis 3. Some combustion reactions can often be classified as synthesis reactions. Ex) S + O 2 → SO 2

Hydrogen Burning Video H O 2  2 H 2 O Synthesis Reaction

exothermic 4. These reactions are usually exothermic, releasing a large amount of energy as light, heat, or sound. CO 2 H 2 O 5. When a hydrocarbon (compound containing H & C) is involved in a combustion reaction, CO 2 and H 2 O are always the products. 6. Ex. __CH 4 + __O 2 → __CO 2 + __H 2 O kJ C 2 H 6 + O 2 → CO 2 + H 2 O + heat

Combustion Reaction

5 Types of Chemical Reactions Video5 Types of Chemical Reactions Video