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Equations & Reactions. 8.1 Describing Chemical Reactions. A. Chemical Changes and Reactions 1. New substances are produced . 2. Chemical reaction – chemical bonds between atoms or ions break , and new bonds form between atoms or ions. B. Evidence of a Chemical Reaction 1. color change
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8.1 Describing Chemical Reactions A. Chemical Changes and Reactions 1. New substances are produced. 2. Chemical reaction – chemical bonds between atoms or ions break, and new bonds form between atoms or ions. B. Evidence of a Chemical Reaction 1. color change 2. formation of a precipitate 3. temperature change 4. formation of a gas
C. Mechanics of a Chemical Reaction 1. Starting Materials – reactants 2. Ending Materials - products 3. reactants→ products Arrow = yieldsor produces 4. Many reactions occur to complete a set of valence electrons. 5. Symbols above the yield sign represent conditions necessary for a reaction to proceed. Ex) = delta = heat = electrolysis
6. Some reactions occur spontaneously. 7. Symbols represent the state of the reactants and products. Liquid = l Gas = g Solid = s Crystal = cr Aqueous = aq (solids in water solution) DEMO Ex) Al(s) + CuCl2(aq) → AlCl3(aq) + Cu(s) Ex) 2Al(s) + 3CuCl2(aq) → 2AlCl3(aq) + 3Cu(s)
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 (X2) List them: N2 O2 F2 Cl2 Br2 I2 H2 • a. Tetra-atomic element = Phosphorous (P4) • b. Octa-atomic element = Sulfur (S8)
8.2 Balancing Equations A. Equations in Chemistry 1. Chemical equation: an expression that uses symbols and formulas to describe a chemical reaction. 2. + means “reacts with” 3. → means produces (Called the yield sign)
B. Balancing Chemical Equations 1. Conservation of mass leads to balancing equations – the number of atoms of each element must be the same before & after the reaction. 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. 3. Therefore the MASS OF THE PRODUCTS = MASS OF REACTANTS 4. Subscript – indicates number of atoms of an element present in a compound. 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. General Rule: If there is an odd number of an element, double everything, but that element. F. NEVER EVER EVER, CHANGE THE SUBSCRIPTS!!!!
H2 + N2 → NH3 P + O2 → P2O5 NaNO3 → NaNO2 + O2 C8H18 + O2 → CO2 + H2O
8.3 Classifying Chemical Reactions A. Synthesis Reactions (direct combination) 1. Two or more elements or compounds combine to form a more complex product. A + B → AB 2. Ex. Fe + S → FeS CaO + H2O → Ca(OH)2 sodium + chlorine → sodium chloride 2 Na + Cl2 → 2 NaCl
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Sodium Metal plus Chlorine Gas Video 2 Na + Cl2 2 NaCl Synthesis Reaction
B. Decomposition Reactions (analysis) 1. A single reactant breaks down into simpler substances. AB → A + B 2. The opposite of a synthesis reaction. 3. Ex. 2 HgO → 2 Hg + O2 CaCO3 → CaO + CO2
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C. Single Replacement Reactions 1. Atoms of an uncombined element replace atoms of another element in a compound. A + BX → AX + B 2. A more active element will replace a less active element. (See activity series.)
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3. An Activity Series is a way of rankingelements (usually metals) in order from greatest to least reactivity. It can be used to predict whether a reaction will occur or not. Fe + CuSO4 → FeSO4 + Cu FeSO4 + Cu → No Reaction Mg + CuSO4 → MgSO4 + Cu MgSO4 + Cu → No Reaction
D. Double-Replacement Reactions Atoms or ions from 2 different compounds replace each other. AX + BY → AY + BX Ex. CaCO3 + 2HCl → CaCl2 + H2CO3 Ex. NaOH + HCl →NaCl + HOH
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E. Combustion Reactions 1. One substance reacts with oxygen, O2 to produce oxide compounds. 2. Occurs during burning. 3. Some combustion reactions can often be classified as synthesis reactions. Ex) S + O2 → SO2
Hydrogen Burning Video Synthesis Reaction 2H2 + O2 2 H2O
4. These reactions are usually exothermic, releasing a large amount of energy as light, heat, or sound. 5. When a hydrocarbon (compound containing H & C) is involved in a combustion reaction, CO2 and H2O are always the products. (also a vast amount of heat.) 6. Ex. __CH4 + __O2 → __CO2 + __H2O + 803 kJ C2H6 + O2 → 2 2 6 4 CO2 + H2O + heat 7 2
