Chapter 6-2 Types of Chemical Reactions Charcoal fuels BBQ grills through combustion of carbon. Release of energy (heat or light) is an important product in combustion reactions There are five basic types of reactions: Combination -, Decomposition -, Single-Replacement -, Double-Replacement -, and Combustion Reactions. (Not all reactions fit into this classifications)

Combination Reaction (Synthesis Reaction) Mg O2 O2- Mg2+ 2 Mg + O2  2 MgO

(“opposite of synthesis or combination reaction) In a synthesis (combination) reaction, two or more substances combine to form a single substance A + B + …  AB.. Examples: Fe (s) + S(s)  FeS (s) iron (II) sulfide SO2 (g) + H2O (l)  H2SO3 (aq) sulfurous acid CaO(s) + H2O (l)  Ca(OH)2 (aq) calcium hydroxide In a decomposition reaction, a single compound is broken down into two or more products AB..  A + B + ... Example: CaCO3 (s)  CaO (s) + CO2 (g) (“opposite of synthesis or combination reaction)

Explosion of dynamite is a powerful decomposition reaction

Single-Replacement Reactions 2 K (s) + 2 H2O (l)  2 KOH (aq) + H2 (g) Potassium + water  potassium hydroxide + hydrogen 2 K (s) + 2 H2O (l)  2 KOH (aq) + H2 (g)

In a single-replacement reaction, one element replaces a second element in a compound AB + C  AC + B Example: Mg (s) + 2 AgNO3 (aq)  Mg(NO3)2 (aq) + 2 Ag (s) but Mg (s) + 2 LiNO3 (aq)  no reaction If one metal will displace another metal from a compound can be determined by the relative reactivity of two metals. The activity series of metals lists metals in order decreasing reactivity. A nonmetal can also displace another nonmetal (usually halogens) reactivity decreases down group 7A.

Activity Series of Metals One metal replaces another metal (or hydrogen) Or one halogen replaces another halogen (F, Cl, Br, I) * Metals from Li to Na will replace H from acids and water; from Mg to Pb they replace H from acids only

Double-Replacement Reactions potassium carbonate + barium chloride  potassium chloride + barium carbonate K2CO3 (aq) + BaCl2 (aq)  2 KCl (aq) + BaCO3 (s)

Double-Replacement Reactions involve an exchange of positive ions between two reacting compounds AB + CD  AD + DB Examples: NaS (aq) + Cd(NO3)2 (aq)  CdS (s) + 2 NaNO3 (aq) NaCN (aq) + H2SO4 (aq)  2 HCN (g) + Na2SO4 (aq) Ca(OH)2 (aq) + 2HCl (aq)  CaCl2 (aq) + 2 H2O (l) Generally a reaction between two ionic compounds, with one product precipitating from the solution (a solid being formed) or escaping as a gas, or water being formed as one product.

methane + oxygen  carbon dioxide + water Combustion Reactions methane + oxygen  carbon dioxide + water CH4 (g) + 2 O2 (g)  CO2 (g) + 2 H2O (g)

In a combustion reaction an element or compound reacts with oxygen, usually releasing energy in form of heat or light. (A + O2  AO + ... + energy) CnHm + O2  CO2 + H2O + energy Examples: 2 C2H6(g) + 7 O2(g)  4 CO2(g) + 6 H2O(g) + energy 2 CH3OH(l) + 3 O2(g)  2 CO2(g) + 4 H2O(g) + energy C (s) + O2 (g)  CO2(g) + energy The complete combustion of a hydrocarbon CnHm produces carbon dioxide and water. Without sufficient oxygen, carbon monoxide and carbon (soot) may be formed.

Types of Reactions 1. A + B  AB 2. AB  A + B 3. A + BC  AC + B 4. AB + CD  AD + CB 5. CnHm + O2  CO2 + H2O

Ionic Charges (oxidation numbers) of main group elements +1 +2 +3 V. -3 -2 -1 0 Vary

start at 7, make a 7, plus hydrogen Most elements are composed of atoms, but 7 exist as diatomic molecules: Nitrogen N2, Oxygen O2, Fluorine F2, Chlorine Cl2, Bromine Br2, Iodine I2, and Hydrogen H2, start at 7, make a 7, plus hydrogen

Activity Series of Metals Single Replacement: One metal replaces another metal (or hydrogen) Or one halogen replaces another halogen (F, Cl, Br, I) * Metals from Li to Na will replace H from acids and water; from Mg to Pb they replace H from acids only

6.3 Nature of Reactions Collision Theory In a chemical reaction the particles involved must collide with enough force to initiate the chemical change. The energy needed for a reaction to occur is called activation energy. The rate of a reaction can often be increased by increasing temperature (energy) or concentration (higher probability of collision).

The reaction rate (speed) can be changed by changing factors like temperature, concentration, or by adding a catalyst (to speed up) or an inhibitor (to slow down). …also surface area, particle size etc.

A reversible reaction can change direction and the products react back into the reactants (example rechargeable batteries). A double arrow is used instead of the usual left –to –right arrow) .

Dynamic Equilibrium Reactants and products are formed at the same rate (no net change of the chemical composition) H2(g) + N2(g) NH3(g)

Le Chatelier’s principle The ratio of reactants and products in the equilibrium (how much of each) is not necessary equal (1:1). Le Chatelier’s principle The equilibrium can be shifted, meaning pushed more to one side by changing reaction conditions, especially concentration and temperature.

Activation energy with and without catalyst