UNIT VI Types of Chemical Reactions Lesson 2

VI.5 TYPES OF CHEMICAL REACTIONS When we classify reactions by how atoms are rearranged, there are 5 MAJOR TYPES of reactions:

VI.5 TYPES OF CHEMICAL REACTIONS 1. Synthesis - is the combination of two or more substances to form a _____________________________. A + B → AB Examples:2H 2 (g) + O 2 (g) → 2H 2 O (g) CaO(s) + H 2 O(l) → Ca(OH) 2 (s) 2Fe(s) + O 2 (g) → 2FeO(s) How to predict the products : assume product is made of the two reactants joined together. =related

VI.5 TYPES OF CHEMICAL REACTIONS 2. Decomposition – is the opposite of synthesis; one substance breaks down to form two or more ________________________________. AB → A + B Examples:CO 2 (g) → C(s) + O 2 (g) 2Ag 2 O(s) → 4Ag(s) + O 2 (g) How to predict the products : the products will be the elements that make up the original compound. lated

VI.5 TYPES OF CHEMICAL REACTIONS 3. Single Replacement – occurs when one element is replaced by another element in a compound. A + BC→ AC + B Examples:Cl 2 (aq) + 2KBr(aq) → 2KCl(aq) + Br 2 (aq) Fe(s) + CuSO 4 (aq) → FeSO 4 (aq) + Cu(s) *This is when an ELEMENT reacts with a compound and REPLACES an element in the compound. How to predict the products : replace metal with metal OR replace nonmetal with nonmetal. CuI 2 + Fe → Fe I 2 + Cu(Cu is replaced by Fe – METAL) CuI 2 + Br 2 → Cu Br 2 + I 2 ( I is replaced by Br – NONMETAL)

VI.5 TYPES OF CHEMICAL REACTIONS 4. Double Replacement – occurs when the elements in a solution of reacting compounds ______________ places, or ________________ each other. AB + XY→ AY + XB Examples: ZnBr 2 (aq) + 2AgNO 3 (aq) → 2AgBr(s) + Zn(NO 3 ) 2 (aq) *This is when TWO COMPOUNDS react – pay close attention to CHARGE on ions.

VI.5 TYPES OF CHEMICAL REACTIONS 4. Double Replacement AB + XY→ AY + XB ZnBr 2 (aq) + 2AgNO 3 (aq) → Zn(NO 3 ) 2 (aq) + 2AgBr(s) Note that compounds are made of ions put together. What you will do is take a positive ion on the reactant side and exchange it with the positive ion on the product side. BE VERY careful about the charges and how you combine the ions together to make a new compound.

VI.5 TYPES OF CHEMICAL REACTIONS A special type of double replacement reaction... Neutralization – reaction between an ACID and a BASE (the H in the acid and the OH in the base form H 2 O) the products are always WATER and SALT Acid + Base → Salt + Water HB + XOH → XB + HOH Examples: HCl(aq) + NaOH(aq) → NaCl(aq) + H 2 O(l)

VI.5 TYPES OF CHEMICAL REACTIONS A special type of double replacement reaction... Neutralization Example:

VI.5 TYPES OF CHEMICAL REACTIONS 5. Combustion – chemical reaction that usually gives off a large amount of energy in the form of heat or light involves the reaction of a substance with OXYGEN (O 2 ) hydrocarbons (compounds made of H and C) are involved carbon dioxide and water are produced C x H y + O 2 → CO 2 + H 2 O Ex: C 3 H 8 (g) + 5O 2 (g) → 3CO 2 (g) + 4H 2 O(g)

VI.5 TYPES OF CHEMICAL REACTIONS 5. Combustion ---Special notes C x H y + O 2 → CO 2 + H 2 O If you have an organic compound with O, you get the same thing + O 2 → CO 2 + H 2 O If you have an organic compound with S, you get: Sulphur dioxide reacts with water to form sulphurous acid(acid rain) -----Nice SUMMARY on p. 118 of your Hebden textbook

VI.5 TYPES OF CHEMICAL REACTIONS Predicting Products 1. Synthesis (2 elements form one compound) Mg + I 2 → 2. Decomposition (1 compound breaks into elements) HgO →

VI.5 TYPES OF CHEMICAL REACTIONS Predicting Products 3. Single Replacement (interchange metal or non- metal) Mg + AlCl 3 → 4. Double Replacement (interchange positive ions in compound) FeCl 2 + K 2 S →

VI.5 TYPES OF CHEMICAL REACTIONS Predicting Products 5. Combustion (products will be carbon dioxide and water) C 6 H 14 + O 2 →

VI.6 ENERGY CHANGES IN CHEMICAL REACTIONS In a chemical reaction, BONDS are broken and new bonds are formed. ENERGY is required to break these bonds. Endothermic Reactions ABSORB heat (energy) from the surroundings Ex: HCl + energy → H + Cl energy appears on the REACTANT side there is more energy at the end of the reaction (as reaction proceeds, more energy is required to break the bonds)

VI.6 ENERGY CHANGES IN CHEMICAL REACTIONS Endothermic Reactions

VI.6 ENERGY CHANGES IN CHEMICAL REACTIONS Exothermic Reactions GIVES OFF heat (energy) to the surroundings Ex: H + Cl → HCl + energy energy appears on the PRODUCT side there is more energy at the beginning of reaction (as reaction proceeds, energy is lost to the surroundings)

VI.6 ENERGY CHANGES IN CHEMICAL REACTIONS Exothermic Reactions

VI.6 ENERGY CHANGES IN CHEMICAL REACTIONS ∆H = change in enthalpy (the heat contained in the system) Endothermic Rxn: ∆H is positive Ex: A + 45kJ → B A → B ∆H = + 45kJ

VI.6 ENERGY CHANGES IN CHEMICAL REACTIONS Exothermic Rxn: ∆H is negative Ex: C → D + 65kJ C → D ∆H = - 65kJ Summary : HEAT on LEFT -----> ∆H = +ve HEAT on RIGHT -----> ∆H = -ve

VI.6 ENERGY CHANGES IN CHEMICAL REACTIONS ∆H of the products and reactants:

L ET US TRY (I NTRO TO UNIT 7 ) C 12 H 22 O O 2  12CO H 2 O kJ How much heat is released during the formation of 9.6 moles of CO 2 ?

H OMEWORK Balancing & Types of rxns: Questions: p. 118 # (every 3 rd ), 67 (every 2 nd one) Endo vs Exo Questions: p. 120 #