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:

1. Synthesis 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) 2Fe(s) + O 2 (g) → 2FeO(s) How to predict the products : assume product is made of the two reactants joined together. =related Ex: CaO(s) + H 2 O(l) → Ca(OH) 2 (s)

2. Decomposition 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

3. Single Replacement 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)

4. Double Replacement 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.

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.

Special D.R (Neutralization) 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)

Special double replacement (Neutralization) A special type of double replacement reaction... Neutralization Example:

5. Combustion 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)

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

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

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 →

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

Types of reactions depending on Energy Changes

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)

Endothermic 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)

Exothermic Reactions

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

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

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

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