Unit 6: Chemical Reactions Chapter 9

Unit 6: Chemical Reactions The Nature of Chemical Reactions (9-1) A. Definition: process in which 1 or more substances are converted to new substances with different chemical and physical properties B. Common Examples: C. Components 1. Reactant: substance that enters a chemical reaction 2. Product: substance that is produced from a chemical reaction

D. Reason for Reactions * Depends on: valence electrons & energy II. Chemical Equations (9-2) A. Definition: sentences that represent reactions B. Indicates: what substances are involved in the reaction & how many

C. Types of Equations 1. Word Equations a. Name chemicals involved in the reaction b. Only a qualitative description Ex: hydrogen + oxygen  water (reactants) (yields) (product) 2. Formula Equations a. Gives the formulas of the chemicals involved and the ratios

b. Gives a quantitative description Ex: H2 + O2  H2O (reactants) (yields) (product) ** This is not balanced, it must be! Why? ** Law of Conservation of Mass/Atoms: in ordinary chemical reactions: 1.) Total mass of reactants is equal to total mass of products

2.) Number of elements for reactants is equal to the number of elements for products ** Atoms are not created nor destroyed, but they simply rearrange. ** Coefficients: numbers in front of the formulas in chemical equations that give the ratios of the substances involved in the reaction

2 molecules 1 molecule 2 molecules Ex: 2H2 + O2 ---> 2H2O 2 molecules 1 molecule 2 molecules or 2 moles 1 mole 2 moles Ex: Interpret the following chemical reaction 2 NaI + Cl2 ---> 2NaCl + I2 ** Note: What is the difference between 2CO and CO2

D. Writing and Balancing Chemical Equations 1. Review: Indicate how many atoms of each element are present in the following: a. 5NH3 ---> b. 4Ca(OH)2 ---> c. 3BaSO4 --->

2. Review: Diatomics (H2,O2,N2,Cl2,F2,Br2,I2) *Exist as a molecule of 2 atoms when they are by themselves (natural/elemental form) 3. Steps to writing chemical equations a. Write the correct chemical formulas for compounds involved in the reaction *All compounds should have a neutral charge- subscripts are used to balance the charge on ionic compounds

b. Balance equation according to the law of b. Balance equation according to the law of conservation of atoms – coefficients are used c. Indicate the phase of each compound (s) ---> solid (l) ---> liquid (g) ---> gas (aq) ---> solid salt dissolved in water d. Show the energy change in the equation Endothermic ---> Energy written on left side Exothermic ---> Energy written on right side

III. Classifying Chemical Reactions (9-3) 1. Synthesis (S), Composition, or Direct Combination – two or more substances combine to form a more complex substance General Form: A + B ---> AB Examples: 2H2 + O2 ---> 2H2O 4Fe + 3O2 ---> 2Fe2O3 (rusting) 2CO + O2 ---> 2CO2

2. Decomposition (D) – a complex substance breaks down to form two or more simple substances General Form: AB ---> A + B Examples: 2H2O ---> 2H2 + 2O2 2KClO3 ---> 2KCl + 3O2 2HgO ---> 2Hg + O2

3. Single Replacement (SR) – one element is replaced in a compound by another element General Form: A + BX ---> AX + B or Y + BX ---> BY + X Examples: Fe + CuSO4 ---> FeSO4 + Cu Cl2 + 2KI ---> 2KCl + I2 Cu + AgNO3 ---> CuNO3 + Ag

4. Double Replacement (DR) or Ionic – ionic compounds swap partners/pairings General Form: AB + XY ---> AY + XB Examples: NaCl + AgNO3 ---> NaNO3 + AgCl Ba(OH)2 + NaBr --> BaBr2 + 2NaOH (NH4)2CO3 + CaCl2 --> 2NH4Cl + CaCO3