Chapter 11 Chemical Reactions

Evidence of a Chemical Reaction Color Change Energy Change (gets cold or hot) Exothermic: a reaction or process that releases energy Endothermic: a reaction or process that requires energy Odor Change Precipitate: solid formed from a mixture of solutions Gas Produced Irreversible Process New Properties

2HCl (aq) + Mg (s)  MgCl2 (aq) + H2 (g) Chemical Equations Chemical Equation: a representation of a chemical reaction Formulas of the reactants (on the left) are connected by an arrow with the formulas of the products (on the right) 2HCl (aq) + Mg (s)  MgCl2 (aq) + H2 (g) Reactants Products

Symbols in chemical equations aqueous solution (aq): Substance dissolved in water

Catalyst Catalyst: a substance or process that speeds up the reaction Homogeneous Catalyst: One in the same physical state as the reactant 2H2O2 (aq) 𝑁𝑎𝐼 𝑎𝑞 2H2O (g) + O2 (g) Heterogeneous Catalyst: One in a different physical state 2H2O2 (aq) 𝑀𝑛𝑂2 𝑠 2H2O (g) + O2 (g) General Examples of Catalysts: Compounds or elements Heat  Light Aqueous Solutions

Steps to balancing chemical equations Write down your given equation Write down the number of atom per element that you have on each side of the equation Use coefficients to balance the elements, one at a time - Coefficient: Whole number written before the molecule - Never balance an equation by changing subscripts Check each atom or polyatomic ion to be sure they are equal Make sure all coefficients are in the lowest possible ratio

Examples _____ Ca (s) + _____ HCl (aq)  _____ CaCl2 (aq) + _____ H2 (g) _____ Al + _____ H2SO4  _____ Al2(SO4)3 + _____ H2 _____ C7H14 + _____ O2  _____ CO2 + _____ H2O

Examples - Answers Ca (s) + 2 HCl (aq)  CaCl2 (aq) + H2 (g) 2 Al + 3 H2SO4  Al2(SO4)3 + 3 H2 2(C7H14 + 10.5 O2  7 CO2 + 7 H2O) = 2 C7H14 + 21 O2  14 CO2 + 14 H2O

Classifying Chemical Reactions 5 Basic Types of Reactions: Synthesis (Combination) Reaction: Two or more substances react to form a single product A + X  AX 4 Al (s) + 3 O2 (g)  2Al2O3 (s) Decomposition Reaction: A single compound broken into two or more products Most require energy to occur AX  A + X 2HgO (s)  2 Hg (l) + O2 (g)

Classifying Chemical Reactions Combustion Reaction: Oxygen reacts with a substance to release energy Some are also synthesis reactions C (s) + O2 (g)  CO2 (g) Most are reactions with hydrocarbons (compounds with carbon and hydrogen) CH4 (g) + 2 O2 (g)  CO2 (g) + 2 H2O (g)

Single Replacement Reaction: An element replaces on part of a compound Activity Series: a list of elements in decreasing reactivity An element will replace anything that is less active Reactions can be predicted based on the series A + BX  AX + B or Y + BX  BY + X Mg (s) + Zn (NO3)2 (aq)  Mg(NO3)2 (aq) + Zn (s) Double Replacement Reaction: Exchange of ions between two compounds May end in a precipitate AX + BY  AY + BX Na2S (aq) + Cd(NO3)2 (aq)  CdS (s) + 2 NaNO3 (aq)

Reaction Mechanisms 1. Reactions that form water: Water is produced, chemicals are neutralized Acid + Base  Salt + Water (Double replacement) - Acid: begins with a hydrogen atom (H) - Base: contains hydroxide (OH) - Salt: ionic compound Example: 2HI + Cu(OH)2  CuI2 + 2H2O HCl + NaOH  NaCl + H2O

Reaction Mechanisms 2. Reactions that form a metal: Metal + Salt  Metal + Salt (Single replacement) Example: 2Fe + 3CuSO4  3Cu + Fe2(SO4)3 Na + AgNO3  Ag + NaNO3

Reaction Mechanisms 3. Reactions that form a gas: A. Hydrogen Gas Metal + Acid  Salt + H2 gas (Single Replacement) 2Na + H2SO4  Na2SO4 + H2 Ca + 2HCl  CaCl2 + H2 B. Hydrogen Sulfide Gas (H2S) Metal Sulfide + Acid  salt + H2S (Double Replacement) MgS + 2HCl  MgCl2 + H2S C. CO2 Gas (bi) carbonate + acid  salt + water + CO2 HCl + NaHCO3  NaCl + H2O + CO2

Reactions in Aqueous Solution Aqueous Solution: a substance dissolved in water Solvent: substance doing the dissolving Water is the universal solvent Solute: substance being dissolved Predicting the product state: Cations and anions will combine and either form a precipitate or no precipitate. When a combination is SOLUBLE it means it was dissolved in water and an aqueous solution formed. INSOLUBLE combinations do not dissolve and instead form a solid precipitate.

Representing reactions in aqueous solutions Molecular equation Shows formulas of all parts that are mixed 2NaOH(aq) + CuCl2 (aq)  2NaCl (aq) + Cu(OH)2 (s) Complete ionic Shows particles as they interact with water **Only aqueous solutions break into charged particles** 2Na+ + 2OH- + Cu2+ + 2Cl-  2Na+ + 2Cl- + Cu(OH)2(s) Net ionic Shows only those particles that are directly involved in the chemical change Eliminate the ions that remain unchanged – spectator ions Net ionic: 2OH- + Cu2+  Cu(OH)2(s)

No Reactions Once you complete the Net Ionic Equation, if all the ions “cancel out” there is no reaction. Occurs when all the reactants and products are aqueous solutions Example: NaCl (aq) + KBr (aq)⟶NaBr (aq) +KCl (aq) Na+ + Cl- + K+ + Br- ⟶ Na+ + Br- + K+ + Cl-