Chemical Reactions

Chemical Reactions Processes in which one or more substances are converted into new substances with different physical and chemical properties Ex: rusted metal, burned toast

Chemical Reactions Parts of a chemical reaction: Reactants – substances before reaction occurs Products – substances produced by the reaction

Chemical Reactions Reason why reactions occur: Increase in stability for products

Types of Reactions Combination - two substances combine to form new substance A + B  AB Ca + Cl2  CaCl2

Types of Reactions Combustion – substance reacts with oxygen A + O2  AO CH4 + O2  CO2 + H2O

Types of Reactions Decomposition – substance breaks into two or more substances AB  A + B MnSO4  MnO + SO3

Types of Reactions Single-replacement – uncombined element “trades places” with an element in a compound A +BX  AX + B Cd + 2 HCl  CdCl2 + H2

Types of Reactions Double-replacement – two binary compounds “trade partners” AX +BY  AY + BX CaCO3 + 2 HCl  CaCl2 + H2CO3

Chemical Equations Sentences representing a chemical reaction 2 types Word equations - gives names of reactants & products Formula equations – shows chemical formulas and symbols

Chemical Equations Formula equations… Symbols used: + reacts/combines with, and  yields/produces  precipitate formed  gas emitted

Chemical Equations Formula equations… Symbols used: (s) solid state (l) liquid state (g) gaseous state (aq) aqueous state

Chemical Equations Diatomic molecule reminder… Hydrogen H2 Nitrogen N2 Oxygen O2 Fluorine F2 Chlorine Cl2 Bromine Br2 Iodine I2

Chemical Equations Unbalanced Examples: Calcium reacts with oxygen to produce calcium oxide Carbon reacts with oxygen to produce carbon dioxide

Chemical Equations Unbalanced Examples: Solid sodium reacts with chlorine gas to produce solid sodium chloride Nitrogen gas reacts with hydrogen gas to produce ammonia gas

Chemical Equations Unbalanced Examples: Sodium hydroxide reacts with hydrogen chloride to produce water and sodium chloride

Chemical Equations How to balance: Write formula equation with correct formulas & symbols Count number of atoms on each side of arrow (make table if needed) Mass must be conserved (number of atoms on each side of arrow are equal), use coefficients to adjust Work with one atom or ion at a time Save H & O for last (they tend to be in more than one place & can be difficult) Recount to ensure that number of atoms are equal on each side (coefficient ratio needs to be lowest possible numbers)

Chemical Equations Balancing examples: NaOH + HCl  H2O + NaCl Ca + O2  CaO

Chemical Equations Balancing examples: N2 + 3 H2  2 NH3 CH4 + 2 O2  CO2 + 2 H2O

Chemical Equations Balancing examples: C2H6 + O2  CO2 + H2O FeCl3 + Na2CO3  NaCl + Fe2(CO3)3 C2H6 + O2  CO2 + H2O

Chemical Reactions Reaction Rates Reactions happen if particles have enough energy.

Chemical Reactions Reaction Rates Factors Temperature Concentration Surface area/Particle Size Catalysts – substances that speed up reaction without being used

Chemical Reactions Reactions are reversible When rate of forward and reverse reaction are equal, system is at equilibrium.

Chemical Reactions Reactions are reversible Equilibrium is described by Le Chatelier’s principle If stress is applied to a system, the system changes to relieve the stress

Chemical Reactions Reactions are reversible Factors affecting equilibrium: Concentration Temperature Pressure Increasing pressure shifts equilibrium to whichever side has fewer particles. Decreasing pressure has opposite effect.

Chemical Reactions Reactions are reversible Equilibrium is described by a constant Keq, which is a ratio of products to reactants. Keq > 1, products favored Keq < 1, reactants favored

Chemical Equations Can include heat Endothermic – heat, from surroundings, enters the system, has positive heat of reaction (DH) value, reactants have lower potential energy than products

Chemical Equations Can include heat Endothermic reaction

Chemical Equations Can include heat Endothermic reaction: C(s) +2 S(s) + 89.3 kJ  CS2(l) C(s) +2 S(s)  CS2(l) DH = 89.3 kJ

Chemical Equations Can include heat Endothermic – heat, from surroundings, enters the system Exothermic – heat exits the system and goes to surroundings, has negative heat of reaction (DH) value , reactants have higher potential energy than products

Chemical Equations Can include heat Exothermic reaction

Chemical Equations Can include heat Exothermic reaction: CaO(s) + H2O(l)  Ca(OH)2(s) + 65.2 kJ CaO(s) + H2O(l)  Ca(OH)2(s) DH = -65.2 kJ

Chemical Equations Can include heat For systems with a constant pressure, heat is also called enthalpy (so DH is also change in enthalpy)

Chemical Equations Can include heat Enthalpy diagrams – visual representation of a thermochemical equation

Chemical Equations Enthalpy diagrams – Endothermic reaction

Chemical Equations Enthalpy diagrams – Exothermic reaction

Chemical Reactions Generally involve heat Hess’s Law – summary of all intermediate steps in a reaction, includes enthalpy changes for each step

Chemical Reactions Generally involve heat Hess’s Law – Can be used to determine heat of process that cannot be directly measured

Chemical Reactions Generally involve heat Hess’s Law – Example: conversion of diamond to graphite is too slow to measure, but combustion can be measured

Chemical Reactions Generally involve heat Hess’s Law – Example: C(s, diamond) + O2(g)  CO2(g) DH = - 395.4 kJ

Chemical Reactions Generally involve heat Hess’s Law – Example: C(s, graphite) + O2(g)  CO2(g) DH = - 393.5 kJ

Chemical Reactions Generally involve heat Hess’s Law – Example: difference between the two DH values will give the DH for converting diamond to graphite

Chemical Reactions Hess’s Law Diagram – -1.9 kJ

Chemical Reactions Hess’s Law (another look) – Determine the enthalpy change for the reaction of nitrogen dioxide becoming dinitrogen tetroxide. 2 NO2(g) → N2O4(g) N2(g) + 2 O2(g) → 2 NO2(g) DH = 67.7 kJ N2(g) + 2 O2(g) → N2O4(g) DH = 9.7 kJ DH = -58.0 kJ

Chemical Reactions Will a reaction occur? A reaction that releases heat is likely to occur

Chemical Reactions Will a reaction occur? A reaction that releases heat is likely to occur Since endothermic reactions occur, there must be something besides enthalpy to determine if reaction happens

Chemical Reactions Will a reaction occur? Reactions are dependent on a combination of enthalpy and entropy

Chemical Reactions Will a reaction occur? Entropy (S) is the measure of disorder in a system Law of Disorder - processes move towards maximum disorder (or chaos)

Chemical Reactions Will a reaction occur? Entropy can be determined by comparing entropy of reactants to entropy of products

Chemical Reactions Will a reaction occur? Spontaneous reactions Reactions that occur naturally and favor products (may be slow or fast reactions), energy will be released Spontaneity or nonspontaneity depends on conditions

Chemical Reactions Will a reaction occur? Spontaneous reaction examples: Heat released & entropy increased Increase in entropy is more than amount of heat absorbed Amount of heat released is more than decrease in entropy

Chemical Reactions Will a reaction occur? Nonspontaneous reactions Reactions that do not favor product formation, so they are not likely to occur (but they can happen) Spontaneity or nonspontaneity depends on conditions

Chemical Reactions Will a reaction occur? Nonspontaneous reaction examples: Heat absorbed & entropy decreased Increase in entropy is less than amount of heat absorbed Amount of heat released is less than decrease in entropy

Chemical Reactions Will a reaction occur? Spontaneous reactions have energy available to do work called free energy.

Chemical Reactions Will a reaction occur? Spontaneous reactions have energy available to do work called free energy. The maximum free energy is called Gibbs free-energy change (DG)

Chemical Reactions Gibbs free-energy change (DG) is calculated as: DG = DH – TDS T is in Kelvins

Chemical Reactions More Gibbs free-energy change (DG)… Spontaneous reactions DG is negative Nonspontaneous reactions DG is positive