UNIT 11: CHEMICAL REACTIONS A. THERMOCHEMISTRY B. CHEMICAL KINETICS

ENERGY OF CHEMICAL REACTIONS THERMOCHEMISTRY= ENERGY OF CHEMICAL REACTIONS

ENERGY AND REACTIONS Exothermic reactions Release heat energy The surroundings become warmer E.g. Combustion Endothermic reactions Absorb heat energy The surroundings become cooler E.g. Activating cold packs ***Energy is measured in joules (J) or calories (cal)

surroundings

Bond Energy in chemical reactions Breaking bonds is endothermic---energy is required Making bonds is exothermic---energy is released

For example: Exothermic: heat is a product A + B → C + energy/heat A + B → C + 6 kJ CH4(g)+2O2(g)  CO2(g) + 2H2O(l) + 890.4 kJ CH4(g)+2O2(g)  CO2(g) + 2H2O(l) ΔH = -890.4 kJ Endothermic: heat is a reactant A + B + energy/heat → C A + B + 4 kJ → C 27 kJ + NH4NO3(s)  NH4+(aq) + NO3-(aq) NH4NO3(s)  NH4+ (aq) + NO3- (aq) ΔH = +27 kJ

ENTHALPY (H) Enthalpy--a measure of heat content of a chemical system Depends on temperature and mass H = change in heat content that accompanies a process Exothermic: loss of heat by system ΔH products have less energy Endothermic: gain of heat by system +ΔH products have more energy

POTENTIAL ENERGY DIAGRAMS Show relationship between time and energy during a chemical reaction. Terms associated with P. E. diagrams: activation energy activated complex endothermic/exothermic reactions forward/reverse reactions

Activation Energy The activation energy (Ea) is the minimum energy needed for a reaction to take place upon proper collision of reactants. activated complex The activated complex: exists while old bonds are breaking and new bonds are being formed.

- low Ea = fast reaction rate - takes less energy for the Activation Energy: - depends on reactants - is always positive - low Ea = fast reaction rate - takes less energy for the reaction to start. Ea

Potential Energy Diagrams Show relationship between time and energy during the course of a chemical reaction. Activated Complex (transition state) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Ea Ea(reverse) Energy (E) in kJ/mol Reactants - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - DH Products - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Forward Rxn (exothermic) Reverse Rxn (endothermic) Course of Reaction (time)

Practice #1 For the energy diagrams provided, label the reactants, products, DH, and Ea. Also, determine the values of DH for the forward reaction and the value of Ea. DHforward = Eproducts – Ereactants 80 60 40 20 -20 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Energy (kJ/mol) products Ea DH - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - reactants Forward Reverse

Practice #1 Continued * DHforward = Eproducts – Ereactants = 55 kJ/mol – (-20 kJ/mol) = 75 kJ/mol What are 2 ways to determine from a potential energy diagram if a reaction is endothermic or exothermic? 80 60 40 20 -20 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Energy (kJ/mol) products Ea DH - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - reactants Forward Reverse

Endothermic Reaction A reaction in which heat is absorbed Products have higher potential energy than the reactants. The pink curve shows the uncatalyzed reaction. The blue curve shows what happens when a catalyst is present. A catalyst lowers the activation energy and the reaction proceeds at a faster rate. The energies and amounts of the products and reactants stays the same, and the DE stays the same. The catalyst just allows the reaction to reach equilibrium faster. Energy Course of Reaction

Exothermic Reactions Reactions in which heat is released. Products have lower potential energy than the reactants. The blue curve shows the uncatalyzed reaction. The red curve shows what happens when a catalyst is present. A catalyst lowers the activation energy and the reaction proceeds at a faster rate. Again, nothing changes but the amount of time it takes for the reaction to reach equilibrium. Exothermic rxns are referred to as “spontaneous” because they can proceed to products without outside intervention. Energy Course of Reaction

Hrxn = Hfinal – Hinitial Hrxn = Hproducts – Hreactants Calculating Hrxn Hrxn = Hfinal – Hinitial Hrxn = Hproducts – Hreactants

Practice Calculate Hrxn for the following reaction. CaCO3(s) → CaO(s) + CO2(g) Hreactants = -1207 kJ Hproducts = -1029 kJ Hrxn = +178 kJ Is it endothermic or exothermic? endothermic

Find Hrxn for the following reaction if the enthalpy of the products is -966 kJ and the enthalpy of reactants is -75 kJ. CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) Hrxn = -891 kJ It is exothermic

HEATS OF FORMATION When compounds are formed, either from the elements that make them up or in a reaction, energy accompanies the formation. This energy is known as the heat of formation Some compounds require energy and the heat of formation is a positive value. It is an endothermic reaction. Some compounds release energy and the heat of formation is a negative value. It is an exothermic reaction. Only compounds have heats of formation. Pure elements are not made from anything and therefore have no heats of formation.

Find the ΔHrxn for the following reaction 2H2S(g) + 3O2(g) → 2H2O(l) + 2SO2(g) ΔHf H2S(g) = -20.63 kJ/mol ΔHf H2O(l) = -285.830 kJ/mol ΔHf SO2(g) = ─296.8 kJ/mol Hrxn = - 1124.06 kJ

B. REACTION RATES (OTHERWISE KNOWN AS CHEMICAL KINETICS)

WHAT FACTORS DRIVE A CHEMICAL REACTION? How does a chemical reaction take place? Collision theory is used to explain that in order for a reaction to occur, particles MUST collide (this is reasonable), but not all collisions result in the formation of a new product

Collision theory States that atoms, molecules, ions must collide in order to react Must collide with: a. correct orientation b. enough energy to form activated complex or transition state

A. Correct orientation

B. Enough energy to break bonds and form activated complex 19

REMEMBER----- A reaction won’t happen if: Insufficient energy to break bonds. N2 O2 N2 O2 Molecules are not aligned correctly. 19

http://www.kentchemistry.com/links/KineticsLinks.htm

The study of reaction rates (speed) Chemical kinetics The study of reaction rates (speed) 17

Fast: Slow: Oxidation: Paper burning Oxidation: Nails rusting REACTION RATE IS DEFINED AS: Speed at which reactant is used up. Speed at which product forms. Fast: Oxidation: Paper burning Slow: Oxidation: Nails rusting Paper turning yellow

Factors that affect reaction rates Concentration Surface area Temperature Catalyst Nature of the reactants

Concentration - high concentration = fast reaction rate - more opportunities for collisions because there are more particles in the same volume that can react. There are less red particles in the same volume so there is less chance of a collision There are more red particles in the same volume so there is more chance of a collision so the reaction goes faster

Surface area high surface area = fast reaction rate more opportunities for collisions Increase surface area by… using smaller particles – if we make the pieces of the reactants smaller, we increase the number of particles on the surface which can react. dissolving in water – gases & dissolved particles can mix & collide freely. Reactions happen rapidly.

Temperature - high temperature = fast reaction rate - high KE - when we increase the temperature, we give the particles energy - this makes the particles move faster - so there are more opportunities for collision - it is easier to reach activation energy

Catalyst substance that increases reaction rate without being consumed in the reaction lowers the activation energy

Nature of the reactants reactant structure(polar vs. nonpolar) physical state of reactants ---ionic compounds dissolve faster than covalent compounds in water (salt dissolves faster than sugar) reactions occur faster in solutions or if reactants are gaseous more active elements will react more vigorously. Cs is more reactive than Na so it will react faster and more vigorously than Na when placed in water. 25

http://www.seilnacht.com/film/english.html http://www.open2.net/sciencetechnologynature/worldaroundus/akalimetals.html