Reaction Energy & Reaction Kinetics Thermochemistry….ch.17 p. 511 Thermochemistry = study of the transfer of heat in chemical reactions and physical changes. Temperature = measurement of kinetic energy of molecules (Ex) fire wood Thermal energy = total energy of matter… measured in joules Heat = transfer of thermal energy. Follows 2nd law of thermodynamics… heat always flow from higher to lower temperature.

exothermic = got hot = released heat Enthalpy change: change of energy absorbed or released as heat during a chemical reaction… exothermic = got hot = released heat endothermic = got colder = absorbed heat Do NaOH & NH3NO3

Exothermic Reaction

Endothermic Reaction

Energy may be consumed or produced (exothermic or endothermic) in a chemical reaction. Enthalpy: change in energy delta H j/mol or kj/mol “throughout nature reactions proceed from.. Higher energy state lower energy state “most chemical reactions in nature are exothermic”

“ the energy needed to get the reaction started” Activation Energy p. 533-534 match Wood log paper Activation Energy: Ea “ the energy needed to get the reaction started”

Potential Energy Diagram / endothermic

Potential Energy diagram / exothermic

Read graph 2 ways just like heating/cooling curves Lecture notes 1 p. 533-534 The line is the energy Read graph 2 ways just like heating/cooling curves Ea = activation Energy for exothermic will be – Ea’ = activation Energy for endothermic will be + deltaE = change in energy also known as deltaH . “ notice delta E or delta H is equal for either direction” Bonds breaking is endothermic Bonds forming is exothermic

Label Ea forward, Ea reverse, delta Eforward, delta Ereverse Label each graph endo / exo in forward reading. Label Ea forward, Ea reverse, delta Eforward, delta Ereverse Determine value for each.

How is height of curve related to ease with which a reaction will occur? What would the activation curve look like for a rapid reaction? What would happen if amount <Ea were supplied?

Reaction Rate: “How fast the reaction proceeds” Reaction Rate depends on : 1. number of molecule collisions 2. efficiency of the collisions 5 factors can effect the rate of reaction: l. Nature of reactants 2. surface area (big chunk vs. powder) 3. temperature ( hot or cold) 4. concentration 5. catalyst (only effects rate does not participate in reaction…can recover.)

“most reactions in nature are toward disorder” Organized disorganized Entropy: a measure of degree of randomness of the molecules. Delta S j/mol*K or kj/mol*K “most reactions in nature are toward disorder” Organized disorganized Gas has more entropy than liquid …liquid more entropy than solid S L g Increase in entropy + delta S …..more crazy random (favored) Decrease in entropy - delta S ……more organized You can tell entropy by looking at the phases of the reaction…….ws. 17.2

To determine if a reaction will occur…. G must be negative in order Free Energy ( G) determines whether a reaction will occur taking into account both enthalpy and entropy and temperature . Reactions are driven to lowest enthalpy, highest entropy To determine if a reaction will occur…. G must be negative in order for a reaction to be probable. Spontaneity: means that a reaction will occur- doesn’t say fast or slow, just that it will happen. Ex: rusting of iron is a spontaneous reaction( even though it will take awhile) because it will happen.

How to determine if a reaction is Spontaneous or Not. Will it work or not? Free Energy G formula G = H - T S G has to negative to be spontaneous H- exothermic S+ more disorder T in Kelvin

H= 176 kj/mol S= 0.285 kj/mol.K T=298.15K 1. Will the following reaction be spontaneous? NH4Cl3(s) NH3 (g) + HCl3(g) H= 176 kj/mol S= 0.285 kj/mol.K T=298.15K The positve value of G shows this reaction will not occur at the stated temp.

At what temp will this be spontaneous? Br2 (l) Br2(g) H= 31.0kj/mol S= 93.0J/mol.K At what temp will this be spontaneous? Hint: try standard room temp first….298K Temp must be above 333K P548 #24, #25,

Reaction Equilibrium Le Chatelier’s Principle: if a system at equilibrium is subjected to a stress then the reaction will be shifted in the direction to relieve the stress. We will learn 4 stress effects : change in pressure change in temperature change in concentration adding a catalysts

Change the pressure: an increase in pressure will favor a shift toward the side with lowest number of gas particles. 3H2 (g) + N2 (g) 2NH3 (g) + heat C (s) + H2 (g) + heat CO (g) + H2 (g) Change the temperature: increasing Temperature will drive the reaction toward the reactants (shift to left) if exothermic. increasing temperature will drive the reaction toward the products (shift to right) if endothermic.

Change in Concentration: adding more reactants always drives the reaction toward the products (shift to the right) adding more products always drives the reaction toward the reactants ( shift to the left) “see-saw ride principle” “Remember reverse psychology”…if take away some product then shift toward the products. 3 H2 + N2 2NH3 CaCO3 CaO + CO2 Ca(OH)2 CaO + H20 Fe2O3 + 3CO 2Fe + 3CO2

does not cause a shift in equilibrium speed up or slow down only Adding a Catalyst: does not cause a shift in equilibrium speed up or slow down only Review lecture notes ch 18 Ws 18.2

Writing Equilibrium equations p.553-557 reversible reaction: A chemical reaction in which the products Can react to reform the the reactants. When is a reaction that is reversible considered to have reached chemical equilibrium? When rate of forward reaction = rate of reverse reaction Equilibrium constant: Keq A numerical ratio of the concentration of the products to the concentration of the reactants

Writing the equilibrium expression: Keq Write the concentration of the products using their coefficients as exponents. Divide the concentration of the reactants using their coefficients as exponents. N2O4 2NO2 CO + 3H2 CH4 + H2O 8H2S 8H2 + S8