College Chemistry - Unit 10: Reactions

chemical change

the chemical identity of the reactants changes
bonds are broken in the reactants, and new bonds are formed in the products

bond enthalpy (bond energies)

energy needed to break a bond (+ΔH to break, -ΔH to form)

enthalpy of reaction [ΔH(rxn)]

change in PE of a reaction

+ΔH(rxn) = endothermic = products higher in PE than reactants
-ΔH(rxn) = exothermic = products are lower in PE than reactants

finding enthalpy of reaction [ΔH(rxn)]

draw the lewis structure
find which bonds were broken and formed
ΔH(rxn) = Σ (the sum of) all bonds broken - Σ bonds formed

energy profile of a reaction

shows the change in PE over the course of the reaction

activation energy [E(a)]

the energy needed to start bond breaking (lower = faster)

activated complex (transition state)

highest energy species in a reaction
occurs at the top of E(a)
structurally halfway between reactants and products

collision theory

reactions happen through collisions of reactants that make physical contact through effective collisions

effective collision

produces a reaction; must collide with enough force to start bond breaking [E(a)]

must collide in the correct orientation (the right parts of each reactant must make contact)

rate of reaction / reaction rate

how fast a reaction is happening
depends on number of effective collisions

how experimental factors affect reaction rate: temperature

increase temperature = increase reaction rate
speeds up particles (increasing KE and more powerful collisions) = more reactant molecules provide E(a)

how experimental factors affect reaction rate: concentration

increase concentration (mol/L) of reactants = increase reaction rate; more frequent collisions

how experimental factors affect reaction rate: pressure

increase pressure of gaseous reactants = increase reaction rate; more frequent collisions between particles

how experimental factors affect reaction rate: surface area

increase surface area of solid reactants = increase reaction rate
more particles available to react (only surface area particles can react)

how experimental factors affect reaction rate: catalyst

make reactions happen faster by lowering E(a)
at a given temperature, more reactants will have sufficient KE to provide E(a)

chemical equilibrium

the amount of reactants and products no longer changes

equilibrium constant [K(eq)]

the value the ratio [amount of products] / [amount of reactants] reaches at equilibrium
can be calculated from concentration [K(c)] or gas pressures [K(p)]

calculating K(eq)

only include aqueous and gaseous reactants and products
[concentration or pressure of products] / [concentration or pressure of reactants]
coefficients of elements become exponents

interpreting K(eq)

K(eq) < 1 = more reactants at equilibrium, the reaction favors reactants
K(eq) > 1 = more products at equilibrium, the reaction favors products

equilibrium positions

the varying amounts of reactants and products that satisfy K(eq)

Le Chatelier's Principle

if a change is made to a reaction at equilibrium, the reaction will shift in a direction (left or right) to minimize the change

Le Chatelier's Principle: adding reactants/products

the reaction will shift to the opposite side of which side was added to to consume added substance

Le Chatelier's Principle: removing reactants/products

the reaction will shift to the side of change

Le Chatelier's Principle: gas pressure change

find number of gas particles on each side (add up coefficients)
when increasing pressure, the reaction shifts to the side with fewer gas particles
when decreasing pressure, the reaction shifts to the side with more gas particles

Le Chatelier's Principle: temperature change

consider temperature as a reactant or product depending on if the reaction is exothermic or endothermic
exothermic: heat is a product
endothermic: heat is a reactant

shift left increases K(eq)
shift left decreases K(eq)