Energy Changes and Rates of Reaction - Standard Formation & Rxn Rates

Standard Enthalpy of Formation

change in enthalpy that accompanies the formation of 1 mole of a compound from its elements

• All elements in the equation (not the substance formed) must be in their standard states

Standard State

the most stable form of the substance under standard conditions (25oC and 100 kPA)

elements w gas as their standard state

noble gases and diatomic elements (except bromine, which is liq)

elements w liq as their standard states

bromine and mercury

stan state for substance in sol is at a concen of

1 mol / L

most elements are ______ in standard state

solid

standard formation formula

writing formation equations

1. Write 1 mol of product in the state that has been specified.

2. Write the reactant elements in their standard states.

3. Balance the equation so that it yields 1 mol of product.

Elementary Step

a step involving one, two, or three collisions that cannot be explained by simpler reactions

Reaction Mechanism

a series of elementary steps by which a chemical reaction occurs

Rate-Determining Step

the slowest step in the reaction – determines the overall rate at which a reaction can proceed

Reaction Intermediate

an entity that is neither a reactant or a product, but is formed and consumed in during the reaction sequence

differences between intermediate and catalyst

intermediate: made and consumed DURING rxn seq

catalyst: present before and after rxn seq

Requirements for Plausible Reaction Mechanisms

• Summing the elementary steps in the reaction mechanism must give the overall balanced equation for the reaction. (hess’s law)

• The reaction mechanism must agree with the experimentally determined rate law

in an pot en diagram, the rxn step that makes the highest peak is the

slow reaction, therefore the rate determining step

the top of a peak

transition state

chemical kinetics

study of REACTION RATES and REACTION MECHANISMS (events at molecular level that control the speed and outcome of a reaction)

rxn rate

a measure of how fast a reaction proceeds and is measured by how fast a reactant is used up, or a product is formed.

• i.e. a change in the concentration of a reactant or product over time

units of rxn rate

mol/L•s

for rxn rate, if you are measuring the disappearance/consumption/decomposition etc of a compound, you need a ____ sign in front

neg

for rxn rate, if you are measuring the appearance/formation etc of a compound, you need a ____ sign in front

pos / no sign

concentration of reactants goes _______ over time

down → neg slope

concentration of products goes ______ over time

up → pos slope

rxn rate is typically not ____________

constant

avg rate of rxn

draw a secant line btwn two points on a concen - time graph and the slope would equal the avg rate of rxn

avg rate of rxn fromula

rate A = (concentration of A at a time t2 – concentration of A at time t1) / t2 – t1

simplified avg rate of rxn formula

rateA = Δ[A] / Δt

• where [A] rep the concen in mol / L, Δ[A] is the change in mol / L and Δt is the change in time

instantaneous rate of rxn

the rate of reaction at a particular time during the reaction (x point on a concen time graph)

to find instantaneous rate

1. Plot the concentration - time graph

2. Draw a tangent to the curve at the specified time

3. Find the slope of the tangent (equal to instantaneous rate

initial rate of rxn

the speed of the reaction the instant the reactants are mixed (t = 0) (tangent line starting from origin

writing stoichiometric relationships for rxn rates

use fractions lol

ways to measure rxn rates experimentally (4)

• a measurement of the amount of gas produced

• a measurement in the change of conductivity of a solution (for reactions that involve ions)

• a measurement in the change in pH

• a measurement of a change in colour

rxns that produce a gas

1. measure change in volume or pressure of gas produced

2. measure change in mass (products or reactants)

rxns that involve ions

measure change in conductivity of solution or change in pH

rxns that change colours

measure change in intensity of colour (concentration) using a spectrophotometer

spectrophotometer

an apparatus for measuring the intensity of light in a part of the spectrum, especially as transmitted or emitted by particular substances.

five key factors of the speed of a rxn rate

1. chem nature

2. temp

3. concen

4. s.a

5. catalyst

collision theory

for a reaction to occur, the reactant particles must first collide with one another

concepts of collision theory (3)

• a system consists of particles in constant motion at speeds proportional to the temp of sample.

• chemical rxn must involve particle collisions...with each other and with the walls of the containers.

• rate of rxn depends on frequency of collisions and the fraction of collisions that are effective

requirements for an effective collision

• correct orientation → collide in a way that the reactive site is involved

• sufficient en

• particles have to collide

activation en

the minimal en needed for a collision to be effective (to break the bonds in the reactant molecule)

• diff btwn n the initial energy of the reactants and the energy of the activated complex at the transition state.

transition state theory involves the

1. activation en (Ea)

2. activated complex

3. transition state

activated complex

an unstable molecule containing partially broken and partially formed bonds representing the maximum potential energy point in the change (top of peak)

trans state

energy maximum where the activated complex is formed

rate of a rxn depends on 2 criteria:

1. freq of collisions

2. frac of collisions that are effective (suff en, correct orien)

chem nature : a factor of rxn rates

• more bonds a compound has, the more en it needs to break all the bonds, thus a slower rxn

• stronger bonds would also slow the rxn

• therefore complicated molecules are less reactive and since the collision must be at the right orientation, it would take longer for the rxn to occur

temp : a factor of rxn rates

increase in temperature causes an increase in collision frequency and the fraction of collisions that are effective

• particles collide more often and with more force

concen : a factor of rxn rates

higher concen → more collisions bc more particles are occupying the fixed space

s.a : a factor of rxn rates

only the particles at the surface, where the substances are in contact, can react

• the amount of exposed surface area, where the two reacting phases are in contact, affects the reaction rate.

• more surface area → higher rxn rate

catalyst : a factor of rxn rates

• can either quicken or slow a rxn

• works by lowering the activation en needed for a rxn to occur

• turns the “slow” rxn steps into fast ones

inhibitor

a catalyst that slows a rxn

homogeneuous vs heterogenous catalyst

hetero → diff state of matter than the reactants and is usually a solid

homo → same phase as the reactants usually gas or liq

biological catalysts

catalysts produced by living organisms to help with biological processes

temp and how it specifically affects rates of rxn

doubled for every 10 celcius degree increase and halved for every 10 celcius degrees decrease (SEE EXAMPLES)