Review for energetics and kinetetics test

Energy and reaction rates

Energy

Energy = ability to do work. Law of conservation of energy = first law of

thermodynamics. Chemical potential energy = stored energy in

bonds. 1 calorie = 4.184 joules = 1/1000 Calories Specific heat = heat to raise 1 gram 1° C.

Energy

Heat = specific heat x mass x change in temperature

q = c x m x ΔTA calorimeter is an insulated device used

to find specific heats.Page 524 gives an example of this in useBe able to use this equation to solve for

any unknown.

Energy

Thermochemistry is the study of heat changes that go with reactions and phase changes.

System = specific area of reactionSurroundings = everything elseUniverse = system + surroundingsEnthalpy is heat content of a system at

constant pressure.

Energy

Change in heat of a reaction is the change in enthalpy = ΔH.

ΔH = Hproducts – Hreactants

Exothermic = -ΔHEndothermic = +ΔH

Energy

A thermochemical equation is a balanced chemical equation with the physical states of all the reactants and products, and energy change.

Enthalpy of combustion (ΔHcomb) is the enthalpy change of the complete burning of one mole of a substance.

Energy

Molar enthalpy (heat) of vaporization (ΔHvap) is the heat to vaporize one mole of liquid.

Molar enthalpy (heat) of fusion (ΔHfus) is the heat needed to melt one mole of a solid.

ΔHvap = -ΔHcond

ΔHfus = ΔHsolid

Energy

Hess’s Law is a way to take intermediate steps to find the overall energy change for a reaction.

Standard enthalpy (heat) of formation (ΔH°f) is the energy needed to form one mole of substance in standard state from the elements in standard state.

ΔH°rxn= ΣΔH°f (products) - ΣΔH°f(reactants)

Energy

Entropy is the number of ways a systems energy can be distributed (directly related to freedom of particles).

Second law of thermodynamics states that spontaneous process happen in a way that increases the entropy of the universe.

Change in entropy (ΔS) is equal to products – reactants.

Energy

You can predict ΔS by examining the process. Change in state can be predicted by change in

movement. Gas dissolved into solvent lowers movement. More gas = more entropy. Solid or liquid into solvent increases entropy. Entropy increases with temperature increase.

Energy

Gibbs free energy equation lets you know how much energy is available to do work.

ΔGsystem = ΔHsystem – TΔSsystem

This is used to tell reaction spontaneity. (– values are spontaneous, + are nonspontaneous) A list of situations is on page 547.

Reaction rates

Reaction rate is a change in concentration in a given time, usually in mol/(L s).

Represented by brackets around a substance. [NO] is molar concentration of NO.

Reaction rate = ΔM/Δt

Reaction rates

Collision theory relates collisions with reaction speed.

Collisions need to be orientated correctly and have enough energy.

When collisions are correct, an activated complex is formed for a brief time.

Activation energy (Ea) is the amount of energy needed to for the activation complex.

Reaction rates

Factors affecting reaction rate:Nature of reactants. (some are more

reactive)Higher concentrations react more readilyMore surface are allows more collisionsHigher temperature allows more collisions

with more energyCatalysts increase speed (lower activation

energy) inhibitors slow speed.

Reaction rates

Rate laws show the relationship between concentration and speed/rate.

General rate law is: rate = k[A]m [B]n

k = rate constantm and n are reaction ordersMethod of initial rates determines the

reaction rate through experimental results of various concentrations. (page 576)

Reaction rates

Instantaneous reaction rates are the slope of the straight line tangent to the curve at the given instant.

This can also be found with concentrations, temperature, rate law and rate law constant at that temperature.

Reaction rates

A reaction mechanism is the complete sequence of steps of a reaction.

Complex reactions have more than one elementary step.

Intermediates are formed in one elementary step and consumed in another.

The rate determining step is the slowest step that holds everything else up.