Thermochemistry

Units for enthalpies of reaction

Units for enthalpies of reaction

kJ/mol

Stoichiometrically, what determines how much heat is released from a reaction?

limiting reactant (use moles of LR to determine heat released from enthalpy of reaction)

1 L-atm =

101.3 J

1 cal =

4.184 J

Work

The energy used to move an object against a force (transfer of energy by any process other than heat)

Heat

The energy used to cause the temp of an object to increase

Kinetic energy

KE = ½mv²

Electrostatic potential energy

U = kQq / r

(U = potential energy, r = distance between particles, q and Q are the charges of particles)

K =

8.99 × 10⁹ J-m/C²

Coulomb’s Law

The relationship between electrostatic force and distance, the closer two charged particles are, the stronger the force between them

Coulomb’s Law equation

F_e = kq₁q₂ / r²

Electrostatic potential is positive for ____ charges

like

Electrostatic potential is negative for ___ charges

opposite

As particles move apart, their electrostatic potential energy approaches _____

zero

As like charged particles come closer together, E_el becomes…

more positive

As opposite charged particles come closer together, E_el, becomes…

more negative

Smaller separation between like charged particles =

greater repulsion, higher (more positive) E

Smaller separation between two opposite charged particles =

greater attraction, lower (more negative) E

1 Cal =

1000 cal = 1 kcal

System

the portion we single out (reactants and products)

Surroundings

Everything else (the container and space beyond it)

Open system

Matter and energy can be exchanged with the surroundings (boiling pot of water on stove)

Closed system

Energy can be exchanged with the surroundings, but matter cannot (piston)

Isolated system

Neither energy nor matter can be exchanged with the surroundings (insulated thermos)

First law of thermodynamics

Energy cannot be created nor destroyed, any energy that’s lost in a system must go to surroundings

Internal energy (E or U)

The sum of all kinetic and potential energies of the components of the system. Includes motions and interactions of particles

+∆E =

system has gained energy from its surroundings

-∆E =

system has lost energy to its surroundings

∆E =

q + w

For q, + means…

gains heat

For q, - means…

loses heat

For w, + means work done…

on system

For w, - means work done…

by system (gas is created and expands to move a piston)

Endothermic

The system absorbs heat

Exothermic

The system loses heat

State function

Value depends only on where you started and ended

List of state functions

Internal energy, potential energy, kinetic energy, chemical energy, volume, pressure, enthalpy

List of path functions

Heat, work, distance

When a system is at constant volume, ∆V =

0

w =

-P∆V

If volume is constant and -P∆V goes to zero, then ∆E =

q_v (subscript v reminds us we’re at constant volume)

At constant pressure, ∆H =

q_p = ∆E + P∆V

Enthalpy

Measure of heat flow, total heat absorbed/released by the system under constant pressure

The coefficients in a balanced thermochemical equation represent the number of moles of reactant and product associated with…

enthalpy change (given enthalpy = enthalpy / moles of compound, they are in proportion with each other)

The magnitude of ∆H depends on the amount of…

reactant consumed in the process (doubling moles of reactant doubles enthalpy)

The ∆H for a reaction is _____ in magnitude, and ______ in sign, to the ∆H of the reverse reaction

equal; opposite

Enthalpy depends on…

states of matter

Enthalpy has a greater magnitude for reactions that go from gas to…

liquid

Greater change in enthalpy =

greater change in matter

Heat capacity

The amount of heat required to raise an objects temp by 1 degree (C or K); the greater the heat capacity, the greater the heat required to produce a given increase in temp

Unit: J/K

Specific heat capacity

The heat capacity of one gram of a substance

Unit: J/g°C

q =

mC_s∆T

q = heat evolved in rxn

m = reactant mass in grams

C = specific heat

∆T = change in temp

q_soln =

-q_rxn

If the temp of the water increases in a coffee cup calorimeter, then the reaction is…

exothermic

Coffee cup calorimetry equation

q_soln = (grams of soln)(specific heat of soln)*∆T = -q_rxn

• Water is surroudings

Coffee cup calorimeters are used to find…

ENTHALPY, because it’s conducted at constant pressure and q_p = ∆H

Bomb calorimetry equation

q = -C_cal∆T

Bomb calorimeters are used to find…

internal energy (∆U) of combustion reactions at constant volume

In a bomb calorimeter, q is calculated in…

amount of heat per grams of substance in the bomb

Hess’s Law

If a rxn is carried out in a series of steps, ∆H for the rxn will be equal to the sum of the enthalpy changes for the steps; we can find ∆H for any process, as long as the ∆H of the component rxns are known, since enthalpy is a state function

Enthalpies of rxn

∆H°_rxn = ∑n∆H°_f (products) - ∑n∆H°_f(reactants)

Standard enthalpy of formation (∆H_f°)

The change in enthalpy for the rxn that forms one mol of a compound from elements in their standard states

Enthalpy of formation for elements and noncompounds

0

If you’re calculating heat released using calorimetry when mixing two reactants, neither of which are water, and they both participate in the reaction, then the mass used in q = mC∆T is…

the combined mass of both reactants because both are in the system

To find the molar heat of a reaction with two reactants, neither of which are water, divide the total heat released by…

the mass in moles of the limiting reactant (LR determines how much product is made and thus how much heat is released)

Enthalpy of formation for Cl2

0 (any molecule that only contains one element is 0)

Enthalpy of breaking bonds

Endothermic (requires energy)

Enthalpy of bond formation

Exothermic (releases energy, negative)

How to calculate enthalpy of rxn from bond enthalpies

sum of bonds broken (reactants) - sum of bonds formed (products)

Steps to find enthalpy of rxn from bond enthalpies

1. Draw Lewis structures for each species in equation

2. Count number of bond types in reactant

3. Multiply number of bonds by the coefficients in equation

4. Do the same for the products

5. Do reactant bonds minus product bonds

How to calculate heat released from a melt

1. If not at the melting point, use q=mCAT to find amount of heat released when temperature is increased to the melting point

2. Multiply enthalpy of fusion by amount of substance using whatever mass units will cancel to get just J or kJ

If you put a hot piece of metal in a cold cup of water, the final temperatures and magnitude of heat evolved from both the metal and water will be the…

same