Chemistry Chapter 10

kinetic energy

energy in motion

thermal energy

energy associated with the random motion of atoms and molecules

potential energy

energy possessed by an object by virtue of its position

chemical energy

energy stored within the structural units of chemical substances

• stored inside the bonds

electrostatic energy

potential energy that results from the interaction of charged particles

• electrons coming loose (ex. rubbing a balloon on your hair)

Law of conservation of energy

• energy can neither be created nor destroyed

• when energy of one form disappears, the same amount of energy reappears in another form/forms

System

a part of the universe that is of specific interest

UNIVERSE = SYSTEM + SURROUNDINGS

Surroundings

constitute for the rest of the universe outside the system

UNIVERSE = SYSTEM + SURROUNDINGS

thermochemistry

study of heat in chemical reactions

Heat

transfer of thermal energy

exothermic process

when heat is transferred from the system to the surroundings

• feels hot because heat is being gained and we are in the surroundings

• heat added within the product

endothermic process

when heat is transferred from the surroundings to the system.

• feels cold because heat is being lost and we are in the surroundings

• heat is added within the reactants

State Functions

properties that are determined by the state of the system

energy = state function

magnitude of change depends only one the initial and final states of the system

• volume

• pressure

• temperature

• energy

First law of therodynamics

energy can be converted from one from to another but can not be created nor destoryed

delta U of system = - delta U of surroundings

delta U of system + delta U of surroundings = 0

• energy goes up = exothermic reaction

• energy goes down = endothermic reaction

thermodynamics

study of the conversion of heat and other kinds of energy

• three different systems

Open system

can exchange energy and mass with the surroundings

Closed system

allows transfer of hear to surroundings but not mass

Isolated System

doesn’t exchange mass nor energy with the surroundings

IDEAL SYSTEM

work and heat: Δu = q+w

the overall charge in the systems internal energy and is given by: Δu=q+w

q = heat

q is positive for endothermic processes - GAINING HEAT

q is negative for exothermic processes - LOOSING HEAT

w = work

w is positive for work done ON the system - VOLUME DECREASES

w is negative for work done BY the system - VOLUME INCREASES

Volume of the system

under constant volume conditions, pressure increases

• the volume of a system can change- if the volume can’t change the pressure changes

LESS VOLUME = MORE PRESSURE

Enthalpy (H)

thermodynamic function of a system

H= U+PV

change in enthalpy / change in heat of system (ΔH)

what we are interested in when a thermochemical reaction occurs. It is equal to the heat exchanged between the system and the surroundings for a constant pressure system.

• change in this = change in internal temperature

ΔH = ΔU + Δ(PV)

constant pressure process- qp=ΔH

qp= ΔU +PΔV

enthalpy of reaction ΔH

the difference between the enthalpies of the products and the enthalpies of the reactants

ΔH = H(products) - H(reactants)

• ΔH > 0 (positive) = endothermic process, heat lost

• ΔH < 0 (negative) = exothermic process, heat gained

ex. H2O(s) → H2O(l), ΔH = +6.01 kJ/mol

heat is absorbed by the system from the surroundings- H2O(s) + 6.01 : kJ/mol → H2O (J)

• ΔH>0 = endothermic process

enthalpy is an extensive property, this means…

it depends on how much there is. It is dependent on the amount of matter involved.

Guidelines for thermochemical equations

1. always specify the physical states of reactants and products because they help determine the actual enthalpy charges

2. when multiplying an equation by a factor (n), multiply the ΔH value by the same factor

• double the whole equation

3. reversing the equation changes the sign but not the magnitude of ΔH

• if the ΔH is positive, it becomes negative

• if the ΔH is negative, it becomes positive

calorimetry

the measurement of heat changes are measured on a device called a caloimeter

s = J / S °C

Specific Heat (s)

the amount of eat required the raise the temperature of one gram of of the substance by one degree Celcius

Heat Capacity (C)

the amount of heat required to raise the temperature of an object by one degree celcius

• the “object” may be given quantity of a particular substance

specific heat capacity units: J/(g*°C)

heat capacity unit: J/°C

q=smΔt

how the heat associated with a temperature change is calculated

m=mass, s= specific heat, ΔT= change in temperature

ΔT= (Tfinal - Tinital)

Transfer of Heat

qp=ΔH

q(sys) = -msΔT, q(surr) = msΔT

q(sys) = -q(surr)

(msΔT)sys = -(msΔT)surr

• the minus sign is used the keep sign conversions consistant

Constant-Volume Bomb

how constant volume calorimetry is carried out

• constant volume calorimeter is an isolated system

• bomb calorimeters are used to determine heats of combustion- q(cal) = -q(rxn)

calculating q(cal)

the heat capacity of the calorimeter must be known

q(cal) =CcalΔT

q(rxn) = -q(cal)

q(rxn) = -CcalΔT

Hess’s Law

the change in enthalpy for a stepwise process is the sim of the enthalpy changes for each of the steps

Steps for applying Hess’s Law

1. manipulate thermochemical equations in a manner that gives the overall desired equation

2. remember the rules for manipulating thermochemical equations

3. add the ΔT for each step after proper manipulation

4. process is useful for calculating enthalpies that cannot be found directly

Standard enthalpy of formation (ΔH°f)

the heat change that results when one mole of a compound is formed from its constituent elements in their standard states

ΔH°rxn = ∑nΔH°f(products) - ∑mΔH°f(reactants)

change in ΔH = sum of products - sum of reactants

Bond Enthalpy

the enthalpy change associated with breaking a bond in one mole of gaseous molecule

ΔH°= ∑BE(reactants) - ∑BE(products)

total energy output (bonds broken) - total energy released (bonds formed)

• breaking takes energy = POSITIVE

• making gives off energy = NEGATIVE