Chemistry- Unit 3: Thermochemistry

Thermochemistry

study of energy changes as matter undergoes chemical/physical change

Energy

ability to do work

work

energy transferred to an object by a force

Heat (Q)

transfer of kinetic energy

Temperature (T)

measure of average kinetic energy of particles in a substance

Enthalpy (H)

measure of total energy in a system

Open System

mass and energy can be exchanged

closed system

only energy transferred

isolated system

no energy transferred

Exothermic

release energy (heat) to environment (-H)

Endothermic

absorbs heat from the surroundings (+H)

Calorimetry

process of measuring thermal energy change in a chemical/physical change

Calorimetry Assumptions

No heat is transferred, any heat absorbed or released is negligible, aqueous solution has density of water

Specific heat capacity (c)

amount of energy required to raise temperature 1 degree celsius of 1 g of substance

Heat of reaction (q)

amount of energy released or absorbed by system

Calorimetry Equation

q= m c (T2-T1)

mL=

g

q before =

q after

molar enthalpy change

measured in a reaction, kJ/mol

# of moles n=

mass (m)/Molar Mass (M)

concentration (C) x volume (V)=

number of moles n

\-H=

q

Change in H

amount of heat given off during a change of state

Change in H (vaporization)

changes from liquid to gas

Change in H (fusion)

change from solid to liquid

Formula for Change in H

H products- H Reactants

Standard state

when a substance is most stable, 0 kJ/mol

Hess’ Law

change in enthalpy is the same reagardless of amount of steps to complete reaction

Reaction Rate

speed a chemical reaction occurs at, rate= quantity/time

concentration change/change in time =

rate of reaction

Average rate of reaction

change in concentration over time over a given period

Instantaneous rate of reaction

rate of reaction at a particular point in time (slope of tangent)

Nature of Reactants

ions react faster than molecules

concentration

effective collisions occur with higher concentration of reactant

Temperature

increase in temperature, increases particles with sufficient energy to overcome Ea

Pressure

number of collisions increase with increased pressure

Surface area

greater exposed surface area means greater chance of collisions

presence of a catalyst

substance that increases reaction rate by lowering Ea

Collision theory

chemical reactions occur when particle collide when orientation is favorable and occur with sufficient energy

Activation Energy (Ea)

minimum amount of energy required to initiate a chemical reaction

Rate Law shows

relationship between reaction rates and concentration of reactants

rate law =

k \[A\]\[B\]

Unit of K for zero order

mol/L x s

Unit of K for first order

1/s

Unit of K for 2nd order

L/mol x s

Unit of K for 3rd order

L^2/mol^2 x s

Reaction Mechanism

series of elementary steps that occur as reactants become products

Intermediates

items that don’t appear in final equation

Rate-determining step

slowest step, reaction cannot proceed faster than slowest step

Enthalpy

measure of total energy in a thermodynamic system

First Law:

energy of a closed system is constant

Second Law:

entropy never decreases

Third Law:

entropy of a pure substance reaches zero as temperature reaches absolute zero

Breaking bonds

require energy

Forming bonds

releases heat