Energy Changes and Rates of Reaction - Thermodynamics

thermochemistry

the study of energy changes that accompany physical and chemical changes in matter

three types of changes that occur in matter

physical (changing state), chemical, nuclear

• all are accompanied by a change in en

fission

large atom splits into two or more smaller atoms

• way stronger than other en sources

• used in nuclear power plants

fusion

joins two or more lighter atoms to make a larger and heavier one

• in development

• way stronger than fission

energy

the ability to do work

three main ways to store en

• kin en

• poten en

• nucl en

kin en

the energy of an object due to its motion (translation, rotation, vibration)

poten en

the energy of a body or system due to its position or composition – found in chemical bonds (released when new bonds form)

nucl en

stored in the nucleus as energy needed to hold the nucleus together

therm en

total potential and kinetic energy of a substance

• often considered a form of kinetic energy due to its connection to the motion of molecules.

heat

the transfer of thermal energy from a warm object to a cooler object

temp

the average kinetic energy of the particles in a sample, measured in °C or K

energy flows between substances because of their ________________ in temperature

difference

law of conservation of energy pts (4)

• The total energy of the universe is constant (1st Law of Thermodynamics)

• Energy can neither be created nor destroyed

• Energy can be transferred from one substance to another

• Can be converted into various forms

chemical system

a set of reactants and products under study, usually represented by a chemical equation

surroundings

all matter around the system that is capable of absorbing or releasing thermal energy

change in en in universe =

change in en system + change in en surrounding = 0

any change in the system is accompanied by an _____________________ change in the surroundings

equal (magnitude) nd opposite (sign)

three types of systems

1. open system

2. closed

3. isolated

open system

A system where both matter and energy can move in and out (Example: Open mug)

closed system

A system where energy can move in and out, but not matter (Example: Coffee cup with lid)

isolated system

An ideal system, where neither energy or matter can move in or out (Example: insulated coffee cup with lid)

total en of a system =

PE + KE of all species in the system

• kin en

  • moving e

  • vibration, rotation and translation of atoms and molecules

• poten en

  • nuclear poten en of protons and neutrons

  • bond en (stored en)

  • intermolecular forces

exothermic rxns

• release therm en

• heat (q) flows from the system to the surroundings, usually causing an increase in the temperature of the surroundings

• q has a negative value (q < 0); i.e. losing heat from the system

endothermic rxns

• absorb thermal energy

• heat (q) flows from the surroundings into the system, usually causing a decrease in the temperature of the surroundings

• q has a positive value (q > 0); i.e. adding heat to the system

calorimetry

technological process of measuring energy changes in a chemical system

• heat of a reaction can be measured in this isolated environment

specific heat capacity

• denoted as “c'“

• units : J/g°C OR J/g°K

• the amount of energy required to raise the temperature of 1 g of a substance by 1°C or one K; depends on type of substance and state of substance

amount of heat transferred formula

q= mcΔT

The amount of heat transferred (q) (can be neg or pos), in Joules (J), depends on:

• mass of sample (m) measured in grams

• temperature change (ΔT) measured in °C or K (can be neg or pos)

• specific heat capacity (c) measured in J/g•°C (or J/g•K)

water specific heat capacity

4.18 J/g*C

q (heat)

• mag of q tells you how much en is involved

• sign of q tells you whether the system lost or gained en from the surroundings

q (cont’d)

q (system) = -q (surroundings)

density

mass (g) per volume (mL)

2nd law of thermodynamics

when 2 objects are in thermal contact, heat is always transferred from the object at a higher temperature to the object at a lower temperature, until the 2 objects are at the same temperature (= “thermal equilibrium”)

aq solutions use the heat capacity of

water (4.18)

energy is a __________________

state function → it is INDEPENDENT of how the change happens and depends only on the states of the initial reactants and the final products in a system

molar enthalpy

energy released or absorbed per mole of a substance

bond energy

measure of bond strength in a chemical compound

why are multiple bonds more energetic than single bonds

The higher the bond enthalpy the more energy is required to break that bond and hence the stronger the bond.

hess’s law

states that the enthalpy change of a process is the same whether the process takes place in one step or in a series of steps

hess’s law formula

H( r ) = H1 + H2 + H3 ….

why is hess’s law helpful?

calculating enthalpies of reactions that are difficult to study in the lab

• difficult because chemicals are expensive, dangerous, or unpredictable

hess’s law 1

When a chemical reaction is reversed, the sign of its ∆H changes

hess’s law 2

When a chemical reaction is multiplied by a coefficient, the same is true of its enthalpy

premise of bond energies

all reactions involve bond breaking and bond making as the atoms recombine

bond breaking

energy is always required to break a bond. Bond breaking is endothermic

bond making

energy is always released when a bond is formed. Bond making is always exothermic

formula for bond energy

potential en diagram w bond en

exothermic potential en diagram (with bond en)

less energy is needed to break the bonds than is released when forming the new compounds

endothermic potential en diagram (with bond en)

more energy is required to break the bonds than is released when forming new compounds

formula for standard formation

products minus reactants

bond dissociation energy

quantity of energy required to break a chemical bond

Bond dissociation energies are ____________ values

positive

reported energies are an _____________ bond en

avg

as the # of bonds increases, the length of the bond ____________

decreases

en must be _________ to break bonds

added (endo, pos)

en is _____________ when forming bonds

released (exo, neg)

units for rate of rxn

mol / L * s

in chemical rxns, both products and reactants have

potential en