CHEM 121 chapter 5: Thermochemistry

Thermodynamics

Study of energy and its transformations

Thermochemistry belongs to

Thermodynamics

Thermochemistry

Relationships between chemical reactions and energy changes that involves heat

Energy

The capacity to do work or transfer heat

Two forms of energy

Kinetic and Potential

Repulsion

Potential energy is above 0

Attraction

Potential energy is less than 0

No attraction or repulsion

Potential energy is 0

Chemical energy originates from

Chemical reactions

Chemical energy is mainly associated with changes in

Potential energy

Electrostatic interactions at the atomic level

Eel = (kQ1 + kQ2)/d

Smaller separation =

Greater separation =

higher repulsion and attraction;

less repulsion and attraction

Ions close together have a high or low potential energy? Ions far apart have a high or low potential energy?

Low; High

The velocity will ______ when the cations and ions move together, causing the KE to ________. Therefore, energy is __________.

Increase, increase, released

Energy is released when a chemical bond is ________; Energy is consumed when a chemical bond is _______.

Formed, Broken

The first law of thermodynamics

Energy cannot be created nor destroyed

Can energy be converted from one form to another?

Yes

The System

The portion we single out for study

The Surroundings

Everything else but the System

The Universe

The system + the surroundings

Internal energy

The sum of all the kinetic and potential energies of the System

Internal energy equation; What is it equivalent to?

Equivalent to first law of thermodynamics;

—> Delta E = q (heat) + w (work)

State functions

Change only depends on initial and final conditions

Path functions

Change depends on “pathway” of the change

Delta E =

= Final Energy - Initial Energy

= Heat - (Pressure x Change in Volume)

= Qv (constant volume)

= q_p- P(change in volume)

If a system loses energy, the energy will _______, and the internal energy will be ________.

Decrease; negetive

If a system gains energy, the energy will _______, and the internal energy will be ________.

Increase; positive

If the initial energy is greater than the final energy, energy is ________ because Delta E is what?

released; less than 0

Delta E = q if?

The change in Velocity = 0

Endothermic

System absorbs heat

Exothermic

System releases heat

Magnitude of work =

= Force x Distance (F x delta h)

= Pressure x Change in volume

Constant volume

Change in volume = 0

Enthalpy

A thermodynamic quantity that equals to heat flow

Enthalpy state function (equation)

H = E + PV

Enthalpy change under constant pressure

Change in Enthalpy = q_p

3 things to remember about Enthalpy in chemical reactions

1. Enthalpy is an extensive property

2. The Enthalpy of a reaction changes its sign of the reaction is reversed

3. The Enthalpy changes with the states of reactants and products

Enthalpy is an _______ property, meaning what?

Extensive; dependent on amount

Enthalpy of a reaction changes sign if the reaction is _______.

reversed

The enthalpy changes with the states of _______ and _______.

Reactants, products

What is Delta Enthalpy?

Change in H = H_final - H_initial

If the enthalpy decreases, the reaction is

Exothermic

Hess’s Law

If a reaction is carried up in a series of steps, the enthalpy of the overall reactions equals the sum of the change in enthalpy of the individual steps

Enthalpy of formation

Enthalpy change associated with the formation of a compound from its constituent elements; Change in H_f