Chem 30: Thermochemistry

Thermodynamics

The science of the interconnection of heat and other kinds of energy

State function

Independent of the path taken 🔼E or 🔼H is always consistent if the reactants and products are the same

Thermal energy

The energy associated with the random motion of atoms and molecules. (Kinetic energy)

Temperature and mass measurements can help us calculate total thermal energy change.

Thermal energy is not

Temperature. Temperature is just a measurement of change in thermal energy.

Thermochemistry is the study of

Thermal energy changes

Chemical energy

Is stored within the structural units of a chemical substance: (potential energy) it quantity is deterred by the type and arrangement of constituent atoms. We consider this potential energy for the purpose of the course

Three types of motion for kinetic energy

Translational (place to place)

Rotational

Vibrational

Heat is

the transfer of thermal energy between two bodies that are at different temperature

Temperature is

Not heat but the measurement of thermal energy of a substance

First law of thermodynamics

Energy can be converted from one form to another, but cannot be created or destroyed

The term Internal energy refers to

all the energy contained within a chemical system.

The flow of heat between a system and it's surroundings involves changes in the internal energy of the system.

In a chemical system, the energy exchange between a system and it's surrounds can be accounted for by heat(q) and work (w) how?

🔼E= q+w

*note: other than heat, all energy transferred is defined as work

An increase in internal energy of a system can take three forms

- an increase in temperature

- a phase change (Ep)

- the initiation of a chemical reaction (activation energy)

A decrease in the internal energy of a system will usually result in:

- a decrease in temperature

- a phase change

- note it would be unusual for a decrease in internal energy to initiate a chemical reaction

While the total internal energy of a system (E) can't be determined, changes in internal energy (🔼E) can be determined. This change in internal energy will be

be the amount of energy exchanger between a system and it's surroundings during a physics or chemical change

Exothermic reaction

When the initial state of the system is higher in energy than the final state of the system. There was energy lost to the surroundings and the change in the internal energy of the system is negative (<0)

Endothermic reaction

When the initial stage of the system is lower in energy then the final state of the system, there was energy gained from the surroundings and the change in internal energy of the system is positive (>0)

Whether energy change, heat and work assume positive or negative values is determined from the perspective of

the system. If a system loses energy to it's surrounds 🔼E= -

Signs:

Heat absorbed by the system from the surrounds

Endothermic process +

Signs: heat absorbed by the surroundings from the system

Exothermic process: -

Work done by the system on the surroundings

Negative -

(Pressure of the system must be higher than the surroundings. Increase in gaseous moles piston moved out

Work done on the system by the surroundings

Positive +

Decrease in gaseous moles, piston moves in

When w and q are opposite signs what is the sign of 🔼E?

Depends on The sizes of q and w

Work refers to?

The force that moves an object over a distance.

Only what kind of work is of significance in chemical systems and only when there is an increase or decrease in the amount of gas present

Only pressure/volume

Pressure- volume work:

When is it positive, when is it negative?

1)When a gas is expanding against an external P, work is a negative quantity since it flows out of the system

2) when a gas is compressed, work is a positive quality, since work or energy flows into the system

If there is no change in the total volume of gas before and after a reaction occurs (🔼V=0) there is ?

No significant work done by or on the system

If the reaction is performed in a rigid container, there may be changes in? But not?

They may be changes in pressure, but if there is no change in volume, the atmosphere outside the container didn't "move" and without movement, no work is done by or on the system.

The term enthalpy (H) is used to represent:

🔼E + P🔼V, occurs at constant external pressure

In way enthalpy can be considered to be change in ? With what taken out?

In a way enthalpy can be considered to be energy with work taken out. Of course, change in enthalpy (🔼H) will represent the exchange of heat between a system and it's surroundings at constant external pressure.

When enthalpy can be considered to be energy with work taken out, change in enthalpy can then be equal to?

Heat exchanged at constant external pressure

For chemical reaction 🔼H can be calculated how? And measured how?

For chemical reaction 🔼H can be calculated theoretically and measured directly

When can change in internal energy be assumed to be same as the change in enthalpy

Often P🔼V is small, even when there is a gas involved, even when there is a change in volume. Theoretically, therefore work is done by of on the system. If P🔼V is sufficiently small, it can be ignored from calculations and the change in internal energy can be assumed to be the same as the change in enthalpy

In another world, if all the change in energy of the system was given off as heat then 🔼E= ?

🔼E= qv

Photosynthesis

Formula? 🔼H=?

Energy+ 6CO2(g) + 6H2O (l) -> C6H12O6 (s) + 6 O2 (g)

🔼H= +

(Cell resp is same thing in reverse 🔼H= -)

Products of combustion in an OPEN system

*Gaseous water and carbon dioxide gas

If they don't specify the type do combustion assume it?

An Open system

Draw page 16 in notes book

Enthalpy change (🔼H)

The difference between the potential energy of the reactants and the potential energy of the products in a process; under condition of constant pressure, is equal to the heat, Q, absorbed or released by a system

🔼H is meassured in?

KJ

🔼H is?

Equal to the heat, Q, absorbed or released by a system

🔼nH/ 🔼rH

Units?

Standard molar enthalpy/ molar enthalpy of reaction

(KJ/mol)

🔼cH

Units?

Standard molar enthalpy of combustion

Enthalpy change for the complete combustion of one mile of a substance

- (KJ/mol)

🔼f H

Units?

Standard molar enthalpy of formation

- ( KJ/mol)

- change in enthalpy when one mole of a compound is formed from it's elements at SATP

See examples of standard molar enthalpy of formation

Page 17

🔼dH

Standard molar enthalpy of decomposition: (flip the sign from formation)

🔼solH

- standard molar entropy of solution

-positive: up in phase

- negative: down in phase

🔼vapH

- standard molar entropy of vaporization

- phase change

- ( l -> g) or (g ->l)

- no change temp

🔼fusH

-standard molar entropy of fusion

- phase change

- ( s -> l) or (l-> s)

- no change temp

See page 26-27

Potential Energy graphs: Enthalpy as two parts

🔼H forward

🔼H reverse

Potential Energy graphs: activation energy

Ea forward, Ea reverse

Catalysts and activation energy

Reactant particles must collide with sufficient energy to begin to break bonds

Activation energy of a reaction is?

The minimum collision energy required for a successful reaction

Specific heat capacity

The amount of heat/energy (in joules) required to increase the temperature of 1.0g of a substance by 1.0 degree Celsius

Every substance has an

Experimentally calculated heat capacity

In order to calculate the energy required to raise the temperature of a substance, you need to include?

the specific heat capacity of that substance in your calculation

http://www.luc.edu/faculty/spavko1/c105/lect/lect-12.pdf

Calorimeter is?

Calorimeter is a device that is used to measure the heat released or absorbed by a chemical or physical process taking place within it.

Energy and bomb calorimeters:

E lost= E gained

simple calorimeter (with two poly Styron cups)

Used for neutralization and dissolving

Simple Calorimeter: (with spirt burner)

- simple combustion

- energy per gram

Ek formulas

1. 🔼H =mc🔼t

(Units J)

2. 🔼H= C🔼t (bomb)

Ep formulas

🔼H= n• 🔼nH

🔼H= C• V • 🔼nH

Hess's Law

The enthalpy change of a physical or chemical process depends only in the initial and final states.

The enthalpy change of the overall process is the sum of enthalpy changes of it's individual steps.

See page 38

When adding a series of reactions what are the options for manipulating?

1. Flip rxn and flip 🔼H sign

2. Multiply Xrxn/X🔼H (both) by the same value.

3. Both

4. Leave it alone

Use Hess's Equation when?

Given a word statement or balanced chemical reaction

🔼H =

🔼H= (Σn • 🔼fH products ) - (Σn • 🔼fH reactants)

Be carful if formation vs bond energies

Formation: products - reactants

Bond energies: reactants- products

Heat of formation of an element is

zero

Units for heat capacity of a calorimeter

KJ/Degrees C