Chapter 06: Thermochemistry

Question 1

Energy

Answer 1

The capacity of matter to do work

Question 2

Work

Answer 2

w

w = F×d
(work = force × distance)

The action of a force through a distance.

Question 3

Heat

Answer 3

q

The flow of energy caused by a difference in temperature

Question 4

Types of energy (4)

Answer 4

Kinetic energy (due to motion)

Thermal energy (associated with temperature)

Potential energy (due to position/composition)

Chemical energy (associated with positions of electrons and nuclei)

Question 5

Heat v. Temperature

Answer 5

Transfer of thermal energy v. measure of thermal energy

Question 6

Thermal equilibrium

Answer 6

Heat flows from matter with high temperature to matter with low temperature until both objects reach the same temperature.

Question 7

Thermochemistry

Answer 7

Study of relationships between chemistry and energy

Question 8

System

Answer 8

Specific part of the universe that is of interest in a study

Question 9

Surroundings

Answer 9

Everything else with which the system can exchange energy

Question 10

Open system

Answer 10

Mass & energy is exchanged

Question 11

Closed system

Answer 11

ONLY energy is exchanged (not mass)

Question 12

Isolated system

Answer 12

Neither mass nor energy is exchanged (no exchange at all)

Question 13

Kinetic energy

Answer 13

KE = 1/2mv²

Directly portional to m and v

Units: kg×m²/s²

(half of mass × velocity squared)

Question 14

1 J = ? KE

Answer 14

1 J = 1 kg×m²/s²

Question 15

1 cal = ? J

Answer 15

1 cal = 4.184 J

Question 16

1 Cal = ? cal

Answer 16

1 Cal = 1000 cal (or 1 kcal)

Question 17

Exothermic reaction

Answer 17

Releases heat by transferring thermal energy from system to surroundings

Question 18

Endothermic reaction

Answer 18

System gains heat from surroundings to react

Question 19

Internal energy

Answer 19

E

Sum of KE + PE of all particles within a system

Question 20

Change in internal energy

Answer 20

ΔE = E_products - E_reactants

Question 21

State function

Answer 21

Function that depends only on intial and final conditions

Question 22

First law of thermodynamics

Answer 22

Energy can be converted from one form to another, but it cannot be created nor destroyed

Total energy of universe is contant

Thus:

ΔE_system = -ΔE_surroundings

and:

ΔE = q + w

*q (heat) is positive when energy is gained
negative when energy is lost

*w is positive when work is done ON the system
negative when work is done BY the system

Question 23

Energy exchange

Answer 23

Accomplished through heat and work between system and surroundings

Question 24

Pressure-volume work

Answer 24

Occurs when the force is caused by a volume change against an external pressure

w = -PΔV

work = the negative of external pressure × change in volume

work done = -w!

Question 25

1 L×atm = ? J

Answer 25

1 L×atm = 101.3 J

Question 26

Enthalpy

Answer 26

H

H = E + PV
(internal energy + pressure×volume)

*State function

Question 27

Enthalpy change

Answer 27

ΔH

The heat (q) evolved in a reaction at constant pressure

ΔH = q_p

ΔH = H_products - H_reactants

Question 28

Heat capacity

Answer 28

C

q = C × Δt

The amount of heat (q) required to raise the temperature of a given quantity (m) of a substance by 1°C

Units: J/°C or J/K

Proportionality constant

System absorbs heat = tempature increase
Directly proportional

Question 29

Specific heat capacity

Answer 29

C_s
units: J/g × °C

A measure of a substance’s instrinsic ability to absorb heat

The amount of heat (q) required to raise the temperature of 1 gram of a substance by 1 °C

q = m × C_s × Δt

Question 30

Molar heat capacity

Answer 30

The amount of heat (q) required to raise the temperature of 1 mole of a substance by 1°C

Units: J/mol °C

Question 31

Bomb calorimeter

Answer 31

Constant-volume calorimetry

Since volume is constant, no work is done, w = 0

Thus:

ΔE_rxn = q_v = q_rxn

-q_rxn = q_cal = C_cal × ΔT

q_cal is heat absorbed by calorimeter
-q_rxn is heat released by reaction
C_cal is heat capacity of calorimeter

Question 32

Coffee-cup calorimetry

Answer 32

Constant-pressure calorimetry

Used to measure enthalpy of a reaction: ΔH_rxn

q_soln = m_soln × C_{s, soln} × ΔT

and

q_rxn = -q_soln = q_p = ΔH_rxn

Question 33

Standard enthalpy of formation

Answer 33

ΔH_f°

The heat change that results when one mole of a compound is formed from its elements at a pressure of 1 atm

ΔH_f° of any element in most stable form is zero

stable liquids: Hg, Br

stable gases: H₂, N₂, O₂, F₂, Cl₂, noble gases

note: C graphite is stable, not C diamond, though
note: S₈ rhombic is stable

Question 34

Standard enthalpy of a reaction

Answer 34

ΔH_rxn°

The enthalpy of a reaction carried out at 1 atm

ΔH_rxn° = Σn_productsΔH_{<span>f</span>}° - Σn_reactantsΔH_{<span>f</span>}°

n = number of moles

Question 35

Hess’ law

Answer 35

When reactants are converted to products, the change in enthalpy is the same whether the reaction takes place in one step or a series of steps

ΔH_rxn° = ΔH₁° + ΔH₂°