Thermochemistry Definitions and Formulas

Question 1

Enthalpy of Reaction

Answer 1

Change in enthalpy, Delta H, for a reaction at a given temperature and fixed pressure (only pressure-volume work)

Example: 2Na(s) + 2H₂O (l) → 2NaOH(aq) + H₂ (g)

Obtained by subtracting the enthalpy of the reactants from the enthalpy of the products.

Delta H = H(products) - H (reactants)

Key Relationship: Delta H = q_p

Question 2

State Function

Answer 2

Property of a system that depends only on its present state, which is determined by variables such as temperature and pressure and which is independent of any previous history of the system

T, H, E, V, U, P

Question 2

Allotrope

Answer 2

one of two or more distinct forms of the element in the same physical state.

Question 3

Law of Conservation of Energy

Answer 3

Energy may be converted from one form to another, but the total quantity of energy remains constant

Question 3

Exothermic process

Answer 3

Chemical reaction or a physical change in which heat is evolved (q is negative)

Question 3

Molar Heat Capacity

Answer 3

Heat capacity for one mole of substance

Amount of heat required to raise the temperature 1 degree celsius

Units = J / (mol • ºC)

Question 5

Thermochemistry

Answer 5

One area of thermodynamics. It concerns the study of the quantity of heat absorbed or evolved (giving-off) by chemical reactions.

Question 5

Kinetic Energy

Answer 5

The energy associated with an object by virtue of its motion

E_k = 1/2 (mass) (velocity)²

1 Joule = kg • m²/s²

Question 5

What is the enthalpy of reaction (delta H), for the formation of tungsten carbide (WC), from the elements?

W(s) + C(graphite) → WC(s)

Answer 5

Question 6

Heat of Reaction

Answer 6

• At a given temperature, is the heat, q, absorbed or evolved from a reaction system to retain a fixed temperature of the system under the conditions specified for the reaction, such as a fixed pressure
• denoted by q_rxn , the qty of heat between the reaction system and its surroundings.

Question 7

First Law of Thernodynamics

Answer 7

Change in internal energy of a system delta U, equals q + w (heat plus work)

Question 9

Internal Energy

Answer 9

The sum of the kinetic and potential energies of the particles making up the substance.

E_tot = E_k + E_p + U

Where: U = internal energy, E_k = Kinetic Energy and E_p = Potential Energy

Question 10

Calorimeter

Answer 10

Device used to measure the heat absorbed or evolved during a physical or chemical change.

Two types of calorimeter:

coffee-cup calorimeter: constant pressure calorimeter

Bomb calorimeter: volume is constant, q (heat of reaction) does not equal to delta H, get a value of Delta E instead.

Question 11

Hess’s Law of heat summation

Answer 11

• For a chemical equation that can be written as the sum of two or more steps, the enthalpy change for the overall equation equals the sum of the enthalpy changes for the individual steps.
• The overall enthalpy change for a reaction is equal to the sum of the sum of the enthalpy changes for the individual steps in the reaction.

Question 13

Energy

Answer 13

The potential or capacity to move matter.

Question 14

Work

Answer 14

An energy transfer into or out of a thermodynamic system, whose effect on the surroundings is equivalent to moving an object through a field of force.

Question 15

Thermodynamic System

Answer 15

The substance or mixture of substances that we single out for study in which a change occurs. Which is commonly called the system.

Question 15

Extensive Property

Answer 15

It depends on the amount of substances in the system.

i.e. doubling the amount of substances, doubles the systems quantity of internal energy.

Question 15

Heat Capacity

Answer 15

denoted by the symbol C, the heat capacity of a sample of substance is the quantity of heat needed to raise the temperature of the sample of substance one degree Celsius

Formula: q = C (Temp_final - Temp_init)

This property is Extensive.

Question 16

Using the standard enthalpies of formation, consider this equation:

Answer 16

Question 18

Enthalpy

Answer 18

of a thermodynamic system, is defined as:

H = U + PV

where: H = enthalpy, U = internal energy, P = pressure, V = volume

1. Is an extensive property
2. is a State function
3. Change in enthalpy of reaction equals heat of reaction at constant pressure (q_p = H_{Change In})

Question 19

Specific Heat Capacity

Answer 19

also known as specific heat.

is the quantity of heat required to raise the temperature of one gram of a substance by one degree Celsius at constant pressure.

q = s • m • (Temp_final - Temp_init)

Where s = specific heat of the substance, m = mass in g and T = degree Celsius

Question 21

Endothermic Process

Answer 21

Chemical reaction or physical change in which heat is absorbed , q is positive

Question 21

Calculate the heat absorbed by 15.0 g of water to rais the temperature from 20.0 °C t 50.0°C at constant pressure.

Specifc heat of water is 4.18 J/(g•°C)

Answer 21

Question 22

Rules for writing Thermochemical equations

Answer 22

1. When a thermochemical equation is multiplied by a factor, multiply the Delta H by the same factor because it is an extensive property
2. When a reaction is reversed, reverse the value of delta H

Question 23

Thermodynamics

Answer 23

The science of the relationships between heat and other forms of energy

Question 25

Potential Energy

Answer 25

The energy an object has by virtue of its position in a field of force.

Ep = mgh

Where: m = mass, g = 9.81 m/s², h = height measured from some standard level.

Question 27

Calorie

Answer 27

Non-SI unit of energy commonly used by chemist, originally defined as the amount of energy required to raise the temperature of one gram of water by one degee Celsius

1 cal = 4.184 J (exact definition)

Question 28

Standard State

Answer 28

• refers to the standard thermodynamic conditions chosen for substances when listing or comparing thermodynamic data: 1 atm pressure and the specified temperature (typically 25 ºC)
• Standard conditions are indicated by a superscript degree sign (º).

Question 29

Standard enthalpy of formation

Answer 29

• Also called the standard heat of formation,
• denoted delta Hº_f
• The enthalpy change for the formation of one mole of the substance in its standard state from its elements in their reference form and in their standard states.
• The standadrd enthalpy change for any chemical reaction id found by subtracting the sum of the heats of formation of all the reactants from the sum of the heats of formation of all products
• delta Hºreaction = ΣnDeltaHº (products) - ΣmDeltaHº (reactants)
• Standard enthalpy of formation of a pure element in its reference form is zero
• Standard enthalpy of formation of an ion are found by calculations because cation and anions are both formed.

Question 30

Reference form

Answer 30

the stablest form (physical state and allotrope) of the element under standard thermodynamic conditions.

Question 31

Surroundings

(Thermodynamics)

Answer 31

Everything in the vicinity of the thermodynamic system

Question 33

Heat

Answer 33

• An energy transfer into or out of a thermodynamic system that results from a temperature difference between the system and its surroundings. Denoted by the symbol q
• convention: q+ : energy going into the system, q- : energy going out of the system
• faster moving atom/molecules collide with slower moving ones causing a transfer of energy
• Speed and temperature becomes equal.
• q = delta H (at fixed pressure and a given temperature)

Question 34

How much heat could you obtain from 10.0g of Methan (CH₄) burning in Oxygen at constant pressure?

Answer 34

Question 35

Thermalchemical Equation

Answer 35

Chemical equation for a reaction (including phase labels) in which the equation is givien a molar interpretation and the enthalpy of reaction for these molar amounts and is written directly after the equation.

2H₂(g) + O₂ (g) → 2H₂O (g); Delta H = -483.7 kJ

Question 36

Pressure-Volume Work

Answer 36

w = -P(V_final - V_initial)

If change in pressure is positive, system expands doing work on its surroundings, and thus the system loses energy, hence the negative sign.