Thermodynamics Calorimetry-Enthalpy Chapter 5.2+5.3

Question 1

Calorimeter (4)

What is does+Endo+Exo reactions+How is does it

Answer 1

• A device used to measure the amount of heat involved in a chemical or physical process.
• An exothermic reaction occurs in solution in a calorimeter, the heat produced by the reaction is absorbed by the solution, which increases its temperature.
• An endothermic reaction occurs, the heat required is absorbed from the thermal energy of the solution, which decreases its temperature .
• The temperature change, along with the specific heat and mass of the solution, can then be used to calculate the amount of heat involved in either case.

Question 2

System

Answer 2

the substance or substances undergoing the chemical or physical change

Question 3

Surroundings

Answer 3

All other matter, including components of the measurement apparatus, that serve to either provide heat to the system or absorb heat from the system

Question 4

In a calormetric determination, there are 2 situations:

*use words such as heat, q, thermal energy, run type…

Answer 4

1. An exothermic process occurs and heat, q, is negative, indicating that thermal energy is transferred from the system to its surroundings
2. An endothermic process occurs and heat, q, is positive, indicating that thermal energy is transferred from the surroundings to the system

Question 5

In a calorimeter heat transfer occur

What is net heat change?

Answer 5

between the two substances, with no heat gained or lost by either its external environment. Under these ideal circumstances, the net heat change is zero

Question 6

Because energy is neither created nor destroyed during a chemical reaction, the heat produced or consumed in the reaction (the “system”), q reaction, plus the heat absorbed or lost by the solution (the “surroundings”), q solution, must add up to

Answer 6

0

Question 7

How do do speific heat capacity questions with acid/solutions:

Answer 7

we can proceed as if it were water in terms of its specific heat and mass values.

Sub in values for water specific heat, mass (add the hcl+naoh concentration) and change in temp of the overall rxn as water’s since its so much as a solvent

Question 8

Bomb Calorimeter calibration (3)

How calibration done? What does it determine?+when is it performed?

Answer 8

• Bomb calorimeters require calibration to which includes the bomb, water, stirrer, thermometer, and entire container.
• The calibration is accomplished using a reaction with a known q, such as a measured quantity of benzoic acid ignited by a spark from a nickel fuse wire that is weighed before and after the reaction. The temperature change produced by the known reaction is used to determine the heat capacity of the calorimeter.
• The calibration is generally performed each time before the calorimeter is used to gather research data.

Question 9

What does Bomb calorimeter operate on?

Answer 9

Constant volume: the system is sealed or isolated from its surroundings, which accounts for why its volume is fixed

Question 10

Bomb calorimeters require calibration to determine the

Answer 10

heat capacity of the calorimeter

Question 11

Different substances have different heat capacities, therefore

Answer 11

different objects pf the same mass can exhibit different temperature changes after absorbing the same amount of heat

Question 12

Internal energy

Answer 12

The total of all possible kinds of energy present in a substance

the sum of all the microscopic kinetic and potential energies of the particles in the system if the system would be at rest and not in a macroscopic energy potential.

Question 13

Energy is transferred into a system when (2)+ex

Answer 13

it absorbs heat (q) from the surroundings or when the surroundings do work (w) on the system

For example, energy is transferred into room-temperature metal wire if it is immersed in hot water (the wire absorbs heat from the water), or if you rapidly bend the wire back and forth (the wire becomes warmer because of the work done on it).

Question 14

Conversely, energy is transferred out of a system when (2)

Use wire example

Answer 14

heat is lost from the system, or when the system does work on the surroundings.

Question 15

First law of thermodynamics

Formula+ what it shows and what does the positive/neg mean?

Answer 15

This is one version of the first law of thermodynamics, and it shows that the internal energy of a system changes through heat flow into or out of the system (positive q is heat flow in; negative q is heat flow out) or work done on or by the system.

The work, w, is positive if it is done on the system and negative if it is done by the system.

Question 16

As a system undergoes a change….

internal energy+what happen to it

Answer 16

its internal energy can change, and energy can be transferred from the system to the surroundings, or from the surroundings to the system.

Question 17

The internal energy, U, of a system can be changed by (2)

Answer 17

Heat flow and work

Question 18

State function

Answer 18

The value of a state function depends only on the state that a system is in, and not on how that state is reached/a property whose value does not depend on the path taken to reach that specific value

Question 19

If a quantity is not a state function, then its value

Answer 19

does depend on how the state is reached.

Question 20

Internal energy is an example of a

Answer 20

state function

Question 21

Tell me which is a state function when climbing a hill (altitude and distance)

Answer 21

Altitude- State function
Distance- Not a state function

Question 22

Enthalpy (H)

formula

Answer 22

the sum of a system’s internal energy (U) and the mathematical product of its pressure (P) and volume (V):

Question 23

Enthalpy is an example of a

Answer 23

State function

Question 24

Measuring enthalpy

Answer 24

Enthalpy values for specific substances cannot be measured directly; only enthalpy changes for chemical or physical processes can be determined.

Question 25

For processes that take place at constant pressure (a common condition for many chemical and physical changes), the enthalpy change (ΔH) is:

Answer 25

at constant pressure, the change of enthalpy equals the change of heat.

Question 26

If a chemical or physical process is carried out at constant pressure with the only work done caused by
expansion or contraction, then the heat flow (qp) and enthalpy change (ΔH) for the process are

Answer 26

equal

Question 27

List of state functions (9)

Answer 27

Pressure
Temperature
Volume
Mass
Internal energy (U)
Gibb’s free energy
Entropy
Enthalpy
Kinectic energy

Question 28

Bomb calorimter internal energy is

Answer 28

the heat measured relates to the change in internal energy (ΔU=q).

Because U=q+work

work=0 since the volume doesnt change.

Question 29

the heat produced by a reaction measured in a bomb calorimeteris not equal to —- because….

Answer 29

ΔH

the closed, constant-volume metal container prevents the pressure from remaining constant (it may increase or decrease if the reaction yields increased or decreased amounts of gaseous species).

Question 30

The following conventions apply when using ΔH

The signs and what they mean. What is required?

Answer 30

• A negative value of an enthalpy change, ΔH < 0, indicates an exothermic reaction; a positive value, ΔH > 0, indicates an endothermic reaction. If the direction of a chemical equation is reversed, the arithmetic sign of its ΔH is changed (a process that is endothermic in one direction is exothermic in the opposite direction).

Question 31

The following conventions apply when using ΔH

What is used to show enthalpy, what is the relationship with it?

Answer 31

• Chemists use a thermochemical equation to represent the changes in both matter and energy. In a thermochemical equation, the enthalpy change of a reaction is shown as a ΔH value following the equation for the reaction. This ΔH value indicates the amount of heat associated with the reaction involving the number of moles of reactants and products as shown in the chemical equation.

Question 32

The following conventions apply when using ΔH

What is used to show enthalpy, what is the relationship with it?

Answer 32

• Chemists use a thermochemical equation to represent the changes in both matter and energy. In a thermochemical equation, the enthalpy change of a reaction is shown as a ΔH value following the equation for the reaction. This ΔH value indicates the amount of heat associated with the reaction involving the number of moles of reactants and products as shown in the chemical equation.

Question 33

If the coefficients of the chemical equation are multiplied by some factor, the enthalpy change must be…. because….

Answer 33

multiplied by that same factor
because ΔH is an extensive property

Question 34

The following conventions apply when using ΔH

What does the enthalpy change depend on in the chemical formula?

Answer 34

The enthalpy change of a reaction depends on the physical states of the reactants and products, so these must be shown. For example, when 1 mole of hydrogen gas and mole of oxygen gas change to 1 mole of liquid water at the same temperature and pressure, 286 kJ of heat are released. If gaseous water forms, only 242 kJ of heat are released.

Question 35

When determining the ΔH for a
chemical reaction, what needs to be taken into account?

Answer 35

Be sure to take both stoichiometry and limiting reactants into account

Question 36

ΔH vs ΔHof

Answer 36

ΔHof- is used to indicate an enthalpy change for a process occurring under standard conditions

ΔH -is used to indicate an enthalpy change for a reaction occurring under nonstandard conditions

Question 37

standard state

Why it is important+ the conditions

Answer 37

a commonly accepted set of conditions used as a reference point for the determination of properties under other different conditions.

materials under a pressure of 1 bar and solutions at 1 M usually at 298.15K

Question 38

Standard enthalpy of combustion ΔHofC

Answer 38

enthalpy change when 1 mole of a substance burns (combines vigorously with oxygen) under standard state conditions; it is sometimes called “heat of
combustion.”

Question 39

The enthalpy of combustion of ethanol, −1366.8 kJ/mol, is the

Answer 39

amount of heat produced when one mole of ethanol undergoes complete combustion at 25 °C and 1 bar pressure, yielding products also at 25 °C and 1 bar (100kpa).

Question 40

Standard enthalpy of formation ΔHoff

Answer 40

An enthalpy change for a reaction in which exactly 1 mole of a pure substance is formed from free elements in their most stable states under standard state conditions.
Depend on product… has to be 1 mole

Question 41

The standard enthalpy of formation of an element in its most stable form is

Answer 41

equal to zero under standard conditions, which is 1 atm for gases and 1 M for solutions.

Ex: graphite is most stable form of Carbon so its 0 bt dimaond has enthalpy formation of 1.88 kj/mol

Question 42

Hess Law states

Answer 42

If a process can be written as the sum of several stepwise processes, the enthalpy change of the total process equals the sum of the enthalpy changes of the various steps.

Question 43

Hess’s law is valid because

Answer 43

enthalpy is a state function: Enthalpy changes depend only on where a chemical process starts and ends, but not on the path it takes from start to finish.

Question 44

When do you use this?

Answer 44

To determine the enthalpy change of any reaction if the corresponding enthalpies of formation of the reactants and products are available.