The Nature of Energy The First Law of Thermodynamics Enthalpy Enthalpies of Reaction Calorimetry

Department of Chemistry and Biochemistry

CHM 101 - Reeves

• The Nature of Energy• The First Law of Thermodynamics• Enthalpy• Enthalpies of Reaction• Calorimetry• Hess’ Law• Enthalpies of Formation

CHM 101 – Chapter Five


CHM 101 - Reeves

CHM 101 – Chapter FiveThe Nature of Energy

Chemical reactions frequently involve large exchanges of energy

Sometimes, the reaction releases large amounts of energy to the surroundings in the form of heat and light.


CHM 101 - Reeves

CHM 101 – Chapter FiveThe Nature of Energy

Chemical reactions frequently involve large exchanges of energy

Less commonly, chemical reactions absorb energy from the surroundings.


CHM 101 - Reeves

CHM 101 – Chapter FiveForms of Energy

Kinetic Energy: Energy associated with motion:

Potential Energy: Energy associated with position:

The energy is d issipated as heat. The tem perature of the water at the bottom of the fa lls

is h igher than at the top


CHM 101 - Reeves


Kinetic Energy & Units of Energy

What is the kinetic energy of a 1500 lb automobile traveling at 100 mph (45 m/s)?

The energy unit Joule is defined as:

Thus, the energy of the automobile is


CHM 101 - Reeves

CHM 101 – Chapter FiveThe First Law of Thermodynamics

The total amount of energy in the universe is constant. Energy cannot be created or destroyed.

If no energy crosses the boundary, the total energy of the system cannot change.


CHM 101 - Reeves


system surroundings

Energy that crosses the boundary from the surroundings into the system increases the energy of the system and is considered positive.

Energy can only cross the boundary in the form of heat (q) or work (w).

Heat flowing into the system or work done on the system is defined as positive.

Heat flowing out of the system or work done by the system is defined as negative.


CHM 101 - Reeves


Thus the First Law is:

What is the change in the energy of a system if 1 kJ of heat is absorbed by the system and 500J of work are done by the system?


CHM 101 - Reeves

CHM 101 – Chapter FiveFunctions of State

A function of state is one that depends only on the state of the system, as defined by its contents and conditions such at temperature and pressure. Internal energy (E) is a function of state.

The change of a state variable such as E depends only on the initial and final states of the system, NOT on the path.


CHM 101 - Reeves


• If the system expands or contracts against a pressure P, then "PV" work will result. Consider the expansion of the cylinder below.

• Work is defined as force times distance in the direction of the force. Since work done by the system is negative:

A

h

Work


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Work may take many forms including mechanical, electrical, etc. In this course, we will not explicitly consider systems that involve work in these forms.

Thus, if a process takes place at constant volume, V = 0

Then, the change in E can be determined directly by measuring the heat that flows in a constant volume process.

However, most processes in chemistry occur at constant pressure, where PV work can occur. Thus we define enthalpy (H) as Then at constant pressure:

PVEH

Enthalpy


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Like internal energy (E), enthalpy (H) is a state function. Thus, to carry out the reverse reaction, 890 kJ must be put into the system, an endothermic process.

CO2(g) + 2H2O(l) CH4(g) + 2 O2(g) H = 890 kJ

Enthalpy


CHM 101 - Reeves

CHM 101 – Chapter FiveEnthalpy

Determine the enthalpy change when 1.5 g of CH4 is burned in excess oxygen.

CH4(g) + 2 O2(g) CO2(g) + 2H2O(l) H = -890 kJ

What mass of oxygen would be consumed if this reaction released 100 kJ of heat?


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If a process such as a chemical reaction is carried out at constant pressure, the heat (q) is a direct measure of the enthalpy change (H)

Calorimetry

5 kg

W ater Bath 10 kg w ater

T in i = 25 oC

If the system is surrounded by a water bath (surroundings), the flow of heat will result in a temperature change of the water bath.


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Heat and temperature are NOT the same; the amount of heat (q) required to induce a temperature change (T) in a substance depends on:

Calorimetry

1. The mass (m) of the substance,

2. The size of the temperature change (T), and

3. The specific heat capacity (CP) or specific heat of the substance.

CP is the amount of heat required to raise the temperature of one gram of the substance by one degree centigrade. The specific heat capacity of water is 4.184

TmCq P

KgJ


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If 200 g of copper metal (CP = 0.39 ) at 100oC is immersed in 100 g of water (CP = 4.184 ) at 20oC in an insulated container, what it the final temperature?

Calorimetry

KgJ

KgJ


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If 200 g of copper metal (CP = 0.39 ) at 100oC is immersed in 100 g of water (CP = 4.184 ) at 20oC in an insulated container, what it the final temperature?

CalorimetryoJg C

oJg C


CHM 101 - Reeves

CHM 101 – Chapter FiveCalorimetry

2 H2(g) + O2(g) 2 H2O(l)

If 1.00 g of hydrogen gas were burned in an open container surrounded by a 10.0 kg water bath originally at 25.00 oC, the water temperature would rise to 28.42 oC. What is the enthalpy change for the combustion of hydrogen?


CHM 101 - Reeves

CHM 101 – Chapter FiveHess’ Law

When two moles of carbon is burned in limited oxygen, carbon monoxide is formed. How might the enthalpy change of this reaction be determined?

The enthalpy change cannot be measured directly because combustion in limited oxygen produces both CO(g) and CO2(g).


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2 C(s) + O2(g) 2 CO(g) H = ?

While this reaction cannot be carried out cleanly, two related combustion reactions can be done: 1) The combustion of carbon in excess oxygen produces carbon dioxide exclusively.

2) The combustion of carbon monoxide produces carbon dioxide exclusively.


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2 C(s) + O2(g) 2 CO(g) H = ?Hess’ Law: If two or more equations can be added together to produce a desired reaction, the enthalpy of the desired reaction will be the sum of their enthalpies.

C(s) + O2(g) CO2(g) H = -394 kJ

CO(g) + O2(g) CO2(g) H = -283 kJ21


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The combustion of nitrogen monoxide forms nitrogen dioxide. Calculate the heat of combustion of nitrogen monoxide given the following:

2 2 2 180.74N g O g NO g H kJ

2 2 22 2 67.68N g O g NO g H kJ


CHM 101 - Reeves

CHM 101 – Chapter FiveEnthalpy of Formation

Hess' Law can be formalized by defining Enthaply of Formation, which is the enthalpy change (Hf

0) that occurs when one mole of a substance is formed from its elements in their standard state at 25oC.

The Standard state of an element is its most stable form at 25oC. For example, carbon is a solid (graphite), oxygen and hydrogen are both diatomic gases.

Thus for methane (CH4) the enthalpy of formation is represented by the equation


CHM 101 - Reeves


Enthalpies of Formation have been determined for a large number of compounds, and are found in tabular form in chemistry reference texts


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Using Hess’ Law, it can be shown the Enthalpies of Formation ( ) can be combined to determine the enthalpy change ( ) for any chemical reaction. Thus:

0 0rxn prod f reac fH n H prod n H reac

0fH

rxnH


CHM 101 - Reeves


Calculate the enthalpy change (H) the reaction of gaseous hydrogen peroxide (H2O2) with hydrogen to form liquid water.

H2O2(g) + H2(g) 2 H2O(l)

Compound Hof kJ/mol)

H2O(l) -285.8

H2O(g) -241.8

H2O 2(l) -187.8

H2O 2(g) -136.1

Appendix C: Thermodynamic Quantities for Selected Compounds at 298K

= -435.5kJ


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To see how enthalpies of formation can be applied, consider the combustion of methane:

CH4(g) + 2 O2(g) CO2(g) + 2 H2O(l)

First, break the reactants down into their elements:


CHM 101 - Reeves


CH4(g) + 2 O2(g) CO2(g) + 2 H2O(l)

Then, make the products from those elements:

The Nature of Energy The First Law of Thermodynamics Enthalpy Enthalpies of Reaction Calorimetry

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