ENTHALPYChemistry 11 - Energetics
Source: Brown and LeMay’s Chemistry the Central Science
Objectives
Define the terms exothermic reaction, endothermic reaction, and enthalpy change of reaction (ΔH)
State that combustion is an exothermic reaction. Apply the relationship between temperature
change, enthalpy change, and the classification of a reaction as endothermic and exothermic.
Deduce, from an enthalpy level diagram, the sign of the enthalpy change of the reaction.
Energy
What is it? How / why is it important? Are there different kinds of energy? How do we measure it?
Chemical reactions
Distinguish between a physical and chemical reaction.
Chemical reactions
A physical reaction involves a change of state or phase but the chemical composition of the substance stays the same.
A chemical reaction involves a change in the composition of the substance.
What are some chemical reactions / processes that you can think of?
Apply what we have learned in bonding to explain what a chemical reaction is.
Energy and chemical reactions
How do you think energy and chemical reactions might be related?
The first law of thermodynamics
There are many types or forms of energy. Name some types of energy that we have
encountered before.
Consider a rock sitting on top of a cliff that is made to fall to ground level. Describe the changes that have occurred in terms of energy.
The first law of thermodynamics
Energy can be converted from one form to the other.
It can neither be created nor destroyed, it can only transform from one type to another.
Is the amount of energy in the universe changing? decreasing? increasing?
The first law of thermodynamics
The first law of thermodynamics states that the amount of energy in the universe is constant.
So why are we always being told to conserve energy if it’s not going anywhere?
The first law of thermodynamics
In this unit, we will often refer to systems, of which there are many different types: isolated closed open
The first law of thermodynamics
A comparison of types of systems
We will often work with isolated systems, because it is the ideal situation. Do we actually work with isolated systems in the “real world”?
Type of system
Can MASS leave or enter?
Can ENERGY leave or enter?
isolated NO NO
closed NO YES
open YES YES
The first law of thermodynamics
Define the internal energy of a system. How would we represent a change in internal
energy?
The first law of thermodynamics
The change in internal energy of a system may be expressed as:
ΔE = Efinal – Einitial
What would a positive ΔE imply?A negative ΔE?
The first law of thermodynamics
Remember that the sign of a quantity only signifies direction.
- ΔE: Energy is lost . + ΔE: Energy is gained.
The first law of thermodynamics
Changes in internal energy
In which process is energy lost? Energy gained?
Which process has + ΔE? – ΔE?
Inte
rnal
ene
rgy,
E
final state
initial state final state
initial state
The first law of thermodynamics
Energy diagram for the interconversion of H2(g), O2(g), and H2O(l)
Inte
rnal
ene
rgy,
E
H2O(l)
H2(g), O2(g)
Which process has a +ΔE?-ΔE?
The first law of thermodynamics
Energy diagram for the interconversion of H2(g), O2(g), and H2O(l)
Inte
rnal
ene
rgy,
E
H2O(l)
H2(g), O2(g)
ΔE > 0 ΔE < 0What does it mean if there are arrows going both ways?
The first law of thermodynamics
Recall two terms that we have discussed before: heat, Q work, W
What is heat? What is work? How do they relate to energy?
The first law of thermodynamics
If heat is added to a system, what happens to internal energy? What if heat is taken away?
If work is done on a system, what happens to internal energy? What if work is done by the system?
The first law of thermodynamics
The effect of heat and work on internal energy
Quantity Symbol Effect on internal energy, E
heat added to sys
heat taken away from sys
work done on sys
work done by sys
The first law of thermodynamics
The effect of heat and work on internal energy
Derive an expression that relates the change in internal energy ΔE to heat Q and work W.
Quantity Symbol Effect on internal energy, E
heat added to sys + Q increase
heat taken away from sys - Q decrease
work done on sys + W increase
work done by sys - W decrease
The first law of thermodynamics
ΔE = Q + W
This is the quantitative definition of the first law of thermodynamics.
The first law of thermodynamics
Two gases, A(g) and B(g), are confined in a cylinder with a moving piston. Substances A and B react to form a solid product:
A(g) + B(g) C(s)
As the reaction occurs, the system loses 1150J of heat to the surroundings. The piston moves downward as the gases react to form a solid. This action requires 480 J of work to be done on the system.
What is the change in internal energy of the system?
The first law of thermodynamics
Review of chemical equations:
A(g) + B(g) C(s)
The first law of thermodynamics
Review of chemical equations:
2H2O(l) 2H2(g) + O2(g)
The first law of thermodynamics
ΔE = Q + WΔE = -1150 + 480
ΔE = -670 J
The first law of thermodynamics
Calculate the change in internal energy of the system for a process in which the system absorbs 140 J of heat from the surroundings and does 85 J of work on the surroundings.
The first law of thermodynamics
ΔE =+55 J
Exothermic vs endothermic
Chemical reactions may be classified as exothermic or endothermic.
One type means it releases heat and the other means it absorbs heat. Which is which?
Exothermic vs endothermic
When a process absorbs heat, it is said to be endothermic.
When a process releases heat, it is said to be exothermic. endo- means “within” exo- means “outside” -therm means “heat”
Can you think of some processes (physical or chemical) that are endothermic or exothermic?
State functions
Consider 50 g of water at 25C in a pot. That system carries with it a certain amount of
internal energy.
What are processes that may have occurred to reach this endpoint?
State functions
The pot of 50 g of water at 25C may have been obtained either by: cooling 50 g of water from 100C to 25C, or melting 50 g of ice and raising the temperature from
0C to 25C
Does the process by which that system reached the endpoint affect its internal energy?
State functions
No, it does not.
The process by which we arrived at that result doesn’t matter. The internal energy of 50 g of water at 25C is the same no matter how it was done.
This is an example of a state function, a quantity for which the magnitude only depends on the present state of the system, not the path the system took to reach that state.
State functions
You are travelling from Manila to Baguio. Manila is at sea level, 0 m, and Baguio is much higher, 1500 m above sea level.
Is the altitude change a state function?Is the distance traveled a state function?
State functions
There are several routes or paths to get to Baguio from Manila: via EDSA or C-5 and Katipunan via NLEX or National Highway via SCTEX or National Highway via Kennon Road or Marcos Highway
Does the path you take affect the altitude change from Manila to Baguio?
Does it affect the distance traveled from Manila to Baguio?
State functions
Quite obviously, the path taken does not affect the altitude change. Baguio is still 1500 m higher than Manila. Therefore, altitude change is a state function.
Again, quite obviously, the path taken will affect the distance you travel. Therefore, distance traveled is not a state function.
Can you think of any other state functions?
State functions
Quite obviously, the path taken does not affect the altitude change. Baguio is still 1000 m higher than Manila. Therefore, altitude change is a state function.
Again, quite obviously, the path taken will affect the distance you travel. Therefore, distance traveled is not a state function.
Can you think of any other state functions?
Enthalpy
Let’s go back to the First law of thermodynamics:ΔE = Q + W
How would you measure heat?How would you measure work?
Enthalpy
Heat can be measured using calorimeters, which we discussed in physics. What are the main parts of a calorimeter?
How is work defined in physics?
Enthalpy
Consider the following reaction:Zn(s) + 2H+
(aq) Zn2+(aq) + H2(g)
Work is done by the system in this reaction. How will we use W = Fd to measure work?
Enthalpy
Zn(s) + 2H+(aq) Zn2+
(aq) + H2(g)
The work is done by the expanding hydrogen gas.
Work can be measured if that expanding gas can be harnessed to exert a force on something.
Enthalpy
W = Fd
for gases, force is related to pressure:
P = F / A
F = PAtherefore:
W = PAd
W = P ΔV
where P is the pressure in the system and ΔV is the change in volume.
Enthalpy
W = P ΔV
What does + ΔV imply? – ΔV?
Enthalpy
Relationship between change in volume and work ΔV
Expansion or compression?
Work done by or on the system?
W
+ΔV
-ΔV
Enthalpy
Relationship between change in volume and work
Therefore the more accurate relationship would be:
W = - P ΔV
ΔVExpansion or compression?
Work done by or on the system?
W
+ΔV expansion by the sys -W
-ΔV compression on the sys +W
Enthalpy
A thermodynamic function called enthalpy, H, (enthalpein means “to warm”) accounts for heat flow in processes at constant pressure, such that:
H = E + PV
where H is enthalpy, E is the internal energy of the system, P is the pressure of the system and V is the
volume of the system.
Why is it important to specify “at constant pressure”?
Enthalpy
H = E + PV
Is internal energy a state function? pressure? volume?
Enthalpy
H = E + PV
Internal energy E, pressure P, and volume V are all state functions, therefore enthalpy H is also a
state function.
Enthalpy
H = E + PV
However, in the context of chemical reactions, changes in these quantities are far more relevant.
Instead of H, we are interested in ΔH. Derive an expression for change in enthalpy ΔH.
Enthalpy
H = E + PVH = Δ Δ(E + PV)
ΔH = ΔE + PΔV
Which of these quantities can be measured?
Enthalpy
ΔE cannot be measured so where do we go from here?
ΔH = ΔE + PΔV
Is ΔE related to any quantities that can be measured?
Enthalpy
ΔH = ΔE + PΔV
H = (Q + W) + PΔ ΔV
H = Q – PΔ ΔV + PΔV(at constant pressure)
H = QΔ P
therefore the change in enthalpy ΔH is the heat that flows into or out of a system at constant pressure.
What does +ΔH imply? –ΔH?
Enthalpy
Classifying reactions as endothermic or exothermic based on change in enthalpy
ΔH Heat flow Exo- or endothermic?
+ΔH
-ΔH
Enthalpy
Classifying reactions as endothermic or exothermic based on change in enthalpy
ΔH Heat flow Exo- or endothermic?
+ΔH into system endothermic
-ΔH out of system exothermic
Enthalpy
We can describe the energy changes in a system during a chemical reaction using either ΔE or ΔH.
What is the advantage of using ΔH to describe energy changes?
Enthalpy
Indicate the sign of the enthalpy change ΔH in each of the following processes carried out at constant pressure:(a) an ice cube melts(b) 1 g of butane is combusted completely to form
carbon dioxide and water
Enthalpy
(a) heat flows into the system, +ΔH, endothermic(b) explosion: heat flows out of the system, -ΔH,
exothermic
Enthalpy
Suppose we confine 1 g of butane and sufficient oxygen in a cylinder with a movable piston. The cylinder is perfectly insulated so no heat can escape to the surroundings. A spark initiates the combustion of the butane which forms carbon dioxide and water vapor. If we used this to measure enthalpy, would the piston rise, fall, or stay the same?
2C4H10(g) + 13O2(g) 8CO2(g) + 10H2O(g)
Enthalpy
Combustion reaction2C4H10(g) + 13O2(g) 8CO2(g) + 10H2O(g)
If it is used to measure enthalpy, pressure must remain constant.
Notice how many moles of gas are on each side of the reaction.
Enthalpy
Combustion reaction2C4H10(g) + 13O2(g) 8CO2(g) + 10H2O(g)
The piston would rise to make room for the additional molecules of gas.
Ideal gas simulation
Enthalpies of reaction
Combustion reaction2H2 (g) + O2(g) 2H2O(g)
Predict the sign of the change in enthalpy ΔH of this reaction.
Enthalpies of reaction
Combustion reaction2H2 (g) + O2(g) 2H2O(g)
This is a combustion reaction so we would expect heat to be released from the system.
Is it an exothermic or endothermic reaction?
Enthalpies of reaction
Combustion reaction2H2 (g) + O2(g) 2H2O(g)
This is an exothermic reaction.ΔH = - 486.3 kJ/mol
Enthalpies of reaction
Combustion reaction2H2 (g) + O2(g) 2H2O(g) ΔH = - 486.3 kJ/mol
ΔH in this case would be referred to as the enthalpy of reaction or the heat of reaction.
The symbol is ΔHrxn
The equation above is called a thermochemical equation. What information does it provide?
Is that a lot of energy?
Enthalpies of reaction
Combustion reaction2H2 (g) + O2(g) 2H2O(g) ΔH = - 486.3 kJ/mol
Enth
alpy
Enthalpies of reaction
Combustion reaction2H2 (g) + O2(g) 2H2O(g) ΔH = - 486.3 kJ/mol
Enth
alpy
ΔH < 0
2H2 (g) + O2(g)
2H2O(g)
Enthalpies of reaction
Combustion reactionCH4 (g) + 2O2(g) CO2(g) + 2H2O(l) ΔH = - 890 kJ/mol
Enth
alpy
Enthalpies of reaction
Combustion reactionCH4 (g) + 2O2(g) CO2(g) + 2H2O(l) ΔH = - 890 kJ/mol
ΔH < 0
Enth
alpy
CH4 (g) + 2O2(g)
CO2(g) + 2H2O(l)
Enthalpies of reaction
Combustion reactionCH4 (g) + 2O2(g) CO2(g) + 2H2O(l) ΔH = - 890 kJ/mol
How much energy would be released by combusting 2 mol of methane?
Enthalpies of reaction
Combustion reactionCH4 (g) + 2O2(g) CO2(g) + 2H2O(l) ΔH = - 890 kJ/mol
Twice the amount of methane combusted would mean twice the amount of energy is released (in sufficient
oxygen).ΔH = - 1780 kJ
The enthalpy of reaction is directly proportional to the amount of reactants consumed in the process.
Enthalpies of reaction
Combustion reactionCH4 (g) + 2O2(g) CO2(g) + 2H2O(l) ΔH = - 890 kJ/mol
Predict the value of ΔH for the following reaction:CO2(g) + 2H2O(l) CH4 (g) + 2O2(g)
Enthalpies of reaction
Reverse of a combustion reactionCO2(g) + 2H2O(l) CH4 (g) + 2O2(g)
Enth
alpy
ΔH = -890 kJ
CH4 (g) + 2O2(g)
CO2(g) + 2H2O(l)
Enth
alpy
ΔH = 890 kJ
CH4 (g) + 2O2(g)
CO2(g) + 2H2O(l)
Enthalpies of reaction
Combustion reactionCH4 (g) + 2O2(g) CO2(g) + 2H2O(l) ΔH = - 890 kJ/mol
Enth
alpy
ΔH = -890 kJ
CH4 (g) + 2O2(g)
CO2(g) + 2H2O(l)
ΔH = +890 kJ
The enthalpy change of a reaction is equal in magnitude but opposite in sign to its reverse reaction.
Enthalpies of reaction
Combustion reactionCH4 (g) + 2O2(g) CO2(g) + 2H2O(l) ΔH = - 890 kJ/mol
Compare the enthalpy of the above reaction with the reaction below:
CH4 (g) + 2O2(g) CO2(g) + 2H2O(g)
Which would have a larger ΔH?
Enthalpies of reaction
Consider the following process:2H2O(l) 2H2O(g)
Is heat absorbed or released?Is this an exothermic or endothermic process?
Enthalpies of reaction
Consider the following process:2H2O(l) 2H2O(g)
The evaporation of water requires energy. Therefore this is an endothermic process.
ΔH is positive.
Enthalpies of reaction
SO
Process ΔH (kJ/mol)CH4 (g) + 2O2(g) CO2(g) + 2H2O(l) - 890
2H2O(l) 2H2O(g) +88
CH4 (g) + 2O2(g) CO2(g) + 2H2O(g) ???
Enthalpies of reaction
SO
The enthalpy change of reaction depends on the state of the reactants and products.
Process ΔH (kJ/mol)CH4 (g) + 2O2(g) CO2(g) + 2H2O(l) - 890
2H2O(l) 2H2O(g) +88
CH4 (g) + 2O2(g) CO2(g) + 2H2O(g) -802
Enthalpies of reaction
1. The enthalpy of reaction is directly proportional to the amount of reactants consumed in the process.
2. The enthalpy change of a reaction is equal in magnitude but opposite in sign to its reverse reaction.
3. The enthalpy change of reaction depends on the state of the reactants and products.
Enthalpies of reaction
How much energy is released when 4.50 g of methane gas (CH4) is burned in a constant pressure system?
CH4 (g) + 2O2(g) CO2(g) + 2H2O(l) ΔH = - 890 kJ/mol
Enthalpies of reaction
CH4 (g) + 2O2(g) CO2(g) + 2H2O(l) ΔH = - 890 kJ/mol
4.50 g of methane would release 250 kJ of heat.
Enthalpies of reaction
Hydrogen peroxide can decompose to water and oxygen by the following reaction:
2H2O2(l) 2H2O(l) + O2(g) ΔH = -196 kJ/mol
Calculate the value of ΔH if 5.00 g of hydrogen peroxide decomposes at constant pressure.
Enthalpies of reaction
Hydrogen peroxide can decompose to water and oxygen by the following reaction:
2H2O2(l) 2H2O(l) + O2(g) ΔH = -196 kJ
ΔH = -14.4 kJ
Objectives
Define the terms exothermic reaction, endothermic reaction, and enthalpy change of reaction (ΔH)
State that combustion is an exothermic reaction. Apply the relationship between temperature
change, enthalpy change, and the classification of a reaction as endothermic and exothermic.
Deduce, from an enthalpy level diagram, the sign of the enthalpy change of the reaction.
Energetics
So what is the relationship between energy and chemical reactions?
Other learning activities
Enthalpy worksheet Applications of chemistry
You will be assigned an article about a particular application of Energetics.
I will ask random people to share with the class on Tuesday. Be prepared.
Enthalpy QUIZ on Tuesday, March 10.