enthalpy chemistry 11 - energetics source: brown and lemay’s chemistry the central science

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.


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 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?


In this unit, we will often refer to systems, of which there are many different types: isolated closed open


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


Define the internal energy of a system. How would we represent a change in internal

energy?


The change in internal energy of a system may be expressed as:

ΔE = Efinal – Einitial

What would a positive ΔE imply?A negative ΔE?


Remember that the sign of a quantity only signifies direction.

- ΔE: Energy is lost . + ΔE: Energy is gained.


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


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?


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?


Recall two terms that we have discussed before: heat, Q work, W

What is heat? What is work? How do they relate to energy?


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 effect of heat and work on internal energyQuantity Symbol Effect on internal energy,

E

heat added to sys

heat taken away from sys

work done on sys

work done by sys


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


ΔE = Q + W

This is the quantitative definition of 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?


Review of chemical equations:

A(g) + B(g) C(s)


Review of chemical equations:

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


ΔE = Q + WΔE = -1150 + 480

ΔE = -670 J


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.


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

ΔVExpansion 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 + PVwhere 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.



This is a combustion reaction so we would expect heat to be released from the system.

Is it an exothermic or endothermic reaction?



This is an exothermic reaction.ΔH = - 486.3 kJ/mol


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?



Enth

alpy



Enth

alpy

ΔH < 0

2H2 (g) + O2(g)

2H2O(g)


Combustion reactionCH4 (g) + 2O2(g) CO2(g) + 2H2O(l) ΔH = - 890 kJ/mol

Enth

alpy



ΔH < 0

Enth

alpy

CH4 (g) + 2O2(g)

CO2(g) + 2H2O(l)



How much energy would be released by combusting 2 mol of methane?



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.



Predict the value of ΔH for the following reaction:CO2(g) + 2H2O(l) CH4 (g) + 2O2(g)


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)



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.



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?


Consider the following process:2H2O(l) 2H2O(g)

Is heat absorbed or released?Is this an exothermic or endothermic process?


Consider the following process:2H2O(l) 2H2O(g)

The evaporation of water requires energy. Therefore this is an endothermic process.

ΔH is positive.


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) ???


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


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.


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


CH4 (g) + 2O2(g) CO2(g) + 2H2O(l) ΔH = - 890 kJ/mol

4.50 g of methane would release 250 kJ of heat.


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.


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.

enthalpy chemistry 11 - energetics source: brown and lemay’s chemistry the central science

Documents

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