chemistry-enthalpy power point

UNIT C: ENERGETICS
CHAPTER ELEVEN

Curricular outcomes:

Today’s agenda:

1.1K: Recall the application
of Q = mcΔt to the analysis
of heat transfer
1.2K: Explain, in a general
way, how stored energy in
the chemical bonds of
hydrocarbons originated
from the sun
1.9k: Identify the reactants
and products of
photosynthesis, cellular
respiration and
hydrocarbon combustion
1.10K: Classify chemical
reactions as endothermic or
exothermic

1) Read pg. 480-482 and complete
Are You Ready pg. 476#1-6, 8-10

2) Intro to Energetics
Ziploc Lab
Calorimetry Review Practice

DO YOU REMEMBER??
THE LAW OF CONSERVATION OF ENERGY
 During physical and chemical

processes, energy may change
form, but it may never be
created nor destroyed.
 If a chemical system gains

energy, the surroundings lose
energy
 If a chemical system loses

energy, the surroundings gain
energy

Examples:
 When octane (C3H8, the main
component of gasoline) is burned
in your car engine, chemical bond
energy (potential energy) is
converted into mechanical energy
(pistons moving in the car engine;
kinetic energy) and heat.
 When we turn on a light switch,

electrical energy is converted into
light energy and, you guessed it,
heat energy.

DO YOU REMEMBER??
EXOTHERMIC

ENDOTHERMIC

 A change in a chemical

 A change in chemical energy

energy where energy/heat
EXITS the chemical system
 Results in a decrease in
chemical potential energy

where energy/heat ENTERS
the chemical system
 Results in an increase in
chemical potential energy

THE LAW OF CONSERVATION OF
ENERGY

An Introduction to Energetics
 Kinetic Energy (Ek) is related to the motion of an entity
 Molecular motion can by translational (straight-line),

rotational and vibrational
 Chemical Potential Energy (Ep) is energy stored in the

bonds of a substance and relative intermolecular forces
 Thermal Energy is the total kinetic energy of all of the

particles of a system. Increases with temperature.
 Symbol (Q), Units (J), Formula used (Q=mcΔT)
 Temperature is a measure of the average kinetic energy

of the particles in a system
 Heat is a transfer of thermal energy. Heat is not possessed by

a system. Heat is energy flowing between systems.

Roll mouse over
for a particle
motion and heat
transfer demo

Do You Remember?
HEATING AND COOLING CURVES
 Why does the graph plateau?
 Remember, a phase change is primarily

changing a substances potential energy.
 Bonds are being broken or formed,

changing the Ep, during the plateau of
the graph.


Remember it takes energy to break bonds, and energy is
released when bonds are formed

 When the graph is climbing, the kinetic

energy of the particles is increasing
because additional thermal energy is
being added.

An Introduction to Energetics
 Which has more thermal energy, a hot cup of coffee

or an iceberg?
 An iceberg!

Put very roughly, thermal energy is related to the amount of
something you have multiplied by the temperature.
Let's assume your iceberg is at the freezing point of water - 0
degrees Celsius (~273 Kelvin). Now your cup of coffee might
be 75 degrees Celsius (~350 Kelvin).
350 isn't a whole lot more than 270, but an iceberg is
thousands of times larger than a cup of coffee. Even though
the iceberg is at a lower temperature, it contains more
thermal energy because the particles are moving and it's
much larger than the cup of coffee.

Thermal Energy Calculations
 There are three factors that affect thermal energy (Q = mcΔt):
 Mass (m)
 Type of substance (c)
 c is the specific heat capacity, the quantity of energy required to

raise the temperature of one gram of a substance by one degree Celsius

 Temperature change (Δt)

 I.e. Consider a bathtub and a teacup of water! All water has the same

specific heat capacity which is 4.19J/g °C. However, the bathtub
would take considerably more energy to heat up!

Thermal Energy Calculations
 Example: Determine the change in thermal energy when 115 mL

of water is heated from 19.6oC to 98.8oC?
MASS = DENSITY X VOLUME
MASS = DENSITY X VOLUME
SHOW HOW L = kg AND mL
SHOW HOW L = kg AND mL
=g
=g

The density of a dilute aqueous solution is the same as that of water;
that is, 1.00g/mL or 1.00kg/L
c water = 4.19J/g °C

or

4.19 kJ/kg °C

or 4.19 kJ/L °C

Thermal Energy Calculations #2
 Example: A sample of ethanol absorbs 23.4 kJ of energy when its

temperature increases by 14.25°C . The specific heat capacity of
ethanol is 2.44 J/g°C . What is the mass of the ethanol sample?
Q = 23.4 kJ
Δt = +14.25°C
c = 2.44 J/g°C
m=?

Q = mcΔt
m = Q/cΔt
m=

23.4 kJ
.
(2.44J/g°C)(+14.25°C)

m = 0.6729 kg = 0.673 kg

How do we measure Q?
With a simple laboratory calorimeter, which
consists of an insulated container made of three
nested polystyrene cups, a measured quantity of
water, and a thermometer.
The chemical is placed in or dissolved in the
water of the calorimeter.
Energy transfers between the chemical system
and the surrounding water is monitored by
measuring changes in the water temperature.
“Calorimetry is the technological process of
measuring energy changes of an isolated system
called a calorimeter”

Includes: Thermometer, stirring rod,
stopper or inverted cup, two
Styrofoam cups nested together
containing reactants in solution

Comparing Q’s
Negative Q value

Positive Q value



An exothermic change



An endothermic change



Heat is lost by the system



Heat is gained by the system



The temperature of the
surroundings increases
and the temperature of the
system decreases



The temperature of the
system increases and the
temperature of the
surroundings decreases



Example: Hot Pack



Example: Cold Pack



Question Tips: “How much
energy is released?”



Question Tips: “What heat is
required?”

Other Calorimetry assumptions. . .
• All the energy lost or gained by the chemical system is gained or lost
(respectively) by the calorimeter; that is, the total system is isolated.
• All the material of the system is conserved; that is, the total system is
isolated.
• The specific heat capacity of water over the temperature range is
4.19 J/(g•°C). (** IN YOUR DATA BOOK)
• The specific heat capacity of dilute aqueous solutions is
4.19 J/(g•°C).
• The thermal energy gained or lost by the rest of the calorimeter (other
than water) is negligible; that is, the container, lid, thermometer, and
stirrer do not gain or lose thermal energy.

chemistry-enthalpy power point

1.1K: Recall the
application of Q = mcΔt
to the analysis of heat
transfer
1.2K: Explain, in a
general way, how stored
energy in the chemical
bonds of hydrocarbons
originated from the sun
1.9k: Identify the
reactants and products
of photosynthesis,
cellular respiration and
hydrocarbon combustion
1.10K: Classify chemical
reactions as endothermic
or exothermic

Today’s homework:
Finish Are You Ready pg. 476#1-6, 8-10
Do you remember how to rearrange
formulas??
Practice rearranging Q=mcΔt for m, c, and Δt

Pg. 487 #3-8
Homework Book pg. 1-3

1.3k: Define enthalpy
and molar enthalpy for
chemical reactions
1.4k: Write balanced
equations for chemical
reactions that include
energy changes
1.5k: Use and interpret
ΔH notation to
communicate and
calculate energy
changes in chemical
reactions
1.8k: Use calorimetry
data to determine the
enthalpy changes in
chemical reactions

Today’s Agenda:
Review homework
Enthalpy PowerPoint
Practice

ENTHALPY
 The total of the kinetic and potential energy within

a chemical system is called its enthalpy.
(Energy possessed by the system)
 Enthalpy is communicated as a difference in

enthalpy between reactants and products, an
enthalpy change, ΔrH
 .
 Units (usually kJ)

ENTHALPY CHANGES
 In a simple calorimetry experiment involving a burning candle and a can of water, the

temperature of 100 mL of water increases from 16.4°C to 25.2°C when the candle is burned
for several minutes. What is the enthalpy change of this combustion reaction?
Assuming: ΔcH = Q (The energy lost by the chemical system, (burning candle), is equal
to the energy gained by the surroundings (calorimeter water)
Assuming: Q = mcΔt then ΔcH = mcΔt
We will use
We will use
ΔcH ==- - mcΔt
mcΔt
ΔcH

•

Is the value of ΔcH going to be positive or negative??

•

If the surroundings gained energy (water), then the system (burning candle) lost it. So
based on the evidence, the enthalpy change of combustion for this reaction is -3.69J

ENTHALPY CHANGES
 When 50 mL of 1.0 mol/L hydrochloric acid is neutralized completely by 75 mL of 1.0

mol/L sodium hydroxide in a polystyrene cup calorimeter, the temperature of the total
solution changes from 20.2°C to 25.6°C. Determine the enthalpy change that occurs in the
chemical system.

Is this an Endothermic or
Is this an Endothermic or
Exothermic reaction??
Exothermic reaction??

 Based upon the evidence available, the enthalpy change for the neutralization of

hydrochloric acid in this context is recorded as -2.83 kJ.

DO YOU REMEMBER??
EXOTHERMIC

ENDOTHERMIC

 A change in a chemical energy

 A change in chemical energy where

where energy/heat EXITS the
chemical system
 Results in a decrease in chemical
potential energy
 ΔH is negative

energy/heat ENTERS the chemical
system
 Results in an increase in chemical
potential energy
 ΔH is positive

MOLAR ENTHALPY
•

Molar enthalpy: ΔrHm the change in enthalpy expressed per mole of a
substance undergoing a specified reaction (kJ/mol)

•

Have we had other quantities expressed per mole? YES!

•

How will we calculate this?

MOLAR ENTHALPY #2
1.

Predict the change in enthalpy due to the combustion of 10.0 g of propane used
in a camp stove. The molar enthalpy of combustion of propane is
-2043.9 kJ/mol.

1.

Predict the enthalpy change due to the combustion of 10.0 g of butane in a
camp heater. The molar enthalpy of combustion of butane is -2657.3 kJ/mol.

MOLAR ENTHALPY AND CALORIMETRY
•

Can we measure the molar enthalpy of reaction using calorimetry?

•

Yes, but indirectly. We can measure a change in temperature, we can then calculate
the change in thermal energy (Q=mct). Then, using the law of conservation of energy we
can infer the molar enthalpy.

•

In doing so, we must assume that the change in enthalpy of the chemicals involved in a
reaction is equal to the change in thermal energy of the surroundings.
From this equation,
any one of the five
variables can be
determined as an
unknown.

MOLAR ENTHALPY #3
1.

In a research laboratory, the combustion of 3.50 g of ethanol in a sophisticated
calorimeter causes the temperature of 3.63 L of water to increase from 19.88°C
to 26.18°C. Use this evidence to determine the molar enthalpy of combustion
of ethanol.

** You don’t have to equate the two formulas to solve this. Instead, you can calculate Q, then use that
value as ΔrH, and solve for either the chemical amount or the molar enthalpy of reaction.

Q = 95.8 kJ = ΔH

ΔcHm = 1.26 x 103 kJ/mol
= 1.26 MJ/mol

chemical reactions
energy changes
ΔH notation to
communicate and
calculate energy changes
in chemical reactions
1.8k: Use calorimetry
data to determine the
enthalpy changes in
chemical reactions

Today’s homework:
Molar Enthalpy Questions
– HW Book pg. 4 and 5
Tomorrow:
Enthalpy of Solution Lab
Finish HW Book pg. 1-5 and study for quiz
tomorrow

COMMUNICATING ENTHALPY
CHANGES
CHAPTER ELEVEN

chemical reactions
energy changes
ΔH notation to
communicate and

Today’s Agenda:
Review homework
Communicating Enthalpy PowerPoint

COMMUNICATING ENTHALPY
•

We will be learning how to communicate enthalpy changes in four ways:

1.

By stating the molar enthalpy of a specific reactant in a reaction

2.

By stating the enthalpy change for a balanced reaction equation

3.

By including an energy value as a term in a balanced reaction equation

4.

By drawing a chemical potential energy diagram

COMMUNICATING ENTHALPY #1
1.


•

Why do we use standard conditions in chemistry (i.e. SATP)?
We use a standard set of conditions so that scientists can create tables of precise, standard
values and can compare other values easily

•

Do we have standard conditions for enthalpy??
Yes, we will be using SATP (but liquid and solid compounds must only have the same initial
and final temperature – most often 25°C)

•

How do we communicate that standard conditions are used for reactants and products?
With a ° superscript, such as ΔfHm° or ΔcHm° (See data booklet pg. 4 and 5)
*For well-known reactions such as formation and combustion, no chemical equation is
necessary, since they refer to specific reactions with the Δf or Δm
** Would the sign for ΔfHm° be the opposite of the sign for ΔdHm° (decomposition)? YES!
*For equations that are not well known or obvious, then the chemical equation must be stated
along with the molar enthalpy.

1.


Example #1:
•

This means that the complete combustion of 1 mol of methanol
725.9 kJ of energy according to the following balanced equation

releases

Example #2:
•

This does not specify a reaction, so a chemical equation must be stated along with the molar
enthalpy.

•

This is not a formation reaction, since not all of the reactants are elements, so this could not have
been communicated with Δf

2.

By stating the enthalpy change beside a balanced reaction equation

•

Do we know how to calculate enthalpy change??

•

The enthalpy change for a reaction can be determined by multiplying the chemical amount (from the
coefficient in the equation) by the molar enthalpy of reaction (for a specific chemical)

Example: Sulfur dioxide and oxygen react to form sulfur trioxide. The standard molar enthalpy of combustion of
sulfur dioxide, in this reaction, is -98.9 kJ/mol. What is the enthalpy change for this reaction?
1)

Start with a balanced chemical equation.

2)

Then determine the chemical amount of SO2 from the equation = 2 mol

3)

Then use

4)

Then report the enthalpy change by writing it next to the balanced equation.

(this is an exact #, don’t use for sig digs)

to determine the enthalpy change for the whole reaction.

2.


•

THE ENTHALPY CHANGE DEPENDS ON THE ACTUAL CHEMICAL AMOUNT OF
REACTANTS AND PRODUCTS IN THE CHEMICAL REACTION. THEREFORE, IF THE
BALANCED EQUATION IS WRITTEN DIFFERENTLY, THE ENTHALPY CHANGE
SHOULD BE REPORTED DIFFERENTLY

Both chemical reactions agree with the
empirically determined molar enthalpy
of combustion for sulfur dioxide

2.


•

THE ENTHALPY CHANGE DEPENDS ON THE ACTUAL CHEMICAL AMOUNT OF
REACTANTS AND PRODUCTS IN THE CHEMICAL REACTION. THEREFORE, IF THE
BALANCED EQUATION IS WRITTEN DIFFERENTLY, THE ENTHALPY CHANGE
SHOULD BE REPORTED DIFFERENTLY

Example 2:
2.

2Al(s) + 3Cl2(g)  2AlCl3(s)

ΔfH° = -1408.0 kJ

What is the molar enthalpy of formation of aluminum chloride?
ΔfHm° = -1408.0kJ = -704.0 kJ/mol AlCl3
2 mol

2.


•

EXAMPLE: The standard molar enthalpy of combustion of hydrogen sulfide is -518.0 kJ/mol.
Express this value as a standard enthalpy change for the following reaction equation:

•

SOLUTION:

3.


•

If a reaction is endothermic, it requires additional energy to react, so is listed along with the
reactants

•

If a reaction is exothermic, energy is released as the reaction proceeds, and is listed
with the products

•

In order to specify the initial and final conditions for measuring the enthalpy change of
reaction, the temperature and pressure may be specified at the end of the equation

along

the

3.


•

EXAMPLE: Ethane is cracked into ethene in world-scale quantities in Alberta. Communicate the
enthalpy of reaction as a term in the equation representing the cracking reaction.

DOES THE +136.4 kJ
MEAN
EXOTHERMIC OR
ENDOTHERMIC?

3.


•

EXAMPLE: Write the thermochemical equation for the formation of 2 moles of methanol from its
elements if the molar enthalpy of formation is -108.6kJ/mol
2 C(s) + 4 H2(g) + O2(g)  2 CH3OH(l) + ___?_____
ΔfH = 2 mol (-108.6 kJ/mol) = -217.2 kJ (Exothermic)

2 C(s) + 4 H2(g) + O2(g)  2 CH3OH(l) + 217.2 kJ

4.

By drawing a chemical potential energy diagram

•

During a chemical reaction, observed energy changes are due to changes in chemical potential
energy that occur during a reaction. This energy is a stored form of energy that is related to the
relative positions of particles and the strengths of the bonds between them.

•

As bonds break and re-form and the positions of atoms are altered, changes in potential energy
occur. Evidence of a change in enthalpy of a chemical system is provided by a temperature change of
the surroundings.

•

A chemical potential energy diagram shows the potential energy of both the reactants and
products of a chemical reaction. The difference is the enthalpy change (obtained from calorimetry)

•

Guidelines: The vertical axis represents Ep. The reactants are written on the left, products on the
right, and the horizontal axis is called the reaction coordinate or reaction progress.


During an exothermic reaction, the enthalpy
During an exothermic reaction, the enthalpy
of the system decreases and heat flows into
of the system decreases and heat flows into
the surroundings. We observe aatemperature
the surroundings. We observe temperature
increase in the surroundings.
increase in the surroundings.

During an endothermic reaction, heat flows
During an endothermic reaction, heat flows
from the surroundings into the chemical
from the surroundings into the chemical
system. We observe aatemperature decrease
system. We observe temperature decrease
in the surroundings.
in the surroundings.

•

EXAMPLE: Communicate the following enthalpies of reaction as a chemical potential energy diagram.

•

The burning of magnesium to produce a very bright emergency flare.

•

The decomposition of water by electrical energy from a solar cell.

chemical reactions
energy changes
ΔH notation to
communicate and

Today’s homework:
Communicating Enthalpy Changes
Hw Book pg. 6 and 7
Pg. 501 #1, 2a), 3, 4

1.7k: Explain and use
Hess’ law to calculate
energy changes for the
net reaction from a
series of reactions

Today’s Agenda:
Review homework
Hess’ Law PowerPoint

HESS’ LAW
•

Do you think it is convenient or possible to use a calorimeter to test all chemical reactions?

•

NO! Sometimes two products are created simultaneously, sometimes a reaction is too small to be
able to measure accurately. So what do scientists do?

•

Theoretically, we assume that the enthalpy change of a physical or chemical process depends only on
the initial and final conditions. It is independent of the pathway, process or number of intermediate
steps required.

•

Illustration: Bricks are being moved from the ground up to the
second floor. But there are two pathways to do this:
•

Move from the 1st to 2nd floor

•

Move to third floor and then carry down one flight

•

In both cases, the overall change in position is the same.

Hess’ Law
•

G.H. Hess suggested in 1840, that “the addition of chemical equations yields a net chemical
equation whose enthalpy change is the sum of the individual enthalpy changes.”
This is called Hess’ Law

•

Hess’s Law can be written as an equation:
The uppercase Greek Letter, Σ (sigma) means “the sum of”

•

Hess’ discovery allows us to determine enthalpy change without direct calorimetry, using two
rules that you already know:
1) If a chemical equation is reversed, then the sign of ΔrH changes
2) If the coefficients of a chemical equation are altered by multiplying or dividing by a
constant factor, then the ΔrH is altered by the same factor

Hess’ Law
Hess's Law
Hess's Law
The enthalpy change for any reaction depends only on
The enthalpy change for any reaction depends only on
the energy states of the initial reactants and final
the energy states of the initial reactants and final
products and is independent of the pathway or the
products and is independent of the pathway or the
number of steps between the reactant and product.
number of steps between the reactant and product.

Hess’ Law #1
•

Example: Use Hess’ Law to determine the enthalpy change for the formation of carbon monoxide.

•

This reaction can not be studied calorimetrically but we are given the following information to help
solve this equation

•

Our job now, is to manipulate the equations so they will add to yield the net equation
•

We need 1 mol of C(s) to start the equation, so leave (1) unaltered

•

However, we want 1 mol of CO as a product, so reverse equation (2) and divide all terms by 2

•

** Remember whatever you do to the equation, affects the ΔH the same way
ΔΔcH = -566.0kJ (original equation)
cH = -566.0kJ (original equation)
1) Reversed equation; ΔH == + 566.0kJ
1) Reversed equation; ΔH + 566.0kJ
2) Divide equation by 2; Divide ΔH by 22 = +283.0kJ
2) Divide equation by 2; Divide ΔH by = +283.0kJ

Hess’ Law #1
•


•

Now cancel and add the remaining reactants and products to yield the net equation.

•

Add the component enthalpy changes to obtain the net enthalpy change.

The process of using Hess’ Law is aacombination of being systematic and
The process of using Hess’ Law is combination of being systematic and
using trial and error. Do what needs to be done to the given equations
using trial and error. Do what needs to be done to the given equations
so they add to get the equation you want.
so they add to get the equation you want.

Hess’ Law #1
•


•

Sketching a potential energy diagram might help you ensure that you have made the
appropriate additions and subtractions

Hess’ Law #2
•

Example: One of the methods the steel industry uses to obtain metallic iron is to react iron(III) oxide
with carbon monoxide
Fe2O3(s) + 3CO(g)  3CO2(g) + 2Fe(s)

ΔrH = ??

1) CO(g) + ½ O2(g)  CO2(g) ΔfH = -283.0 kJ
2) 2Fe(s) + 3/2O2(g)  Fe2O3(s) ΔfH = -822.3 kJ

3( CO(g) + ½ O2(g)  CO2(g))
reverse

ΔfH =3(-283.0 kJ) = -849.0 kJ

Fe2O3(s)  2Fe(s) + 3/2O2(g)

ΔfH = -822.3 kJ

Fe2O3(s) + 3CO(g)  3CO2(g) + 2Fe(s)

= +822.3 kJ

ΔrH = -26.7 kJ

Hess’ Law #4
•

Example: What is the standard enthalpy of formation of butane? ΔfHm° = ???

•

First, we need to be able to write this balanced formation equation.
4C(s) + 5H2(g)  C4H10(g)

•

The following values were determined by calorimetry:

•

What will we need to do to get our net equation?
-Reverse equation (1) and
-Reverse equation (1) and
change the ΔH sign
change the ΔH sign
-Multiply equation (2)
and its ΔH by 44
and its ΔH by
and its ΔH by 5/2
and its ΔH by 5/2

ΔfHm° = -125.6 kJ/1 mol = -125.6 kJ/mol
C4H10

1.7k: Explain and use
Hess’ law to calculate
energy changes for the
net reaction from a
series of reactions

Today’s Homework:
Pg. 505 #1-4
Hw Book pg. 8 and 9
Tomorrow:
Hess’s Law Lab
HW Book pg. 10

MOLAR ENTHALPIES OF
FORMATION
CHAPTER ELEVEN

1.6k: Predict the
enthalpy change for
chemical equations
using standard
enthalpies of formation

Today’s Agenda:
Review homework
Standard Enthalpies of Formation
PowerPoint
Practice

Molar Enthalpy of Formation
•

Molar enthalpies of formation are defined as the enthalpy change when one mole of a
compound forms from its elements

•

NOTE: The enthalpy of formation for an element is 0 kJ

•

Examples: Using your data booklet, find the following:

•

Δf Hm ° CH4(g) = -74.6 kJ/mol

•

Δf Hm ° O2(g) = 0 kJ/mol (Δf H elements = 0)

•

Δf Hm ° CO2(g) = - 393.5 kJ/mol

•

Δf Hm ° H2O(g) = - 241.8kJ/mol

MOLAR ENTHALPY OF FORMATION
•

Why do we care about the standard molar enthalpies of formation, ΔfH° ???

•

Because we are going to use them to predict standard enthalpy changes for chemical
reactions. How? Using this crazy formula!!

•

What does it mean? The net enthalpy change for a chemical reaction, ΔrH°, is equal to
the sum of the chemical amounts times the molar enthalpies of formation of the
products, ΣnΔf pHm °, minus the chemical amounts times the molar enthalpies of
formation of the reactants, ΣnΔf RHm °

•

Clear as mud?? Basically, the equation says that the change in enthalpy is the total
chemical potential energy of the products minus the reactants. Epproducts – Epreactants

•

We will need to use an example to figure this out.

•

Calculate the molar enthalpy of formation for two moles of carbon monoxide from its
elements.
2C(s) + O2(g)  2CO(g)
ΔfHm = 2 mol(-110.5 kJ) - 2 mol(0 kJ) + 1 mol(0 kJ)
mol
mol
mol
= -221.o kJ
2 mol
= -110.5 kJ/mol

•

Methane is burned in furnaces and in some power plants. What is the standard molar
enthalpy of combustion of methane? Assume that water vapour is a product.

•

Need a balanced chemical equation: CH4(g) + O2(g)  CO2(g) + 2H2O(g)

•

Use the formula and the data booklet to calculate the ΔcH°
We found all of the Δf Hm for the compounds two slides ago

Are we finished with -802.5 kJ?? NO!

•

Methane is burned in furnaces and in some power plants. What is the standard molar
enthalpy of combustion of methane? Assume that water vapour is a product.

•

This can also be communicated as an enthalpy change diagram. Note that the labeling
of the y-axis is different from that in a chemical potential energy diagram.

Epproducts – Epreactants


Go to Hw Book pg. 12 – we will do #2 together

1.6k: Predict the
enthalpy change for
chemical equations
using standard
enthalpies of formation

Today’s Homework:
Hw Book pg. 11 (#1-3) and 12

MULTI-STEP CALCULATIONS
CHAPTER ELEVEN

MULTI-STEP CALCULATIONS
•

We are going to practice using all of our Energy Calculations. Each question will
require more than one step, hence “multi-step”.

•

We will be going through HW Book pg. 13 in class.

•

Then working at your own pace through HW Book pg. 14-15 (answers included)

•

Multi-step Quiz coming up!

ACTIVATION ENERGY
CHAPTER TWELVE

2.1k: Define activation energy
as the energy barrier that must
be overcome for a chemical
reaction to occur
2.2k: Explain the energy
changes that occur during
chemical reactions, referring
to bonds breaking and forming
and changes in potential and
kinetic energy
2.3k: Analyze and label energy
diagrams of a chemical
reaction, including reactants,
products, enthalpy change and
activation energy
2.4k: Explain that catalysts
increase reaction rates by
providing alternate pathways
for changes, without affecting
the net amount of energy
involved; e.g., enzymes in
living systems.

Today’s Agenda:
Kinetics, Bond Energy and Catalysts

KINETICS = RATES OF REACTION
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Collision-Reaction Theory
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A chemical sample consists of entities (ions, atoms, molecules) that are in constant,
random motion at various speeds.

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For a reaction to proceed, reactants must collide

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An effective collision requires sufficient energy to react and the correct orientation,
so that bonds can be broken and new bonds formed

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The more collisions there are, the greater the potential for effective collision.

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Collision-Reaction Theory
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Ineffective collisions involve entities that rebound and do not rearrange and form
new substances.

Factors affecting Reaction Rate:
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Concentration: more reactant particles in a given volume increases
the number of collisions per second

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Surface Area: more opportunity for collisions, the more collisions
there will be

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Temperature: the faster the particles are moving, the more energy
they have to create an effective collision

ACTIVATION ENERGY OF A REACTION
Activation Energy – (EA)
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The minimum collision energy required
for effective collision

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Dependant on the kinetic energy of the
particles (depend on T)

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Analogy: If the ball does not have
enough kinetic energy to make it over
the hill – the trip will not happen.
Same idea, if molecules collide without
enough energy to rearrange their
bonds, the reaction will not occur.
(ineffective collision)


The activated complex
occurs at the at the
maximum potential
energy point in the
change along the
energy pathway.

Is this an exothermic or
endothermic change?
Exothermic. This means
the initial energy absorbed
to break the nitrogenoxygen bond is less than
the energy released when a
new carbon-oxygen bond
forms.


In general, the greater the EA, the slower the reaction.
It takes longer for more particles to achieve kinetic
energy necessary for effective collision.

What does this diagram
indicate?
At Temperature 2, a
greater number of
particles will have the
activation energy
required
Will this increase the
rate of the reaction?
Yes


Is this an exothermic or
endothermic change?
Endothermic. A continuous
input of energy, usually heat,
would be needed to keep the
reaction going, and the
enthalpy change would be
positive.

Bond Energy and Enthalpy Changes
 Bond energy is the energy required to break a chemical bond; it is also the energy

released when a bond is formed.

- bonded particles + energy  separated particles
- separated particles  bonded particles + energy

 The change in enthalpy represents the net effect from breaking and making bonds.
 ΔrH = energy released from bond making – energy required for bond breaking
 Exothermic reaction: making > breaking (ΔrH is negative)
 Endothermic reaction: breaking > making (ΔrH is positive)

LET’S SEE IF YOU GET IT
Draw energy pathway diagrams for general endothermic and a general exothermic reaction. Label
the reactants, products, enthalpy change, activation energy, and activated complex.

CATALYSTS AND REACTION RATE
 A catalyst is a substance that increases the rate of a chemical reaction without

being consumed itself in the overall process.
 A catalyst reduces the quantity of energy required to start the reaction, and

results in a catalyzed reaction producing a greater yield in the same period of time
than an uncatalyzed reaction.
 It does not alter the net enthalpy change for a chemical reaction

Catalysts lower the activation
energy, so a larger portion of
particles have the necessary
energy to react = greater yield

 How do catalysts work??
 Scientists do not really understand the actual mechanism. Catalysts are also usually

discovered through trial and error.
 What they do know is that they provide an alternative, lower energy pathway from

reactants to products.
 Most of the catalysts (enzymes) for biological reactions work by shape and orientation.

They fit substrate proteins into locations on the enzyme as a key fits into a lock, enabling
only specific molecules to link or detach on the enzyme.
 Almost all enzymes catalyze only one specific reaction

 Reaction Mechanisms
 Steps making up the overall reaction
 Each step = elementary reaction
 Reaction intermediates: substances formed in one elementary reaction and consumed in another

The rate-determining step of a
reaction is the step with the
highest activation energy.
It is called the ratedetermining step because it is
the slowest step.

2.1k: Define activation energy
as the energy barrier that must
be overcome for a chemical
reaction to occur
2.2k: Explain the energy
changes that occur during
chemical reactions, referring
to bonds breaking and forming
and changes in potential and
kinetic energy
2.3k: Analyze and label energy
diagrams of a chemical
reaction, including reactants,
products, enthalpy change and
activation energy
2.4k: Explain that catalysts
increase reaction rates by
providing alternate pathways
for changes, without affecting
the net amount of energy
involved; e.g., enzymes in
living systems.

Today’s Homework:
HW Book 16-18

chemistry-enthalpy power point

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