16.1 Thermochemistry

Summary

This educational video script covers thermochemistry, focusing on energy transfer in chemical reactions. It explains the difference between heat and temperature, introduces specific heat capacity, and the heat equation. The script also discusses enthalpy of reaction, including exothermic and endothermic processes. It further explains concepts like enthalpy of formation and combustion, and uses Hess's Law to solve a practice problem, demonstrating how to calculate the enthalpy of formation for methane.

Takeaways

• 🔥 Thermochemistry involves the study of energy transfer in chemical reactions, particularly as heat.
• 🌡️ Temperature measures the motion of molecules, whereas heat is the transfer of energy.
• 💧 The bomb calorimeter is used to measure the total energy transferred to water molecules during an exothermic reaction.
• 🌡️ The Kelvin scale is used to measure temperature, and energy changes are measured in kilojoules (kJ) or calories.
• 🔥 Specific heat capacity (Cp) is defined as the energy required to heat 1 gram of a substance by 1 Kelvin.
• 💧 The heat equation (Q = mcΔT) relates the total energy transfer to the specific heat, mass, and temperature change of a substance.
• 🔄 Enthalpy of reaction (ΔH) measures the change in energy between reactants and products.
• 🔥 Exothermic reactions release energy, resulting in a negative ΔH, while endothermic reactions absorb energy, resulting in a positive ΔH.
• 🌐 Thermochemical equations show the relationship between the reactants, products, and energy changes in a reaction.
• 🔍 Hess's Law allows for the calculation of the enthalpy of formation of a compound by combining and manipulating known thermochemical equations.
• 🔥 The enthalpy of formation helps determine the stability of a compound, with more stable compounds having lower (more negative) values.

Q & A

What is thermochemistry?

Thermochemistry is the study of energy transfer in chemical reactions, typically measured as heat.

What is the difference between heat and temperature?

Temperature measures the motion of molecules, such as how fast water molecules oscillate in a bomb calorimeter, while heat is the transfer of energy, like the energy released during an exothermic reaction.

Why is the specific heat capacity important in thermochemistry?

Specific heat capacity is crucial because it determines how much heat is required to change the temperature of a substance. It varies between materials, affecting how efficiently they can be heated or cooled.

What units are used to measure temperature and heat in the context of this script?

Temperature is measured in degrees Kelvin (K), and heat is measured in joules (J).

How is the heat equation formulated?

The heat equation is formulated as Q = mcΔT, where Q is the total energy transfer, m is the mass of the material, c is the specific heat capacity, and ΔT is the change in temperature.

What is enthalpy of reaction?

Enthalpy of reaction, ΔH, represents the change in energy during a chemical reaction, which is the difference between the energy of the products and the energy of the reactants.

How does the enthalpy of reaction relate to the stability of a compound?

A compound with a negative enthalpy of formation is more stable than its constituent elements because it releases energy when formed, indicating a lower energy state.

What is the difference between an exothermic and an endothermic reaction?

An exothermic reaction releases energy (ΔH is negative), while an endothermic reaction absorbs energy (ΔH is positive).

What is the significance of Hess's Law in thermochemistry?

Hess's Law allows the calculation of the enthalpy change for a reaction by combining known thermochemical equations, even if the reaction does not occur in one step.

How is the enthalpy of formation used to determine the stability of a compound?

The enthalpy of formation is used to determine the stability of a compound by comparing its energy content to that of its constituent elements. A negative enthalpy of formation indicates a more stable compound.

What is the enthalpy of combustion and how is it measured?

The enthalpy of combustion is the energy released when one mole of a substance is burned in excess oxygen. It is measured as the heat released per mole of reactant.

Outlines

00:00

🔥 Thermochemistry Basics

This paragraph introduces the concept of thermochemistry, focusing on the transfer of energy during chemical reactions, typically measured as heat. It distinguishes between heat and temperature, with the latter being the motion of molecules and the former being the transfer of energy. The script uses the example of a bomb calorimeter to explain how heat energy from a reaction can be transferred to water, causing its molecules to move faster. The measure of temperature is in Kelvin, and heat is measured in joules. The paragraph also covers specific heat, which varies by material, and how it impacts the amount of heat transferred. The heat equation is introduced, showing the relationship between the energy transfer (Q), specific heat, mass, and temperature change.

05:00

🌡 Understanding Heat Transfer

The script delves into the factors affecting heat transfer during reactions, which include the material's specific heat, mass, and the change in temperature. It explains that different materials have different specific heats, affecting how much energy is needed to change their temperature. The heat equation is expanded upon, showing how these factors combine to calculate the total energy transfer. The concept of enthalpy of reaction is introduced, which measures the change in energy between reactants and products. The script uses the example of hydrogen combustion to illustrate an exothermic reaction, where energy is released. The paragraph concludes with a discussion of thermochemical equations, which combine the chemical reaction with the energy change, and the concept of exothermic and endothermic reactions.

10:03

🔬 Enthalpy of Reaction and Formation

This section discusses the enthalpy of formation, which is the energy change when elements are combined to form a compound. It explains that the enthalpy of formation can indicate the stability of a compound, with more stable compounds having lower values. The script contrasts this with the enthalpy of combustion, which measures the energy released when a substance is ignited in the presence of oxygen. The paragraph also introduces Hess's Law, which is used to calculate the enthalpy of formation of methane from carbon and hydrogen. The process involves manipulating known thermochemical equations to find the desired reaction's enthalpy change, taking into account the direction of reactions and the coefficients of reactants and products.

Keywords

💡Thermochemistry

Thermochemistry is the study of energy transfer in chemical reactions, typically measured as heat. It is central to the video's theme, as it sets the stage for understanding how chemical reactions involve the transfer of energy. The script discusses how heat is different from temperature and how it is measured in relation to chemical reactions, such as the combustion of hydrogen.

💡Heat

Heat, in the context of the video, is defined as the transfer of energy. It is a key component in thermochemistry and is used to measure the energy changes during chemical reactions. The script uses the example of a bomb calorimeter to illustrate how heat is transferred to water molecules, causing them to move faster.

💡Temperature

Temperature is described as a measure of the motion of molecules. It is distinct from heat, as it pertains to the speed of molecular oscillation. The video uses the bomb calorimeter to explain how temperature changes are measured in relation to the energy transferred during reactions.

💡Specific Heat

Specific heat is a property of a material that indicates how much heat is required to raise the temperature of a unit mass of the material by one degree Celsius (or Kelvin). It is crucial in thermochemistry for calculating the amount of heat absorbed or released by substances. The script explains that water has a high specific heat, making it harder to heat up compared to iron.

💡Heat Equation

The heat equation is a formula that relates the total energy transfer (Q) in a reaction to the specific heat of a material, its mass, and the temperature change. It is a fundamental equation in the video that demonstrates how to calculate the energy change in a chemical reaction. The script uses this equation to show the relationship between heat transfer, mass, and temperature change.

💡Enthalpy

Enthalpy is a measure of the total energy of a thermodynamic system. It is a key concept in the video, as it is used to describe the change in energy during a reaction. The script explains how enthalpy of reaction (ΔH) is calculated as the difference between the energy of the products and the energy of the reactants.

💡Thermochemical Equation

A thermochemical equation is a chemical equation that includes the enthalpy change of the reaction. It is used in the video to illustrate how the energy change is represented alongside the chemical reaction. The script mentions that these equations are essential for understanding the energy transfer in chemical processes.

💡Exothermic Reaction

An exothermic reaction is a process that releases energy, usually in the form of heat. The video uses the combustion of hydrogen as an example of an exothermic reaction, where a significant amount of energy is released, making the reaction violent, with heat, sound, and light.

💡Endothermic Reaction

An endothermic reaction is one that absorbs energy from its surroundings. The video contrasts this with exothermic reactions and uses electrolysis as an example where energy is required to break down water into hydrogen and oxygen, making it an endothermic process.

💡Enthalpy of Formation

Enthalpy of formation is the change in enthalpy during the formation of 1 mole of a compound from its elements in their standard states. The video explains its importance in determining the stability of a compound. For instance, carbon dioxide has a negative enthalpy of formation, indicating it is more stable than its elemental forms.

💡Hess's Law

Hess's Law states that the total enthalpy change in a chemical reaction is the same, regardless of the number of steps or the pathway taken. The video uses this law to solve a practice problem, demonstrating how known reactions can be manipulated to find the enthalpy of formation for a compound like methane.

Highlights

Thermochemistry is about the transfer of energy in chemical reactions, often measured as heat.

Temperature measures the motion of molecules, while heat is the transfer of energy.

Heat transfer is measured in degrees Kelvin and energy in kilojoules (kJ).

Specific heat capacity (Cp) is the energy required to heat 1 gram of a substance by 1 Kelvin.

Heat transfer depends on the material, its mass, and the change in temperature.

The heat equation is Q = mcΔT, where Q is energy transfer, m is mass, c is specific heat, and ΔT is temperature change.

Enthalpy of reaction (ΔH) represents the change in energy between products and reactants.

Exothermic reactions release energy, while endothermic reactions require energy input.

Thermochemical equations show the energy change associated with a chemical reaction.

Enthalpy of formation is the energy change when one mole of a compound is formed from its elements.

Enthalpy of combustion is the energy released when one mole of a substance is burned in excess oxygen.

Enthalpy of formation helps determine the stability of compounds.

Hess's Law allows the calculation of unknown enthalpy changes using known reactions.

Reversing a reaction equation changes the sign of its enthalpy change.

Multiplying coefficients in an equation scales the enthalpy change proportionally.

By manipulating known equations, you can derive the enthalpy of formation for methane from carbon and hydrogen.

The enthalpy of formation for methane is -74.3 kJ/mol.

Transcripts

all right so this video is going to be

dealing with chapter 16 section one

which is all about thermochemistry and

thermochemistry is basically about the

transfer of energy in chemical reactions

usually measured as heat and so as a

first uh sort of intro we're going to be

discussing the difference between heat

and temperature so temperature as we've

already studied is basically the motion

of molecules so if you have these water

molecules in this bomb calorimeter that

I'll explain later it's basically a

measure of how fast they oscillate back

and forth right whereas heat is defined

as the transfer of energy so if you were

to you know have some sort of fire or

some sort of exothermic reaction

releasing energy inside this bomb

calorimeter what it would do is that

energy would eventually transfer into

the water making these molecules move

faster and faster and then you can

measure the total energy

transferred that is how much energy was

created in here and then transferred to

the Water by how much heat is released

and as a quick clarification we're going

to be using uh degrees Kelvin for

measure of temperature which basically

equal de C plus

273 uh de and heat we're going to be

using kogs

or

calories moving on now we're going to be

discussing the concept of specific heat

and before we go into that we have to

discuss the factors that go into how

much heat is transferred during reaction

so the heat transfer depends on three

main things first is the material and

this is where specific heat comes in for

example it's much harder to heat up

water than it is to heat up iron and

that's due to different properties in

the two materials like how Iron is a

good conductor of heat it depends on the

Mass of material so for example it's a

lot easier to heat up 1 G of water than

1 kilog and the final thing is the total

change in temperature so if you're going

to heat something up by 100 Kelvin it's

going to take a lot less energy than if

you were to heat it up by 1 Kelvin now

this first Factor all depends on what is

known as the specific heat of the

material which is usually given by CP

the C meaning that at a constant

pressure it has a certain specific heat

and it's defined as the energy required

in jewels to heat up 1 G of

material by 1° Kelvin and for water that

number is

4.18

jewles per gam degree Kelvin bringing

all these factors together now we get

what's known as the heat equation which

basically says that Q the total energy

transfer

is equal to the specific heat of the

material times the total mass m of the

material times the total temperature

change so T and you'll notice that this

is in jewles per gam degree Kelvin this

is in grams and this is in kelvin so you

cancel out the grams and the Kelvin and

you get that the total energy change is

in fact in Jews so dimensionally it all

sense moving on now we're going to be

discussing what's known as the enthalpy

of reaction which basically gives the

change in energy that is the Delta h of

a reaction so it's how much energy the

products have stored in them minus how

much energy the reactants initially had

so if we look at uh the combustion of

hydrogen in the presence of oxygen down

here uh yes we can see what turns into

what but we don't really know how much

energy is produced and for that you have

to go over here to the product side

and if you've ever seen the reaction of

uh hydrogen in the presence of oxygen

you'll note that it's very violent it

releases a lot of heat a lot of sound

and a lot of light when you light it on

fire basically and that is all

contributing to this change in energy

over here now with this complete story

both the chemicals involved as well as

how much heat is transferred we get what

is known as a thermochemical equation in

other words Thermo meaning heat or

energy and chemical as in the chemicals

involved in the process it should be

noted as well that the energy released

over here is completely proportional to

how much goes into the reaction so if

you have four moles of hydrogen and 2

moles of oxygen in other words you

double how much you put into the

reaction you too have to double uh the

energy output over here so if you were

to react four moles of hydrogen in the

presence of two moles of oxygen you'd

release

9672 K rather than

4836 because this releases energy as a

product over here this is what is known

as an exothermic reaction so if you were

to reverse it in other words if you were

to take 2 moles of water and add

4836 K to the system through

electrolysis or what have you you could

decompose it into two moles of hydrogen

and a mole of oxygen and that would then

be an endothermic process because it

requires energy to take in usually

however you don't give the energy in

these blank spaces within the reaction

normally what you do is you just write

the reaction like I have right here and

then beside it you'll note the change in

energy the Delta H in other words so for

this the change in energy would be uh

4836 K because it released those 483

83.6 and likewise the Delta H for this

reaction the synthesis reaction the

endothermic one would be a positive

4836 so exothermic the Delta H is

negative and for

endothermic the Delta H is positive you

can think of it basically as how much

energy is going into the reaction so if

you put in energy as in an endothermic

reaction it's going to be positive if it

releases energy or you take out energy

it's going to be

negative so a good way to visualize uh

the difference between exothermic and

endothermic reactions is with what is

known as a reaction pathway so here we

have an exothermic reaction pathway in

which the reactants start with a high

amount of energy and then the reaction

takes place and they end up with this

low amount of energy and this transfer

from reactant two products is the Delta

H and as you can see it has a negative

value if we were to have it so

that the pathway went

upwards so up here you could

see here we start with the reactants

here we have the products the Delta H

would then be positive and that would be

of course an endothermic reaction moving

on now we're going to be discussing a

specific enthalpy PES of reaction for

example the enthalpy of

formation is defined as the specific

enthalpy of reaction for composition

reactions so when you take two elements

and synthesize some sort of compound

it's the enthalpy

change and this zero just means that

it's in its standard state so for at

room temperature and one atmosphere that

means that water is a liquid you know

oxygen is a gas Etc now this enthalpy of

formation is basically defined as the

energy required to synthesize one mole

of the material so why do we measure the

enthalpy of formation it's basically to

determine how stable a compound is in

its current state so Elements by

definition you know oxygen Etc

have a enthalpy of formation of zero

because nothing can form

uh an element so for all of them they

have no enthalpy of formation

essentially however some compounds are

significantly more stable than the

elements that comprise them for example

carbon dioxide has an enthalpy of

formation of

NE

393.7 K in other words when you uh

combust carbon in the presence of oxygen

to create carbon dioxide you release

this much energy in doing so likewise it

would take that much energy to

decompose carbon dioxide into its

constituent elements so carbon dioxide

is necessarily more stable than its

Elemental form and those with

positive uh enthalpies of formation so

when they're greater than zero tend to

be very unstable because they're already

above the energy equilibrium it just

takes a small thing to sort of tip them

over to the edge into rapid

decomposition our next special case is

is enthalpy of combustion and that is

basically defined as the energy released

when you ignite some sort of uh reactant

let's say hydrogen gas in the presence

of excess oxygen and because you have

all this excess it's hard to determine

exactly how much product you create so

the enthalpy of combustion is defined as

one mole or really the energy released

by the ignition of one mole of reactant

in the presentence of all that oxygen so

now we're going to do a practice problem

uh using hess's law to determine the

enthalpy of formation of a compound in

this case the enthalpy of formation of

methane from carbon and hydrogen and

there are two main rules you should know

first thing is that if you have all

these equations which uh involve the

various steps involved in creating

methane or using methane in the case of

this last combustion equation uh if you

reverse the direction of the equation

then you also change the sign so you

change from positive to negative Etc the

second thing is you can multiply the

coefficients of known equations to fit

um the steps

necessary to determine the enthalpy of

formation so in other words you see this

1/2 here in front of the oxygen you can

multiply all the coefficients in this

equation by two to make that a

102 so the biggest thing in figuring out

how to do one of these thermochem

equations is getting your products and

reactants on the right side in other

words you see how methane is over here

on the left side right now we need to

move it over onto the product side and

we can do that by changing the sign oh

that's originally supposed to be

negative by changing the sign of that

89.8 K to a positive sign

and then rewriting the equation

similarly because this oxygen is 1/2 we

can multiply all the coefficients by two

and simply double this number so now

having reversed this equation as is

shown down here at the bottom of the red

final equation and multiplied this all

through by two including the energy down

here we can then solve for the desired

reaction by canceling out the two sides

so if you see we have two o2s over here

and we can cross those out with O2 over

there on the product side we have a CO2

over here and a CO2 over here we can

eliminate and finally we have 2 H2O on

the left and 2 H2O on the right and from

here it's just a

matter of rewriting what we have left in

for the desired

reaction and adding up the total energy

that's over on this side so the Delta h

in standard form of

formation ends up being -

74.3 kles for methane