Thermochemistry. Topic: Specific Heat and Calorimetry Objectives: Day 1 of 2 To understand specific...

Thermochemistry

Topic: Specific Heat and Calorimetry

Objectives: Day 1 of 2

• To understand specific heat and it’s units

• To understand calorimetry and and the equation q = (m)(Cp)(ΔT)

• To understand the difference between endo and exothermic changes

• To understand the first law of thermodynamics

Unit: Thermochemistry

Heat Transfer & Specific Heat

• If you recall, heat is the energy transferred between objects that have different temperatures

• Heat is always transferred from an object with a higher temperature to an object with a lower temperature

• Temperature is a measure of the average kinetic energy

What is Heat?

• The transfer of kinetic ______from a hotter object to a colder object

• When heat is abosrbed molcules move more _______

Answer BankGram

Absorbsinto

Energyconstant

HeatsystemColder

1transferjoules

energy

rapidly

• The temperature change of an object depends on:– The amount of heat that is transferred to or from

the object– The mass of the object– The specific heat of the object

Heat Transfer & Specific Heat

Specific Heat

• Specific heat is the amount of heat needed to raise the temperature of 1 gram of a substance by 1ºC

• We represent specific heat with symbol (Cp)

• It is a constant for a given substance at a certain pressure

• The units of specific heat are (J/g•ºC)– Where J is the SI unit for energy, the joule (J)

Specific Heat

• Think of specific heat as how much heat a substance can hold

• Substances with a high specific heat act like a “heat sponge”

• They can absorb a lot of heat with just a small change in temperature

What is specific heat?

• The amount of ______needed to raise the temperature of 1 _____ of a substance by ___ºC

• It is symbolized by the symbol Cp

• It is a constant for a given substance at a certain pressure

• Units are in (J/g•ºC)

Answer BankGram

Absorbsinto

Energyconstant

HeatsystemColder

1transferjoules

heat

gram

1

Calorimetry

• Calorimetry is the measure of heat transfer Suppose a chemist want s to calculate the amount of heat transferred when the temperature of a sample changes by so many degrees

• The amount of heat absorbed or released by a sample is symbolized by the letter qq

• It is calculated using the following equation: q = (m)(q = (m)(CCpp)()(ΔΔT)T)

What is calorimetry?

• A measure of heat _______ given by the equation: q = (m)(Cp)(ΔT)

Answer BankGram

Absorbsinto

Energyconstant

HeatsystemColder

1transferjoules

transfer

Calorimetry• Here is what each symbol means for the

equation: q = (m)(q = (m)(CCpp)()(ΔΔT)T)

q = (m)(q = (m)(CCpp)()(ΔΔT)T)

q is the symbol for heatIf q is positive heat is absorbed

by the system & the reaction is endothermic

Cp is the specific heat of the Substance it is a constant

M is the mass of the sample in grams

If q is negative heat is lost by the system & the reaction

is exothermic

ΔT is the change in temperature

Calorimetry• If energy flows into the system (endothermic) q

is positive(+)

• On the other hand if energy is flowing out of the

system (exothermic) qq is negative(-)

System

Surroundings

Surroundings

Surroundings

Energy

System

Surroundings

Surroundings

Surroundings

Energy

What is the difference between endo- and exothermic changes?

• An endothermic change ______ heat (q+) and energy flows ____the system

• An exothermic change releases heat (q-) and energy flows out of the_____

Answer BankGram

Absorbsinto

Energyconstant

HeatsystemColder

1transferjoules

absorbs

into

system

Thermodynamics

• The study of energy is called thermodynamics

• The law of conservation of energy is often called the first law of thermodynamics and is stated as follows:

The energy in the universe is constant

What is the First Law of Thermodynamics?

• The First Law of Thermodynamics states that the energy in the universe is________

Answer BankGram

Absorbsinto

Energyconstant

HeatsystemColder

1transferjoules

constant

Practice:What amount of heat is needed to increase the

temperature of 2006 grams of Mercury (Hg) by 7.5ºC? The specific heat of mercury is 0.139 J/g•ºC.

• Use the equation q = (m)(Cp)(ΔT) and Solve for q

• q = (m)(Cp)(ΔT)• q = (2006 g)(0.139 J/g•ºC)(7.5ºC)• q = 2090 Joules

First Law of Thermodynamics

• The First Law of Thermodynamics states that the energy in the universe is constant

• This is another way of stating the law of conservation of energy

The First Law of Thermodynamics

• Suppose you want to measure the amount of heat transferred from a hot piece of metal (system) to a sample of cooler water (surroundings)

• If you know the specific heat of water, the water’s mass and the water’s temperature change, you can calculate the amount of heat absorbed by the water

• This equals the amount of heat released by the metal System

Surroundings

System

Practice:A 118 gram piece of tin at 85ºC is dropped into a

100 grams of water at 35ºC. The final temperature of the mixture is 38ºC The specific heat of water is 4.18 J/g•ºC

1) What amount of heat is absorbed by the water? 2) What amount of heat is released by the tin?3) What is the specific heat of tin?• Solving for the specific heat of water we get: • q = (100g)(4.18 J/g•ºC)(3.0ºC) = 1254 J• The value is positive because heat is absorbed• This same amount of heat is released by tin, and when

heat is released the q value is negative

• qwater = -qtin • 1254 J = -1254 J

Visualize:• What is hotter, the water or

piece of tin?

• In which direction will heat move?

System

Surroundings

System

118 gram piece of tinat 85o Celsius

100 grams of waterat 35o Celsius

Heat moves from the metalTo the water

(system surroundings)qwater Is positive (+)

heat is gainedqmetal Is negative (-)

heat is lost

Practice:• Because the same amount of heat that is absorbed by water

is also released by tin, we can use 1254 J for to solve for the amount of heat released by tin

• However when heat is released, it’s value is negative and we get -1254 J, So, the amount of heat released by tin is -1254 J

• Substituting -1254 in for q, we can solve for specific heat (Cp)

• qtin = -1254 J = (118g)(Cp)(-47ºC)

• Cp = __-1254___

(118g)(-47ºC)

• Cp = 0.23 J/g•ºC

Summarize:• In the equation, q = (m)(Cp)(ΔT), q

represents the heat ____or_____, m is the ____ in grams, Cp is the _____heat, and ΔT is the change in _____

• An endothermic change ______ heat (q+) and An exothermic change ______heat (q-)

• In the last practice problem the amount heat lost by the ____ is equal to the amount of heat ______by the water (Or q = -q)

Answer BankgramMass

gainedAbsorbsreleases

temperaturespecificmetal

systemlost

Topic: Enthalpy and Reaction Rates

• Objectives:

• To understand enthalpy and how it applies to a reaction

• To understand how reactions proceed and the factors that effect them

Enthalpy

• So far we have seen that some reactions are exothermic and other reactions are endothermic

• Chemists also like to know exactly how much energy or heat is produced or absorbed by a given reaction

• We will use the term enthalpy to describe the change in heat that occurs during a reaction

• ΔH = Heat

Enthalpy

• Enthalpy (H) is the amount of heat a sample has at a certain temperature and pressure

• It is the sample’s heat content

• Enthalpy cannot be measured, only changes in enthalpy can be measured

• As a reaction takes place and atoms are rearranged, heat is either released or absorbed

What is Enthalpy?

• Enthalpy (H) is the amount of ______a sample has at a certain temperature and pressure

• Think of it as a sample’s heat____

Answer BankenzymereleasedContent

activationAbsorbedminimumproducts

Heatchemical

Enthalpy

• The change in heat that takes place during a reaction is called the heat of reaction (ΔH )

• It is the amount of heat released or absorbed in a chemical reaction, given 1 mol of a reactant or product

• ΔHrxn is the difference between enthalpy of the reactants and products

• In other words,

ΔHrxn = (H of products) – ( H of reactants)

What is the heat of reaction?

• The amount of heat _______or _______in a chemical reaction, given 1 mol of a reactant or product represented by ΔH

• Where ΔHrxn = (H of______ ) – ( H of reactants)



Heatchemical

Practice:• Classify the reactions below as wither exo-

or endothermic:

CaCO3 CO2 + CaO ΔHrxn = +178.4 kJ/mol CaCO3

CH4 + 2O2 CO2 + 2H2O ΔHrxn = -890.2 kJ/mol CH4

ΔHrxn is positive so the reaction is endothermic

ΔHrxn is negative so the reaction is exothermic

Practice:• When 1 mol of methane is burned at

constant pressure, -890 kJ of energy is released as heat. Calculate ΔH for a process in which a 5.8 gram sample of methane is burned at constant pressure

q = ΔH = -890 kJ/mol CH4

The negative sign indicates an exothermic processkJ/mol is our conversion factor

First convert grams to moles using the molar mass of methane 16 g/mol,

then use our conversion factor of -890 kJ/mol

1 mol CH4 -890kJ5.8 g CH4

16.0 g CH41 mol CH4

= -320 kJ CH4

Practice:• 2SO2+O2 2SO3 ΔHrxn = -98.9 kJ/mol SO3

• Consider the above equation:

• If 8.9 mol of SO2 reacts with excess oxygen, how much heat is released?

ΔHrxn = -98.9 kJ/mol SO3 The negative sign indicates an exothermic process

kJ/mol is our conversion factor

We are given the ΔH for SO3 So, we need to use

our mol ratio to go from mol of SO2 to mol SO3

2 mol SO3 -98.9kJ8.9 mol SO2

2 mol SO21 mol SO3

= -880.2 kJ SO3

Enthalpy and Reaction Rates(Reaction Coordinates)

• Chemists often use diagrams to show how energy is transferred during reactions

• Remember, a change in energy always accompanies a reaction whether positive or negative

Enthalpy and Reaction Rates• The diagram to the right

illustrates how energy changes from reactants to products

• The activation energy is the minimum energy needed for the reaction to take place

• Once the activation energy is met, the reaction can proceed

Enthalpy and Reaction Rates• A common analogy is pushing a boulder

over a hill. Actually over a "pass" .• The reactants are on one side like the

boulder.

• The energy needed to push the boulder to the crest of the hill is like the activation energy.

• The products are like the condition when the boulder is at the bottom of the far side of the "pass".

What is Activation Energy?

• The _______energy needed for the reaction to take place



Heatchemical

En

erg

y

Reaction Rate (time)

Reactants

Products

EA= Activation energyminimum amount of energy

needed for a chemical reaction to take place

ΔH

ΔH = The amount of heat released or absorbed in a chemical reaction

(heat change)

EA

En

erg

y


ReactantsH2O + CO2

ProductsC6H12O6 + O2



ΔH = 1274.5 endothermic (+)

EA

En

erg

y


ReactantsCH4 + O2

ProductsCO2 + H2O



ΔH = -890 exothermic (-)

Catalyst• In order to lower the

activation energy, some reactions utilize what’s know as a catalyst

• By lowering the activation energy, a catalyst (substance) accelerates a chemical reaction without itself being affected

Catalyst

• A catalyst is a substance that lowers the activation energy and accelerates a chemical reaction without itself being affected

• A biological catalyst is called an enzyme

• Enzymes help orientate molecules or atoms in specific manner so they are more likely to react

• Your body has millions of enzymes!!

What is a catalyst?

• A substance that lowers the _____energy and ______ a reaction without itself being affected

• A biological catalyst is called an _____



HeatChemical

accelerates

Review:

• Is the reaction endothermic or exothermic?

• What number represent the activation energy?

• What number represents the products?

Summarize:• Review:• ______ is a measure of the average kinetic

energy of a molecular motion• As heat is added, molecules move ______• The amount of heat released or absorbed in a

chemical reaction is the _____ of reaction• A substance that lowers the activation energy is

called a _____• Draw & Label the diagrams below:

En

erg

y


Reactants

CH4 + O2

Products

CO2 + H2O

EA= Activation energy

ΔH = - 890 exothermic (-)

En

erg

y


Reactants H2O + C O2

Products C6H12O6 + O2

ΔH = +1274.5 endothermic (+)

EA

Answer Bankcatalystfaster

activationproducts

Heattemperature

Temperature rises as we

add heat

At a certain temperature(00C) the ice melts

and becomes water, The temperature

stabilizes. This is the Molar Heat of Fusion

H fusion

The energy required to melt 1 mole of

ice into water

Now that the ice has melted, the temperature

of the water continues to rise

If we continue to add heat, the Temperature Of the steam will rise

The energy required to

change one mol of

a liquid into vapor

At a certain temperature

(100 0C) Water becomes

steam and once again the

temperature stabilizes.

This is the

Molar Heat of Vaporization

H vaporization

Empirical Formula

• Empirical Formula - A formula that gives the simplest whole-number ratio of atoms in a compound.

• For example:

• If carbon and hydrogen are present in a compound in a ratio of 1:2, the empirical formula for the compound is CH2.

En

erg

y


ReactantsCH4 + O2

ProductsCO2 + H2O

EA= Activation energy

ΔH = -890 exothermic (-)

En

erg

yReaction Rate (time)

Reactants H2O + C O2

Products C6H12O6 + O2



ΔH = 1274.5 endothermic (+)

EA

Thermochemistry. Topic: Specific Heat and Calorimetry Objectives: Day 1 of 2 To understand specific...

Documents

Transcript of Thermochemistry. Topic: Specific Heat and Calorimetry Objectives: Day 1 of 2 To understand specific...