1 Calorimetry. Calorimetry - the accurate and precise measurement of heat change for chemical and...

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Calorimetry

• Calorimetry - the accurate and precise measurement of heat change for chemical and physical processes.

• The device used to measure the absorption or release of heat in chemical or physical processes is called a Calorimeter

Calorimetry

• Foam cups are excellent heat insulators, and are commonly used as simple calorimeters

A Cheap Calorimeter

• For systems at constant pressure, the heat content is the same as a property called Enthalpy (H) of the system

Calorimetry

• Changes in enthalpy = H• q = H These terms will be used

interchangeably in this textbook• Thus, q = H = m x C x TH is negative for an exothermic

reactionH is positive for an endothermic

reaction

Calorimetry

• Calorimetry experiments can be performed at a constant volume using a device called a “bomb calorimeter” - a closed system

In terms of bonds

Breaking this bond will require energy.

Making these bonds gives you energy.In this case making the bonds gives you more energy than breaking them.

Exothermic

• The products are lower in energy than the reactants

• Releases energy

2Al (s) + 3Cl2 (g) --> 2 AlCl3 (s) + 1408 kJ

∆H=1408 kJ

C + O2 CO2E

Reactants Products

C + O2

+ 395 kJ

Endothermic

• The products are higher in energy than the reactants

• Absorbs energy

2 H2O + 575 kJ ------> 2 H2 + 1 O2 (g)

∆H = + 572 kJ

CaCO3 CaO + CO2E

Reactants Products

CaO + CO2

176 kJ

CaCO3 + 176 kJ CaO + CO2

Chemistry Happens in

MOLES• An equation that includes energy is called

a thermochemical equation

• CH4 + 2O2 CO2 + 2H2O + 802.2 kJ

• 1 mole of CH4 releases 802.2 kJ of energy.

• When you make 802.2 kJ you also make 2 moles of water

• What is the molar enthalpy of CO2 (g) in the reaction for the burning of butane below?

2 C4H10 +13 O2 8 CO2 +10 H2O

∆H=-5315 kJ

Answer: Molar enthalpy is the enthalpy change in equation divided by the balance of CO2

• Molar enthalpy, ∆H substance = 5315 kJ ÷ 8 mol

• = 664 kJ / mol.

• For each of the following rewrite the equation in " H " notation, for one mole of the underlined substance.

Fe2O3 (s)+3CO(g)→3CO2(g)+2Fe(s)+25kJ

• Answer:1/3 Fe2O3 (s)+CO(g)CO2(g)+2/3 Fe(s) ∆H = - 8.3 KJ

4 NH3(g)+5O2 (g)→4 NO(g)+6H2O(l)+1170kJ

2 HCl (g)+96 KJ → H2 (g)+Cl2 (g)

N2 (g)+3 H2 (g) → 2 NH3 (g)+92 KJ

2 CO2 (g)+566 KJ →2 CO (g)+ O2 (g)

4 Al (s) +3 O2 (g) →2 Al2O3 (s)+3360 KJ

Thermochemical Equations

• A heat of reaction is the heat change for the equation, exactly as written– The physical state of reactants and

products must also be given.

– Standard conditions for the reaction is 101.3 kPa (1 atm.) and 25 oC

CH4 + 2 O2 CO2 + 2 H2O + 802.2 kJ

• If 10. 3 grams of CH4 are burned completely, how much heat will be produced?

10. 3 g CH4

16.05 g CH4

1 mol CH4

802.2 kJ

= 514 kJ

CH4 + 2 O2 CO2 + 2 H2O + 802.2 kJ

• How many liters of O2 at STP would be required to produce 23 kJ of heat?

• How many grams of water would be produced with 506 kJ of heat?

• How much heat will be released if 65 grams of butane is burned in a lighter according the equation:

2 C4H10 +13 O2 8 CO2 +10 H2O

∆H=-5315 kJ

= 2976.4 kJ

= 3.0 MJ

104104 2

HmolCHgC

• Calculate the heat released when 120 grams of Iron (III) oxide is formed by the following equation

2 Fe2O3 (s) → 4 Fe(s)+3 O2 (g)

∆H=1625 kJ

OmolFeOgFe

70.159

1120 32

= 610.5 kJ= 610 kJ

• Q = n ∆H (substance)

Where n = # of moles

• What mass of carbon dioxide must form to create 1200 kJ of heat when the following reaction occurs?

C6H12O6(s)+6O2(g)→6CO2(g)+6H2O(l)

∆H=- 2808kJ

Answer: 110 grams

• 3) What mass of oxygen is needed to completely react and release 550 kJ of heat in the following reaction?

• 4Fe (s)+3O2 (g) → 2 Fe2O3 (s)

∆H=- 1625 kJ

Answer: 32 grams

Summary, so far...

Enthalpy• The heat content a substance has at a

given temperature and pressure• Can’t be measured directly because

there is no set starting point• The reactants start with a heat content• The products end up with a heat content• So we can measure how much enthalpy

changes

Enthalpy• Symbol is H• Change in enthalpy is H (delta H)• If heat is released, the heat content of

the products is lowerH is negative (exothermic)

• If heat is absorbed, the heat content of the products is higherH is positive (endothermic)

Reactants Products

Change is down

H is <0

Reactants Products

Change is upH is > 0

Heat of Reaction• The heat that is released or absorbed in a

chemical reaction• Equivalent to H

• C + O2(g) CO2(g) + 393.5 kJ

• C + O2(g) CO2(g) H = -393.5 kJ

• In thermochemical equation, it is important to indicate the physical state

• H2(g) + 1/2O2 (g) H2O(g) H = -241.8 kJ

• H2(g) + 1/2O2 (g) H2O(l) H = -285.8 kJ

Heat of Combustion• The heat from the reaction that

completely burns 1 mole of a substance

• OBJECTIVES:– Classify, by type, the heat changes

that occur during melting, freezing, boiling, and condensing.

• OBJECTIVES:– Calculate heat changes that occur

during melting, freezing, boiling, and condensing.

Heats of Fusion and Solidification

• Molar Heat of Fusion (Hfus) - the heat absorbed by one mole of a substance in melting from a solid to a liquid

• Molar Heat of Solidification (Hsolid) - heat lost when one mole of liquid solidifies

Heats of Fusion and Solidification

• Heat absorbed by a melting solid is equal to heat lost when a liquid solidifies– Thus, Hfus = -Hsolid

Heats of Vaporization and Condensation

• When liquids absorb heat at their boiling points, they become vapors.

• Molar Heat of Vaporization (Hvap) - the amount of heat necessary to vaporize one mole of a given liquid.

• Condensation is the opposite of vaporization.

• Molar Heat of Condensation (Hcond) - amount of heat released when one mole of vapor condenses

• Hvap = - Hcond

The large values for Hvap and Hcond are the reason hot vapors such as steam is very dangerous– You can receive a scalding burn from

steam when the heat of condensation is released!

• H20(g) H20(l) Hcond = - 40.7kJ/mol

Heat of Solution

• Heat changes can also occur when a solute dissolves in a solvent.

• Molar Heat of Solution (Hsoln) - heat change caused by dissolution of one mole of substance

• Sodium hydroxide provides a good example of an exothermic molar heat of solution:

Heat of Solution

NaOH(s) Na1+(aq) + OH1-

Hsoln = - 445.1 kJ/mol

• The heat is released as the ions separate and interact with water, releasing 445.1 kJ of heat as Hsoln thus becoming so hot it steams

H2O(l)

1 Calorimetry. Calorimetry - the accurate and precise measurement of heat change for chemical and...

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