Thermochemistry

Some say the world will end in fire,

Some say in ice,

From what I’ve tasted of desire

I hold with those who favor fire.Robert Frost

Fire and Ice

Principles of Heat Flow

• Thermochemistry is the study of heat flow

• System: the part of the universe on which we are focused

• Surroundings: exchange energy with system (in close contact with system)

State Properties

• Composition• Temperature• Pressure• (remind you of stp?)

• Heat flow (q)• q is + when heat flows

into the system from the surroundings (heating of a cup of coffee)

• q is - when heat flows out of the system into the surrounds (cooling of a cup of coffee)

if X is a state property, then ∆X = Xfinal-Xinitial

Heat Flow is NOT a state property

Endo- and Exothermic Processes

• An endothermic process (q>0) in which heat flows form the surroundings into the reaction system.

• And exothermic process(q<0) in which heat flows from the reaction system into the surroundings

system

surroundings

IN

OUT

endothermicexothermic

Magnitude of Heat Flow

q = C x ∆t

∆t = tfinal - tinitial

For a pure substance

q = m x c x ∆t

m is mass

c is specific heat

Specific heat is the amount of energy necessary to raise the temperature of 1 gram of a substance 1 degree C.

t is temperature

q is heat flow

C is heat capacity

q is measured in joules

Example

• How much heat is needed to raise the temperature of a 3000 gallon pool from 10 C to 20 C?

The specific heat of water is 4.18 J/goC

1. Q = m x c x ∆t

2. Plug and chug

Calorimetry

• Calorimeter: device that measures the heat flow in a reaction

• qreaction = -q calorimeter

• qreaction = -Ccal x ∆t

• “old” heat unit is the calorie

• 1 cal = 4.184 J (c)• 1 kcal = 4.184 kJ (C)

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Coffee Cup Calorimeter

• Heat from the reaction inside the innermost cup flows out to the water in the outer cup.

• Ccal = mwater x cwater

• qreaction =-mwater x cwater x ∆t

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Bomb Calorimetry

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Example

The reaction between H and Cl :

H2(g) + Cl2(g) --> 2HCl(g)

can be studied in a bomb calorimeter. It is found that when a 1.00 g sample of H2 reacts completely the temperature rises from 20.00 C to 29.82 C. Taking the heat capacity of the calorimeter to be 9.33 kj?oC, calculate the amount of heat evolved in the reaction.

• Which equation to use?

qreaction = -Ccal x ∆t

Plug and chug

Answer: -91.6 kJ

exothermic

Example

When 1.00 g of CaCl2 is added to 50.0 g of water in a coffee-cup calorimeter, it dissolves

CaCl2(s) --> Ca2+(aq) + 2Cl-(aq)

And the temperature rises from 25.00 C to 28.51 C. Assuming that all the heat given off by the reaction is transferred to the water, calculate q for the reaction system.

• Which equation?

•exothermic

Answer: -734 J

Plug and chug

qreaction = -mwater x cwater

Enthalpy (H)

• Enthalpy is a type of chemical energy (heat content)

• Under constant P, heat flow for a reaction system is equal to the difference in enthalpy between products and reactants

qreaction= ∆H=

Hproducts- Hreactants

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Enthalpy continued

• For an exothermic reaction

q = ∆H< 0

• For an endothermic reaction

q = ∆H> 0

The enthalpy of a substance, like its volume, is a state property

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Thermochemical Equations

• A chemical equation which shows the enthalpy relation between products and reactants

NH4NO3(s) --> NH4+

(aq)+ NO3-(aq)

∆H = +28.1 kJ

The sign of ∆H indicates endothermic (+) or exothermic (-)

H2(g)+ Cl2(g) --> 2HCl ∆H = -185kJ The value quoted for H applies when products and reactants are at the same temp (usually 25C)

The phases must be specified--significant differences at different phases

The coefficients represent numbers of moles

Rules of ThermochemistryThe magnitude of ∆H is

directly proportional to the amount of reactant or product

The ∆H for a reaction is equal in magnitude but opposite in sign to ∆H for the reverse reaction.

Heat of fusion : heat absorbed when a solid meltsHeat of vaporization: heat absorbed when a liquid vaporizes

-185 kJ 1 mol H2

1 mol Cl2 -185 kJ

-185 kJ

2 mol HCl

H2(g)+ Cl2(g) --> 2HCl ∆H = -185kJ

2HCl (g)--> H2 (g) + Cl2 (g) ∆H = 185 kJ

H2(g)+ Cl2(g) --> 2HCl(g)

∆H = -185 kJ

Example

Enthalpy of Formation

Example

Example