Chemical Reactions and Energy

8/8/2019 Chemical Reactions and Energy

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Chemical Energy &

Reactions


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Thermochemistry

y Study of heat changes occuring during reactionsy Potential

y Stored energy due to chemical composition

y Kineticy is the energy of motion

y Heat/thermaly Energy transferred from one substance to another due to temperature

differences.

y Always from hot ---> cold

y Chemicaly Energy stored within bonds of molecules (intramolecular) and between

molecules (intermolecular)

y Nuclear Energyy Change in the nucleus of the atom to form different elements


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Energy and changes

y System

y Defined as the part of the universe on which we focus ourattention.

y The SURROUNDINGS include everything else in the universe.


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Temperature vs Heat

y Temperaturey Measure of how hot or cold a substance is.y Higher T indicates higher molecular motiony Measured in oC or K

y Heaty Form of energy contained in a substancey Unit in J (Joule) or KJ (Kilojoule)y Compare T and heat content of a cup of coffee vs. icebergy

Quantity of heat in a substance depends on its:y Temperaturey Massy Specific Heat


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y Enthalpy (H)y Energy content (KJ) of a substance

y Excluding nuclear energy

y

Heat of Reaction ((

H)y Amount of energy (KJ) released or absorbed during a chemicalreaction or physical change

y Heat of formationy Energy involved when a substance is produced

y Heat of combustiony Energy released when a substance is burned


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Types of changes

y Molar heat of

y Refers to the amount of energy released or absorbed per moleof a substance involved in a chemical reaction.

y

Each type of change absorbs or releases energy.y Least in physical- most in nuclear (by far)


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Exothermic reaction

y A process that releases heat to the surroundings (becomeshot to touch)

y Ex- combustion, condensation

yRepresented in equations by2H2 + O2 ---> 2H2O + 340kJ

2H2 + O2 ---> 2H2O (H = -340kJy Negative sign denotes exothermic


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Exothermic Graphical Representation

y Products (P) have LESSenergy or enthalpy

content than reactants(R), since there is a netLOSS of energy tosurroundings during

reaction


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Endothermic Reactions

y Involve a net absorption of energy by the reactants.

y Ex- Recharging a battery, evaporation

y Represented by:y > N2 + 2O2 + 67.6 kJ --> 2NO2y > N2 + 2 O2 ---> 2NO2 (H = +67.6kJ

y Positive sign indicates an endothermic reaction


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Endothermic Reaction Graphical

Representation

y Products have GREATERenthalpy since energy is

absorbed from thesurroundings and storedin products


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Specific Heat Capacity (c)

y Amount of heat energy required to raise or lower thetemperature of 1g of a substance by 10C.

y Ex-y H

2O= 4.19 J/goC

y Iron = 0.45 J/goC

y Aluminum = 0.89 J/goC

y Copper = 0.387 J/goC

y Brass = 0.38 J/goC

y Zinc = 0.39 J/goC


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How to find heat absorbed or lost

y Use Q = m x c x (T

y Wherey m = mass of substance (g)

y Q = heat energy (J)

y (T = Change in temperature (Tf-Ti) (celsius)

y

C = specific heat of substance


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Q = mc(T Example

y A 30g block of Aluminum at 25OC absorbs 2.508 kJ ofenergy.What was its final temperature? (cAl = 0.89 J/g

0C)

(T = Q = 2508J = 94OC

mc 30x0.89

Tf= (Ti + (T) = (25 + 94) = 119oC


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Calorimetery An instrument used to measure the amount of heat energy

released or absorbed during a chemical reaction

y If the reaction is EXOthermic, heat will be released into thewater and the temperature of the H2O (TH2O) will rise

y If the reaction is ENDOthermic, heat will be absorbed fromthe H2O and TH2O drops.


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Example #1

y Julie dissolves 3.7g of Ca(OH)2 in 300g of H2O at 15.6OC. The

solutions temperature rises to 21OC.

A) How much heat was released?

B)What is Ca(OH)2s molar heat of solution?

A) Q=mc(T m= mass of substance that absorbs the heat (water)

NOT what released it (Ca(OH)2)

Q= 300 x 4.19 x 5.4 = 6787.8 J or 6.79 kJ

b) 3.7g x 1mol/74g = 0.05 mol Ca(OH)2

Molar Heat = 1 mol x 6.79kJ/0.05mol = 135.8kJ


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Example #2

y When 2.4g of CH4 is burned in a calorimeter, the 850 mL ofH2O at 19

OC rises to 25.3OC.What is the molar heat of

combustion of CH4?

Q = mc (T = 850 x 4.19 x 6.3 = 22 437.5 J or 22.44 kJ/2.4g

2.4g x 1mol/16g = 0.15 mol CH4

1 mol x 22.44 kJ/0.15 mol = 149.6kJ


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Stoichiometry

y Example #1

2CO2 + 43.8kJ --> 2CO + O2N.B. 43.8 kJ/2 mol CO2 or 43.8 kJ/1mol O2

If 6.7L of CO2 at 35OC and 112 kPa is decomposed, how much heat energyis absorbed?

n= PV = 112 x 6.7 = 0.293 mol CO2

RT 8.31 x 308

0.293 mol x 43.8 kJ = 6.42 kJ

2 mol CO2


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Example #2

y Using N2 + 3H2 ---> 2 NH3 (H= -34.2kJ

y If 22.23 kJ of heat is released in a reaction, how many grams ofH2 must be reacted?

22.23 kJ x 3molH2 = 1.95 mol H2

34.2kJ

1.95 mol H2

x 2g/mol = 3.9g of H2

must be reacted


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Heat Transfery Heat always transfers from hot to cold

y Heat will continue to transfer from the hot object to the coldobject (as long as they are in contact) until the same finaltemperature is reached.

y Heat LOST by hot = heat GAINED by cold

y -Qlost hot = +Qgained coldy -(mh x (Th x ch) = +(mc x (Tc x cc)


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Example

y A 450g sample of water at 80OC is mixed with 360g of waterat 12oC.What is the final temperature of the mixture?

-[450g x (Tf- 80OC) x 4.19] = + [360g x (Tf- 12

OC) x 4.19]

4.19s cancel eachother out

-450Tf+ 36000 = 360Tf- 4320

-810Tf= -40320

Tf= 49.78OC


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Heat ofneutralization problem

y When 50 mL of a 0.25M NaOH solution is neutralized by100 mL of an HCl solution, the temperature of the mixturerises by 1.5OC.What is the molar heat of neutralization of

NaOH? (assume densities and specific heat of the solutionsare same as water)


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Solution

y Heat released into mixture:

Q=mc(T = 150g x 1.5OC x 4.19J/gOC = 942.8 J

Mols of NaOH neutralized : (molarity =mol/Vol)

mol = molarity x Volume

0.25 mol/L x 0.05L = 0.0125 mol NaOH

Molar heat of neutralization of NaOH:

y 1 mol NaOH x 942.8J/0.0125 mol = 75420 J/mol

or

75.42 kJ/mol

Chemical Reactions and Energy

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