reaction energy and kinetics

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Reaction Energy and Kinetics Chapter 17

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Reaction Energy and Kinetics. Chapter 17. Temperature = Thermal Energy. 90 0 C. 40 0 C. Energy Changes in Chemical Reactions. Heat is the transfer of thermal energy between two bodies that are at different temperatures. Temperature is a measure of the thermal energy. - PowerPoint PPT Presentation

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Page 1: Reaction Energy and Kinetics

Reaction Energy and Kinetics

Chapter 17

Page 2: Reaction Energy and Kinetics

Heat is the transfer of thermal energy between two bodies that are at different temperatures.

Energy Changes in Chemical Reactions

Temperature is a measure of the thermal energy.

900C400C

greater thermal energy

Temperature = Thermal Energy

Page 3: Reaction Energy and Kinetics

The specific heat (s) of a substance is the amount of heat (q) required to raise the temperature of one gram of the substance by one degree Celsius.

The heat capacity (C) of a substance is the amount of heat (q) required to raise the temperature of a given quantity (m) of the substance by one degree Celsius.

C = ms

Heat (q) absorbed or released:

q = mst

q = Ct

t = tfinal - tinitial

Page 4: Reaction Energy and Kinetics

How much heat is given off when an 869 g iron bar cools from 940C to 50C?

s of Fe = 0.444 J/g • 0C

t = tfinal – tinitial = 50C – 940C = -890C

q = mst = 869 g x 0.444 J/g • 0C x –890C = -34,000 J

Page 5: Reaction Energy and Kinetics

Constant-Volume Calorimetry

No heat enters or leaves!

Page 6: Reaction Energy and Kinetics

Constant-Pressure Calorimetry

No heat enters or leaves!6.5

Page 7: Reaction Energy and Kinetics

Chemistry in Action:

Fuel Values of Foods and Other Substances

C6H12O6 (s) + 6O2 (g) 6CO2 (g) + 6H2O (l) H = -2801 kJ/mol

1 cal = 4.184 J

1 Cal = 1000 cal = 4184 J

Page 8: Reaction Energy and Kinetics

14.5

Reaction Mechanisms

The overall progress of a chemical reaction can be represented at the molecular level by a series of simple elementary steps or elementary reactions.

The sequence of elementary steps that leads to product formation is the reaction mechanism.

2NO (g) + O2 (g) 2NO2 (g)

N2O2 is detected during the reaction!

Elementary step: NO + NO N2O2

Elementary step: N2O2 + O2 2NO2

Overall reaction: 2NO + O2 2NO2

+

Page 9: Reaction Energy and Kinetics

14.5

Elementary step: NO + NO N2O2

Elementary step: N2O2 + O2 2NO2

Overall reaction: 2NO + O2 2NO2

+

Intermediates are species that appear in a reaction mechanism but not in the overall balanced equation.

An intermediate is always formed in an early elementary step and consumed in a later elementary step.

The molecularity of a reaction is the number of molecules reacting in an elementary step.

• Unimolecular reaction – elementary step with 1 molecule

• Bimolecular reaction – elementary step with 2 molecules

• Termolecular reaction – elementary step with 3 molecules

Page 10: Reaction Energy and Kinetics

A + B C + D

Exothermic Reaction Endothermic Reaction

The activation energy (Ea) is the minimum amount of energy required to initiate a chemical reaction.

14.4

Page 11: Reaction Energy and Kinetics

A catalyst is a substance that increases the rate of a chemical reaction without itself being consumed.

uncatalyzed catalyzed

ratecatalyzed > rateuncatalyzed

Ea < Ea‘ 14.6

Page 12: Reaction Energy and Kinetics

The Rate Law

The rate law expresses the relationship of the rate of a reaction to the rate constant and the concentrations of the reactants raised to some powers.

aA + bB cC + dD

Rate = k [A]x[B]y

reaction is xth order in A

reaction is yth order in B

reaction is (x +y)th order overall

Page 13: Reaction Energy and Kinetics

The experimental rate law for the reaction between NO2 and CO to produce NO and CO2 is rate = k[NO2]2. The reaction is believed to occur via two steps:

Step 1: NO2 + NO2 NO + NO3

Step 2: NO3 + CO NO2 + CO2

What is the equation for the overall reaction?

NO2+ CO NO + CO2

What is the intermediate?

NO3

What can you say about the relative rates of steps 1 and 2?

rate = k[NO2]2 is the rate law for step 1 so step 1 must be slower than step 2