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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.

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slide2

Temperature = Thermal Energy

900C

400C

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.

greater thermal energy

slide3

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 = msDt

q = CDt

Dt = tfinal - tinitial

slide4

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

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

q = msDt

= 869 g x 0.444 J/g •0C x –890C

= -34,000 J

slide5

Constant-Volume Calorimetry

No heat enters or leaves!

slide6

Constant-Pressure Calorimetry

No heat enters or leaves!

6.5

slide7

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

Chemistry in Action:

Fuel Values of Foods and Other Substances

1 cal = 4.184 J

1 Cal = 1000 cal = 4184 J

slide8

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

Elementary step:

NO + NO N2O2

+

Elementary step:

N2O2 + O2 2NO2

Overall reaction:

2NO + O2 2NO2

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.

N2O2 is detected during the reaction!

14.5

slide9

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

14.5

slide10

A + B C + D

Endothermic Reaction

Exothermic Reaction

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

14.4

slide11

uncatalyzed

catalyzed

Ea< Ea

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

ratecatalyzed > rateuncatalyzed

14.6

slide12

aA + bB cC + dD

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.

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

slide13

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:

Step 2:

NO2 + NO2 NO + NO3

NO2+ CO NO + CO2

NO3 + CO NO2 + CO2

What is the equation for the overall reaction?

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