Kinetics

Kinetics

• Kinetics - rates of chemical reactions and the mechanisms by which they occur

• Rate of a chemical reaction - change in the concentration of products and reactants in a given time

Collision Theory

• Molecules must have effective collisions in order to react– they must have the

correct amount of energy

– correct orientation

Collision Theory

• Not all collisions are successful

• How to increase the rate of a reaction?– Increase the number of collisions– Increase the effectiveness of the collisions

Factors that Affect Reaction Rates

• Nature of the Reactants

• Anything that increases the number of collisions will increase the reaction rate– Concentration – Temperature– Catalyst– Surface Area– Pressure (only for gases)

Nature of the ReactantsNature of the Reactants

• Reactions involving ions (or ionic Reactions involving ions (or ionic compounds) are generally faster compounds) are generally faster than those involving covalently than those involving covalently bonded substancesbonded substances

• Covalently bonded substances Covalently bonded substances have more bonds that must be have more bonds that must be broken before the reaction can broken before the reaction can occuroccur

Nature of the ReactantsNature of the Reactants

Energy required to break bonds is Energy required to break bonds is proportional to the stability of the bondproportional to the stability of the bond – More stable bonds (stronger bond) More stable bonds (stronger bond)

require more energy to break, slower require more energy to break, slower reaction, less reactivereaction, less reactive

– Weaker bonds are broken with less Weaker bonds are broken with less energy, faster reaction, more reactiveenergy, faster reaction, more reactive

Concentration• When the concentration

of one or more of the reactants is increased, the reaction proceeds faster– As concentration

increases, there are more particles, more likely to collide

Pressure of a Gas• When the pressure of a gas increases,

volume decreases

• Making the particles closer together– Therefore there will be more collisions

P = 1.0 atm

P = 2.0 atm

Temperature

• An increase in temperature increases the rate of a reaction– Higher temperatures cause particles to

move faster and have more kinetic energy

– Therefore, more collisions and more effective collisions, due to the increased kinetic energy

CatalystCatalyst

Addition of a Addition of a catalyst lowers catalyst lowers the activation the activation energy, energy, speeding up speeding up the reactionthe reaction

Catalysts are Catalysts are NOTNOT used up used up in a reaction in a reaction

Surface AreaSurface Area

More surface area, faster reactionMore surface area, faster reaction– With more area exposed, there will With more area exposed, there will

be more collisionsbe more collisions

Example:Example: Both samples represent 2.5g of Both samples represent 2.5g of Mg, which would react faster with 25mL of Mg, which would react faster with 25mL of 1.0M HCl? 1.0M HCl?

Sample A

Sample B

Entropy

• Measure of randomness/disorder• Greater disorder, higher entropy• Spontaneous reactions tend to go towards

higher entropy (more disorder)Examples:

1. Solid Liquid Gas (low entropy medium entropy high entropy)

2. Messy bedroom = high entropy3. More moles = more disorder = higher entropy (only look at the moles if the phases are all the same)

Entropy Examples

Given the following reactions, indicate if entropy increases, decreases, or remains the same

1. H2O(l) H2O(g)

2. 3H2(g) + N2(g) 2NH3(g)

3. 4Al(s) + O2(g) 2Al2O3(s)

4. 2H2(g) + O2(g) → 2H2O(l)

Reaction coordinate

1

5

2

43

500kJ

300kJ

700kJ

Pot

enti

al E

nerg

y

Potential Energy DiagramsPotential Energy of the Reactants =

Potential Energy of the Products =

Activation Energy =

Heat of Reaction =

Potential Energy of Activated Complex =

Activation Energy, Reverse Reaction =

Heat of Reaction, Reverse =

Activation EnergyActivation Energy

Minimum energy required to initiate a Minimum energy required to initiate a chemical reaction (energy to break bonds)chemical reaction (energy to break bonds)

Equal to the difference between the Equal to the difference between the Potential Energy of activated complex and Potential Energy of activated complex and potential energy of the reactants potential energy of the reactants difference from the starting point to the topdifference from the starting point to the top

The Larger the Activation Energy, the The Larger the Activation Energy, the slower the reactionslower the reaction

Activated ComplexActivated Complex

Highest point on Potential Energy Highest point on Potential Energy curvecurve

It represents a transition state between It represents a transition state between the products and reactantsthe products and reactants

Heat of Reaction (Enthalpy, H)Heat of Reaction (Enthalpy, H)

Difference between the potential energy of Difference between the potential energy of the products and the potential energy of the products and the potential energy of the reactantsthe reactantsEnergy given off or absorbed by the Energy given off or absorbed by the reactionreactionFound on Reference Table I Found on Reference Table I H = Hp - HrH = Hp - Hr

Heat of ReactionHeat of Reaction

If If H is negativeH is negativeThe reaction is The reaction is EXOTHERMICEXOTHERMICThe Potential Energy of the products is less then The Potential Energy of the products is less then the Potential Energy of the reactantsthe Potential Energy of the reactants

Energy is leaving (exiting) the systemEnergy is leaving (exiting) the system Surroundings will feel warmSurroundings will feel warm

Examples:Examples:

2H2H22(g) + O(g) + O22(g) (g) 2H 2H22O(l)O(l) H = -571.6 kJH = -571.6 kJ

2H2H22(g) + O(g) + O22(g) (g) 2H 2H22O(l) + 571.6kJO(l) + 571.6kJ

Heat of ReactionHeat of Reaction

If If H is positiveH is positiveThe reaction is The reaction is ENDOTHERMICENDOTHERMIC

The Potential Energy of the products is more The Potential Energy of the products is more then the Potential Energy of the reactantsthen the Potential Energy of the reactants

Energy is entering the system (gain of heat) Energy is entering the system (gain of heat)

Surroundings will feel coldSurroundings will feel coldExamples: Examples:

NN22(g) + 2O(g) + 2O22(g) (g) 2NO 2NO22(g) (g) H = +66.4 kJH = +66.4 kJ

NN22(g) + 2O(g) + 2O22(g) +66.4kJ (g) +66.4kJ 2NO 2NO22(g)(g)

Table I ExamplesTable I Examples

1.1. What is the heat of reaction for the What is the heat of reaction for the synthesis of 2 moles of Alsynthesis of 2 moles of Al22OO33? ?

2.2. What is the heat of reaction for the What is the heat of reaction for the synthesis of 1 mole of Alsynthesis of 1 mole of Al22OO33? ?

3.3. What is the heat of reaction for the What is the heat of reaction for the decomposition of 1 mole of NHdecomposition of 1 mole of NH33(g)? (g)?

4.4. As sodium hydroxide ionizes in water, As sodium hydroxide ionizes in water, what happens to the temperature of the what happens to the temperature of the water? water?

Does this graph show an exothermic or endothermic reaction?

Classify the following examples as Classify the following examples as Endothermic or Exothermic?Endothermic or Exothermic?

1.1. CHCH44 + 2O + 2O22 → CO → CO22 + H + H22O + 890.4KJO + 890.4KJ

2.2. 2C + 2H2C + 2H22 + 52.4KJ → C + 52.4KJ → C22HH44

3.3. 4Al + 3O4Al + 3O22 → 2Al → 2Al22OO33 + 3351KJ + 3351KJ

4.4. HH++(aq) + OH(aq) + OH-- (aq) → H (aq) → H22O +55.8KJO +55.8KJ

5.5. KNOKNO33 +34.89KJ → K +34.89KJ → K++(aq) + NO(aq) + NO33--(aq)(aq)

Spontaneous Reactions

• A reaction is most likely to occur when there is a:– Change to a condition of less energy

• Exothermic (lower AE), Negative H

– Change to a condition of greater randomness• Greater entropy