The total heat gained by the calorimeter + water contents = 4.28 x 10 3 J x 10 3 J = x 10 4 J So we can write the following equivalence statement: g benzoic acid x 10 4 J The molar mass of benzoic acid = 122 g/mol Therefore the amount of heat released when 1 mol of benzoic is burned is: 121

= 1 mol benzoic acid x = 2.39 x 10 3 kJ 122

= 1 mol benzoic acid x = 2.39 x 10 3 kJ In this case, since the volume is constant, we have actually determined the change in internal energy of benzoic acid to be x 10 3 kJ/mol. 123

124

125

A short list of common “reaction” types Heat of “Reaction” Example 126

A short list of common “reaction” types Heat of “Reaction” Example Enthalpy of solution: NH 4 NO 3(s) NH 4 + (aq) + NO 3 - (aq) H 2 O 127

A short list of common “reaction” types Heat of “Reaction” Example Enthalpy of solution: NH 4 NO 3(s) NH 4 + (aq) + NO 3 - (aq) H 2 O Enthalpy of dilution: H 2 SO 4(aq) H 2 SO 4(aq) H 2 O 128

A short list of common “reaction” types Heat of “Reaction” Example Enthalpy of solution: NH 4 NO 3(s) NH 4 + (aq) + NO 3 - (aq) H 2 O Enthalpy of dilution: H 2 SO 4(aq) H 2 SO 4(aq) H 2 O Enthalpy of fusion: H 2 O (s) H 2 O (l) 129

A short list of common “reaction” types Heat of “Reaction” Example Enthalpy of solution: NH 4 NO 3(s) NH 4 + (aq) + NO 3 - (aq) H 2 O Enthalpy of dilution: H 2 SO 4(aq) H 2 SO 4(aq) H 2 O Enthalpy of fusion: H 2 O (s) H 2 O (l) Enthalpy of vaporization: H 2 O (l) H 2 O (g) 130

A short list of common “reaction” types Heat of “Reaction” Example Enthalpy of solution: NH 4 NO 3(s) NH 4 + (aq) + NO 3 - (aq) H 2 O Enthalpy of dilution: H 2 SO 4(aq) H 2 SO 4(aq) H 2 O Enthalpy of fusion: H 2 O (s) H 2 O (l) Enthalpy of vaporization: H 2 O (l) H 2 O (g) Enthalpy of reaction: MgCl 2(s) + 2 Na (s) 2 NaCl (s) + Mg (s) 131

Applications of thermochemistry 1. BB 2. SHSC 132

Chemical Kinetics 133

Chemical Kinetics: The study of rates and mechanisms of chemical reactions. 134

Chemical Kinetics: The study of rates and mechanisms of chemical reactions. The word rate means the change of a certain quantity with time. In the present case, it will be a change of concentration of a reactant or product with time that will be of interest. 135

Two Key Questions If a reaction goes, how fast? 136

Reaction Rate: A measure of how rapidly a reaction occurs. It is the change of a reactant or product concentration divided by the time interval required for the change to occur. 137

Reaction Rate: A measure of how rapidly a reaction occurs. It is the change of a reactant or product concentration divided by the time interval required for the change to occur. Reaction Mechanism: The sequence of elementary steps that lead to product formation. 138

Factors that affect the reaction rate. 139

Factors that affect the reaction rate. 1. Nature of the reactants: Elements and compounds in general, have differing reactivities. That is, different tendencies toward bond formation and bond breaking. 140

Factors that affect the reaction rate. 1. Nature of the reactants: Elements and compounds in general, have differing reactivities. That is, different tendencies toward bond formation and bond breaking. 2. The ability of the reactants to meet: The gas phase and liquid phase allow the possibility for reactants to intermingle on the molecular level. 141

Factors that affect the reaction rate. 1. Nature of the reactants: Elements and compounds in general, have differing reactivities. That is, different tendencies toward bond formation and bond breaking. 2. The ability of the reactants to meet: The gas phase and liquid phase allow the possibility for reactants to intermingle on the molecular level. The solid phase is generally a very poor medium for chemical reactions. 142

The effect of surface area on reaction rate. 143

If all the reactants are in the same phase (e.g. all in solution) the reaction is called a homogeneous reaction. 144

If all the reactants are in the same phase (e.g. all in solution) the reaction is called a homogeneous reaction. If all the reactants are not in the same phase (e.g. a gas reacting with a solid surface) the reaction is called a heterogeneous reaction. 145

If all the reactants are in the same phase (e.g. all in solution) the reaction is called a homogeneous reaction. If all the reactants are not in the same phase (e.g. a gas reacting with a solid surface) the reaction is called a heterogeneous reaction. Factors such as molecular shape have an extremely important bearing on whether reactive centers in different reactants can meet. 146

3. The concentration of the reactants. This is obviously closely linked to number 2 above. 147

3. The concentration of the reactants. This is obviously closely linked to number 2 above. 4. The temperature of the system. 148

3. The concentration of the reactants. This is obviously closely linked to number 2 above. 4. The temperature of the system. 5. The presence of catalysts. 149

3. The concentration of the reactants. This is obviously closely linked to number 2 above. 4. The temperature of the system. 5. The presence of catalysts. A catalyst is defined as follows: A substance that increases the rate of reaction without being used up. 150

Note: This definition of a catalyst does not exclude the possibility that the catalyst undergoes some chemistry. If it does in some step, it has to be regenerated in a sequent step in the overall reaction scheme. 151

Rate of Reaction The reaction rate is the change in concentration of a particular reactant or product. The unit of concentration most commonly employed is mol/liter, that is the molar concentration unit. Two common time units are seconds or minutes. 152

Rate of Reaction The reaction rate is the change in concentration of a particular reactant or product. The unit of concentration most commonly employed is mol/liter, that is the molar concentration unit. Two common time units are seconds or minutes. The most common unit of reaction rate is thus: (mol/liter)/second = mol l -1 s -1 = Ms

Consider the reaction: O CH 3 C Cl + H 2 O CH 3 CO 2 H + HCl acetyl chloride 154

Consider the reaction: O CH 3 C Cl + H 2 O CH 3 CO 2 H + HCl acetyl chloride The rate of the reaction can be defined as the change of the reactant concentration over a certain time interval. 155

Consider the reaction: O CH 3 C Cl + H 2 O CH 3 CO 2 H + HCl acetyl chloride The rate of the reaction can be defined as the change of the reactant concentration over a certain time interval. 156

Consider the reaction: O CH 3 C Cl + H 2 O CH 3 CO 2 H + HCl acetyl chloride The rate of the reaction can be defined as the change of the reactant concentration over a certain time interval. That is: 157

Delta notation: 158

Delta notation: Often the initial time is taken as zero seconds. 159

Note that the negative sign in the definition is to ensure that the rate is a positive quantity. 160

Note that the negative sign in the definition is to ensure that the rate is a positive quantity. The concentration of the CH 3 COCl is decreasing, so is a negative quantity, hence the negative sign is needed to make the rate positive. 161

When the stoichiometric coefficients are not equal to unity, they need to be explicitly taken into account in the definition of the rate. 162

When the stoichiometric coefficients are not equal to unity, they need to be explicitly taken into account in the definition of the rate. For the generic reaction: a A + b B c C + d D 163

When the stoichiometric coefficients are not equal to unity, they need to be explicitly taken into account in the definition of the rate. For the generic reaction: a A + b B c C + d D the rate is given by 164

Example: 2 HI (g) H 2(g) + I 2(g) 165

Example: 2 HI (g) H 2(g) + I 2(g) The rate of reaction in terms of HI is 166

Kinetic data for the hydrolysis of acetyl chloride. Time (sec) [CH 3 COCl] [CH 3 CO 2 H] From the above date, the rate over the first 4 second interval is: 167

(0.93 – 1.20) M rate = s = Ms

(0.93 – 1.20) M rate = s = Ms -1 If you calculate the rate over the same time interval at later times you will find the rate of reaction is not constant. 169

(0.93 – 1.20) M rate = s = Ms -1 If you calculate the rate over the same time interval at later times you will find the rate of reaction is not constant. For most reactions, the rate constant does not remain constant as the reaction progresses. 170

(0.93 – 1.20) M rate = s = Ms -1 If you calculate the rate over the same time interval at later times you will find the rate of reaction is not constant. For most reactions, the rate constant does not remain constant as the reaction progresses. The above calculation of the rate constant is unsatisfactory in the sense that it only gives us average rates. 171

C 2 H 4(g) + O 3(g) C 2 H 4 O (g) + O 2(g) 172

In practice, we are interested mainly in the rate of a reaction at a specific time, and not in the average rate, which is arbitrary – because its value depends on the time interval we choose. 173

In practice, we are interested mainly in the rate of a reaction at a specific time, and not in the average rate, which is arbitrary – because its value depends on the time interval we choose. We can eliminate this arbitrariness by calculating the rates over smaller and smaller time intervals. 174

In practice, we are interested mainly in the rate of a reaction at a specific time, and not in the average rate, which is arbitrary – because its value depends on the time interval we choose. We can eliminate this arbitrariness by calculating the rates over smaller and smaller time intervals. When the time interval is made infinitesimally small, the rate becomes the slope of the concentration versus time plot (at a specific time). 175

The hydrolysis of acetyl chloride can also be studied by monitoring the build-up of acetic acid with time. In this case the rate is given by: 176

The hydrolysis of acetyl chloride can also be studied by monitoring the build-up of acetic acid with time. In this case the rate is given by: Note that for a rate expressed in terms of a product concentration, we do not need a minus sign on the right-hand side of the equation. 177

The hydrolysis of acetyl chloride can also be studied by monitoring the build-up of acetic acid with time. In this case the rate is given by: Note that for a rate expressed in terms of a product concentration, we do not need a minus sign on the right-hand side of the equation. In this case is a positive quantity (the concentration of acetic acid is increasing as the time increases). 178

Rate Laws 179

Rate Laws The rate of a reaction can be expressed in a second way. For the hydrolysis of acetyl chloride, we can write 180