Chemistry 104 1

Two Key Questions 1. Will a chemical reaction go? 2

Two Key Questions 1. Will a chemical reaction go? 2. If a reaction goes, how fast? 3

Two Key Questions 1. Will a chemical reaction go? (thermodynamics) 2. If a reaction goes, how fast? 4

Two Key Questions 1. Will a chemical reaction go? (thermodynamics) 2. If a reaction goes, how fast? (kinetics) 5

Thermochemistry 6

Thermochemistry: Deals with energy changes in chemical reactions. 7

Thermochemistry Thermochemistry: Deals with energy changes in chemical reactions. Energy: Capacity to do work or transfer heat. 8

Thermochemistry Thermochemistry: Deals with energy changes in chemical reactions. Energy: Capacity to do work or transfer heat. Work: 9

Thermochemistry Thermochemistry: Deals with energy changes in chemical reactions. Energy: Capacity to do work or transfer heat. Work: Kinetic energy: 10

Potential energy: 11

Potential energy: Energy units: In the SI system of units (International System of units, from the French Système International d’Unités) the unit of energy is the joule. 12

A common non SI unit of energy is the calorie: 1 cal = J 13

A common non SI unit of energy is the calorie: 1 cal = J The energy unit used for food is the Calorie: 1 Cal = 1000 cal 14

Some definitions Thermodynamics : The scientific discipline that deals with the interconversion of heat and other forms of energy. Thermochemistry is a sub-branch of thermodynamics. 15

Some definitions Thermodynamics : The scientific discipline that deals with the interconversion of heat and other forms of energy. Thermochemistry is a sub-branch of thermodynamics. System: Any predefined part of the universe that is of interest to us. 16

Some definitions Thermodynamics : The scientific discipline that deals with the interconversion of heat and other forms of energy. Thermochemistry is a sub-branch of thermodynamics. System: Any predefined part of the universe that is of interest to us. Surroundings: Everything in the universe except for the part that has been defined as the system. 17

Open System: A system that can exchange both mass and energy with its surroundings. 18

Open System: A system that can exchange both mass and energy with its surroundings. Closed system: A system that allows energy but not mass transfer with its surroundings. 19

Open System: A system that can exchange both mass and energy with its surroundings. Closed system: A system that allows energy but not mass transfer with its surroundings. Isolated system: A system that does not allow energy or mass transfer with its surroundings. 20

Open System: A system that can exchange both mass and energy with its surroundings. Closed system: A system that allows energy but not mass transfer with its surroundings. Isolated system: A system that does not allow energy or mass transfer with its surroundings. State of a system: The macroscopic variables such as composition, volume, pressure, temperature, etc., that define a particular system. 21

State Function: Any property of a system that is fixed by the state the system is in. A change in a state function is independent of the path followed – it depends only on the initial and final states of the system. 22

Heats of reaction 23

Heats of reaction Most physical and chemical processes take place under constant pressure conditions. 24

Heats of reaction Most physical and chemical processes take place under constant pressure conditions. The heat change (absorbed or evolved) of any reaction carried out at a constant pressure is called the enthalpy change. 25

Heats of reaction Most physical and chemical processes take place under constant pressure conditions. The heat change (absorbed or evolved) of any reaction carried out at a constant pressure is called the enthalpy change. Enthalpy is represented by the symbol H. 26

Heats of reaction Most physical and chemical processes take place under constant pressure conditions. The heat change (absorbed or evolved) of any reaction carried out at a constant pressure is called the enthalpy change. Enthalpy is represented by the symbol H. Change in enthalpy is represented by the symbol. 27

The symbol delta is used to denote a change. So 28

The symbol delta is used to denote a change. So Example: For a phase change, such as liquid water water vapor that is, H 2 O (l) H 2 O (g) 29

The symbol delta is used to denote a change. So Example: For a phase change, such as liquid water water vapor that is, H 2 O (l) H 2 O (g) 30

The symbol delta is used to denote a change. So Example: For a phase change, such as liquid water water vapor that is, H 2 O (l) H 2 O (g) (at 25 o C) (think about a steam burn) 31

For the combustion of methane: CH 4(g) + 2 O 2(g) CO 2(g) + 2 H 2 O (l) 32

For the combustion of methane: CH 4(g) + 2 O 2(g) CO 2(g) + 2 H 2 O (l) enthalpy of CO 2(g) + 2 enthalpy of H 2 O (l) - enthalpy of CH 4(g) - 2 enthalpy of O 2(g) 33

For the combustion of methane: CH 4(g) + 2 O 2(g) CO 2(g) + 2 H 2 O (l) enthalpy of CO 2(g) + 2 enthalpy of H 2 O (l) - enthalpy of CH 4(g) - 2 enthalpy of O 2(g) that is, is the enthalpy of the products - enthalpy of reactants, with the stoichiometric factors included. 34

For the combustion of methane: CH 4(g) + 2 O 2(g) CO 2(g) + 2 H 2 O (l) enthalpy of CO 2(g) + 2 enthalpy of H 2 O (l) - enthalpy of CH 4(g) - 2 enthalpy of O 2(g) that is, is the enthalpy of the products - enthalpy of reactants, with the stoichiometric factors included. Note: that it is important to specify the state of the reactant, i.e. is it solid, liquid, or gas. 35

Keep the following points in mind: 1.The stoichiometric coefficients always refer to the number of moles. 36

Keep the following points in mind: 1.The stoichiometric coefficients always refer to the number of moles. 2. We must always specify the physical state of the reactants and products. 37

Keep the following points in mind: 1.The stoichiometric coefficients always refer to the number of moles. 2. We must always specify the physical state of the reactants and products. 3. The enthalpy of a substance increases with temperature. It follows that the enthalpy change for a reaction must also depend on the temperature. 38

Enthalpy changes are most commonly tabulated at a temperature of 25 o C. Temperature changes during a reaction present no problem. If the products form at a temperature higher than 25 o C, they will eventually cool down to 25 o C, and the heat evolved on cooling will become part of the enthalpy change for the reaction. 39

Endothermic process: If a reaction (or other process) is accompanied by absorption of heat from the surroundings, is positive, and the reaction (or process) is said to be endothermic. Example: H 2 O (s) H 2 O (l) 40

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Exothermic process: If a reaction (or other process) produces heat, is negative, then the reaction (or process) is said to be exothermic. 42

CH 4(g) + 2 O 2(g) CO 2(g) + 2 H 2 O (l) + heat We can think of the heat very loosely as a “reactant” or “product”. In the case of the reaction above, heat is produced in the reaction. Hence, the reaction is exothermic. 43

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Whenever a reaction is reversed, the magnitude of remains the same, but its sign is reversed. 45

Whenever a reaction is reversed, the magnitude of remains the same, but its sign is reversed. Example: H 2 O (l) H 2 O (g) 44.0 kJ H 2 O (g) H 2 O (l) kJ 46

If we multiply both sides of a chemical equation (or more general process relationship, e.g. a phase transition) by a factor n, the must also change by the same factor. 47

If we multiply both sides of a chemical equation (or more general process relationship, e.g. a phase transition) by a factor n, the must also change by the same factor. Example: H 2 O (l) H 2 O (g) 44.0 kJ 48

If we multiply both sides of a chemical equation (or more general process relationship, e.g. a phase transition) by a factor n, the must also change by the same factor. Example: H 2 O (l) H 2 O (g) 44.0 kJ 2 H 2 O (l) 2 H 2 O (g) 88.0 kJ 49

If we multiply both sides of a chemical equation (or more general process relationship, e.g. a phase transition) by a factor n, the must also change by the same factor. Example: H 2 O (l) H 2 O (g) 44.0 kJ 2 H 2 O (l) 2 H 2 O (g) 88.0 kJ ½ H 2 O (l) ½ H 2 O (g) 22.0 kJ 50

Standard enthalpies of formation 51

Standard enthalpies of formation The enthalpy of formation of a compound is the heat change that occurs when one mole of a compound is synthesized from its elements under constant pressure conditions. 52

Standard enthalpies of formation The enthalpy of formation of a compound is the heat change that occurs when one mole of a compound is synthesized from its elements under constant pressure conditions. This quantity varies with changes in experimental conditions, so the following definition is adopted. 53

Standard enthalpy of formation: The standard enthalpy of formation of a compound is the heat change when one mole of the compound is formed from its component elements in their standard states under constant pressure conditions. 54

Standard enthalpy of formation: The standard enthalpy of formation of a compound is the heat change when one mole of the compound is formed from its component elements in their standard states under constant pressure conditions. The standard state of a substance is the most stable form at 1 bar. Formally, this was 1 atm. (1 atm = bar, so the new and old definitions are very close.) 55

Usually a reference temperature of 25 o C is selected (for the purposes of data tabulation). Symbol employed: (common usage) (IUPAC recommended) 56

Usually a reference temperature of 25 o C is selected (for the purposes of data tabulation). Symbol employed: (common usage) (IUPAC recommended) The superscript refers to standard state. The subscript refers to the formation of 1 mole of the compound. 57

Usually a reference temperature of 25 o C is selected (for the purposes of data tabulation). Symbol employed: (common usage) (IUPAC recommended) The superscript refers to standard state. The subscript refers to the formation of 1 mole of the compound. Examples: C (graphite) + O 2(g) CO 2(g) = kJ/mol 58

Usually a reference temperature of 25 o C is selected (for the purposes of data tabulation). Symbol employed: (common usage) (IUPAC recommended) The superscript refers to standard state. The subscript refers to the formation of 1 mole of the compound. Examples: C (graphite) + O 2(g) CO 2(g) = kJ/mol C (graphite) + ½ O 2(g) CO (g) = kJ/mol 59

General reaction: Enthalpy change Consider the reaction a A + b B c C + d D 60