Physical and Organic Chemistry PDF

20 | Physical and Organic Chemistry 1.1 CONCEPT OF ENTHALPY HEAT IN AL-QURAN Allah SWT has created the world for us, in which a lot of natural resources are available to be used by mankind. Unfortunately, man does not a...

20 | Physical and Organic Chemistry 1.1 CONCEPT OF ENTHALPY HEAT IN AL-QURAN Allah SWT has created the world for us, in which a lot of natural resources are available to be used by mankind. Unfortunately, man does not appreciate these gifts as a lot of damages have been done and resources are taken without maintaining the sustainability. We have used petroleum to generate electricity in cars, whereby in this process chemical energy is transformed into work. But when the reaction is incomplete (as something is wrong with the engine), most of the energy is changed into thermal and light energy. This will heat up the earth. There are too many heat supplied by the sun, as well as too many work done by the machines created by human. The world climate is getting hot, and thus ice melts easily (Figure 1.1). By then, the heat absorbed by the earth is much greater that the heat released to the outer space. Allah SWT has arranged the planet we live in to be in balanced and harmony. But when this phenomena happens the kinetic energy of the mother earth will decrease as the weight decreases, making the momentum lower. What will happen is, our planet will move slower around the sun on its orbit. As the earth moving slower, it will skip the orbit and move closer to the sun. This event will lead to the end of our Figure 1.1: The world is getting hot every day. planet (Doomsday), as described in Surah At-Takwir (81:1-6), which means; When the sun is wrapped up (in darkness). And when the stars fall, dispersing. And when the mountains are removed. And when the full term she-camels are neglected. And when the wild beasts are gathered. And when the seas are filled with flame. In this verse, flame represents heat. Heat accumulated in time as north and south pole are melting. At a moment determined by Allah SWT in the future, everything will go haywire as described in these verses. One of the early signs of Doomsday is increasing temperature of the earth. In this chapter we are going to calculate the amount of heat released by a combustion reaction in a calorimeter. HEAT AND ENTHALPY Energy is very important in our lives. The food we eat supplies the energy for the body to think and move. All physical and chemical processes involve the transfer of energy. In thermodynamics the energy change is studied, whereby the term “thermo” here refers to the heat. Physical and Organic Chemistry 7 x 10.indb 20 23/01/2021 10:08 AM  Thermochemistry | 21 Energy is the capacity to do work or to transfer heat. In physics, you have learnt about kinetic energy and potential energy. In chemistry, an electron in an atom has potential energy because of the electrostatic force on it that is due to the positively charged nucleus and the other electrons in the atom and surrounding atoms. Energy can be transformed into many other forms such as electrical energy, radiant energy (light), nuclear energy and chemical energy. The chemical energy in the fuel or food comes from potential energy stored in atoms due to their arrangements in the molecules by virtue of chemical bonds. This stored chemical energy can be released when compounds undergo chemical changes such as those that occur in combustion and metabolism. Reactions that release energy in the form of heat are called exothermic reactions. Hydrocarbons including methane, the main component of natural gas, undergo combustion with excess of O2 to produce CO2 and H2O. CH4(g) + 2O2(g) CO2(g) + 2H2O(l) ∆H = –890 kJ Exothermic reaction can be explained using the energy profile diagram as shown below in Figure 1.2. Enthalpy reactants ΔH = –ve products Reaction pathway Figure 1.2: An exothermic process. A process that absorbs energy from its surrounding is called endothermic. For example the melting of one mol of ice at 0oC at constant pressure must be accompanied by the absorption of 6.02 kJ of energy. Notice here that energy changes also can be accompanied by physical changes. H2O(s) H2O(l) ∆H = +6.02 kJ For an endothermic reaction, the ∆H is shown in the energy profile diagram in Figure 1.3. The enthalpy of reaction, ∆H is the difference between the enthalpies of the products and the enthalpies of the reactants. ∆Hrxn = ∑H(products) – ∑H(reactants) Physical and Organic Chemistry 7 x 10.indb 21 23/01/2021 10:08 AM  22 | Physical and Organic Chemistry Basically, ∆H represents the heat given off or absorbed during a reaction. The standard condition used during this reaction is at a temperature of 25oC and a pressure of 1 atm. Enthalpy products ΔH = +ve reactants Reaction pathway Figure 1.3: An endothermic process. There are many types of enthalpies, in which we are going to go through the definition, such as enthalpy of formation, combustion, solution, hydration, neutralisation, atomisation, ionisation energy, electron affinity, lattice energy and enthalpy of sublimation.  Thermochemical equation is a chemical reaction equation that includes (a) phase (b) heat of reaction, ∆H For reverse equation, the magnitude of ∆H for the equation remains the same but its sign changes. EXAMPLE 1.1 NH3 reacts with F2 to form HF and NF3. The standard enthalpies of formation of NH3, HF and NF3 are –46 kJ mol─1, –269 kJ mol─1 and –114 kJ mol─1 respectively. Calculate the enthalpy change for the reaction. The standard enthalpy of formation of any element in its standard state is zero by definition. For example, although oxygen can exist as ozone, O3, atomic oxygen, O, and molecular oxygen, O2(g), O2 is the most stable form at 1 atm pressure and 25°C. Similarly, hydrogen is H2(g), not atomic hydrogen, H. Physical and Organic Chemistry 7 x 10.indb 22 23/01/2021 10:08 AM  Thermochemistry | 23 EXAMPLE 1.2 When 1 mol of liquid water is formed from 1 mol of ice at 0°C, the enthalpy change is +6.02 kJ. Write the thermochemical equation for this process. EXAMPLE 1.3 When 1 mol of ice is formed from 1 mol of liquid water at 0°C, the enthalpy change is –6.02 kJ. Write the thermochemical equation for this process. TYPES OF ENTHALPIES (a) Enthalpy of Formation Standard enthalpy of formation is the the heat change when 1 mol of a compound is formed from its constituent elements in their standard states. For example, H2(g) + ½O2(g) H2O(l) ∆Hf = –286 kJ mol–1  The standard enthalpy of formation of any element in its most stable form is zero. For example, ∆H(O2) = 0 and ∆H(Cl2) = 0. If the value of enthalpy of formation is positive, it means that the compound does not exist under standard condition. For example, the enthalpy of formation of NaCl2 is +2195 kJ mol–1. Therefore, this compound does not exist under standard conditions. (b) Enthalpy of Combustion Enthalpy of combustion is the heat released when 1 mol of substance is burnt completely in excess oxygen under standard conditions. All combustion reactions are exothermic. For example, C(s) + O2(g) CO2(g) ∆Hc = –393 kJ mol–1 (c) Enthalpy of Solution/Solvation When 1 mol of KCl dissolves in water, 690 kJ of heat will be absorbed from the surroundings. We would explain this effect by observing the beaker that contains the solution becomes slightly colder. Physical and Organic Chemistry 7 x 10.indb 23 23/01/2021 10:08 AM  24 | Physical and Organic Chemistry Depending on the nature of the cation and anion involved, ∆Hsoln for an ionic compound may be either negative (exothermic) or positive (endothermic). Hence, enthalpy of solution is the heat changes when 1 mol of a substance dissolves in water to form a very dilute solution under standard conditions. For example, KCl(s) K+(aq) + Cl–(aq) ∆Hosoln= +690 kJ mol–1 (d) Enthalpy of Hydration Enthalpy of hydration is the heat released when 1 mol of gaseous ion is hydrated in water under standard conditions. Heat of hydration is a negative quantity for cations and anions. Na+ (g) Na+(aq) ∆Hohydr = –406 kJ mol–1 Cl–(g) Cl–(aq) ∆Hohydr = –363 kJ mol–1 The solvation and hydration process of ionic crystal in water can be explained further. Water is a polar compund. When an ionic compound dissolves in water, the positive end of water molecule will attract the negative ion and the negative end of water will attract the positive ion (Figure 1.4). Figure 1.4: Example of hydration process. Electrostatic forces between the ions and water molecules will release hydration energy. This phenomenon is called hydration and the enthalpy change associated among them is called enthalpy of hydration. (e) Enthalpy of Neutralisation Enthalpy of neutralisation is the heat change when 1 mol of water, H2O is formed from the neutralisation of acid and base under standard conditions. For example, NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l) ∆Hn = –58 kJ mol–1 (f) Enthalpy of Atomisation The enthalpy of atomization is the heat change when 1 mol of gaseous atoms is formed from its element under standard conditions. For monoatomic substances such as metals, the sublimation process atomises the solid. Therefore, the term enthalpy of sublimation is the same as the enthalpy of atomisation. For example, Physical and Organic Chemistry 7 x 10.indb 24 23/01/2021 10:08 AM  Thermochemistry | 25 Na(s) Na(g) ∆Ha = +109 kJ mol–1 ½Cl2(g) Cl(g) ∆Ha = +123 kJ mol–1 (g) Ionisation Energy, IE Ionisation energy is the energy required when 1 mol of electron is removed from 1 mol of its gaseous atom. For example, First ionisation energy of Mg, Mg(g) Mg+(g) + e– IE1 = +740 kJ mol–1 Second ionisation energy of Mg, Mg+(g) Mg2+(g) + e– IE2 = +1400 kJ mol–1 (h) Electron Affinity, EA Heat changes when 1 mol of gaseous atom gains 1 mol of electron to form 1 mol of gaseous ion. For example, First electron affinity of O, O(g) + e– O–(g) EA1 = –142 kJ mol–1 Second electron affinity of O, O–(g) + e– O2–(g) EA2 = +844 kJ mol–1 (i) Enthalpy of Sublimation The enthalpy of sublimation is defined as the heat absorbed when 1 mol of a solid sublimes to produce 1 mol of vapour at a constant temperature and pressure, as shown in the example below: Na(s) Na(g) ΔHosubl = +107.8 kJ mol–1 However, for some other substances, such as I2(s) and CO2(s), the process of sublimation may not necessarily produce atoms. I2(s) I2(g) ∆Hosubl = +106 kJ mol–1 (j) Lattice Energy Lattice energy is the heat change when 1 mol ionic compound in its solid state is formed from its constituent gaseous ions. It is a measure of the strength of the ionic bond in an ionic compound. For example, the lattice energy of sodium chloride corresponds to the following change, Na+(g) + Cl–(g) NaCl(s) ΔHlatt = –771 kJ mol–1 Lattice dissociation energy is the measure of the forces of attraction between the positive and negative ions. Lattice dissociation enthalpy is the enthalpy change needed to convert 1 mol of an ionic solid into its constituent gaseous ions. For example, Physical and Organic Chemistry 7 x 10.indb 25 23/01/2021 10:08 AM  26 | Physical and Organic Chemistry NaCl(s) Na+(g) + Cl–(g) ΔHlatt = +771 kJ mol–1 The factors that influence lattice energy are the charge and size of its ions. The lattice energy changes are large, and the values are negative because more energy is needed to be released to form the ionic solids. The higher the charge and/or the smaller the size of the ions, the stronger the ionic bond and the more negative the lattice energy. For example,  The lattice energy of NaCl and NaBr is –771 kJ mol–1 and –733 kJ mol–1 respectively. Bromide ions are bigger than chloride ions, and so the distance between the centres of the positive and negative ions in the lattice is greater in NaBr. Increasing distance weakens the forces of attraction between them, and hence the less negative the lattice energy.  The lattice energy of NaCl and Na­2O is –771 kJ mol–1 and –2478 kJ mol–1 respectively. Oxide ions has higher negative charge compared to chloride ions. The higher the charge of the ions, the smaller the ions, the more negative the lattice energy.  The lattice energy of calcium fluoride, CaF2 and calcium oxide, CaO is –2560 kJ mol–1 and –3640 kJ mol–1 respectively. The lattice enthalpy of CaO is much greater (more negative) than that of CaF2. The main that affect this observation is the charge on the ions. In CaO, both positive and negative ions carry two charges (calcium oxide, 2+ ions are attracting 2– ions). In CaF2, they carry two and one respectively (the attraction is between 2+ and two 1– ions). The strength of the attractions is much greater in CaO than in CaF2.  The lattice energy of NaCl and MgO is –771 kJ mol –1 and –3795 kJ mol –1 respectively. The lattice enthalpy of magnesium oxide is much greater (more negative) than that of sodium chloride. Magnesium ions are smaller than sodium ions, and oxide ions are smaller than chloride ions. That means that the distance between the positive and negative ions is quite a lot less in MgO than in NaCl, and so the forces of attraction will be greater in MgO. But the main factor is the charge on the ions. In MgO, both positive and negative ions carry two charges (magnesium oxide, 2+ ions are attracting 2– ions). In NaCl, they only carry one (the attraction is only between 1+ and 1– ions). The strength of the attractions is much greater in MgO than in NaCl. In fact, the strength of the attractions is proportional to the charges on the ions. If you double the charges on both positive and negative ions, the strength of the attractions will go up four times. Physical and Organic Chemistry 7 x 10.indb 26 23/01/2021 10:08 AM EXAMPLE 1.4 Compare the lattice energy for the pair of the compounds below and explain. (a) KCl and CaCl2 (b) Li2O and LiF PRACTICE 1.1 Write balance thermochemical equation for each case below. (a) Combustion of ethylene, C2H4(g) when the ∆Hc = ─1411 kJ mol─1. (b) Formation of Na2SO4(s) (∆Hf = ─1385 kJ/mol). (c) Formation of ethanol, C2H5OH(l) (ΔHf = ─278 kJ mol─1). (d) Standard enthalpy of formation of H2O(g) is ─245.1 kJ mol─1. (e) Neutralisation reaction of Ba(OH)2(s) with HNO3 (aq) (ΔHn = ─116.4 kJ mol─1).