Chemical Energetic Guided by Mr. CB ANG Tel: 017-480 5531 STPM

Chemical Energetics Exothermic reaction is a reaction that releases heat energy into its surroundings. The enthalpy of the products is less than the enthalpy of the reactants, hence enthalpy change (∆H) is negative. CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (g) ∆H = - 892 kJ

Endothermic reaction is a reaction that absorbs heat from the surroundings. The enthalpy of the products is higher than the enthalpy of reactants. The enthalpy change (∆H) is positive. C (s) + 2S (g) CS2 (l) ∆H = + 107 kJ

Standard conditions refer to: (i) Temperature = 25oC or 298K (ii) Pressure = 1 atm (iii) concentration of solution = 1.0 mol dm-3 (iv) reactants and products are at their normal physical states at 25oC and 1 atm Q = MCΘ

Examples of Exothermic Reactions : a)Combustion of fuels b)Oxidation of food in the respiration process c)The rusting of Iron d)Dissolving soluble bases in water e)Neutralization Reactions

Examples of Endothermic Reactions a) The dissolving of crystalline salts such as

CuSO4.5H2O, MgSO4.7H2O and Na2CO3.10H2O

b) Melting process c) Evaporation and boiling process d) Thermal decomposition of salts

Definition of enthalpy of - Formation - The standard enthalpy of combustion of an element is the enthalpy change when 1.0 mole of the substance is completely burnt in oxygen under standard condition of 298K and 1 atm. Combustion - The standard enthalpy of combustion of an element is the enthalpy change when 1.0 mole of the substance is completely burnt in oxygen under standard condition of 298K and 1 atm. Hydration - The standard enthalpy of hydration of an ion is the heat energy liberated when 1.0 mole of the gaseous ions is dissolved in water to form a solution at infinite dilution under standard conditions of 298 K and 1 atmosphere Solution - The heat evolved or absorbed when 1.0 mole of solute dissolves in water to form an infinitely dilute solution Neutralization-The heat evolved when one mole of H+ ions reacts with one mole of OH- ions to form one mole of water molecules under standard conditions of 298 K and 1 atm, Atomization - The standard enthalpy of atomization (∆Hatomθ) of an element is the enthalpy change when one mole of gaseous atoms is formed from the elements in the standard state. Bond energy -Bond energy (bond enthalpy) is the enthalpy change when 1 mole of covalent bonds between 2 atoms are broken in the gas phase Ionization energy - First ionization energy for an element is the minimum energy required to lose one electron valence from 1 mole of gaseous atom to produce 1 mole of uni-positive gaseous ion under standard conditions Electron affinity- First affinity electron is the energy evolved when 1 mole of gaseous atom received electron to form 1 mole of uni-negative gaseous ion under the standard conditions

Chemical Energetic Guided by Mr. CB ANG Tel: 017-480 5531 STPM

Enthalpy of Formation Note: 1. Enthalpy of formation for element to element is 0 2. The state of the elements most be from the most stable state at standard condition [STPM 2002] The N204 molecule is the dimer for the N02 molecule. The enthalpy of formation of N204 and N02 are +9.67 kJ mol-1 and +33.86 kJ mol-1 respectively. Calculate the standard enthalpy change for the formation (in kJ mol-1) of N204 from N02. A-58.05 kJ mol-1 B-24.19 kJ mol-1 C+24.19 kJ mol- D +43.53 kJ mol-1 [STPM 2008] In industry, ethanoic acid can be prepared from the oxidation of ethanol with oxygen. The standard enthalpies of formation of ethanoic acid, water and ethanol are – 487 kJ mol–1, –286 kJ mol–1 and –278 kJ mol–1 respectively. The standard enthalpy change, in kJ mol–1, for the preparation is A –1051 B –495 C –209 D +495 [STPM 2011] The standard enthalpy of formation of ethanol is -278 kJ per mole. Which equation relates to the formation of ethanol under standard conditions? A. 2C(g) + 3H2(g) + 1/2O2 (g) C2H5OH(l) B. 2C(s) + 3H2(g) + 1/2O2 (g) C2H5OH(l) C. 2C(g) + 3H2(g) + 1/2O2 (g) C2H5OH(l) D. 4C(g) + 6H2(g) + O2 (g) 2C2H5OH(l) [STPM 2013] The standard enthalpy of formation of CH3NHNH2, CO2 and H2O are +53 kJ mol-1, -393 kJ mol-1 and -286 kJ mol-1 respectively. The standard enthalpy change for the following reaction is -5116 kJ. 4 CH3NHNH2 (l) + 5 N2O4 (l) → 4 CO2 (g) + 12 H2O (l) + 9 N2 (g) What is the standard enthalpy of formation of N2O4? A - 20 kJ mol-1 B - 100 kJ mol-1 C + 20 kJ mol-1 D + 100 kJ mol-1

Enthalpy of Hydration & Solution // Solubility of ionic solid in liquid Note: 1. Magnitude of the standard enthalpy of hydration is directly proportional to the charge on the ion but inversely proportional to the ionic radius.

∆H hyd α

2. ∆Hsol = -Lattice energy + ∆H hyd [STPM 2002] When one mole of sodium bromide dissolves in water, the enthalpy change is -8 kJ mol-1. If the lattice energy of sodium bromide and the hydration energy of the Na+ ion are -736 kJ mol-1 and -406 mol-1 respectively, what is the hydration energy of the Br- ion? A-1150 kJ mol-1 B -338 kJ mol-1 C-322 kJ mol-1 D+1 150 kJ mol-1 [STPM 2004] Which of the following cations has the highest hydration energy? A Li+ B Na+ C K+ D Rb+ [STPM 2007] The radii of the four ions are as following: Ion Radius/nm U+ 0.345 V- 0.151

W2+ 0.085

X2+ 0.069 Which ion has the largest hydration energy ? A. U+ B.V- C. W2+ D. X2+ [STPM 2013] The energy cycle for sodium bromide is shown below.

Which enthalpy change represents the enthalpy of solution of sodium bromide?

A H1 B H2 C H3 D H4

Chemical Energetic Guided by Mr. CB ANG Tel: 017-480 5531 STPM

Enthalpy of Combustion Note : 1. Must be considered in excess oxygen 2. final product are usually carbon dioxide and water, and exothermic 3. Enthalpy of combustion are usually exothermic reaction [STPM 2004] The standard enthalpy of combustion of methanol, carbon and hydrogen is -715 kJ mol-1, -394 kJ mol-1 and -286 kJ mol-1 respectively. What is the standard enthalpy of formation, in kJ mol of methanol? A -251 B -35 C +35 D +251 [STPM 2005]Standard combustion enthalpies for 1,2-ethanediol (HOCH2CH2OH), carbon and hydrogen are -1180.0 kJmol-1, -393.7 kJ mol-1 and -285.9 kJ mol-1 respectively. What is the standard formation enthalpy, in kJ mol-1, for 1,2-ethanediol? A —500.4 B —465.1 C +465.1 D +500.4 [STPM 2010] The standard enthalpy change, ∆H for several reactions are shown below : Reaction ∆H0/kJ H2(g) + ½ 02(g) H20(l) -286 CH3CH3 + 7/2O2(g) 2CO2(g) + 3H2O(l) -1560 C2H2(g) + 5/2 O2(g) 2CO2(g) + H2O -1299 C2H4(g) + H2(g) C2H6(g) -157 The value of enthalpy change C2H2 (g) + H2(g) C2H6(g) is …… A.-132kJ B. -154kJ C. -486kJ D. -3302KJ

Enthalpy of Atomization Note : 1. Changes from compound at the most stable state at standard condition to gaseous 2. No changes in the number of electron or proton [STPM 2000] Which of the following equations correctly represents the enthalpy of atomization of sodium? A Na(s) Na(l) B Na(s) Na(g) C Na(l) Na(g) D Na(l) Na+(g)+ e- E Na(g) Na+(g)+ e- [STPM 2006]

Chemical Energetic Guided by Mr. CB ANG Tel: 017-480 5531 STPM

Enthalpy of Neutralization Note: 1. Neutralization between acid and alkaline to produce salt and water 2. Usually are exothermic reaction and depend on the strength of acid 3. Stronger the acid, higher the heat release for neutralization 4. Weak acid, less exothermic because some of the heat are absorb back to complete the dissociation of acid [STPM 2008] The standard enthalpies of neutralization for the reactions between an aqueous solution of sodium hydroxide and several hydrohalic acids are given below. Hydrohalic acid Standard enthalpy of neutralization/kJ mol–1 HF –68.6 HCl –57.6 HBr –57.3 HI –57.2 The standard enthalpy of neutralization of HF is the most negative because …………. A HF is the strongest acid. B HF has the strongest covalent bond. C the fluorine atom has the highest electronegativity. D the fluorine ion has the most exothermic enthalpy of hydration. [STPM 2013] In an experiment, 0.500 g of magnesium metal was dissolved in an excess of dilute hydrochloric acid in a calorimeter. The calorimeter recorded a temperature rise of 23.3 °C. The total heat capacity of the calorimeter and its contents is 410 J 0C-1. Calculate the heat released in the experiment, and determine the standard enthalpy of reaction between magnesium metal and dilute hydrochloric acid. [4]

Ionization energy & Affinity electron Note: 1. Involve changes in the number of electron only 2. Ionization energy – removal of electron 3. Affinity of electron – acceptance of electron [STPM 2001]The diagram below shows the Born-Haber cycle for the formation of sodium fluoride

What is the electron affinity, in kJ mol-1, of chlorine? A -246 B -275 C -328 D -820 [STPM 2008]Which equation illustrates the first ionization energy of the given element? A C(g) C+(g) + e– B Na(s) → Na+(g) + e– C Cl(g) + e– → Cl–(g) D O(g) + e– → O–(g) [STPM 2009 ]

Chemical Energetic Guided by Mr. CB ANG Tel: 017-480 5531 STPM

Hess’s Law The overall enthalpy change is equal to the sum of the enthalpy changes for the individual steps in a reaction

Born Haber Cycle Using the following data given, generate the Born – Haber energy cycle and thus calculate the lattice energy for sodium chloride. Standard enthalpy of atomization for Na = + 108 kJ mol-1 Standard enthalpy of atomization for Cl = + 121 kJ mol-1 First ionization energy for Na = + 500 kJ mol-1 First electron affinity for Cl = - 364 kJ mol-1

[STPM 2005]

[STPM 2006]