HEAT TRANSFER & ENTHALPY CHANGE

1. HEAT TRANSFER & ENTHALPY CHANGE Chapter 5.1 & 5.3

2. Energy Changes at molecular level • Each particle in a system (chemical reaction mixture) has potential (Ep)and kinetic energy (Ek) • Both forms of energy are inter-convertible i.e. Ep=Ek • When energy is transferred from one object to another, it appears as work or as heat. • The total energy of a system is sum total of all K.E & P.E (or internal energy) of its component parts i.e. electrons, nuclei, atoms and molecules. • Different chemicals have different Ekand Ep because they are made of different types & numbers of particles.

3. Energy Changes at molecular level • Overall chemical kinetic energy is a result of: • molecules moving through space (Ek(translational)) • molecules rotating (Ek(rotational)) • bound atoms vibrating Ek(vibrational)) • electron moving within each atom Ek(electron)) • Overall chemical potential energyis a result of: • forces between bound atoms vibrating (Ep(vibration)) • forces between nucleus & electrons and between electrons in each atom (Ep(atom)) • forces between protons & neutrons in each nucleus (Ep(nuclei)) • forces between nuclei & shared electron pair in each bond (Ep(bond))

4. Energy Changes at molecular level • When chemical materials change, their Ek and Ep also change. That is what we see as heat or light. • In exothermic reactions reactants have >Epstored in their bonds than products. • Extra energy is released to surroundings

5. Energy Changes at molecular level In Endothermic reactions: • Reactants have lower chemical Epstored in their bonds than products • Extra energy is gained from the surroundings

6. Heat TRANSFER & ENTHALPY CHANGE • Enthalpy (H): total internal energy of a substance at a constant pressure. • Researchers have not found a way to measure the total internal energy of a system. Therefore, they measure the enthalpy change. • Enthalpy change (H): energy absorbed from or released to the surroundings when a system changes from reactants to products. OR • The difference in enthalpies of reactants and products during a change. ΔH = Hfinal - Hinitial Δ H = Hproduct - Hreactants

7. Exothermic & Endothermic revisited • In endothermic changes: • Hfinal > Hinitial, therefore ΔH is positive MgCO3(s)MgO(s) + CO2(g) ΔH°=117.3 kJ • In exothermic changes: • Hfinal < Hinitial, therefore Δ H is negative H2O2(l) H2O(l) + 1/2 O2(g) H = -98.2 kJ Enthalpies of Reaction (Δ𝐇𝐫𝐱𝐧): the enthalpy change of a chemical reaction; Varies depending on conditions (temperature and pressure)

8. Representing enthalpy changes in chemistry (CH# 5.3) • Two ways of representing: • Thermochemical Equations & • Potential Energy diagrams Writing Thermochemical Equations: • Can show physical or chemical change • A balanced equation that states the heat of reaction (Δ𝐇𝐫𝐱𝐧) • Can show heat of reactant or product • Can show heat as separate Δ𝐇𝐫𝐱𝐧

9. STEPS in writing a thermochemical equation • The combustion of Sulphur (S) STEP#1: Write the chemical equation for the change S + O2 SO2 STEP#2: Balance the equation if required S + O2 SO2 STEP #3: Show heat as a product for exothermic change S + O2 SO2 + 296.9 kJ OR Show heat as a separate expression S + O2 SO2H = – 296.9 kJ

10. Thermochemical Equations • If the reaction is reversed the value of H remains the same but the sign is reversed SO2 S + O2H = + 296.9 kJ • Also, if numbers in the equation change (or equation is multiplied by a number), the amount of energy produced/absorbed also changes accordingly: 2S + 2O2  2SO2 H = – 593.8 kJ • Answer Q# 1 (a) – (c) on page 319 of the textbook. Only write the thermochemical equations for them.

11. EnthalpY of some important chemical changes • Heat of formation: Quantity of energy associated with the formation of ONE mole of a substance from its elements. S + O2 SO2Hf = – 296.9 kJ • Heat of combustion: heat released or absorbed when ONE MOLE of a substance reacts with oxygen S + O2 SO2Hcomb = – 296.9 kJ • Heat of neutralization: heat released or absorbed when 1 mole of acid or 1 mole of a base react with a base or an acid. 2 NaOH(aq) + H2SO4(aq) 2Na2SO4(aq) + 2H2O(l) Hneut= – 100.0 kJ

12. Enthalpy changes for some physical changes • Heat of Fusion: Heat released or absorbed when 1 mole of substance melts or freezes. H2O(s)  H2O(l) Hfusion = + kJ Endothermic H2O(l)  H2O(s) Hfusion = - kJ Exothermic • Heat of Vaporization: Heat released or absorbed when 1 mole of substance vaporizes or condenses. H2O(l)  H2O(g) Hvap = + kJ Endothermic H2O(g)  H2O(l) Hvap = - kJ Exothermic • Heat of solution: Heat released or absorbed when 1 mole of substance dissolves in water. NH4Cl(s) NH4+(aq) + Cl-(aq)  Hsol = + kJ Endothermic

13. Practice Question • Write the following as a thermochemical equation: 2 mol H2 gas reacts with 1 mol O2 gas to produce 2 mol H2O(l). At 25°C and 1 atm of pressure this reaction releases 571.8 kJ of heat. 2H2 (g) + O2(g) → 2H2O (l) ΔH° = –571.8 kJ OR 2H2 (g) + O2(g) → 2H2O (l) + 571.8 kJ

14. Potential energy diagrams • Horizontal lines indicate initial and final states • Vertical line is the energy axis • H is the difference between the heights of the two lines • Tail of arrow always point at initial state (reactants) • Head of arrow always point at final state (products) • Arrow signifies direction of energy change H

15. Practice Question • Answer Q#1 (a) – (b) on page 320 of textbook.