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1.4 Energetics

Introduction to Energetics. a. demonstrate an understanding of the term enthalpy change, Δ H b. construct simple enthalpy level diagrams showing the enthalpy change

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1.4 Energetics

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  1. Introduction to Energetics • a. demonstrate an understanding of the term enthalpy change, ΔH • b. construct simple enthalpy level diagrams showing the enthalpy change • recall the sign of ΔH for exothermic and endothermic reactions, eg illustrated by the use of exo- and endothermic reactions in hot and cold packs • evaluate the results obtained from experiments using the expression: • q = mcΔT • and comment on sources of error and assumptions made in the experiments. The following types of experiments should be performed: • i. experiments in which substances are mixed in an • insulated container and the temperature rise measured 1.4 Energetics Crowe 2010 Connector – Draw energy level diagrams for an exothermic and an endothermic reaction. On your diagram show the activation energy and the energy change for the reaction.

  2. Exothermic & Endothermic reactions Endothermic reaction Energy is absorbed.Reaction vessel becomes cooler.Temperature inside reaction vessel decreases. Exothermic reaction Energy is released.Reaction vessel becomes hotter.Temperature inside reaction vessel increases.

  3. Enthalpy • The heat content of a chemical system is called the enthalpy(H) • The enthalpy change (ΔH) is the amount of heat released or absorbed when a chemical reaction occurs at constant pressure. ΔH = H (products) – H (reactants) units kJ mol-1 (kJ/mol) • IfΔH is negative the reaction is exothermic • IfΔH is positive the reaction is endothermic. • Energy changes are measured under standard laboratory conditions (Room Temperature Pressure - RTP): 25oC (298K) & 101.3kPa (1 atmosphere)

  4. Practical 1.4 – Measuring some enthalpy changes See separate slide show

  5. Working out enthalpy changes in solutionsPart 1 Calculating the heat absorbed by the solution q = m x c xΔT E.g. When 50cm3 of 1.0M HNO3 was neutralised by 50cm3 of 1.0M NaOH the following results were obtained: Starting temperature of solns. = 19.0oC Maximum temperature reached = 25.1oC Calculate the heat absorbed by the solution, given that: the mass of 1cm3 of water = 1g; and that the specific heat capacity of water = 4.2 JK-1g-1. q = m x c xΔT q = 100 x 4.2 x 6.1 = 2562 J

  6. Working out enthalpy changes in solutionsPart 2 Calculating the enthalpy change • Write a balanced equation for the reaction between nitric acid and sodium hydroxide. • What are the units for ΔH? • What is the definition of the enthalpy of neutralisation? HNO3(aq) + NaOH (aq) NaNO3(aq) + H2O (l) kJ mol -1 Enthalpy of neutralisation is the enthalpy change when 1 mole of H+ ions  from an acid is neutralised by 1 mole of OH- ions       Use the information given in the question and your answer from q = mcΔT to work out the enthalpy of neutralisation for the above reaction.

  7. Accuracy • Remember to give numerical answers in line with that of the least accurate measurement recorded – don’t just write your calculator’s result with lots of decimal places! • % error = uncertainty in measurement x 100 • reading E.g.1 a balance has an uncertainty of 0.01 g when read to the second decimal place. What is the % error for a reading of 2.64g? Note: measurements to 2dp so answer to 2dp % error = 0.01 x 100 = 0.38% 2.64 E.g.2 the temperature rise was 6.1oC, while the thermometer could be read to ± 0.5oC. What was the % error? (Hint: how many readings are required to determine a temperature change?) Note: measurements to 1dp so answer to 1dp % error = 2 x 0.5 x 100 = 16.4% 6.1

  8. Enthalpy Change in Chemical Equations • If a chemical equation is reversed, the sign of ΔH is also reversed. • The value of ΔH given as kJ mol-1 refers to kJ per 1 mole of reactant or product as written in the equation. N2(g) + 3H2(g) -----> 2NH3(g)          ΔH = - 92.4 kJ/mol The formation of ammonia is exothermic reaction (ΔH is negative) and 92.4 kJ of energy per mole of nitrogen gas is released. 2NH3(g) -----> N2(g) + 3H2(g)         ΔH = + 92.4 kJ/mol The decomposition of ammonia is an endothermic reaction (ΔH is positive), and 92.4kJ of energy per mole of nitrogen gas is absorbed by the reaction.

  9. Manipulating the Enthalpy Change Term • The value of ΔH given as kJ mol-1 refers to kJ per 1 mole of reactant or product as written in the equation. • N2(g) + 3H2(g) -----> 2NH3(g)               H = - 92.4 kJ/mol • 92.4 kJ of energy is released for every 1 mole of N2(g) • 92.4 kJ of energy is released for every 3 moles of H2(g) • 92.4 kJ of energy is released for every 2 moles of NH3(g) produced. • How much energy is released if only 1 mole of ammonia (NH3) gas is produced? • How much energy is released if 10 moles of nitrogen (N2) gas and 30 moles of hydrogen (H2) gas is used in the reaction? • How much energy is released if 5 moles of hydrogen (H2) gas and 5/3 mole of nitrogen (N2) gas is used in the reaction? 46.2 kJ, 924kJ, 154 kJ

  10. Useful information • Usually one reagent is in excess to ensure a complete reaction • So calculations should be based on the fully reacted reagent. • Certain assumptions are made during the calculation • The density of the solution and its specific heat capacityis that of water. • That no heat is lost to the surroundings. • The main source of error in these experiments is • heat loss to the surroundings – (atmosphere & equipment) • Other sources of error include: • incorrect measurements • solution concentrations • mass of reactants

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