Chemistry_Energetics_Lecture_1.Chemistry_Energetics_Lecture

Chemistry_Energetics_Lecture_1

1. FC304 Chemistry Theme: 3 Hour: 1 Energetics 1

2. The lecture notes included in this presentation have been adapted from the resources accompanying the module textbooks: Lister, T and Renshaw, J (2015) AQA Chemistry AS Student’s book, London: Nelson Thornes Lister, T and Renshaw, J (2015) AQA Chemistry A2 Student’s book, London: Nelson Thornes

3. Introduction to Theme 3 In the this theme we will cover: Heat, specific heat and enthalpy (Session 1) Standard enthalpy changes and Hess’s law (Session 2) Bond enthalpies (Session 3) Born-Haber cycles (Session 5) Enthalpy of solution (Session 6) Entropy and Free Energy (Session 7) Exam style questions (Sessions 4 and 8)

4. Introduction to Theme 3 In this theme we will cover the following academic literacies: Find, evaluate and synthesise data from multiple sources Find and explain patterns in, interpret, evaluate and draw conclusions from data Use subject specific vocabulary effectively Demonstrate numeracy skills of benefit to study and future employability

5. At the end of this lesson you should be able to: Relate heat and specific heat. Calculate ∆H from calorimetric data.

6. Thermochemistry Thermodynamics is the science of the relationship between heat and other forms of energy. Thermochemistry is the area of Thermodynamics related to the heat changes involved in chemical reactions.

7. All chemical reactions obey the First Law of Thermodynamics (also known as The Law of Conservation of Energy): “ Energy may be converted from one form to another, but the total quantities of energy remain constant ”. Energy and Chemical Reactions

8. When a chemical reaction happens: energy is required to break bonds energy is released when bonds are formed. Energy and Chemical Reactions

9. Energy and Chemical Reactions In chemical reactions, energy is often transferred from the “system” to its “surroundings,” or vice versa. The substance or mixture of substances under study in which a change occurs is called the thermodynamic system (or simply system .) The surroundings are everything outside the thermodynamic system.

10. Heat Energy and Chemical Reactions Heat can be defined as the energy that flows into or out of a system because of a difference in temperature between the system and its surroundings. The Heat energy is transferred from a region of higher temperature to one of lower temperature; once the temperatures become equal, heat flow stops.

11. Heat Energy and Chemical Reactions Exothermic Heat “out of” a system Endothermic Heat “into” a system Energy System Surroundings Energy System Surroundings

12. Enthalpy, H The heat absorbed or evolved by a reaction depends on the conditions under which it occurs, such as pressure. Instead of the term “heat”, scientists prefer to refer to a related absolute property: Enthalpy. The symbol used for enthalpy is H

13. Enthalpy and Enthalpy Change When a reaction happens, reactants change into products. The reactants have a particular enthalpy (energy) and the products have a different enthalpy (energy). The difference between these two energies is the “heat of reaction” or “enthalpy of the reaction” Δ H Δ H = H products - H reactants

14. There is no way of measuring the enthalpy of any single substance directly. For this reason we can only discuss enthalpy changes in a reaction and this can be measured as an amount of heat given out (exothermic reaction) or taken in (endothermic reaction). Enthalpy and Enthalpy Change

15. Enthalpy and Enthalpy Change We can represent the level of enthalpy possessed by reactants and products on a “Enthalpy Diagram”. Enthalpy, H Progress of Reaction Reactants Products Δ H

16. Exothermic reactions If the potential energy diagram shows that the energy of the reactants is higher than that of the products, the reaction will release energy. Δ H is negative. It will be an Exothermic reaction. Enthalpy, H Progress of Reaction Reactants Products Δ H is -ve

17. Enthalpy Change - Exothermic E.g. When concrete is mixed, the main reaction is between calcium oxide and water. This reaction gives out a large amount of heat – so much that when building a large concrete structure such as a dam, cooling pipes must be included to carry away the heat. CaO(s) + H 2 O(l) → Ca(OH) 2 (s) ΔH = −65.2 kJ mol -1

18. Endothermic reactions If the potential energy diagram shows that the energy of the reactants is lower than that of the products, the reaction will take in energy. Δ H is positive. It will be an Endothermic reaction. Enthalpy, H Progress of Reaction Reactants Products Δ H is +ve

19. Enthalpy Change - Endothermic One example of an endothermic reaction is: 2NH 4 NO 3(s) + Ba(OH) 2 .8H 2 O (s)  2NH 3(g) + 10H 2 O (l) + Ba(NO 3 ) 2( aq ) ΔH = +17.44 kJ mol -1 Watch this at: http://www.youtube.com/watch?v=GmiZ0huvZzs

20. Obviously, these amounts of heat energy released or absorbed must be dependent on the quantity of substances reacting so we must define the enthalpy changes in terms of; “energy changes (kilojoules) per amount of substance (the mole)”. Therefore, the units of enthalpy and enthalpy change are kJ mol -1 Enthalpy and Enthalpy Change

21. Thermochemical equations It is important to give the exact reaction equation when quoting the associated energy change. 2Mg(s)+ O 2 (g)  2MgO(s) Δ H ⦵ = - 1204 kJ mol -1 Mg(s)+ O 2 (g)  2MgO(s) Δ H ⦵ = - 602 kJ mol -1  

22. Thermochemical equations Note that you must also always give the states of the reactants and products when quoting  H for a reaction. Pb (s)+ Cl 2 (g)  PbCl 2 (s)  H ⦵ = - 359 kJ mol -1 There would be a large extra energy change needed to change Pb (s) into Pb (g) !

23. Standard State Enthalpy Changes In order that chemists worldwide can compare notes on thermochemical experiments, a series of standard conditions have been agreed. The term “ standard state enthalpy” refers to an enthalpy change for a reaction in which reactants and products are considered to be in their standard states Standard states are ( most stable state of the substance) at a specified temperature 298 K (25 ° C) and pressure (one atmosphere )

24. Standard State Enthalpies The enthalpy change for a reaction in which reactants are in their standard states is denoted as; ∆H ⦵ You will see enthalpy changes without the circle but you can assume that all enthalpy changes in this module are standard state unless you are specifically told otherwise.

25. Measuring Enthalpy changes Enthalpy changes are calculated by measuring the heat required to change the temperature of a surrounding substance (usually water). The heat required to raise the temperature of a substance is its heat capacity .

26. Specific heat capacity The specific heat capacity, c , of a substance is: The amount of heat needed to raise the temperature of one gram of a substance by one degree Substance Specific heat capacity (J g -1 K -1 ) Aluminium 0.901 Iron 0.449 Water 4.18

27. Measuring Enthalpy Changes: Calorimetry A bomb calorimeter is a device used to measure the heat absorbed or released during a physical or chemical change. Everything inside the “bomb” is the system . The water is the surroundings.

28. Bomb calorimeter

29. Allow the reaction to heat (or cool) a known mass of water, measure the temperature change of the water then calculate the energy required using the formula; Q = c m ∆T Q= Heat /energy change (J) ∆T = the temperature change (K) ( final temperature – initial temperature ) m = mass of water (g) c = specific heat capacity of water ( 4.18 J g -1 K -1 OR kJ K -1 kg -1 ) Measuring the Enthalpy Change

30. Step to find enthalpy by Calorimetric Method Use q = c m ∆T , to calculate the given quantities' energy change . Divide the answer of step 1 by a thousand to convert J to kJ Work out the moles of the reactants used Divide q by the number of moles of the reactant, to give Δ H Add a sign and unit (kJmol -1 ) Measuring the Enthalpy Change

31. Example 0.253g of ethanol is burned in a calorimeter. The temperature of the surrounding 150g of water rises 10K. Calculate the enthalpy of combustion for 1 mole of ethanol: Step1: Heat is given out by the ethanol Q = c m Δ T = 4.18 x 150 x 10 = 6270 J Step 2: 6270 J (6.27 kJ) Heat has been given out so the reaction is exothermic = - 6.27 kJ

32. Example Step 3: The molar mass of ethanol = 46g Moles of ethanol used = 0.253/46 = 0.0055 moles Step 4: Assuming all the heat produced by the combustion was absorbed by the water: 0.0055 moles of ethanol burns to give -6.27kJ of heat. Therefore 1 mole of ethanol should burn to give (-6.27 x 1/0.0055) kJ = -1140 kJ heat Step 5: Estimated enthalpy of combustion (∆ H c ) = - 1140 kJ mol -1

33. Question Enthalpy changes are not just for combustion (burning) reactions – the heat evolved when a substance dissolves can be measured this way too. Calculate the enthalpy of solution for dissolving 0.80 g of Sodium Hydroxide in 25cm 3 of water if the temperature rise observed is 5.2°C. (c = 4.18 J g -1 K -1 ) Note that the density of water = 1g cm -3

34. Summary Reactions can be exothermic or endothermic. Laboratory methods can be used for determining enthalpy changes using the equation: energy transferred = c m ∆T.

35. Self-study and homework Complete Heat and Enthalpy worksheet