Thermochemistry

1. Thermochemistry Energy Energy is defined as the ability to do work. There are several forms of energy Kinetic energy – energy due to motion EK = 1/2mv2 Potential energy – the energy due to the position of a particle in a field e.g. Gravitational, electrical, magnetic etc.

2. Thermochemistry Energy The unit of energy is the Joule (J) and 1 J = 1 kgm2s-2 Thermochemistry is the study of chemical energy and of the conversion of chemical energy into other forms of energy. It is part of thermodynamics – the study of the flow of heat.

3. Thermochemistry Thermochemically, we define the system as the part of the universe under study and the surroundings as everything else. Systems come in three forms: Open The system can exchange matter and energy with the surroundings Closed The system can exchange energy only with the surroundings Isolated There is no exchange of matter or of energy with the surroundings

4. Thermochemistry Matter is continually in motion and has an internal energy that is composed of several different types There is Translation Rotation Vibration Potential between molecules and inside molecules. The internal energy is written as U

5. Thermochemistry Matter is continually in motion and has an internal energy that is composed of several different types There is Translation Rotation Vibration Potential between molecules and inside molecules. The internal energy is written as U The internal energy is directly connected to heat and the transfer of heat.

6. Thermochemistry Heat is the transfer of internal energy between the surroundings and the system or between systems. The direction of the heat flow is indicated by the temperature – heat flows along a Temperature gradient from high temperature to low temperature. When the temperature of the system and that of the surroundings are equal, the system is said to be in thermal equilibrium

7. Thermochemistry Energy is the capacity to do work but what is work? Work is the action of a force over a distance. To be able to do work, we must be able to exert a force over a distance. During this process, energy is expended. w = F x d where w is the work, F is the force and d is the distance. Work is measured in Joules.

8. Thermochemistry PV work When a gas expands against an external pressure, for example in a cylinder, against a constant weight (weight being a force.....) the work done can be written as w = F x d As P = Fthen F = PA A Thus w = PAd and as Ad = Vfinal – Vinitial = V Then w = PV

9. Thermochemistry PV work By convention, the work done when a gas expands is negative, Thus w = - PV for an expanding gas

10. Thermochemistry State Functions The state of a system is defined by the precise conditions of the system: The quantity and type of matter present The temperature and pressure The molecular structure of the system As 1 mole = 6.02 x 1023 particles, defining the state of a system uniquely is experimentally impossible in an absolute sense.

11. Thermochemistry State Functions and U The internal energy, U, of a system is a function of the state of the system. Although we cannot measure the absolute state of a system, we can measure changes in the state of the system in a relative way, by measuring the work and the heat that takes place during a chemical change. As U is a function of the state of the system, it does not depend on the way the state of the system is prepared – it is independent of the path.

12. Thermochemistry State Functions and U U is therefore a state function of the system. It depends only on the present state of the system and not on the previous history or the path by which the system was prepared. Because we have no measure of the state of a system, or of the internal energy, we can only measure the change in the state, through the observation of work and transfers of heat into and out of the system.

13. Thermochemistry State Functions and U U is therefore a state function of the system. It depends only on the present state of the system and not on the previous history or the path by which the system was prepared. Because we have no measure of the state of a system, or of the internal energy, we can only measure the change in the state, through the observation of work and transfers of heat into and out of the system.

14. Thermochemistry Energy Energy is defined as the ability to do work. There are several forms of energy Kinetic energy – energy due to motion EK = 1/2mv2 Potential energy – the energy due to the position of a particle in a field e.g. Gravitational, electrical, magnetic etc.

15. Thermochemistry Energy The unit of energy is the Joule (J) and 1 J = 1 kgm2s-2 Thermochemistry is the study of chemical energy and of the conversion of chemical energy into other forms of energy. It is part of thermodynamics – the study of the flow of heat.

16. Thermochemistry Thermochemically, we define the system as the part of the universe under study and the surroundings as everything else. Systems come in three forms: Open The system can exchange matter and energy with the surroundings Closed The system can exchange energy only with the surroundings Isolated There is no exchange of matter or of energy with the surroundings

17. Thermochemistry Matter is continually in motion and has an internal energy that is composed of several different types There is Translation Rotation Vibration Potential between molecules and inside molecules. The internal energy is written as U

18. Thermochemistry Matter is continually in motion and has an internal energy that is composed of several different types There is Translation Rotation Vibration Potential between molecules and inside molecules. The internal energy is written as U The internal energy is directly connected to heat and the transfer of heat.

19. Thermochemistry Heat is the transfer of internal energy between the surroundings and the system or between systems. The direction of the heat flow is indicated by the temperature – heat flows along a Temperature gradient from high temperature to low temperature. When the temperature of the system and that of the surroundings are equal, the system is said to be in thermal equilibrium

20. Thermochemistry Energy is the capacity to do work but what is work?

21. Thermochemistry Energy is the capacity to do work but what is work? Work is the action of a force over a distance. To be able to do work, we must be able to exert a force over a distance. During this process, energy is expended.

22. Thermochemistry Energy is the capacity to do work but what is work? Work is the action of a force over a distance. To be able to do work, we must be able to exert a force over a distance. During this process, energy is expended. w = F x d where w is the work, F is the force and d is the distance. Work is measured in Joules.

23. Thermochemistry PV work When a gas expands against an external pressure, for example in a cylinder, against a constant weight (weight being a force.....) the work done can be written as w = F x d As P = Fthen F = PA A Thus w = PAd and as Ad = Vfinal – Vinitial = V Then w = PV

24. Thermochemistry PV work By convention, the work done when a gas expands is negative, Thus w = - PV for an expanding gas

25. Thermochemistry State Functions The state of a system is defined by the precise conditions of the system: The quantity and type of matter present The temperature and pressure The molecular structure of the system As 1 mole = 6.02 x 1023 particles, defining the state of a system uniquely is experimentally impossible in an absolute sense.

26. Thermochemistry State Functions and U The internal energy, U, of a system is a function of the state of the system. Although we cannot measure the absolute state of a system, we can measure changes in the state of the system in a relative way, by measuring the work and the heat that takes place during a chemical change. As U is a function of the state of the system, it does not depend on the way the state of the system is prepared – it is independent of the path.

27. Thermochemistry State Functions and U U is therefore a state function of the system. It depends only on the present state of the system and not on the previous history or the path by which the system was prepared. Because we have no measure of the state of a system, or of the internal energy, we can only measure the change in the state, through the observation of work and transfers of heat into and out of the system.

28. Thermochemistry State Functions and U U is therefore a state function of the system. It depends only on the present state of the system and not on the previous history or the path by which the system was prepared. Because we have no measure of the state of a system, or of the internal energy, we can only measure the change in the state, through the observation of work and transfers of heat into and out of the system.

29. Thermochemistry Internal Energy, U and State Functions Energy, and therefore the capacity to do work is present in all matter. This internal energy is stored in translational, rotational, vibrational and potential forms or modes in the material. The exact distribution of energy defines the state of the system, together with external variables such as pressure, temperature.

30. Thermochemistry Internal Energy, U and State Functions U is a function of the state of the material only, not of the history of the sample or the path taken to prepare the state of the sample. Heat is the transfer of energy between the surroundings and the sample - the symbol for heat is q Work is the result of a force acting over a distance - the symbol for work is w

31. Thermochemistry Internal Energy, U and State Functions Heat and work are the only two ways of changing the internal energy of a system. Temperature is defined by the direction of the flow of heat, which is always from high temperature to low temperature. When the the temperature of the system and the surroundings are the same, the system is at thermal equilibrium with it’s surroundings.

32. Thermochemistry The sign conventions of thermochemistry When the internal energy of the system rises, this energy change has a positive sign. - The energy of the system rises when heat is absorbed - The energy of the system rises when work is done on the system e.g. a gas is compressed - in these cases, q is positive w is positive

33. Thermochemistry The sign conventions of thermochemistry When the internal energy of the system lowers, this energy change has a negative sign. - The energy of the system lowers when heat is leaves the system - The energy of the system rises when the system does work e.g. a gas expands against an external pressure - in these cases, q is negative w is negative

34. Thermochemistry Internal energy rises: q > 0 w > 0 Internal energy drops: q < 0 w < 0

35. Thermochemistry The First Law of Thermodynamics Energy can be exchanged but cannot be created or destroyed. It is a statement of the Law of Conservation of Energy U = Ufinal – Uinitial = q + w

36. Thermochemistry Chemical applications of the 1st Law Any chemical change can be characterized as an Endothermic change or an Exothermic change. In an exothermic reaction, internal chemical energy is converted into heat, which leaves the system if the system is not isolated or causes the temperature to rise if the system in isolated.

37. Thermochemistry Chemical applications of the 1st Law In an endothermic reaction, heat is required to drive the chemical reaction and in an isolated system, the temperature will fall. In an non-isolated system, heat is absorbed from the surroundings. Exothermic T rises (isolated) q negative (non-isolated) Endothermic T falls (isolated) q positive (non-isolated)

38. Thermochemistry Reactions at constant pressure and constant volume At constant volume, V = 0 and so UV = qV- PV UV = qV + 0 = qV When the system can do PV work, i.e. a system at constant pressure, UP = qP- PV where w = - PV

39. Thermochemistry Most reactions take place at constant pressure and therefore we define a new function, which is a state function in the same way that U is a state function Rearranging UP = qP- PV UP + PV = qP We term qP the enthalpy of the reaction qP = H = UP + PV

40. Thermochemistry Enthalpy is an extensive property – one that depends on the quantity of the material present in the reaction. This follows directly from the fact that the enthalpy is the heat generated by a reaction – there is more energy released from 1000 kg of methane when it burns than from 1 g.

41. Thermochemistry Enthalpies and internal energies are measured in kJ mol-1 and the stoichiometry of a reaction is directly applicable to the enthalpy – half the quantity of the reaction results in half the enthalpy change taking place.

42. Thermochemistry We can characterize reactions as endothermic or exothermic using the enthalpy, H. If the enthalpy change is negative, the reaction is exothermic and heat is given out by the system

43. Thermochemistry We can characterize reactions as endothermic or exothermic using the enthalpy, H. If the enthalpy change is negative, the reaction is endothermic and heat is absorbed by the system

44. Thermochemistry Using the enthalpy, we can account for the heat entering a reaction at constant pressure – in the same way that we account for the products and reactants in a reaction. In an endothermic reaction, the energy absorbed by the system can be considered as a reactant. Conversely, an exothermic reaction, one which evolves heat, has the energy as a product.

45. Thermochemistry Enthalpies and internal energies are measured in kJ mol-1 and the stoichiometry of a reaction is directly applicable to the enthalpy – half the quantity of the reaction results in half the enthalpy change taking place.

46. Thermochemistry Heat Capacities When a definite quantity of energy is absorbed by materials, the temperature rises.With different materials, the temperature rise, T, is different. The quantity of energy required to raise a quantity of material by 1 K is termed the heat capacity. Mathematically, C = q T where C is the heat capacity, q is the heat.

47. Thermochemistry Heat Capacities The specific heat is the heat per gram of sample and the molar heat capacity is the heat capacity per mole.

48. Thermochemistry Specific Heats, Molar Heats and Calorimetry The heat capacity is the quantity of heat required to raise a given quantity of a substance by 1 K The specific heat 1 gram though 1 K The molar heat 1 mole through 1 K The units of heat capacity are Jg-1K-1 (specific heat) or Jmol-1K-1 (molar heat)

49. Thermochemistry Specific Heats, Molar Heats and Calorimetry To calculate the heat transferred to a sample we use q = quantity x heat capacity xT For the specific heat q = mCsTwhere m = mass For the molar heat q = nCmTwhere n = no. of moles Make sure that the units of the heat capacity matches the units of quantity that is in the heat equation

50. Thermochemistry Specific Heats, Molar Heats and Calorimetry To measure the heat capacity, a calorimeter is used. A calorimeter measures heat transfers, heats of reaction or heats of dissolution.