1 / 100

Thermochemistry

Learn about the energy involved in chemical reactions, including thermal energy, potential energy, and the conservation of energy. Explore specific heat capacity and temperature changes. Discover how energy is exchanged between a system and its surroundings.

auxier
Download Presentation

Thermochemistry

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Thermochemistry Chapter 16

  2. Question to think about: • Imagine for a moment that you live in one of those places where you have to heat your home. • Most people heat their home with fossil fuels (ex. Natural gas). • When you turn on the heat, what factors go into determining how much the temperature in your house will rise?

  3. Thermochemistry • The study of energy involved during chemical reactions. Energy sources: ~ chemical ~ nuclear ~ solar ~ geothermal ~ wind/water

  4. Nature of Energy • even though Chemistry is the study of matter, energy effects matter • energy is anything that has the capacity to do work • work is a force acting over a distance • Energy = Work = Force x Distance • energy can be exchanged between objects through contact • collisions Tro, Chemistry: A Molecular Approach

  5. Classification of Energy • Kinetic energy (Ek) is energy of motion or energy that is being transferred • thermal energy is kinetic Tro, Chemistry: A Molecular Approach

  6. Classification of Energy • Potential energy (EP) is energy that is stored in an object, or energy associated with the composition and position of the object • energy stored in the structure of a compound is potential Tro, Chemistry: A Molecular Approach

  7. Law of Conservation of Energy • energy cannot be created or destroyed • First Law of Thermodynamics • energy can be transferred between objects • energy can be transformed from one form to another • heat → light → sound Tro, Chemistry: A Molecular Approach

  8. Main Forms of Energy We Are Concerned With • Thermal Energy (Heat) • kinetic energy associated with molecular motion • Chemical • potential energy in the attachment of atoms or because of their position • “stored” energy Tro, Chemistry: A Molecular Approach

  9. Energy • Heat: the energy of motion of molecules • Kinetic molecular theory: • substances are composed of particles that are continually moving and colliding with other particles to create reactions. • Kinetic energy: energy of motion • Potential energy: stored energy

  10. Units of Energy • joule (J) is the amount of energy needed to move a 1 kg mass a distance of 1 meter • 1 J = 1 N∙m = 1 kg∙m2/s2 • calorie (cal) is the amount of energy needed to raise one gram of water by 1°C • kcal = energy needed to raise 1000 g of water 1°C • food Calories = kcals Tro, Chemistry: A Molecular Approach

  11. Energy Flow and Conservation of Energy • we define the system as the material or process we are studying the energy changes within • we define the surroundings as everything else in the universe • Conservation of Energy requires that the total energy change in the system and the surrounding must be zero • DEnergyuniverse = 0 = DEnergysystem + DEnergysurroundings • D is the symbol that is used to mean change • final amount – initial amount

  12. Energy Exchange • energy is exchanged between the system and surroundings through either heat exchange or work being done Tro, Chemistry: A Molecular Approach

  13. Internal Energy • the internal energy is the total amount of kinetic and potential energy a system possesses • the change in the internal energy of a system only depends on the amount of energy in the system at the beginning and end • a state function is a mathematical function whose result only depends on the initial and final conditions, not on the process used • DE = Efinal – Einitial • DEreaction = Eproducts - Ereactants Tro, Chemistry: A Molecular Approach

  14. State Function Tro, Chemistry: A Molecular Approach

  15. Temperature: • A measure of the average kinetic energy of particles • Proportional to their speed • Measured with a thermometer

  16. A temperature change is explained as a change in kinetic energy • Temperature depends on the quantity of heat (q) flowing out or in of the substance.

  17. Heat (Q) Q=mc ∆t • Q=heat • m=mass • ∆t=change in temperature (tf-ti) • c=specific heat capacity (J/(g oC) Specific heat capacity is the quantity of heat required to raise the temperature of a unit mass of a substance by one degree Celsius.

  18. Specific Heat Capacity • measure of a substance’s intrinsic ability to absorb heat • the specific heat capacity is the amount of heat energy required to raise the temperature of one gram of a substance 1°C • Cs • units are J/(g∙°C) • the molar heat capacity is the amount of heat energy required to raise the temperature of one mole of a substance 1°C • the rather high specific heat of water allows it to absorb a lot of heat energy without large increases in temperature • keeping ocean shore communities and beaches cool in the summer • allows it to be used as an effective coolant to absorb heat Tro, Chemistry: A Molecular Approach

  19. Table of Specific Heats

  20. Law of conservation of energy • ∆E universe = O • The total energy of the universe is constant, it is not created or destroyed. • However, it can be transferred from one substance to another. • ∆E universe = ∆E system + ∆E surroundings

  21. First Law of thermodynamics • Any change in energy of a system is equivalent by an opposite change in energy of the surroundings. • ∆E system = - ∆E surroundings • According to this law, any energy released or absorbed by a system will have a transfer of heat, q. • So, q system = -q surroundings

  22. Sample Problem • 15 g of ice was added to 60.0g of water. The Ti of water was 26.5 oC, the final temperature of the mixture was 9.70 oC. How much heat was lost by the water? • q=mc ∆t q=(60.0g) (4.18 J/g oC) (9.70-26.5 oC) q= - 4213.44 J =-4.21 kJ

  23. Calculate the amount of heat needed to increase the temperature of 250g of water from 20oC to 56oC. q = m x c x (Tf - Ti)q = 250g x 4.18 J/g oC x (56oC - 20oC)q = 37 620 J = 38 kJ

  24. Calculate the specific heat capacity of copper given that 204.75 J of energy raises the temperature of 15g of copper from 25oC to 60oC. • q = m x c x (Tf - Ti)q = 204.75 Jm = 15gTi = 25 oCTf = 60 oC 204.75 J = 15 g x c x (60oC - 25oC)204.75 J= 15 g x c x 35oC204.75 J= 525 g oC x cc = 204.75 J ÷ 525 g oC = 0.39 J/ g oC

  25. Watch flash about heat flow http://www.mhhe.com/physsci/chemistry/animations/chang_7e_esp/enm1s3_4.swf

  26. Homework: • Handout

  27. Enthalpy (H) • Total kinetic and potential energy (internal energy) of a system under constant pressure. • System: area where reaction takes place • Its particles average motion define its properties. • Surroundings: outside of the system

  28. Open-system • both matter and energy can freely cross from the system to the surroundings and back. • Ex: an open test tube

  29. Closed-System • energy can cross the boundary, but matter cannot. • Ex: a sealed test tube

  30. Isolated-System • neither matter nor energy can cross between the system and the surroundings. • Ex: The universe • there are no surroundings to exchange matter or energy with (as far as we know!)

  31. The internal energy of a reactant or product cannot be measured, but their change in enthalpy (heat of reaction) can. ∆ H = Hproducts – Hreactants A change in enthalpy occurs during phase changes, chemical reactions and nuclear reactions. ∆ H system = q surroundings

  32. If energy is released out of the system then it is considered to be an exothermic enthalpy change (exit). If energy is absorbed into the system then it is considered to be an endothermic enthalpy change (enter).

  33. Endothermic Reactions

  34. Endothermic Reactions Method 2: 2 HgO (s) 2 Hg (l) + O2(g)ΔH=181.67 kJ Method 3: 2 HgO (s) + 181.67 kJ 2 Hg (l) + O2(g)

  35. Exothermic Reactions

  36. Exothermic Reactions Method 2: 4Al(s)+ 3O2(g) 2 Al2O3(g)ΔH=-1675.7 kJ Method 3: 4 Al(s) + 3O2(g) 2 Al2O3(g) +1675.7 kJ

  37. Calorimeter • Instrument used to measure amount of energy involved in a chemical reaction. • It is equivalent to an isolated or closed system. (nothing may enter or exit the system) • The energy change is not measured within the system, but the energy transferred to its surroundings.

  38. A basic calorimeter • Two styrofoam cups nestled within one another (insulation), then filled with a specific quantity of water. • A chemical reaction or phase change takes place inside and a thermometer is placed within to measure any change in temperature that occurs to the system.

  39. Assumptions • It is an isolated or closed system and there is no heat transfer between the calorimeter and its surroundings. • The amount of heat absorbed or released by the calorimeter itself is too small to influence calculations. • Any dilute solutions involved in the reaction are treated as if they are water.

  40. Bomb calorimeter • Cannot use basic design for combustion reactions. • Used in research for H for fuels, oils, food, explosives… • Larger and more sophisticated • The reaction container is strong enough to with stand an explosion, hence the name “bomb”

  41. Have fixed components, like volume of water, thermometer… • Heavily insulated or vacuum insulated so no convection or conduction can occur affecting the enthalpy of the system. • Use the equation: q=CΔt

  42. A Bomb Calorimeter

  43. Problems • Pg 638 # 5-7 • Handout Questions 1-10

  44. Standard Molar Enthalpy of Formation • Quantity of energy released (-) or absorbed (+) when one mole of a compound is formed directly from its elements at standard temperature and pressure. • We use a table to find them. • Unit for ΔHf: kJ/mol • Watch your states!

  45. Calculating enthalpy changes • Amount of a substance reacting matters, so can use q= nΔH. • Remember n=amount of moles. • If you are given a mass (g) and molar mass (g/mol), then you can solve for n by dividing mass by molar mass. (review from chem 11 stoichiometry section)

  46. Sample Problem: • Show the formation reaction of methanol. • If I had 10.5 g of CH3OH(l), how much energy would be released? C(s) + 2H2(g) + 1/2O2(g)  CH3OH(l) ΔHf = -239 kJ/mol mm= (12.01) +(4 x 1.01) + (16.00) = 32.05 g/mol q = nΔH = (10.5 g / 32.05 g/mol) (-239 kJ/mol) = -78.3 KJ

  47. On Your Own: • I have 125.6 g of NaOH(s) formed at standard temperature and pressure, how much energy will be released? Answer: - 1336.4 kJ

  48. Standard Molar Enthalpy of Combustion • Energy changes involved with combustion reactions of one mole of a substance. • Remember that these reactions are only measured once cooled to 25oC • Combustion is a reaction with oxygen as a reactant

More Related