Department of Chemistry CHEM1020 General Chemistry *********************************************** Instructor: Dr. Hong Zhang Foster Hall, Room 221 Tel:

CHEM1020/General Chemistry _________________________________________ Chapter 14. (L32)-Energy Today’s Outline …Review: Concept of energy …Energy and the first law: Energy is conserved …Energy and the second law: Entropy …Energy and chemical reactions …Our ultimate energy source: Solar energy …Energy and life-support system

Chapter 14. (L32)-Energy Review: Concept of energy …Concept of energy: Energy is the ability to do work. Examples: automobile airplane making a piece of furniture carry something upstairs … …Units for energy: joule (J) calorie (cal) 1000 cal = 1kcal = 4184 J

Chapter 14. (L32)-Energy Review: Kinetic energy and potential energy …Kinetic energy: The energy of motion A moving object has kinetic energy …Potential energy: The energy associated with position An object at a certain position can have potential energy. When the change of the position of the object will either release or require energy Example: gravitational energy spring energy chemical energy nuclear energy

Chapter 14. (L32)-Energy Energy and the first law of thermodynamics …The law of conservation of energy: Energy can neither be created nor destroyed Nothing is free. If somewhere the energy is gained, then somewhere else, the energy is lost. Loss = Gain The first law: “We can’t win.” Energy is conserved, but the energy in useful form is not always there available.

Chapter 14. (L32)-Energy Energy and the second law of thermodynamics …Some properties of energy: -energy can be changed from one form to another. -not all forms of energy are equal; in other words, not all forms of energy are equally useful or available. -high-grade forms of energy are constantly being degraded into low-grade forms. Energy flows downhill.

Chapter 14. (L32)-Energy Energy and the second law This law states that energy does not flow spontaneously from a cold object to a hot one. Refrigerator: You need electrical energy to cool or to do the opposite (the natural process or tendency) Energy loss in un-useful forms (wasted energy) Example: engine with heat energy loss

Chapter 14. (L32)-Energy Energy and the second law Another form of the second law states that natural change will go towards more randomness or less order. Entropy: An measure of disorder High entropy: less order or more disorder Low entropy: more order or less disorder Natural changes tend to go from low entropy to high entropy spontaneously. The second law: “We are doomed to lose”.

Chapter 14. (L32)-Energy Energy and the thermodynamic laws The first law: “We can’t win.” The second law: “We are doomed to lose.” This is according to thermodynamics which is about equilibrium. But, at non-equilibrium state, we have kinetic steady state at which the drama of life is going on and on, powered by solar energy. At non-equilibrium, the change from low order to high order (or from high entropy to low entropy), or order out of chaos, can happen, as long as energy is supplied to sustain the change, and to maintain the steady state. So, life is a steady state at which the change from low order to high order is occurring, driven by solar energy.

Chapter 14. (L32)-Energy Energy and chemical reactions …Chemical reactions and temperature Generally, the higher T, the faster the reaction Temperature and kinetic energy of molecules kinetic-molecular theory

Chapter 14. (L32)-Energy Energy and chemical reactions …Chemical reactions and energy changes Energy (heat) may be released or required during a reaction. The energy change involved in a reaction is quantitatively related to the quantity of the chemicals (molecules or moles) involved.

Chapter 14. (L32)-Energy Energy and chemical reactions …Chemical reactions and energy changes Example: CH 4 (g) + 2O 2 (g)  CO 2 (g) + 2H 2 O(g) kJ 1 mole2 mole Problem: Burning 1.0 mole of propane releases 2201 kJ. How much energy can be released by burning 10 mole propane? The energy = (2201 kJ/mole)x10 mole = kJ

Chapter 14. (L32)-Energy Energy and chemical reactions …Chemical reactions and energy changes -Exothermic reactions: Chemical reactions that result in release of heat to the surroundings. Example: CH 4 (g) + 2O 2 (g)  CO 2 (g) + 2H 2 O(g) kJ -Endothermic reactions: Chemical reactions that require energy input to proceed. Example: 2H 2 O + 573kJ  2H 2 (g) + O 2 (g)

Chapter 14. (L32)-Energy Our ultimate energy source: Solar energy …The sun is a nuclear fusion reactor converting H to He SI unit for power: 1 W = 1 J/s the sun power output: 4x10 26 W the earth receives ~1.73x10 17 W (= ~115 million nuclear power plants) In 3 days, the earth receives energy from the sun equivalent to all our fossil fuel reserves. But, this is only about one part in 50 billion of the sun’s output.

Chapter 14. (L32)-Energy Energy and life-support system …A tiny portion of solar energy is converted to bio- energy to sustain life via photosynthesis enterprise: 6CO 2 + 6H 2 O  C 6 H 12 O 6 + 6O 2 -chlorophyll absorbs visible sunlight to drive photosynthesis to catch solar energy -animals get solar energy through eating carbohydrates generated by photosynthesis -the solar energy thus keeps the order in life (low entropy). Without the energy, life will fall apart and come back to disorder (death)

Chapter 14. (L32)-Energy Quiz Time TBA