Chapter 10 Energy Chemistry B2A

Energy MatterEnergyEmpty space Universe Energy: ability to do work or produce heat.

Energy Kinetic energy (KE): energy of motion KE = ½ mV 2 V: velocity Potential energy (PE): stored energy Law of conservation of energy (Position & Composition)

Kinetic and Potential Energy A B B A Kinetic energy Potential energy Change in potential energy for ball A (change in level) Work: change the position of ball B Heat: friction between surface & ball Frictional heating (heat is a form of energy). T of hill increases

Heat and Temperature Temperature: measure of random motions of the components of substance. T ↑ move faster Kinetic energy ↑

Energy is transferred from high T to low T. Heat: Flow of energy due to a T difference. Heat and Temperature Cold Water Hot Water T final = T hot initial + T cold initial 2

Heat units of heat: calorie (cal) English system joule (J) SI system 1 cal = J Joule: Energy (heat) required to raise T of one gram of water by 1  C.

Amount of heat = specific heat × mass × change in temperature Amount of heat = SH × m × (T 2 – T 1 ) T 2 = final temperature T 1 = initial temperature SH = Specific heat capacity (cal/g °C) Heat

Heat of reaction C 3 H 8 (s) + 5O 2 (g)  3CO 2 (g)+ 4H 2 O(l) + heat (energy) 2HgO(s) + heat (energy)  2Hg(l) + O 2 (g) Endothermic reaction Exothermic reaction All combustion reactions are exothermic.

Heat of reaction Surroundings System Energy Exothermic Endothermic  (PE) Reactant Product Energy released to the surroundings as heat Surrounding Exothermic (burning)

Thermodynamics Thermodynamics: study of energy Law of conservation of energy: energy of the universe is constant. Internal energy (E): sum of the kinetic and potential energies.  E = q + w  “delta”: change Work Heat The first law of thermodynamics:

Surroundings System Energy Exothermic Endothermic  E  0  E  0 Thermodynamics  E = q + w Energy flows into system via heat (endothermic): q = +x Energy flows out of system via heat (exothermic): q = -x

Enthalpy For a reaction in constant pressure, the change of enthalpy is equal to energy that flows as heat.  H p = heat Constant pressure “-” heat or  H p : exothermic: heat flows out of the system. “+” heat or  H p : endothermic: heat flows into the system. Enthalpy (Thermochemistry): heat of chemical reactions.

Calorimetry Calorimeter: A device to measure the heat energy released or absorbed by a reaction.  T   H

Hess’s Law State function: a property of system that changes independently of its pathways. Enthalpy is a state function. In a chemical reaction, change of enthalpy is the same whether the reaction takes place in one step or in a series of steps. N 2 (g) + 2O 2 (g)  2NO 2 (g)  H 1 = 68 kJ N 2 (g) + O 2 (g)  2NO(g)  H 2 = 180 kJ 2NO(g) + O 2 (g)  2NO 2 (g)  H 3 = -112 kJ N 2 (g) + 2O 2 (g)  2NO 2 (g)  H 2 +  H 3 = 68 kJ 1 Step 2 Steps

Two rules about enthalpy 1. If a reaction is reversed, the sign of  H is also reversed. N 2 (g) + 2O 2 (g)  2NO 2 (g)  H 1 = 68 kJ 2NO 2 (g)  N 2 (g) + 2O 2 (g)  H 1 = -68 kJ 2. If the coefficients in a balanced reaction are multiplied by an integer, the value of  H is also multiplied by the same integer. N 2 (g) + 2O 2 (g)  2NO 2 (g)  H 1 = 68 kJ 2N 2 (g) + 4O 2 (g)  4NO 2 (g)  H 1 = 2  68 kJ = 136 kJ  2

Law of conservation of energy Quality-Quantity of Energy Why are we concerned about energy? Gasoline + O 2  CO 2 + H 2 O + energy Spread in universe Concentrated energy Spread energy Use of energy to do work Quantity Quality Heat death

Energy and Our World Sun Woody plants Coal Natural gas Petroleum Photosynthesis Source of energy 6CO 2 + 6H 2 O + energy of sun C 6 H 12 O 6 + 6O 2 Photosynthesis glucose

1. Natural gas –90 to 95 percent methane. –5 to 10 percent ethane, and a mixture of other low-boiling alkanes. 2. Petroleum –A thick liquid mixture of thousands of compounds, most of them hydrocarbons. Energy and Our World Fossil Fuels: formed from the decomposition of marine plants and animals. (C 5 -C 10 ) (C 10 -C 18 ) (C 1 -C 4 ) (C 15 -C 25 ) (  C 25 )

3. Coal –Was formed from the remains of plants that were buried (under high P and T). –20% of our energy. –Expensive, dangerous, and produces pollution (CO & SO 2 ). Energy and Our World

Greenhouse Effect

Driving forces Energy spread: concentrated energy is dispersed widely. (Exothermic process) Matter spread: molecules of a substance are spread out and occupy a larger volume. Dissolving is endothermic process, but because of matter spread, it occurs. heat

Entropy (S) A measure of disorder or randomness. Energy spread  Faster random motions of the molecules in surroundings. Matter spread  Components of matter are dispersed (occupy a larger volume). A Spontaneous process is one that happens in nature on its own. (because of increasing entropy) Dissolving The second law of thermodynamics: The entropy (S) of the universe is always increasing. We run towards a disorder (heat death of universe).