Energy Chapter 10

10.1 The Nature of Energy Energy – the ability to do work or produce heat Energy – the ability to do work or produce heat Potential Energy – due to position or composition Potential Energy – due to position or composition Kinetic Energy – due to motion Kinetic Energy – due to motion Depends on the mass of the object (m) and its velocity (v) Depends on the mass of the object (m) and its velocity (v) KE = ½ mv 2 KE = ½ mv 2

The Law of Conservation of Energy Energy can be converted from one form to another but can be neither created nor destroyed Energy can be converted from one form to another but can be neither created nor destroyed The energy in the universe is constant The energy in the universe is constant

Work Work = Force x distance Work = Force x distance W = Fd W = Fd Frictional Heating – 2 surfaces in contact with each other Frictional Heating – 2 surfaces in contact with each other Depends on surface and force pushing the surfaces together Depends on surface and force pushing the surfaces together

State Function The property of the system that changes independently of its pathway The property of the system that changes independently of its pathway The pathway is how you get there The pathway is how you get there Example Example If you travel from Chicago to Denver what are state functions? If you travel from Chicago to Denver what are state functions? The route you take to get there is your pathway, so it is not a state function The route you take to get there is your pathway, so it is not a state function Change in elevation doesn’t depend on how you get there so it is a state function Change in elevation doesn’t depend on how you get there so it is a state function

10.2 Temperature and Heat Temperature – Measure of the random motion of the components of a substance Temperature – Measure of the random motion of the components of a substance

Heat – The flow of energy due to a difference in temperature Heat – The flow of energy due to a difference in temperature

10.3 Exothermic and Endothermic Processes System – part of universe we are looking at System – part of universe we are looking at Surroundings – everything else Surroundings – everything else Exothermic – energy flows out of a system Exothermic – energy flows out of a system Endothermic – energy flows into a system Endothermic – energy flows into a system

Where does energy as heat come from in exothermic reactions? Where does energy as heat come from in exothermic reactions? It depends on the potential energy between the products and reactants It depends on the potential energy between the products and reactants

10.4 Thermodynamics Law of Conservation of Energy (a.k.a. The First Law of Thermodynamics) Law of Conservation of Energy (a.k.a. The First Law of Thermodynamics) Energy can neither be created nor destroyed under normal conditions Energy can neither be created nor destroyed under normal conditions The energy of the universe is constant The energy of the universe is constant

E = internal energy E is the sum of the kinetic energy and the potential energy E is the sum of the kinetic energy and the potential energy Can be changed by the flow of work, heat, or both Can be changed by the flow of work, heat, or both ∆ = change in; called “delta” ∆ = change in; called “delta” w = work w = work q = heat q = heat ∆E = q + w ∆E = q + w Change in internal energy equals heat plus work Change in internal energy equals heat plus work

Thermodynamic quantities are made up of a number that shows magnitude and a sign that shows whether energy is flowing into the system (endothermic = + ) or out of the system (exothermic = - ) Thermodynamic quantities are made up of a number that shows magnitude and a sign that shows whether energy is flowing into the system (endothermic = + ) or out of the system (exothermic = - )

10.5 Measuring Energy Changes calorie = amount of energy required to raise the temperature of 1 gram of water by one degree Celsius calorie = amount of energy required to raise the temperature of 1 gram of water by one degree Celsius 1000 calories (1 kilocalorie) is what we refer to as a “Calorie” with a capital C 1000 calories (1 kilocalorie) is what we refer to as a “Calorie” with a capital C 1 calorie = joules 1 calorie = joules 1 cal = J 1 cal = J To go from calories to joules multiply by To go from calories to joules multiply by To go from joules to calories divide by To go from joules to calories divide by 4.184

And now for a problem! How much heat, in joules, is required to raise the temperature of 7.40 g water from 29.0 °C to 46.0 °C? How much heat, in joules, is required to raise the temperature of 7.40 g water from 29.0 °C to 46.0 °C? We know we need J of energy raise 1 g of water 1 °C We know we need J of energy raise 1 g of water 1 °C We have 7.40 g of water so it will take 7.4 g x J to raise it 1 °C We have 7.40 g of water so it will take 7.4 g x J to raise it 1 °C We also need to raise the temperature 17 °C so 17.0 °C x 7.4 g x J/ g x °C We also need to raise the temperature 17 °C so 17.0 °C x 7.4 g x J/ g x °C So we need 526 J of energy So we need 526 J of energy Now try this Now try this Calculate the joules of energy required to heat 454 g of water from 5.4 °C to 98.6 °C? Calculate the joules of energy required to heat 454 g of water from 5.4 °C to 98.6 °C?

So we know that the amount of energy we need to raise the temperature of a substance depends on the amount of substance and the change in temperature So we know that the amount of energy we need to raise the temperature of a substance depends on the amount of substance and the change in temperature But the substance also plays a big part But the substance also plays a big part Specific Heat Capacity = the amount of energy needed to raise the temperature of 1 g of a substance 1 °C Specific Heat Capacity = the amount of energy needed to raise the temperature of 1 g of a substance 1 °C

Specific Heats Liquid water = J Liquid water = J Aluminum = 0.89 J Aluminum = 0.89 J Gold = 0.13 J Gold = 0.13 J This explains why certain things heat up faster than others This explains why certain things heat up faster than others The pot heats up faster than the water in it The pot heats up faster than the water in it The water in the pool is colder that the cement around it The water in the pool is colder that the cement around it

Now for another equation The amount of energy required = the specific heat x mass x change in temperature The amount of energy required = the specific heat x mass x change in temperature Q = m x C p x ∆T Q = m x C p x ∆T Try this sample Try this sample A 1.6 g sample of metal that looks like gold requires 5.8 J of energy to change its temperature from 23 °C to 41 °C. Is the metal gold? (Hint – you are finding what s is and comparing to what you know about gold’s specific heat) A 1.6 g sample of metal that looks like gold requires 5.8 J of energy to change its temperature from 23 °C to 41 °C. Is the metal gold? (Hint – you are finding what s is and comparing to what you know about gold’s specific heat) Answer = No; Gold’s s = 0.13 J/ g °C but this substance has an s = 0.20 J / g °C Answer = No; Gold’s s = 0.13 J/ g °C but this substance has an s = 0.20 J / g °C

10.6 Thermochemistry (Enthalpy) Enthalpy (symbol = H) is the same as the flow of heat Enthalpy (symbol = H) is the same as the flow of heat ∆H p = heat ∆H p = heat P tells us it occurred under constant pressure P tells us it occurred under constant pressure ∆ means “change in” ∆ means “change in” So the enthalpy for a reaction at constant pressure is the same as heat So the enthalpy for a reaction at constant pressure is the same as heat

Calorimetry Calorimeter = device used to determine the heat associated with a chemical reaction Calorimeter = device used to determine the heat associated with a chemical reaction Reaction is run in calorimeter and temperature change is observed Reaction is run in calorimeter and temperature change is observed We can use calorimeter to find ∆H We can use calorimeter to find ∆H Once we know ∆H for some reactions we can use those to calculate ∆H for other reactions Once we know ∆H for some reactions we can use those to calculate ∆H for other reactions

10.7 Hess’s Law The change in enthalpy for a given process is independent of the pathway for the process (this means it is a state function) The change in enthalpy for a given process is independent of the pathway for the process (this means it is a state function) Hess’s Law states that the change in enthalpy from reactants to products in a reaction is the same whether it takes place in one step or a series of steps Hess’s Law states that the change in enthalpy from reactants to products in a reaction is the same whether it takes place in one step or a series of steps N 2 + 2O 2 → 2NO 2 ∆H = 68 kJ N 2 + 2O 2 → 2NO 2 ∆H = 68 kJor N 2 + O 2 → 2NO ∆H = 180 kJ N 2 + O 2 → 2NO ∆H = 180 kJ 2NO + O 2 → 2NO 2 ∆H = -112 kJ 2NO + O 2 → 2NO 2 ∆H = -112 kJ So 180 kJ + (-112 kJ) = ∆H = 68 kJ So 180 kJ + (-112 kJ) = ∆H = 68 kJ

Characteristics of Enthalpy Changes If a reaction is reversed, ∆H is reversed If a reaction is reversed, ∆H is reversed Xe + 2F 2 → XeF 4 ∆H = -251 kJ Xe + 2F 2 → XeF 4 ∆H = -251 kJ XeF 4 → Xe + 2F 2 ∆H = +251 kJ XeF 4 → Xe + 2F 2 ∆H = +251 kJ Magnitude of ∆H is proportional to quantities of reactants and products Magnitude of ∆H is proportional to quantities of reactants and products Xe + 2F 2 → XeF 4 ∆H = -251 kJ Xe + 2F 2 → XeF 4 ∆H = -251 kJ 2(Xe + 2F 2 → XeF 4 ) ∆H = -502 kJ 2(Xe + 2F 2 → XeF 4 ) ∆H = -502 kJ

10.8 Quality versus Quantity of Energy One of the most important characteristics is that it is conserved One of the most important characteristics is that it is conserved Eventually all energy will take the form of heat and spread evenly throughout the universe and everything will be the same temperature Eventually all energy will take the form of heat and spread evenly throughout the universe and everything will be the same temperature This means work won’t be able to be done and universe will be dead; called “heat death” This means work won’t be able to be done and universe will be dead; called “heat death” We care more about what kind of energy (quality) than the amount of energy (quantity) We care more about what kind of energy (quality) than the amount of energy (quantity)

10.9 Energy and Our World Fossil Fuels formed by decaying products of plants Fossil Fuels formed by decaying products of plants Petroleum Petroleum Natural Gas Natural Gas Coal Coal Greenhouse Effect – Visible light travels through atmosphere, converted to infrared radiation (heat) which is absorbed by certain molecules, H 2 0 and CO 2 mainly, which radiate it back to earth Greenhouse Effect – Visible light travels through atmosphere, converted to infrared radiation (heat) which is absorbed by certain molecules, H 2 0 and CO 2 mainly, which radiate it back to earth

10.10 Energy as a Driving Force Energy Spread – in any given process, concentrated energy is dispersed widely Energy Spread – in any given process, concentrated energy is dispersed widely Happens with every exothermic reaction Happens with every exothermic reaction When gas is burned, energy stored is dispersed into surrounding air When gas is burned, energy stored is dispersed into surrounding air Matter Spread – molecules of a substance are spread out and occupy a larger volume Matter Spread – molecules of a substance are spread out and occupy a larger volume Salt dissolves in water due to matter spread Salt dissolves in water due to matter spread These 2 processes are important driving forces that cause events to occur These 2 processes are important driving forces that cause events to occur

Entropy Invented function that keeps track of disorder Invented function that keeps track of disorder Entropy (S) is a measure of disorder or randomness Entropy (S) is a measure of disorder or randomness So a cube of ice has a a lower S value than steam So a cube of ice has a a lower S value than steam Energy spread and Matter spread lead to greater entropy Energy spread and Matter spread lead to greater entropy The entropy in the universe is always increasing The entropy in the universe is always increasing