Chemistry 30

Thermochemistry Notes

A. Energy

30-A1.2k  explain, in a general way, how stored energy in the chemical bonds of hydrocarbons originated from the sun

  • we use hydrocarbons in many areas of our lives

eg) ______

______

  • the energy stored in the chemical bonds of hydrocarbons originally comes from the ______
  • ancient plants captured the energy of the sun during ______, ancient animals ate the plants, then death and trapping in rock formations as oil, gas, oil sand etc.
  • there are two fundamental types of energy:
  1. ______: the energy of ______(of particles)
  2. ______: energy that is ______(in chemical bonds)
  • the First Law of Thermodynamics states that:
  • the total energy of the universe is ______
  • energy can be ______however the ______of any systemis ______(it ______ be created or destroyed)
  • the Second Law of Thermodynamics states that:
  • in the absence of energy input, a system becomes more ______
  • heat will always transfer from ______

______until ______ is reached

B. Temperature Change

30-A1.1k  recall the application of Q = mct to the analysis of heat transfer

  • ______is a measure of the ______

______ of the particles of a substance

  • the ______ the particles are moving, the ______ the temperature
  • the amount of energy needed to heat a substance depends on three factors:
  1. the ______ of the substance
  2. the ______
  3. the ______of substance (heat capacity)
  • heat capacity is heat required tochange the temperature of ______of a substanceby ______

where:Q = heat energy in J

m = mass in g

Δt = change in temperature in C

c = specific heat capacity in J/gC

***found on pg 3 of data booklet

Example

Find the heat required to change 2.50 g of water from 10.0C to 27.0C.

Your Assignment: pg 1 “Heat Calculations” #1-6

C. Energy and Enthalpy

30-A1.3k  define enthalpy and molar enthalpy for chemical reactions

  • ______ is thesum of all forms of kinetic and potential energy in a system (EK + EP)
  • unfortunately, the enthalpy of individual substances cannot be measured directly (EK can with a thermometer but how do you measure EP?)
  • changes in enthalpy occur wheneverheat is ______

______in a ______

______…fortunately, this can be measured

  • ______is measured in ______
  • subscripts are sometimes used to denote the type of process, for example “r” for “reaction”

______= enthalpy of reaction

  • the “” symbol is used to denote changes taking place at ______conditions

______= standard enthalpy of reaction (at SATP – 100 kPa and 25C)

  • ______ is the enthalpy change______of a substance
  • molar enthalpy is also designated as Halthough we will be using just ______for our formulas
  • molar enthalpy is measured in ______

***this can be confusing so pay close attention to the context…wording and units!!!

D. Energy Change in Chemical Reactions

30-A1.10k  classify chemical reactions as endothermic or exothermic, including those for the processes of photosynthesis, cellular respiration and hydrocarbon combustion

30-A1.9k  identify that liquid water and carbon dioxide gas are reactants in photosynthesis and products of cellular respiration and that gaseous water and carbon dioxide gas are the products of hydrocarbon combustion in an open system

  • changes in ______ take place during chemical reactions when ______

______

  • chemical bonds are sources of ______ potential energy
  • ______bonds ______ energy
  • ______bonds ______ energy
  • if the energy added to break the bonds is ______ than the energy released when the new bonds are formed then the reaction is ______

eg)

  • endothermic changes are designated as ______since energy is being ______ to the system
  • if the energy added to break the bonds is ______ than the energy released when the new bonds are formed then the reaction is ______

eg)

  • exothermic changes are designated as ______since energy is being ______by the system
  • the amount of energy lost or gained is ______

______ to the amount of substances that react

ie) if 100 g of a substance burning will release ______energy compared to when 50 g of that same substance burns

E. Communicating Enthalpy Changes

30-A1.4k  write balanced equations for chemical reactions that include energy changes

30-A1.5k  use and interpret H notation to communicate and calculate energy changes in chemical reactions

30-A1.3s  compare energy changes associated with a variety of chemical reactions through the analysis of data and energy diagrams

Exothermic Reactions

1. rH Notation

  • the heat of reaction can be given as a rH value ______of the equation
  • the sign on rH is ______since the enthalpy of the system is ______

eg)

2. Inside the Equation

  • in an exothermic reaction enthalpy is included as a ______

eg)

3. Potential Energy Diagrams

  • shows the potential energy of the ______and the ______of a chemical reaction
  • reactants have ______potential energy than the products in an exothermic reaction
  • the difference between the reactants and products is the ______

Endothermic Reactions

1. rH Notation

  • the sign on rH is ______since the enthalpy of the system is ______

eg)

2. Inside the Equation

  • in an endothermic reaction enthalpy is included as a ______

eg)

3. Potential Energy Diagrams

  • reactants have ______potential energy than the products in an endothermic reaction
  • the difference between the reactants and products is the ______

Your Assignment: pgs 1-2 “Energy Changes”

F. Calculating Enthalpy Changes

30-A1.5k  use and interpret H notation to communicate and calculate energy changes in chemical reactions

  • molar enthalpy, in kJ/mol, and the number of moles of a substance can be used to calculate the enthalpy change of a phase change:

where:H =

n =

rH =

  • the number of moles can either come from the ______ in the chemical reaction or from the formula
  • always have a sign on H or H
  • ______for ______ (energy absorbed)
  • ______for ______ (energy released)

Example 1

Calculate the molar enthalpy of combustion for oxygen given the following information:

2 C2H6(g) + 7 O2(g)  4 CO2(g) + 6 H2O(g)cH = 2856.8 kJ

Example 2

Find the enthalpy change when 5.50 g of pentane burns. cH = 3244.8 kJ/mol

Example 3

When methane is burned, oxygen is consumed. Determine the mass of oxygen consumed if the change in enthalpy is 250 kJ and the molar enthalpy of reaction for oxygen is 401.3 kJ/mol.

Your Assignment: pgs 2-3 “Enthalpy Changes” #1-7

G. Calorimetry

30-A1.8k  use calorimetry data to determine the enthalpy changes in chemical reactions

  • calorimetry is a technological process of ______

______

  • the isolated system used to determine theheat involved in a phase change or in a chemical reaction is called a ______

Steps for Using a Simple Calorimeter

  1. Measure the ______ of the water in the calorimeter.
  2. Add the ______to the calorimeter.
  3. Allow reaction to proceed, ______ the solution to ensure even temperature.
  4. Measure the ______ of the water in the calorimeter (maximum temperature for exothermic reactions, minimum temperature for endothermic reactions)
  • calorimetry is based on the First and Second Law of Thermodynamics…energy is conserved and energy is transferred from hotter objects to cooler objects until thermal equilibrium is reached
  • it is assumed that ______is ______

______by the systemexcept for the energy required or released by the ______

  • calculations are based on the Principle of Heat Transfer:
  • you must use a ______with you H values (either enthalpy change or molar enthalpy)

endothermic = ______ value

exothermic = ______value

Example 1

A chemical reaction in a calorimeter causes the temperature of 500 g of water to increase in temperature from 10.0C to 52.0C. Calculate the heat released by this reaction. Give your answer in kJ.

Example 2

An 8.40 g sample of N2(g) is reacted with pure oxygen in a bomb calorimeter containing 1.00 kg of water to produce N2O. The temperature of the water dropped by 5.82C. What is the molar heat of reaction of N2(g) in kJ/mol?

Example 3

A student built a simple calorimeter with a 25.0 g tin can and 150 mL of water. Calculate the molar enthalpy of combustion of ethanol in kJ/mol if 0.166 g of this fuel increased the temperature of the calorimeter by 7.00C. Remember to include not only the heat gained by the water but also by the calorimeter.

Example 4

A student mixes 100.0 mL of 0.500 mol/L HBr(aq) with 100.0 mL of 0.500 mol/L KOH(aq). The initial temperature of both solutions is 21.00C and the highest temperature reached after mixing is 24.40C. Calculate the molar enthalpy of neutralization in kJ/mol for the HBr(aq). Assume both solutions have the density and heat capacity of pure water.

Your Assignment: pg 3 “Calorimetry” #1-7

H. Industrial Bomb Calorimeters

  • industrial calorimeters are used in ______ to measure the heat ofcombustion of food, fuel, oil, crops, and explosives
  • modern calorimeters have ______ eg) volume of water used, container (bomb) material, stirrer and thermometer
  • in calculating the energy of combustion, you take all components of the calorimeter into account:

Etotal = mct (H2O) + mct (stirrer) + mct (thermometer) + mct (bomb)

  • all of the “mc” parts are constant so they are replaced by ______

______the ______of the ______in ______

  • you can also be asked to calculate ______ instead of kJ/mol in calorimetry questions
  • you use the formula ______ instead on nH to give kJ/g

Example 1

A 1.50 g sample of methane is completely burned in a calorimeter with a heat capacity of 11.3 kJ/C. The temperature increased from 20.15C to 27.45C. Calculate the molar enthalpy of combustion for methane. Convert your answer to kJ/g.

Example 2

When 3.00 g of butter is burned in a bomb calorimeter with a heat capacity of 9.22 kJ/C the temperature changes from 19.62C to 31.89C. Calculate the specific enthalpy of combustion in kJ/g.

Your Assignment: pg 4 “Industrial Bomb Calorimetry” #1-3

I. Hess’ Law: Predicting Enthalpy (rH) Changes

30-A1.7k  explain and use Hess’ Law to calculate energy changes for a net reaction from a series of reactions

  • because of the law of conservation of energy, the heat of reaction is the ______whether the reactants are converted to the products in a ______or in a ______
  • G.H. Hess (1840) suggested that if two or more ______

______are ______to give a final equation then the ______can be added to give the ______

  • sometimes the heat of reaction for a chemical change is not easily measured due to time of reaction, cost, rarity of reactants etc. so we use Hess’s Law to calculate rH

Steps:

  1. Write the ______, if it is not given.
  2. ______the given equations so they will ______to yield the ______.
  3. if you multiply or divide an equation, multiply or divide the H by the ______
  4. if you flip an equation, ______ the sign on H
  5. ______the reactants and products where possibleto ______(you must end up with your net equation!)
  6. ______the component enthalpy changes to get the ______

______

Example 1

Find the enthalpy change and draw the EP diagram for C(s, di)  C(s, gr) using the following reactions:

C(s, gr) + O2(g)  CO2(g) H = 393.5 kJ

C(s, di) + O2(g)  CO2(g)H = 395.4 kJ

Example 2

Find the enthalpy change for H2O2(l)  H2O(l) + ½ O2(g) using the following reactions:

H2(g) + O2(g)  H2O2(l) H = 187.8 kJ

H2(g) + ½ O2(g)  H2O(l) H = 285.8 kJ

Example 3

Find the heat of reaction for C(s) + 2 H2(g)  CH4(g) using the following reactions:

C(s) + O2(g)  CO2(g) H = 393.5 kJ

H2(g) + ½ O2(g)  H2O(l) H = 285.8 kJ

CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(l) H = 890.5 kJ

Your Assignment: pgs 4-6 “Hess’s Law of Additivity”

J. Standard Heats of Formation fH

30-A1.6k  predict the enthalpy change for chemical equations using standard enthalpies of formation

30-A2.3s  draw and interpret enthalpy diagrams for chemical reactions

  • sometimes it is not easy to measure the heat change for a reaction (too slow/expensive)
  • in this case, H can be determined using ______

______

  • heats of formation (fH) are the changes in EP that occur when ______
  • Hf for elements cannot be directly measured therefore they are designated as ______…all other fH values are in reference to this…see pages 4-5 in data booklet
  • the fH is an indirect measure of the ______of a compound
  • the more ______, the more ______(this means you have to add that energy to decompose it)

eg) List the following compounds in order from most stable to least stable.

H2O(l)fH =

C2H4(g)fH =

N2O4(g)fH =

PCl3(l)fH =

Al2O3(s)fH =

  • Hess’s Law formula states that the ______is the difference between the standard heats of formation of the ______and the ______

rH =

Example 1:

Calculate the standard heat of combustion for 2 CO(g) + O2(g)  2 CO2(g) and draw the EP diagram for this reaction.

Example 2:

Find the heat of combustion of ethane and draw the EP diagram for this reaction. The products of combustion are gases.

Your Assignment: pg 7 “Heats of Formation” #1

Example 3

Calculate the molar enthalpy of combustion for ethane. The products of combustion are gases.

Your Assignment: pg 7 #2-4

Example 4

Calculate the energy released when 25.0 g of methanol is burned. The products of combustion are gases.

Your Assignment: pg 7 #5-7

Example 5

Calculate the molar heat of formation for ethylene glycol given the following information:

(CH2OH)2() + 5/2 O2(g)  2 CO2(g) + 3 H2O() H = 1178.0 kJ

Your Assignment: pg 7 #8-10; pg 8

K. Energy and Efficiency

30-A1.1sts  explain that the goal of technology is to provide solutions to practical problems

30-A1.2sts  explain that technological problems often require multiple solutions that involve different designs, materials and processes and that have both intended and unintended consequences

  • most of Canada’s energy (electricity) comes from ______processes such as the combustion of ______
  • electricity is also generated through ______processes
  • both methods involve changing ______(a ______change) which turns turbines to generate electrical energy
  • how does the energy from physical, chemical and nuclear processes compare:
  • ______changes involve the breaking and forming of ______forces (______kJ/mol)
  • ______changes involve the breaking and forming of ______(______kJ/mol)
  • ______changes involve changes within the ______(______of kJ/mol)

______is the ratio of ______produced (energy ______) to ______in its production (energy ______)

  • we have developed many technologies that help us to solve practical problems
  • it is important to be as ______as possible with appliances and vehicles
  • saving energy ______and it helps to ______ (greenhouse effect and acid rain)
  • in any process, the ______that take place, the ______the process because of ______ in transfer
  • gas furnace (natural gas) is about ______ efficient since it is used to directly supply heat
  • natural gas power plant is only about ______ efficient because there are several energy conversions that take place before electricity is generated (water to steam to kinetic energy to mechanical energy to electrical energy)

Cheetos Combustion Lab

L. Fuelling Society

30-A1.2sts  explain that technological problems often require multiple solutions that involve different designs, materials and processes and that have both intended and unintended consequences

30-A2.2sts  explain that the appropriateness, risks and benefits of technologies need to be assessed for each potential application from a variety of perspectives, including sustainability

  • we must assess the ______ of relying on any fuel source
  • when selecting an energy source, efficiency is not the only consideration
  • the ______ must also be considered

Advantages vs. Disadvantages of Fossil Fuels

Advantages / Disadvantages

30-A2.3sts  explain that the products of technology are devices, systems and processes that meet given needs; however, these products cannot solve all problems

______processes ______produce ______

______but they do produce ______

______which lasts for thousands of years

  • ______turbines use a ______ energy source (sun indirectly) but are not free of problems…they are ______

______

  • ______power is also ______but damming rivers ______on both sides of the dams
  • the amount of ______ released by a fuel determines how ______ it is
  • fuels that use renewable energy sources (______

______etc.) and nuclear power are considered the ______

  • ______is the ______fossil fuel and ______is the ______
  • regardless of which source of energy we use, we must think about the impact that our fuels have on the environment

M. Bond Energy and Activation Energy

30-A2.1k  define activation energy as the energy barrier that must be overcome for a chemical reaction to occur

30-A2.2k – explain the energy changes that occur during chemical reactions, referring to bonds breaking and forming and changes in potential and kinetic energy

  • bond energy is the energy ______

______or the energy ______

______

  • the ______ of a reaction represents the ______from ______ the bonds in the reactant(s) and ______ the bonds of the product(s)
  • in ______ reactions, bond breaking absorbs ______ energy than the bond formation gives off, resulting in a ______

Exothermic: H2(g) + Cl2(g)  2 HCl(g) + energy

  • in ______ reactions bond breaking absorbs ______ energy than the bond formation gives off, resulting in a ______

Endothermic: 2 H2O() + energy  2 H2(g) + O2(g)

  • the energy barrier that must be overcome for a chemical reaction to occur is called the ______
  • the atoms in the ______have to be ______ in order for them to bond in a different configuration and become the products
  • the activation energy is always ______ than the energy contained in the reactants and the products, however the amount of activation energy necessary is dependent on the reaction

30-A2.3k  analyze and label energy diagrams of a chemical reaction, including reactants, products, enthalpy change and activation energy

  • the ______ of the activation energy barrier on a potential energy diagram represents the ______ of the reaction
  • in both endothermic and exothermic reactions, the molecules of the reactants are moving with a certain amount of ______

______

  • when the reactants ______ with each other, the kinetic energy is transformed into ______
  • this potential energy is then stored in bonds of the chemical species that exists at the top which is called the ______
  • this is a transitional species that is neither a reactant nor a product which has ______and is ______
  • when the partial bonds of the activated complex re-form as chemical bonds in the products, the stored potential energy is converted back into ______as the product molecules ______

N. Catalysts

30-A2.4k  explain that catalysts increase reaction rates by providing alternate pathways for changes, without affecting the net amount of energy involved

  • a catalyst is a substance that ______of a chemical reaction ______ by the reaction
  • catalysts provide ______ for chemical reactions
  • they ______the ______ required for a reaction to take place which results in the production of a ______ of products in a given length of time (even at a lower temperature)
  • catalyzed reactions can be shown on EP diagrams:

O. Catalysts in Industry

30-A2.1sts  explain that the goal of technology is to provide solutions to practical problems

 explain how catalysts, such as catalytic converters on automobiles, reduce air pollution resulting from the burning of fuels

  • catalysts are often used in industry to speed up the reactions and obtain a reasonable reaction rate under reasonable conditions
  • in cars, we have ______ that use Pt(s), Pd(s) and Rh(s) to speed up the combustion of exhaust gases so that more of the products are ______(______instead of ______, ______instead of ______)
  • the oil and gas industry uses catalysts (Pt(s), HF(aq), H2SO4(aq) etc) in the ______of ______and ______ to make more marketable fuels like gasoline

P. Enzymes

  • compounds that act as catalysts in living systems are called ______
  • called ______ catalysts
  • chemical reactions in the body occur at very ______ temperature (37C) and without catalysts many would be too ______

Chem 30 Thermochemistry1Jill Agnew