exothermic and endothermic reactions linking energy profile diagrams to thermometer readings
TRANSCRIPT
Exothermic and Endothermic Reactions
Linking Energy Profile Diagrams to Thermometer Readings
Candidates should be able to:
(a) describe the meaning of enthalpy change in terms of exothermic (ΔH negative) andendothermic (ΔH positive) reactions
(b) *represent energy changes by energy profile diagrams, including reaction enthalpychanges and activation energies (see 6.1(c))
(c) describe bond breaking as an endothermic process and bond making as anexothermic process
(d) *explain overall enthalpy changes in terms of the energy changes associated withthe breaking and making of covalent bonds
(e) describe combustion of fuels as exothermic, e.g. wood; coal; oil; natural gas;hydrogen
(f) describe hydrogen, derived from water or hydrocarbons, as a potential fuel for usein future, reacting with oxygen to generate electricity directly in a fuel cell (details of the constructionand operation of a fuel cell are not required)
(g) name natural gas, mainly methane, and petroleum as sources of energy
THE CONCEPT OF ENTHALPY CHANGE
We can measure the change in energy,
we call this change the enthalpy change
(or heat of reaction).
In chemical reactions, we can measure changes in terms of energy.
We cannot measure the total amount of energy in the beginning, we also cannot measure the total amount of energy in the end.
Exothermic Reaction
25oC
25oCInitial temperature
Reactants +
Products
Exothermic Reaction
25oC
Exothermic Reaction
temperature
time
25C
32C
Reactants give out energy to the surroundings
Exothermic ReactionReactants give out energy to the surroundings
Temperature of surroundings increaseEnergy of reactants decrease
Energyof reactants
ReactantsΔH is negative
Temperature(surroundings)
25C
32C
Products
Energy LevelsEnergy Levels
Exothermic reactions
• Energy is given out
• The products have less energy than the reactants
Combustion and neutralisation are exothermic
Energy Level DiagramsEnergy Level Diagrams
Exothermic reactions
energy
course of reaction
Energy Level DiagramsEnergy Level Diagrams
Exothermic reactions
energy
course of reaction
reactants
Energy Level DiagramsEnergy Level Diagrams
Exothermic reactions
energy
course of reaction
reactants
products
Energy Level DiagramsEnergy Level Diagrams
Exothermic reactions
energy
course of reaction
reactants
products
energy given out
∆H is negative
Endothermic Reaction
25oC
25oCInitial temperature
Reactants +
Products
Endothermic Reaction
25oC
Endothermic Reaction
temperature
time
19C
25C
Reactants take in energy from the surroundings
Endothermic ReactionReactants take in energy from the surroundings
Temperature of surroundings decreaseEnergy of reactants increases
Energyof reactants
Reactants
ΔH is positive
Temperature(surroundings)
19C
25C
Products
Energy Level DiagramsEnergy Level Diagrams
Endothermic reactions
• Energy is taken in
• The products have more energy than the reactants
The energy is taken in from the surroundings
Energy Level DiagramsEnergy Level Diagrams
Endothermic reactions
energy
Energy Level DiagramsEnergy Level Diagrams
Endothermic reactions
energy
course of reaction
Energy Level DiagramsEnergy Level Diagrams
Endothermic reactions
energy
course of reaction
reactants
Energy Level DiagramsEnergy Level Diagrams
Endothermic reactions
energy
course of reaction
reactants
products
Energy Level DiagramsEnergy Level Diagrams
Endothermic reactions
energy
course of reaction
energy taken in∆H is positivereactants
products
Summary TableSummary Table
Exothermic reactions
Endothermic reactions
Energy is given out to the surroundings
Energy is taken in from the surroundings
∆H is negative ∆H is positive
Products have less energy than reactants
Products have more energy than reactants
EXOTHERMIC & ENDOTHERMIC PROCESSES
1. Solid Liquid Gas
2. Dissolving of ammonium chloride or ammonium sulphate crystals in water
3. Thermal decomposition
CaCO3 CaO + CO2
Endothermic processes
1. Gas Liquid Solid
2. Combustion of fuels (e.g. wood; coal; natural gas)
3. Neutralisation reactions ( acid + base)
Exothermic processes
Examples:
∆∆HH
How much energy is given out or taken in?
• Energy is needed to break chemical bonds
• Energy is given out when bonds are made
∆H is the difference between the energy needed to break the bonds in the reactants, and the energy given out when new bonds are made in the products
Bond energiesBond energies
The energy needed to break a chemical bond• Different chemical bonds have different
bond energies
Chemical bond Bond energy, kJ/mole
H―H 436
O=O 498
O―H 464
Bond energiesBond energies2H2 + O2 2H2O Chemical
bondBond energy, kJ/mole
H―H 436
O=O 498
O―H 464
H H O O
H H
Stage 1: Bond breaking
+436kJ
+436kJ
+498 kJ
H H O O
H H
Stage 2: Bond forming
H H O O
-464kJ -464kJ
-464kJ-464kJ
Working out ∆HWorking out ∆H
Summary
• The energy values have units of kJ/mole
• In the exam, you will be given the energy values and all the bonds to make or break• Energy goes in to break bonds
• Energy goes out when bonds are made