Chapter 2 Energy From Fuels Flashcards
What do all substances have?
Chemical energy
What is chemical energy comprised of?
Potential energy + kinetic energy in the substance
What does chemical energy result from? (MARRV)
Movement of electrons Attractions between electrons and protons Repulsion between nuclei Repulsion between electrons Vibrations and rotations around bonds
Define chemical energy
Chemical energy is the sum of chemical potential energy and kinetic energy in a substance. Chemical energy is stored within the bonds between atoms and molecules, The energy results from things such as MARRV
Where is chemical energy stored?
Bonds between atoms and molecules
What is chemical energy sometimes referred to?
Enthalpy or heat content
How is enthalpy denoted
delta H
Define enthalpy
Also known as heat content. It is the sum of the chemical potential energy and kinetic energy in a substance.
Define enthalpy change
The difference in the total enthalpy of the products and the total enthalpy of the reactants denoted delta H. AKA heat of reaction.
Define enthalpy of combustion
The enthalpy change that occurs when mole of a compound is burned completely in oxygen, given the symbol delta Hc
What happens in chemical reaction most basically
Energy can be either absorbed from the surroundings or released to the surroundings
State the law of conservation of energy
Energy cannot be created or destroyed, however, it can change forms (Energy transformations - see fuels - chap 1 )
State the rule governing the law of conservation of energy.
TCE reactants = TCE products
Define the system
Chemical reaction
Define the surroundings
Everything excluding the system. Everything excluding the reaction itself.
Define an exothermic reaction
A reaction where energy is released to its surrounds; delta H is negative.
Define an endothermic reaction
A reaction where energy is absorbed from its surrounds; delta H is positive.
State the rule governing an exothermic reaction
TCE products < TCE reactants
Give the rule governing an endothermic reaction
TCE products > TCE reactants
List 4 examples of endothermic reactions
- Melting
- Evaporation
- Sublimation
- Decomposition
List 2 examples of exothermic reactions
- Condensation
2. Freezing
How are endothermic and exothermic reactions represented
Using energy profile diagrams
In an energy profile diagram, in terms of arrows, what way does the arrow point in an endothermic reaction?
Up as energy is absorbed (more is gained)
In an energy profile diagram, in terms of arrows, what way does the arrow point in an exothermic reaction?
Down as energy is released (energy is lost)
On an energy profile diagram for an endothermic reaction or exothermic reaction, where can the:
i) activation energy
ii) Delta H be found
i) Between reactants and highest peak
ii) Between reactants and products
Define the activation energy
The minimum amount of energy required to break the bonds of all reactants to start a reaction.
What to do in reverse reactions?
Swap everything (products = reactants, reactants = products, and exo=endo, endo=exo) Switch delta H sign too
Define a thermochemical equation
A chemical equation that includes enthalpy change of the reaction
What is the delta H in a thermochemical equation based/dependent on?
Stoichiometric rations in equation
What is the unit for delta H
Kj/mol
When is enthalpy measured
SLC
- 25 degrees
- 100Kpa
- 298K
What does a thermochemical equation show?
energy absorbed or energy released in a chemical reaction
Define a combustion reaction
Combustion reactions are rapid reactions with oxygen accompanied by the release of large amounts of energy.
Are all combustion reactions exothermic?
Yes, they are! They all release heat/thermal energy.
Is combustion a redox reaction?
Yes!
What is a redox reaction?
Involves the simultaneous transfer of electrons from one species to another and thus oxidation numbers change)
When does complete combustion occur?
When O2(g) is in excess
What are the products of complete combustion?
CO2 and H20
When does incomplete combustion occur?
When O2(g) is limited
What are the products of incomplete combustion?
CO and/or C + H20
Define the heat of combustion
The enthalpy change that occurs when a specified amount if the fuel burns completely in oxygen.
How is the heat of combustion measured?
Using a calorimeter
When is the heat of combustion measured?
Under SLC
Which substances have the units kj/mol and Kj/g respectively for the heat of combustion?
Pure and then impure substances
How is energy found?
E=n x delta Hc
How do you find the energy content by gram
delta Hc per mole / Molar mass
How do you find the energy content by tonne
1 Kj/g = 10^3 MJ/tonne
