Reactivity 1 Flashcards
What happens to the energy in an exothermic reaction?
Excess energy is given out to the surroundings as heat
What happens to the energy in an endothermic reaction?
Excess energy required is taken in from the surroundings
Do exothermic reactions have a positive or negative ∆H value?
Negative
Do endothermic reactions have a positive or negative ∆H value?
Positive
What is ΔH⦵ measured in?
kJ mol^-1
What is ΔH⦵?
Standard enthalpy change
Is energy required to form a chemical bond?
No
Is energy required to break a chemical bond?
Yes
What is the formula to calculate enthalpy changes?
ΔH⦵ = ΔH⦵(bonds broken) - ΔH⦵(bonds formed)
If you are doing a simultaneous equation and you rearrange the formula, what happens to ΔH?
Reverse the sign
Arrange from strongest to weakest: single covalent bond, double covalent bond, triple covalent bond
Triple covalent, double covalent, single covalent
What is the idea with the Hess’ Law problems for the indirect determination of an enthalpy change? (triangles)
A going directly to C or through B
How do you find the bond enthalpy in the indirect determination of an enthalpy change?
Figure out the number of bonds broken, and divide previous answer by said number
Specific energy of a fuel formula
energy released from the fuel/mass of fuel consumed
Units for specific energy of a fuel
kJ kg^-1
If you know the enthalpy of combustion, how do you find the specific energy?
Dividing by the molar mass
Three main fossil fuels
Coal, crude oil, natural gas
How is oil and natural gas produced?
Anaerobic decomposition of marine organisms
How is coal produced?
Terrestrial vegetation
Advantages of natural gas
Clean, easily transported, higher specific energy than coal or oil
Disadvantages of natural gas
Contributes to global warming, limited lifespan, uneven distribution
Advantages of oil
Easily transported, feedstock for organic chemicals, convenient fuel for vehicles
Disadvantages of oil
Contributes to global warming, limited lifespan, risk of marine pollution (oil spills)
Advantages of coal
Present in large quantities, feedstock for organic chemicals, releases more energy than renewable sources, longer lifespan
Disadvantages of coal
Contributes to global warming, not as easily transported, mining coal is dangerous, dirty
What is the greenhouse effect?
Greenhouse gases absorbing some of the infrared heat from the earth and re-radiating it back into the earth.
What is a carbon footprint?
Total amount of greenhouse gases produced during human activities
What energy is converted in photosynthesis?
Light energy to chemical energy
What is the photosynthesis equation?
6CO2 + 6H20 → C6H12O6
What is biomass?
Organic material that can be used as an energy source
How is liquid fuel obtained from biomass?
Fermentation
What is the equation for obtaining liquid fuel from biomass?
C6H12O6 → 2C2H5OH + 2CO2
What is gasohol composed of?
10% ethanol and 90% gasoline
What is a common use for gasohol?
Used in unmodified cars, making them less reliant on crude oil.
Advantages of biomass
Renewable and always available, doesn’t significantly contribute to the greenhouse effect, safer to produce, reduces landfill amount
Disadvantages of biomass
Uses up land that could be used for animals, increases the use of fertilisers which could pollute water, removes nutrients from the soil, methods of conversion are small scale.
In what form is the energy given out during the combustion of hydrogen in oxygen to form water?
Electricity (not heat)
Do hydrogen fuel cells need an external source of electricity?
No
What is the product of hydrogen fuel cells?
Water
What are the electrolytes used in alkaline fuel cells?
NaOH or KOH
What is used as the electrolyte in an acidic fuel cell?
An acid
What is the overall reaction equation in alkaline and acidic fuel cells?
2H2 + O2 → 2H2O
What are the products of a methanol fuel cell?
Carbon dioxide and water
If a reaction is reversed, what happens to the ΔH value?
It switches (positive/negative, negative/positive)/changes the sign
