isat test 2 ch. 6 (new): energy from combustion

work

movement against force

• force x distance

are all renewable energy sources sustainable?

NO

what were energy sources pre 1800s?

• wood (heating, cooking)

• whale oil (lighting)

• manpower & horsepower for all other work

steam engine

powered beginning of industrial revolution

• factories

• transportation

• powered by coal

coal

widely used fossil fuel

• based on photosynthesis, from a long time ago (400 million years)

• original source of energy is the sun

• not renewable/sustainable (limited supply)

electricity

thomas edison connected steam engine to world’s first electric generator

• powered factories

• transportation

• changed society

• originally powered by coal, then oil, then gas

• greatly increased energy demand

pros & cons about energy

• prosperity: coinciding with industrial revolution, wealth of nations & individuals has grown dramatically

• at a cost: cost to climate & environment

• no free lunch

what is the biggest thing to consider with energy use?

• THERE IS NO FREE LUNCH

  • has to come from somewhere to convert it to useful forms

energy

capacity to do work

potential energy

energy due to position/composition

kinetic energy

eergy due to movement

heat

thermal energy, characterized by flow from hotter to colder object

• consequence of motion at molecular level

• heat of steam makes molecules move faster, & kinetic energy moves piston

temperature

determines direction of heat flow

• measure of average kinetic energy of molecules

joule (J)

SI unit of measurement for energy

calorie

1 cal is amount of heat required to raise temperature of 1g of water by 1°C

• 1 cal = 4.184 J

what is the most efficient in energy compared to calories?

gas

why fossil fuels?

really good storehouses of potential energy

• fossil fuels are hydrocarbons

• energy is stored in carbon-carbon & carbon-hydrogen bonds

first law of thermodynamics

energy is neither created nor destroyed, may be transformed from one form to another

second law of thermodynamics

entropy (degree of disorder) of the universe is increasing

• whenever energy is transferred from one form to another, not all energy is converted to useful work

• not lost (1st law), but converted to heat

power plant efficiency

• entropy (randomness) of universe is increasing

• not all thermal energy can be transformed into ordered work → theoretical efficiency

how does energy flow?

flows spontaneously from hot to cold (& spreads out in the process)

three requirements to generate a fire (combustion)

• initial source of heat

• fuel

• oxidizer (usual oxygen)

what do most fuels consist of?

hydrocarbonds

hydrocarbons

compounds made of only hydrofen & carbon atoms

combustion

combination of fuel with oxygen to form products where potential energy of the reactants is greater than that of the products

exothermic reaction

change of energy is a negative value

• heat is released from the reaction

• energy products < energy reactants

activation energy

energy required to initiate the reaction

what are the fuels with the highest heats fo combustion called?

hydrocarbons

• as ratio of hydrogen-to-carbon decreases, heat of combustion decreases

• as amount of oxygen in fuel increases, heat of combustion decreases

endothermic reaction

change in energy is a positive value

• energy products > energy reactants

• heat is absorbed in reaction

why does energy change?

due to breaking & forming bonds

• REQUIRES ENERGY

bond energy

energy required to break a specific chemical bond(/energy released when bond is formed)

what do you need to determine when it comes to energy changes?

amount of energy used to BREAK bonds in REACTANTS v. amount of energy used to CREATE bonds in PRODUCTS

steps on calculating heats of combustion

1. determine balanced chemical equation

2. write lewis structures for all reactants & products

3. count bonds in reactants & products

4. total bond energies for reactants

5. total bond energies for products

6. heat of reaction = reactants’ energy - products’ energy