Energy / Chemical energy
"Releasing the chemical energy" in plants!
What is energy?
Something has energy ...
- ...if it has the potential to physically change itself or its environment.
One (dramatic) kind of _change to the environment_ is damage or destruction!
Energy is not an atom or an object. But an object can possess energy.
Heat is energy that can raise the temperature of matter.
We might include a tank of gasoline, a battery, water backed up behind a dam, a stretched out piece of elastic material, or a flying brick.
Decomposition chemical compounds and chemical energy
electrolysis of water
- Hook a battery to the circuit pictured, and electricity flows.
- At each of the platinum electrodes bubbles form.
- But at one of them, exactly twice as much gas (by volume) accumulates as at the other one.
We describe this situation with a chemical reaction equation: $$2H_2O_{\text{(l)}} \rightarrow 2H_\text{2 (g)} + O_\text{2 (g)}$$
The hydrogen ($H_2$) and oxygen ($O_2$) gas that appear cannot be decomposed further into other substances, so we say that hydrogen and oxygen are "elements".
It takes energy (from the battery) to make this reaction happen. The amount of energy depends on how many molecules you have. Sometimes this energy is put into the reaction equation explicitly like this: $$2H_2O_{\text{(l)}} +\Delta H \rightarrow 2H_\text{2 (g)} + O_\text{2 (g)}$$ where $\Delta H$=33.9 kcal / (gram of hydrogen). To put this number in context, see this table of "Heats of combustion" of some common hydrocarbons.
But the reverse reaction: $$ 2H_\text{2 (g)} + O_\text{2 (g)} \rightarrow 2H_2O_{\text{(l)}}+\Delta H $$ releases energy.
The energy released, which is exactly 33.9 kcal / (gram of hydrogen), comes out (dramatically) as heat:
This was
the tragic burning of the hydrogen-filled Hindenburg zeppelin in 1937
That reverse reaction doesn't automatically start. You need a spark to get it started. But once started, it will proceed spontaneously--no energy needs to be supplied to keep it going. The reaction will proceed until either the hydrogen or the oxygen is exhausted.
"Combustion" is the chemical term for what we commonly call "burning". It involves re-combining the atoms in molecules into new compounds that contain oxygen.
Some chemical bonds are "tighter" than others. When atoms are re-arranged in a chemical reaction, the new chemical bonds may take more or less energy to form than the old ones. $\Delta H$ is the difference in energy between the substances on the left and the right of a chemical equation and is also called "chemical energy".
How much gasoline...?
An average woman needs about 2000 kilocalories each day (=2000 "C"alories) to keep her weight constant.
When gasoline is burned, it releases approximately $\Delta H =$ 127 MJ (megajoules) / 1 US gallon. The main constituent of gasoline is 'octane'. $$2\,C_8H_18+ 25\,O_2 \rightarrow 16\,CO_2+ 18\,H_2O.$$
- Google to find the number of kilocalories in 127 MJ. Write this below as _____ kilocalories = 1 gallon of gas .
These Google queries will tell you the number of kcal in 127 MJ. The answer is 30,353 Kilocalories:
- 127 MJ in kcal
- 127 megajoules to kilocalories
In class I also described using a conversion factor. The idea is to think about the number you *have* in units of Megajoules and the number you *want* in units of kilocalories, and use algebra to imagine "cancelling out" the MJ and being left with kcal like this: $$127 \text{ MJ}*\frac{\text{[?] kcal}}{\text{[?] MJ}}=\text{___ kcal}.$$ The fraction in the expression above is a "conversion factor". For example you might look up how many kcal there are per MJ: kcal per MJ. Google will tell you that 1 Megajoule=239 kcal. So, we could fill in that fraction as: $$127 \text{ MJ}*\frac{\text{239 kcal}}{\text{1 MJ}}=(127*239)/1=\text{30,353 kcal}.$$ Or, you could look up MJ per kcal and find that 1 kcal = 0.004184 MJ. Then you would fill in the fraction a different way... $$127 \text{ MJ}*\frac{\text{1 kcal}}{\text{0.004184 MJ}}=127*1/0.004184=\text{30,354 kcal}.$$ Notice that using either of the conversion factors gives you the same answer!
You were asked to write down the number of kcal in 1 gallon of gasoline in units of kcal / 1 gallon (of gasoline). We could think of this also as a conversion problem: $$\frac{\text{127 MJ}}{\text{1 gal gasoline}} * \frac{\text{239kcal}}{\text{1 MJ}}=\frac{\text{30,353 kcal}}{\text{1 gal gasoline}}.$$
- How many gallons of gasoline would release 2000 kilocalories when burned? That is, start with 2000 kCal, and multiply by conversion factor(s) to find gallons of gasoline
This is also a "ratio" question. We can say $x$ represents the number of gallons of gasoline that we're looking for and say these two fractions have to be equal: $$\frac{\text{30,353 kcal}}{\text{1 gal gasoline}}=\frac{\text{2,000 kcal}}{x\text{ gal gasoline}}$$ Using algebra, solve for $x$: $$\begineq x \text{ gal gasoline} &=\text{2,000 kcal}\frac{\text{1 gal gasoline}}{\text{30,353 kcal}}\\ &=\frac{2,000*1}{30,353}\approx \frac{2}{30}=\frac{1}{15}=\text{0.067 gal gasoline}. \endeq$$
- What is the approximate cost of this much gasoline? Start with the amount of gasoline and convert it to dollars.
"Convert" 0.067 gallons of gas to the equivalent number in \$, using the price of gas (about \$2.15 per 1 gal of gasoline) as the conversion factor: $$\text{0.067 gal gas}\frac{\$3.50}{\text{1 gal gas}}=0.067*3.50/1 = \$0.23.$$ So, it would cost about a quarter to buy an amount of gasoline that contains 2000 kcal of energy (the same amount of food calories needed by a typical person in one day).
- Approximately how far could a typical car (~30 mpg) travel on this much gasoline? Start with your number of gallons of gas, and convert to miles.
"Convert" 0.067 gallons into "miles" using a typical gas mileage of 30 MPG, which is a "conversion factor" of 30 miles / 1 gallon of gasoline: $$\text{0.067 gal gas}\frac{\text{30 miles}}{\text{1 gal gas}}=0.067*30=\text{2 miles}.$$
Every 2 miles you drive, you're burning $\approx$2000 kilocalories: the same amount of energy it takes to power a human being for 24 hours!
