Calculate specific heat of an unknown substance if 1638 J raises the temperature of 125 g from 25°C to 52.6°C.
\Delta T = 52.6 °C - 25.0 °C = 27.6 °C
Rearrange equation: c = \frac{Q}{m \times \Delta T}
Q = 1638 J
m = 125 g
\Delta T = 27.6 °C
c = \frac{1638 J}{(125 g) \times (27.6 °C)}
c = 0.475 J/g°C
The substance could be a metal due to its low specific heat.
Energy in Chemical Reactions
Chemical reactions either absorb or release energy.
Redox reactions can produce electrical energy (batteries).
Combustion releases heat and light.
Some chemical changes consume energy (e.g., electrolysis of water).
Plants use solar energy to convert CO2 into sugars (photosynthesis).
Laws of Energy
Law of conservation of energy: Energy cannot be created nor destroyed.
Energy can be lost through inefficient processes (dissipated as heat).
Decomposition of water absorbs energy; H2 and O2 have higher potential energy.
Burning hydrogen releases energy; water has lower potential energy.
Potential energy in molecules is held in chemical bonds; breaking bonds releases energy, forming bonds stores energy.
Energy Sources
Petroleum products, coal, and water sources.
All energy originally came from the sun.
Petroleum is a fossil fuel from plant materials broken down over time.
Hydrocarbons: Compounds containing carbon and hydrogen (e.g., gasoline, natural gas).
Common Hydrocarbons
Methane (CH_4)
Propane (C3H8) - used in fireplaces, water heaters, and stoves
Butane (C4H{10}) - used in lighters
Pentane (C5H{12}), Hexane (C6H{14}), Heptane (C7H{16}), Octane (C8H{18}) - found in gasoline and used to rate burning qualities in gasoline combustion engines
Kerosene - with more numbers of carbons and hydrogens
Natural Gas and Coal
Natural gas: mainly methane with small amounts of higher carbon fuels mixed in.
Coal: formed from plants stored under high pressure for many years.
Higher carbon content = more energy.
Reaction of carbon in coal with oxygen produces CO2 and energy.