We briefly touched on reactions in the previous chapter, and again defined them on the opening page of this chapter. We also introduced the concepts of products, reactants, and state changes. If you are not familiar with these ideas, revisit the What is a Reaction? page in the "Background Information" chapter before continuing.
Based on the examples in the previous pages, it should be obvious that reactions surround us. From electric charges in our neurons to the thermonuclear fusion occurring in the sun, we are dependant on reactions for our very existence. Therefore, in order to explain the behavior of the world around us, it is essential to understand the chemical reaction.
In this chapter, we will assume (for the most part) that reactions occur at STP--standard temperature and pressure, 0° Celsius (273 kelvins) and 1 atmosphere. Any departures from STP will be noted.
One additional concept should be mentioned before we delve into this chapter: heat changes. We know that burning wood releases heat, while boiling water requires heat. Most reactions either consume or release heat energy, so knowing the terminology to describe them is important. Reactions that transfer heat to the surrounding environment to the reaction system are called exothermic, while reactions that transfer heat from the surroundings to the reaction are endothermic. If one reverses a reaction (for instance, condensing water instead of boiling it), its heat change also reverses.
Example Problem 1
Identify whether each reaction or process is exothermic or endothermic.
A. The combustion of hydrogen
B. Activating a chemical cold pack
C. Passing electric current through a filament in a light bulb
D. Melting ice into water
E. Freezing water into water
A. Burning hydrogen is very exothermic (just look at the Hindenburg disaster!)
B. A cold pack absorbs heat from the surrounding area, meaning it is endothermic
C. Light bulbs are exothermic, as you know if you've ever touched one that's on
D. Melting ice requires an input of heat, so it is an endothermic reaction
E. It may not seem like it, but freezing is an exothermic process; it gives off heat energy. Since melting ice requires heat, it follows that freezing water would give off heat
Heat changes in reactions are measured in joules or kilojoules using the symbol ΔH, meaning "change in heat," since the Greek capital delta means "change." You may also see this written out as "Delta-H." A negative ΔH value indicates an exothermic reaction (because heat is being released out of the reaction), while ΔH is positive in endothermic reactions (since heat is flowing into the reaction). This is only a brief introduction to heat changes; the "Thermodynamics" chapter will detail the energy changes that take place in chemical reactions in much greater depth.
Finally, note that the units of reactants and products in a reaction are symbolic. For instance, in the reaction that follows, the coefficients simply mean that ethane and oxygen will react in a 2:7 ratio, producing carbon dioxide and water in a 4:6 ratio.
2 C2H6 (g) + 7 O2 (g) --> 4 CO2 (g) + 6 H2O (g)
For instance, if you have .287 moles of ethane, you need 1.0045 moles of oxygen, and you will get 0.574 moles of carbon dioxide and 0.861 moles of water. If you had twice as many reactants, you would get twice as many products. Half the number of moles of reactants would yield half as many products.
Example Problem 2
Using the reaction above, predict how many moles of oxygen will be necessary to react with 1.288 moles of ethane. How many moles of carbon dioxide will be produced?
Since ethane and oxygen react in a 2:7 ratio, 4.508 moles of oxygen are necessary. In addition, 4 moles of water are produced for every 2 moles of ethane consumed; therefore, 2.576 moles of water will be produced.