Chemical reactions. There are five general types of ordinary chemical reactions, meaning we don't count nuclear reactions in these. Also, these are all chemical changes, not physical changes like ripping up paper or dissolving salt and water that we talked about first semester. Finally, not all reactions fit nicely into one category, some will fit into two categories. The first type of reaction is a combination or a synthesis reaction. In a combination or synthesis reaction, two or more substances combine to form a single substance. Synthesis is another name for this type of reaction because synthesis is a synonym, or means the same as, combine. Notice in each synthesis reaction there is only one compound to the right of the arrow. In each case, two or more reactants combine together to make one product. Picture this like elements getting married. There is now a chemical bond between them. The second type of chemical reaction is called a decomposition. In a decomposition reaction, a single compound is broken down into two or more products. Think of the word decompose, which means to break down. In every decomposition reaction, there is only one compound to the left of the arrow, and it breaks apart into two or more things. It's the exact opposite of a synthesis reaction. So staying with our relationship analogy, it's like a divorce where there is no more chemical bonds between those atoms. In a single replacement reaction, one element replaces another in a compound. In terms of Hollywood relationships, Brad Pitt and Jennifer Aniston were married. Then, Brad met Angelina during the movie Mr. and Mrs. Smith and left Jen to be with Angelina. Brad and Angelina later got divorced as well, but that's a whole bunch of Hollywood drama I am not qualified to teach, but you get the general idea. In chemistry terms, sodium and bromine had a chemical bond, but then chlorine comes in and forms a bond with sodium and kicks out the bromine. Here's another example. Iron and nitrogen were in a compound, and then aluminum came in and formed a bond with nitrogen and kicked out the iron. Now we have magnesium kicking out hydrogen to form a bond with chlorine. Notice in all three examples that you have an element and a compound. Remember, elements have one capital letter and compounds have two or more capital letters. That's how we know it's a single replacement versus a double replacement, which we'll get to in a second. The fourth type of chemical reaction is called a double replacement, where two compounds break apart and the cation of one compound goes with the anion of the other ionic compound. In this example, magnesium and sodium switch places, so magnesium goes with the fluorine while sodium goes with the chlorine. The same idea is happening here. The two cations switch places. So zinc started out with sulfur, but bonds with chlorine while hydrogen, who had been with chlorine, goes with sulfur. Last example, iron, which had been with the nitrate anion, goes with oxygen while magnesium goes with the nitrate. In double replacements, we have two compound switching places, two sets of two or more capital letters. Not just an element and a compound like a single replacement. We can use my character from my version of twilight. In my younger days, I had been with Edward while Bella was with Jake. But then I met Jake and realized that I actually didn't like Edward. Bella and Edward got together and are looking quite salty now that Jake and I are happy. The last type of reaction is called a combustion reaction. It's a specific type of single replacement where you have a hydrocarbon, meaning some compound with hydrogen and carbon, react with oxygen gas, O2, and will always 100% of the time make carbon dioxide CO2 and water. Close the video and try these practice problems. The first problem is a decomposition. Notice how there is only one compound to the left of the arrow. Every time there is one compound to the left of the arrow, it's a decomposition because there's nothing else for it to do but to break apart and to two or more things after the arrow. In this case, silver and sulfur broke their chemical bond. They got divorced. The second problem is a single replacement. Magnesium booted hydrogen out of the compound and is now happy with chlorine while hydrogen is back single and ready to mingle. Number three is a synthesis. There's only one compound to the right of the arrow. Two or more things came together to form a compound. Sodium and bromine are now chemically bonded together. They're in a relationship now. It's Facebook official. Number four is clearly a combustion because you have a hydrocarbon, meaning carbon and hydrogen bonded together, plus oxygen gas making carbon dioxide and water. And the last one is a double replacement. In this case, birium goes with the sulfate while hydrogen goes with the chlorine.