Scientific Background A chemical reaction occurs when substances, called reactants, are mixed and changed to a new set of substances, or products. In this process, chemical bonds between atoms are broken and new bonds are formed, but the atoms themselves remain intact. The law of conservation of matter states that in a chemical reaction, no matter is created or destroyed. (Note: In a nuclear reaction, some matter is converted to energy.) In most cases, a certain amount of energy, called activation energy, is required to get the reaction started. The reaction often releases energy as well. If the activation energy is greater than the released energy, the reaction is endothermic. An exothermic reaction releases more energy than is required to start the reaction. Chemical reactions can be classified into five categories. In a synthesis reaction (also called a combination reaction), two or more substances combine to form a single substance. In a decomposition reaction, a single substance breaks down to form two or more substances. In a single replacement reaction, a compound reacts with an element to form a new compound and a different element. In a double replacement reaction, two compounds exchange parts. Combustion reactions involve burning a fuel (often an organic compound) in oxygen, usually producing carbon dioxide and water. Balancing chemical reactions is useful because it allows you to calculate how much of each reactant you will need to produce a desired amount of product. Scientists use the mole (Mol) to measure amounts of substances. A mole of a substance has a mass in grams that is equal to the atomic mass of one molecule of the substance. For example, water has an atomic mass of 18 amu (atomic mass units), so a mole of water is 18 grams of water. Based on the balanced equation: 2H2 O2 → 2H2O You can deduce that it requires 2 moles of hydrogen gas (4 grams) and 1 mole of oxygen gas (32 grams) to produce 2 moles of water (36 grams). Question 1 Not complete Points out of 1.00 Flag question Questio
