Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Compounds What is a compound? Compounds • A compound is a substance made of two or more elements. Water is made of hydrogen and oxygen. • A compound always has the same ratio (a fixed ratio) of elements. Water always has two atoms of hydrogen for each atom of oxygen. • The elements are not just mixed together—they are combined through a chemical reaction. Writing Compounds • Some compounds, like water and sugar, have there own names. Most do not. • All compounds have names that tell what they are made of. Salt is made of a sodium ion and a chloride ion, so we call it sodium chloride. • Often, we will just write the symbols. NaCl • If a compound needs more than one of an atom, sometimes its name can tell us. Carbon dioxide has one carbon atom and two oxygen atoms. Writing Compounds • When we write the symbols the number of atoms needed goes after the symbol as a subscript (below the line). • Carbon dioxide is written CO2. • The way we write down the symbols for a compound tells us how it is made, so we call it the chemical formula for that compound. • • • • • • • • • • • • Carbon dioxide: CO2 Water: H2O Sodium chloride (salt): NaCl Hydroxide: OHGlucose: (sugar) C6H12O6 Ammonia: NH3 Iron oxide (rust): Fe2O3 Hydrochloric acid: HCl Octane (in petrol) C8H18 Ethanol (alcohol) C2H6O Methane (natural gas) CH4 Propane (cooking gas) C3H8 Molecules • Molecules are groups of atoms. • Since compounds are made of more than one element, the smallest piece of a compound is a molecule. • Some non-metals form molecules made of only that element. Chemical Reactions • Chemical reaction happen when two or more atoms come together to form a new substance. • Oxidation is one of the most common chemical reactions. • A burning piece of wood, a piece of apple turning brown, and a piece of iron rusting are all examples of oxidation. Writing Chemical Reactions • When we write down a chemical reaction, we write the substances we start with (the reactants) on the left, with “+” between each. Usually we write only the chemical symbols. • The substances we finish with (the products) go on the right, with plusses in between. • The reactants and products are separated by a “→”. We say “yields” when we read the reaction. Na+ + Cl- → NaCl Sodium plus chloride yields sodium chloride (salt). Writing Chemical Reactions • Anytime we write a chemical formula, the number of atoms of an element is written as a subscript after the symbol for that element. NaOH + HCl → NaCl + H2O Sodium hydroxide plus hydrochloric acid yields sodium chloride (salt) and water. Writing Chemical Reactions • When we write a chemical reaction, it is like writing the recipe for a cake. • We must tell how much of each thing we need. • We write the number of atoms or molecules of each substance in front of the symbol, unless we only need one of that atom or molecule. Ca2+ + 2F- → CaF2 Calcium plus two fluoride ions yields calcium fluoride. Writing Chemical Reactions • Sometimes we want to keep track of certain molecules even after the chemical reaction. • We do this by putting parentheses “( )” around the formula for that molecule. Ca2+ + 2(OH-) → Ca(OH)2 Calcium plus two hydroxide ions yields calcium hydroxide. Conservation of Mass • Matter can not be create or destroyed in a chemical reaction. • When a chemical reaction happens there is no change in mass. • The mass of all the reactants equals the mass of all the products. 2g H2 + 16g O2 → 18g H2O 40g NaOH + 36g HCl → 58g NaCl + 18g H2O