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Chemistry: Chemical Reactions Notes What is a chemical reaction and why does it happen? • Elements and/or compounds (called reactants) are changed to create one or more new substances (called products). If new compounds aren’t formed then no reaction has occurred. •
During a chemical reaction the atoms that make up the molecules of the starting compounds (reactants) are rearranged in different combinations to form the molecules of the new compounds (products). As part of this rearranging of atoms, existing chemical bonds are broken and new chemical bonds are formed. •
Elements and compounds usually, but not always, undergo reactions to become more stable. What makes one compound more stable than another compound is beyond the scope of this course. •
Mass: While compounds must change during a chemical reaction, the Conservation of Mass Law dictates that the number of atoms of each element remains the same before and after a chemical reaction takes place. Look at the example below. New compounds are formed, but the number of atoms of each element does not change. 2 Al + 6 HCl → 2 AlCl3 + 3 H2 2 Al atoms 6 H atoms & 6 Cl atoms 2 Al atoms & 6 Cl atoms 6 H atoms •
Energy: Chemical reactions follow the Conservation of Energy Law. Energy is stored in the chemical bonds of elements and compounds. The total energy before and after the reaction must stay the same. o
Exothermic Reactions: “Exo” means exit and “thermic” means energy. When the chemical bonds of the products have less energy than the chemical bonds of the reactants; energy is released during the reaction. If the reaction is fast enough and releases enough energy, we have an explosion. All explosions are exothermic reactions, but not all exothermic reactions are explosions. The formation of water from hydrogen and oxygen is exothermic and explosive! Hydrogen + Oxygen → Water + Energy o
Endothermic Reactions: “Endo” means in and “thermic” means energy. When the chemical bonds of the products have more energy than the chemical bonds of the reactants; energy is required to keep the reaction going. The decomposition of water into oxygen and hydrogen (the reverse of the above reaction) is endothermic and requires energy in the form of an electrical current. Water + Energy → Hydrogen + Oxygen STOP -­‐ Review the key vocabulary and symbols on page 3 before continuing. 1. What is a chemical reaction? 2. What is a reactant? STOP – Take Quick Quiz #1 online. What is a chemical equation? • An expression that describes the changes that occur during a chemical reaction. •
Types of chemical equations: o
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Word equation – a verbal/written expression that gives the names of the chemical species involved in the reaction. This type of equation gives qualitative, descriptive information. hydrogen + oxygen → (yields) water Formula equation – the names of products and reactants are replaced with their formulas. This type of equation gives qualitative and quantitative information. H2 + O2 → H2O (unbalanced equation) The above equation is not balanced, because there are two oxygen atoms on the reactant side of the equation, but only one oxygen atom on the product side of the reaction. Formula equations must follow the Law of Conservation of Mass. The total mass of the reacting substances is equal to the total mass of the products formed. This means there has to be the same number of each type of atom on both sides of the equation. 2H2 + O2 → 2H2O (balanced equation) The above equation is balanced, because there are four hydrogen atoms and two oxygen atoms on the reactant side of the equation, and four hydrogen atoms and two oxygen atoms on the product side of the reaction. Why is a chemical equation important? • It captures or records the changes that occur during a reaction. •
It shows the quantitative proportions of the reactants and products. In the above example, o 2 molecules of hydrogen react with 1 molecule of oxygen to form 2 molecules of water, or o 2 moles of hydrogen react with 1 mole of oxygen to form 2 moles of water, or o 4 grams of hydrogen react with 32 grams of oxygen to form 36 grams of water. 3. Circle the equations below that are balanced and obey the Conservation of Mass Law: a. C + O2 → CO2 d. 2 Ca + O2 → 2 CaO b. C + O2 → CO e. CH4 + 2 O2 → CO2 + 2 H2O c. Ca + O2 → CaO f.
4. How are each of these symbols used in a chemical equation: 2 CH4 + 2 O2 → 2 CO2 + 2 H2O
a. + b. → c. (l) STOP – Take Quick Quiz #2 online. Key Vocabulary & Symbols •
Conservation of Mass – Mass is neither created nor destroyed during chemical reactions. •
Conservation of Energy – Energy is neither created nor destroyed during chemical reactions. •
Reactants – chemical compounds that are used up in a reaction, they are found on the left side of the arrow. •
Products – chemical compounds that are created in a reaction, they are found on the right side of the arrow. •
Coefficients – small whole numbers that go in front of a chemical compound or formula. In the equation from the previous page the coefficients are the 2 in front of the H and the 2 in front of the H 2O. Coefficients are used to balance the number of each type of atom on both sides of the arrow. •
Subscripts – small whole numbers that go to the lower right of a chemical element or to the lower right of the parentheses of a polyatomic ion. In the equation from the previous page the subscript is the 2 after the H and O on the left side of the arrow and the 2 after the H on the right side of the arrow. Subscripts are used to create neutral compounds. Writing & Balancing Equations: A 3-­‐Step Process 1. If you are given a word equation with only reactants finish the word equation by writing the chemical names of the products. Remember positive ions keep the same name as their neutral element (Ca2+ is calcium) and negative ions end in –ide (Cl1-­‐ is chloride). The exception to this rule is polyatomic ions. You need to use the charge sheet to get the names. 2. Write the chemical formulas for the reactants and products. All the products need to have a neutral charge. Use the correct symbols from the periodic table with the correct subscripts. a. For elements the subscript is one, unless it is a diatomic molecule then the subscript is 2. The diatomic molecules end in “gen” or “ine”, they are nicknamed the Aunt Genine molecules. They are hydrogen (H2), nitrogen (N2), oxygen (O2), fluorine (F2), chlorine (Cl2), bromine (Br2), and iodine (I2). b. For ionic compounds, use the charge sheet to determine the positive or negative charge of each ion. Then use the diagonal rule to get the correct subscripts for each formula. Examples, for calcium oxide, the ions are Ca2+ and 02-­‐, the subscripts are Ca2O2 which should be simplified to CaO; for aluminum sulfate, the ions are Al3+ and (SO42-­‐ , the subscripts are Al2(SO4)3 . c. For molecular compounds, combine the elements so each element has a full valence shell of electrons (satisfies the Octet Rule). d. Once you have the correct chemical formulas you cannot change the subscripts. The subscripts must stay the same. 3. Balance the equation by using multipliers (also known as coefficients) to adjust the number of molecules of reactants and products. a. The total number of atoms for each element is the same on both sides of the equation. b. You cannot change the subscripts of a chemical formula to balance an equation, because changing the subscripts changes the compound. Example: hydrogen + oxygen → water H2 + O2 → H2 O If we adjusted the subscripts to balance the equation we would write: H2 + O2 → H2 O2 but this would be wrong, because H2 O2 is hydrogen peroxide not water. We use multipliers (coefficient) to change the number of moles of the compound without changing the compound itself. The balanced equation is 2 H2 + O2 → 2 H2 O 5. Balance the following equations: a. __ S + __ O2 → __ SO2 c. __ N2 + __ H2 → __ NH3 b. __ S + __ O2 → __ SO3 d. __ Zn + __ HCl → __ ZnCl2 + __ H2 6. Write a balanced formula equation for sodium chloride decomposing to form sodium metal and chlorine gas. STOP – Show the teacher your work.