Student Exploration: Chemical Equations

Name: Date: Student Exploration: Chemical Equations Directions: Follow the instructions to go through the simulation. Respond to the questions and prompts in the orange boxes. Vocabulary: Avogadro’s number, chemical equation, chemical formula, chemical reaction, coefficient, combination, combustion, conservation of matter, decomposition, double replacement, molar mass, mole, molecular mass, molecule, product, reactant, single replacement, subscript Prior Knowledge Questions (Do these BEFORE using the Gizmo.) A candle is placed on one pan of a balance, and an equal weight is placed on the other pan. What would happen if you lit up the candle and waited for a while? The side of the weight would go down and the candle would go up. Suppose the candle was placed in a large, sealed jar that allowed it to burn for several minutes before running out of oxygen. The candle and jar are balanced by an equal weight. In this situation, what would happen if you lit up the candle and waited? They would stay balanced or equal. Gizmo Warm-up Burning is an example of a chemical reaction. The law of conservation of matter states that no atoms are created or destroyed in a chemical reaction. Therefore, a balanced chemical equation will show the same number of each type of atom on each side of the equation. To set up an equation in the Chemical Equations Gizmo, type the chemical formulas into the text boxes of the Gizmo. First, type in “H2+O2” in the Reactants box and “H2O” in the Products box. This represents the reaction of hydrogen and oxygen gas to form water. Check that the Visual display is chosen on each side of the Gizmo, and count the atoms. A. How many hydrogen atoms are on the Reactants side? 2 Products side? 2 B. How many oxygen atoms are on the Reactants side? 2 Products side? 1 2. Based on what you see, is this equation currently balanced? No, this equation is not balanced. Get the Gizmo ready: Activity A: Erase the chemical formulas in each text box. Check that the Interpreting chemical formulas Visual displays are selected. Introduction: To balance a chemical equation, you first need to be able to count how many atoms of each element are on each side of the equation. In this activity, you will practice counting the atoms that are represented in chemical formulas. Question: How do we read chemical formulas? Observe : Type “H2” into the Reactants box and hit Enter on your keyboard. Note that the formula is shown as H 2 below. The small “2” in H 2 is a subscript . A. What does the “2” in H2 represent? It represents how many hydrogen atoms there are. B. In general, what do you think a subscript in a chemical formula tells you? C. Try typing in other subscripts next to the H, such as 3, 4, This statement above is still true because on the and 5. Is your answer to question B still true? Explain. reactants side it shows how many elements there are. Count : Clear the Reactants It would tell me the amount of an element in a compound. box, and type in a more complex chemical formula: “ Ca( OH)2.” Look at the number of atoms shown. A. How many of each type of atom do you see? B. In general, what happens when a subscript is found outside of parentheses? Try typing in other subscripts next to the (OH), such as C. 3, 4, and 5. Is your answer to question B still true? Explain. Ca: O: H: It tells me how many of the elements are in parentheses match the subscript number. The statement above is still true because the atoms/elements in parentheses match the subscripts number Practice : For each of the real chemical formulas below, calculate how many of each element there are. Check your answers for the first three formulas using the Gizmo. Activity B: Get the Gizmo ready: Erase the chemical formulas in each text box. Balancing equations Introduction: In a chemical reaction, the reactants are the substances that enter into the reaction, and the products are the substances that are made in the reaction. A chemical reaction is balanced if the numbers of reactant atoms match the numbers of product atoms. Goal: Learn to balance any chemical equation. Observe : To model how hydrogen and oxygen react to make water, type “H2+O2” into the Reactants box and “H2O” into the Products box. As the equation is written, which element is not in balance? Explain. oxygen There are 2 oxygens on the left and 1 on the right. Balance : To balance a chemical equation, you are not allowed to change the chemical formulas of the substances involved in the reaction. You are allowed to change the number of molecules of each substance by adding coefficients in front of the formulas. To balance the oxygen atoms, add a “2” in front of the “H2O” in the Products box. How many oxygen atoms are found on each side of the equation now? 2 To balance the hydrogen atoms, add a “2” in front of the “H2” in the Reactants box. How many hydrogen atoms are found on each side of the equation now? 4 Is this equation currently balanced? . Apply : Now enter a more complex chemical reaction: Ca( OH) 2 + HBr ? CaBr 2 +H 2 O. List the numbers of each element in the tables below: Yes Click Show if balanced to check Ca O H Br 1 2 3 1 Ca O H Br 1 1 2 2 Products A. Which elements are out of balance? B. Add coefficients to balance first the bromine (Br) and then the hydrogen (H) atoms. When the equation is balanced, write the complete formula. Oxygen, hydrogen and bromine Ca(OH)2+2HBr CaBr2+2H2O Practice : Chemical reactions are generally classified into five groups, defined below. Balance each equation, using the Gizmo for help. Combination (or synthesis) – two or more elements combine to form a compound. Na + O 2 ? Na 4Na + O2 2Na2O 2 O La 2 O 3 +H 2 La2O3 + 3H2O 2La(OH)3 O? La( OH) 3 N 2 O 5 +H N205 + H2O 2HNO3 2 O ? HNO 3 Decomposition – a compound breaks down into elements and/or simpler compounds. KNO 3 ? KNO 2 2KN03 2KNO2+O2 +O 2 NaN 3 ? Na + N 2NaN3 2Na + 3N2 2 NH 4 NO 3 ?N NH4No3 N2O +2H2O 2 O+H 2 O Combustion – a fuel reacts with oxygen to release carbon dioxide, water, and heat. CH 4 +O 2 ? CO CH4+ 202 CO2 + 2H2O 2 +H 2 O C 3 H 8 +O 2 ? CO 2 +H 2 O C3H8+ 5O2 3CO2 + 4H2O C 6 H 12 O 6 +O C6H12O6 + 6O2 6C02 + 6H2O 2 ? CO 2 +H 2 O Single replacement – an element replaces another element in a compound. KCl +F 2 2KCI + F2 2KF + CI2 ? KF + Cl 2 Mg + HCl ? MgCl 2 +H 2 Mg + 2HCI MgCI2 + H2 Cu + AgNO 3 ? Cu( NO Cu + 2AgNO3 Cu (No3)2 + 2Ag 3 ) 2 + Ag Double replacement – two compounds switch parts with one another. AgNO 3 +K 2 SO 4 ? Ag 2 SO 4 + KNO 3 2AgNO3 +K2SO4 Ag2SO4 + 2KNO3 Mg( OH) 2 + HCl ? MgCl 2 MG(OH)2 + 2HCI MgCI2 +2H2O +H 2 O Al( OH) 3 +H 2 SO 4 ? Al 2AI(OH)3 + 3H2SO4 AI2(SO4)3 + 6H2O 2 (SO 4 ) 3 +H 2 O Get the Gizmo ready: Activity C: Erase the chemical formulas in each text box. In the middle menu, select Molar mass Molar mass . Introduction: Chemists are often interested in obtaining a certain mass of product from a chemical reaction without wasting any reactants. But how is this done? To calculate the masses of reactants needed for a desired mass of product, it is necessary to understand a unit of quantity called the mole. Question: How do chemists know how much of each substance to mix? Observe : The mass of a molecule of a substance is its molecular mass (M). Molecular mass is measured in universal mass units (u). One universal mass unit (1 u) is approximately the mass of a proton. Hydrogen gas has a molecular mass of 2.0158 u. Type the formula “H2” into the Reactants box. What is the molar mass of hydrogen gas, H 2 ? 2.0158 g What is the relationship between the molecular mass and the molar mass of a substance? They both have the same value. Gather data : The balanced equation to synthesize water is: 2H 2 +O 2 ? 2H 2 O. Use the Gizmo to find the molar masses of each substance in this equation: Analyze : Based on the molar masses, how can you tell that an equation is balanced? When the reactants and products have the same mass. Apply : Suppose you had one mole of oxygen (O 2 ). How many moles of hydrogen (H 2 ) would react completely with the oxygen, and how many moles of H 2 O would be produced? 2 moles of hydrogen would react with 1 mole of oxygen, producing 2 moles of water Calculate : Suppose you had 2.0158 grams of hydrogen (H 2 ). A. How many moles of hydrogen do you have? 1 mole B. How many moles of oxygen would react with this much hydrogen? 1 half mole C. What mass of oxygen would you need for this reaction? 15.9 g D. How many grams of water would you produce? 18.0 g Challenge yourself : Suppose you wanted to make 100 grams of water. A. What is the molar mass of water (H2O)? 18.0 g B. How many moles of water are in 100 grams? 5.5 moles C. How many moles of hydrogen will you need? 5.5 moles D. How many moles of oxygen will you need? 2.8 moles Hydrogen – 11.2 g E. How many grams of hydrogen and oxygen will you need? Oxygen – 88.8 g F. Is your answer reasonable? Why or why not? Yes