Stoichiometry Introduction PDF

​ Introduction to Stoichiometry Stoichiometry is the part of chemistry that studies the relationships between the amounts of reactants and products in chemical reactions. The word stoichiometry comes from the Greek words stoichio, meaning element, and metri, meaning measure or measurement. Under...

​ Introduction to Stoichiometry Stoichiometry is the part of chemistry that studies the relationships between the amounts of reactants and products in chemical reactions. The word stoichiometry comes from the Greek words stoichio, meaning element, and metri, meaning measure or measurement. Understanding Coefficients in Balanced Equations In a balanced equation, coefficients are used to ensure that the number of atoms of each element is the same on both the reactant and product sides. For example, in the equation 2CO + O2 → 2CO2, the coefficients 2, 1, and 2 indicate the ratio of molecules of each substance that react or are produced. Meaning of Coefficients The coefficients in a balanced equation represent the ratio of: Moles of each substance Volumes of gases at the same temperature and pressure Particles (molecules or atoms) of each substance Relationship Between Coefficients and Grams However, the coefficients do not directly represent the ratio of grams of each substance. This is because the molar mass of each substance is different. Substance Molar Mass (g/mol) CO 28.0 O2 32.0 CO2 44.0 Law of Conservation of Mass Although the coefficients do not represent the ratio of grams, the law of conservation of mass still applies. This means that the total mass of the reactants is equal to the total mass of the products. The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction, only converted from one form of matter to another. Example: Bread and Cheese Sandwiches To illustrate the concept of stoichiometry, consider a recipe for bread and cheese sandwiches: 2 breads + 1 cheese → 1 sandwich. If you have 12 slices of bread, how many slices of cheese do you need? Using Proportions to Solve Problems You can use proportions to solve problems like this. For example: 12 is to 2 as x is to 1  12 × 1 = 2 × x x=6 The Bridge Method A shortcut for solving proportions is the bridge method, which involves dividing and multiplying: Divide by the coefficient of the starting substance Multiply by the coefficient of the ending substance For example, to find the number of slices of cheese needed for 12 slices of bread: 12 ÷ 2 = 6 6×1=6 Examples Using the Bridge Method Here are some more examples using the bridge method: 9 slices of cheese: 9 ÷ 1 = 9, 9 × 2 = 18 slices of bread 11 sandwiches: 11 ÷ 1 = 11, 11 × 1 = 11 slices of cheese, 11 ÷ 1 = 11, 11 × 2 = 22 slices of bread Summary In summary, stoichiometry is the study of the relationships between the amounts of reactants and products in chemical reactions. Coefficients in balanced equations represent the ratio of moles, volumes, and particles, but not grams. The law of conservation of mass still applies, and problems can be solved using proportions or the bridge method.## Stoichiometry Basics Stoichiometry is the part of chemistry that studies amounts of substances that are involved in reactions. To solve stoichiometry problems, we use bridges. There are two types of bridges: the little bridge and the big bridge. Little Bridge The little bridge is used when dealing with moles, liters, or particles. It involves dividing first and then multiplying. The formula for the little bridge is: givencoefficient of given×coefficient of unknown×molar mass of unknown coefficientofgiven given ​ ×coefficientofunknown×molarmassofunknown However, when using the little bridge with moles or liters, the molar mass is not needed. Big Bridge The big bridge is used when dealing with grams. It involves dividing twice and then multiplying twice. The formula for the big bridge is: given masscoefficient of given×molar mass of givenmolar mass of given×coefficient of unknown×molar mass of unknown coefficientofgiven givenmass ​ × molarmassofgiven molarmassofgiven ​ ×coefficientofunknown×molarmassofunknown The law of conservation of mass states that matter cannot be created or destroyed in a chemical reaction. This means that the total mass of the reactants is equal to the total mass of the products. Examples of Stoichiometry Problems Here are some examples of stoichiometry problems: If we have 3 moles of oxygen, how many moles of carbon monoxide do we need? Using the little bridge: 31×2=6 1 3 ​ ×2=6 moles of carbon monoxide If we have 4 moles of carbon monoxide, how many moles of oxygen do we need? Using the little bridge: 42×1=2 2 4 ​ ×1=2 moles of oxygen If we have 12 liters of CO2, how many liters of O2 do we need? Using the little bridge: 122×1=6 2 12 ​ ×1=6 liters of O2 Grams Problems When dealing with grams, we need to use the big bridge. Here are some examples: Given Unknown Calculation 20 grams of bread grams of cheese 202×1010×1×15=15  2 20 ​ × 10 10 ​ ×1×15=15 grams of cheese 202×1010×1×35=35 2 20 ​ 20 grams of bread grams of sandwich × 10 10 ​ ×1×35=35 grams of sandwich  602×1010×1×15=45 2 60 ​ 60 grams of bread grams of cheese × 10 10 ​ ×1×15=45 grams of cheese 602×1010×1×35=105 2 60 ​ 60 grams of bread grams of sandwich × 10 10 ​  ×1×35=105 grams of sandwich Note that the big bridge involves dividing twice and then multiplying twice. This can be seen in the calculations above. Balancing Equations To solve stoichiometry problems, we need to have a balanced equation. The balanced equation for the reaction between CO and O2 is: 2CO+O2→2CO2 2CO+O2→2CO2 We can use this equation to solve problems involving moles, liters, particles, and grams.## Big Bridge Method The Big Bridge method is used to solve problems involving grams. This method involves a series of division and multiplication operations to find the answer. To use the Big Bridge method, follow these steps: Divide the given number by the first factor Divide the result by the second factor Multiply the result by the third factor Multiply the result by the fourth factor The order of operations is not important, as long as you divide in the column you start and multiply in the column you end. Examples The following examples illustrate the use of the Big Bridge method:  Given Number Factors Result 112 divided by 2, divided by 28, times 32, times 1 64 112 divided by 2, divided by 28, times 44, times 2 176 48 divided by 1, divided by 32, times 28, times 2 84 48 divided by 1, divided by 32, times 44, times 2 132 22 divided by 2, divided by 44, times 32, times 1 8 22 divided by 2, divided by 44, times 28, times 2 14 Double Checking To double check your answer, you can use the following equation: If the result of the Big Bridge method is added to the result of another calculation, the sum should equal the expected answer. For example: 112 + 64 = 176 84 + 48 = 132 14 + 8 = 22 Little Bridge Method Note that if you are using molecules, liters, or moles, you would use the Little Bridge method instead of the Big Bridge method. The Little Bridge method is a shortcut for doing proportions to answer the question of how much. The key difference between the Big Bridge and Little Bridge methods is the units being used. Big Bridge method: used for grams Little Bridge method: used for molecules, liters, or moles