Chapter 9 Chemical quantities

Using Chemical Equations Section 9.1 Using Chemical Equations

Consider the following analogy… Assume you are in charge of making deli sandwiches at a local fast-food restaurant. A particular type of sandwich requires 2 pieces of bread, 3 slices of meat, & 1 slice of cheese. How would you represent this situation as a chemical equation? Your boss asks you to make 50 sandwiches. How much of each ingredient do you need?

Notice that the numbers 100:150:50 correspond to the ratio 2:3:1 2 pieces of bread + 3 slices of meat + 1 slice of cheese  1 sandwich 50(2 pieces of bread) + 50(3 slices of meat) + 50(1 slice of cheese)  50 sandwich Notice that the numbers 100:150:50 correspond to the ratio 2:3:1

A chemical equation gives you the same type of information. It indicates the relative numbers of reactant and product molecules required for the reaction to take place.

Balance the following equation 𝐶𝑂 (𝑔) + 𝐻 2 (𝑔) → 𝐶𝐻 3 𝑂𝐻 (𝑙) 𝐶𝑂 (𝑔) + 2 𝐻 2 (𝑔) → 𝐶𝐻 3 𝑂𝐻 (𝑙)

Why do we balance chemical formulas? Because atoms are just rearranged (not created or destroyed) in a chemical reaction, we must always balance a chemical equation. It’s important to recognize that the coefficients in a balanced equation give the relative numbers of molecules.

Mole Ratio The ratio of moles of one substance to moles of another substance in a balanced chemical equation.

Mole-Mole Relationships

We can use a balanced chemical equation to predict the moles of product that a given number of moles of reactants will yield.

Chapter 9 practice problems

Using Chemical Equations to Calculate Mass Section 9.2 Using Chemical Equations to Calculate Mass

Mass Calculations Remember that moles represent numbers of molecules, and we cannot count molecules directly. In chemistry we count by weighing.

Balance the equation for the reaction Steps for Calculating the Masses of Reactants & Products in Chemical reactions Balance the equation for the reaction Convert the masses of reactants or products to moles Use the balanced equation to set up the appropriate mole ratio(s). Use the mole ratio(s) to calculate the number of moles of the desired reactant or product. Convert from moles back to mass.

Let’s consider the combustion of propane… 𝐶 3 𝐻 8 (𝑔) + 𝑂 2 (𝑔) → 𝐶𝑂 2 (𝑔) + 𝐻 2 𝑂 (𝑔) What mass of Oxygen will be required to react exactly with 4.41g of propane?

Stoichiometry The process of using a balanced chemical equation to determine the relative masses of reactants and products involved in a reaction.

Mass Calculations: Comparing Two Reactions

Let’s consider the relative effectiveness of two antacids. Baking Soda & Milk of Magnesia are used to neutralize excess hydrochloric acid secreted by the stomach. Which acid can consume the most acid? 1.00g 𝑁𝑎𝐻𝐶 𝑂 3 (Baking Soda) or 1.00g 𝑀𝑔(𝑂𝐻 ) 2 ?

First we need a balanced chemical equation 𝑁𝑎𝐻𝐶 𝑂 3 +𝐻𝐶𝑙 →𝑁𝑎 𝐶𝑙 (𝑎𝑞) + 𝐻 2 𝑂 (𝑙) +𝐶 𝑂 2(𝑔) 𝑀𝑔(𝑂𝐻 ) 2 +2𝐻𝐶𝑙→2 𝐻 2 𝑂+𝑀𝑔 𝐶𝑙 2 Second we need to calculate the molar mass of both compounds

The third step is to calculate the moles for both compounds

Limiting Reactants & Percent Yield Section 9.3 Limiting Reactants & Percent Yield

Recall the sandwich example Now assume that you came to work one day and found the following quantities of ingredients: 20 slices of bread 24 slices of meat 12 slices of cheese How many sandwiches can you make? What will be left over?

20 𝑠𝑙𝑖𝑐𝑒𝑠 𝑏𝑟𝑒𝑎𝑑 × 1 𝑠𝑎𝑛𝑑𝑤𝑖𝑐ℎ 2 𝑠𝑙𝑖𝑐𝑒𝑠 𝑜𝑓 𝑏𝑟𝑒𝑎𝑑 =10 𝑠𝑎𝑛𝑑𝑤𝑖𝑐ℎ𝑒𝑠 24 𝑠𝑙𝑖𝑐𝑒𝑠 𝑚𝑒𝑎𝑡 × 1 𝑠𝑎𝑛𝑑𝑤𝑖𝑐ℎ 3 𝑠𝑙𝑖𝑐𝑒𝑠 𝑜𝑓 𝑚𝑒𝑎𝑡 =8 𝑠𝑎𝑛𝑑𝑤𝑖𝑐ℎ𝑒𝑠 12 𝑠𝑙𝑖𝑐𝑒𝑠 𝑐ℎ𝑒𝑒𝑠𝑒 × 1 𝑠𝑎𝑛𝑑𝑤𝑖𝑐ℎ 1 𝑠𝑙𝑖𝑐𝑒 𝑜𝑓 𝑐ℎ𝑒𝑒𝑠𝑒 =12 𝑠𝑎𝑛𝑑𝑤𝑖𝑐ℎ𝑒𝑠 How many sandwiches can I make? Which ingredient is the limiting reactant? You can make 8 sandwiches. The meat is the limiting reactant, even though you have more meat to begin with each sandwich requires 3 slices of meat.

Stoichiometric Mixture One that contains the relative amounts of reactants that matches the numbers in the balanced equation.

To determine how much product can be formed from a given mixture of reactants, we have to look for the reactant that is limiting – the one that runs our first and thus limits the amount of product that can form.

Limiting Reactant or Limiting Reagent The reactant that is completely used up when a reaction is run to completion. We cannot assume that a given mixture of reactants is a stoichiometric mixture, so you must determine whether one of the reactants is limiting.

Remember… In real life you can’t count the molecules directly – you can’t see them and, even if you could, there would be far too many to count. Instead, you must count by weighing.

Steps for Solving Stoichiometry Problems Involving Limiting Reactants Write & balance the equation for the reaction. Convert known masses of reactants to moles. Using the numbers of moles of reactants & the appropriate mole ratios, determine which reactant is limiting. Using the amount of the limiting reactant & the appropriate mole ratios, compute the number of moles of the desired product. Convert from moles of product to grams of product, using the molar mass (if this is required by the problem).

Theoretical Yield The maximum amount of a given product formed when the limiting reactant is completely consumed.

Percent Yield The actual yield of a product as the percentage of the theoretical yield. 𝐴𝑐𝑡𝑢𝑎𝑙 𝑌𝑖𝑒𝑙𝑑 𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑌𝑖𝑒𝑙𝑑 ×100=𝑃𝑒𝑟𝑐𝑒𝑛𝑡 𝑌𝑖𝑒𝑙𝑑

Example If a reaction of Nitrogen actually gave 6.63g of nitrogen instead of the predicted 10.6g, the percent yield of nitrogen would be 6.63𝑔 𝑁 2 10.6𝑔 𝑁 2 ×100=62.5%