Stoichiometry Messin with recipes Recipes and balanced equations

Stoichiometry!!! Messin’ with recipes

Recipes and balanced equations So we’ve seen recipes like… 1 cup sugar + 1 cup Nutella + 1 egg 24 Nutella cookies Ratio: 1 sugar: 1 Nutella: 1 Egg: 24 cookies 1 cup sugar + 2 cups flour + 1 egg + (Some other stuff) 30 Snickerdoodle cookies Ratio: 1 sugar: 2 flour: 1 egg: 30 cookies These recipes are giving us ratios And in cooking (chemistry), you need to follow the ratios in order to carry out reactions properly.

Recipes and reactions Here a couple more recipes… er… reactions, as we call them in chemistry class Ca. C 2(s)+2 H 2 O(l) C 2 H 2(g) + Ca(OH)2(aq) Ratio: 1 Ca. C 2: 2 H 2 O: 1 C 2 H 2: 1 Ca(OH)2 2 H 2(g) + O 2(g) 2 H 2 O(l) Ratio: 2 H 2: 1 O 2 : 2 H 2 O 2 C 2 H 6(g) + 7 O 2(g) 4 CO 2(g) + 6 H 2 O(l) Ratio: 2 C 2 H 6: 7 O 2: 4 CO 2: 6 H 2 O

Getting the ratio right In cooking, getting the ratio right determines whether something tastes good or not. In chemistry, getting the ratio right can mean life or death. 2 Na. N 3(s) 2 Na(s) + 3 N 2(g) What if you started with too much sodium azide?

Ratios in equations What are the ratios in these equations? 2 H 2(g) + O 2(g) 2 H 2 O(l) 2 moles hydrogen (H 2)/1 mole oxygen (O 2) or 2: 1 2 moles hydrogen (H 2)/2 moles water (H 2 O) or 1: 1 1 mole oxygen (O 2)/2 moles water (H 2 O) or 1: 2 2 C 2 H 6(g) + 7 O 2(g) 4 CO 2(g) + 6 H 2 O(l) 2 moles ethane (C 2 H 6)/7 moles oxygen (O 2) or 2: 7 2 moles ethane (C 2 H 6)/4 moles carbon dioxide (CO 2) or 1: 2 2 moles ethane (C 2 H 6)/6 moles water (H 2 O) or 1: 3 7 moles oxygen (O 2)/4 moles carbon dioxide (CO 2) or 7: 4 7 moles oxygen (O 2)/6 moles water (H 2 O) or 7: 6 4 moles carbon dioxide (CO 2)/6 moles water (H 2 O) or 2: 3

How do I measure a mole? By counting molecules? 6. 02 x 1023? Really? Mass or volume, right?

Mass in chemical reactions So if I use mass to measure how much of a chemical I have… But my equation has mole ratios… I’m going to need to do mass to mole conversions to figure out how much of each reactant will be needed to produce a certain amount of product.

2 H 2(g) + O 2(g) 2 H 2 O(l) If I start with 64 grams of oxygen (and excess hydrogen), how much water will I produce? Convert mass to moles 64 grams X 1 mole = 2 moles oxygen 1 Molar mass of O 2 32 grams Remember our ratio of oxygen to water is 1 mole oxygen/2 moles water? Convert moles oxygen to moles water 2 moles oxygen X 2 moles water 1 = 4 moles water 1 mole oxygen Convert moles to mass Mole ratio!!! 4 moles water X 18 grams = 72 grams of water 1 1 mole Molar mass of water

Boom! We just did stoichiometry! Here’s what we did (do) to figure out masses in chemical equations. 1. 2. 3. Convert the given mass to given moles using molar mass of the substance given Change the given moles to moles wanted using ratios from the balanced equation Change the moles wanted to grams wanted using the molar mass of the substance wanted

Let’s try this… 2 C 2 H 6(g) + 7 O 2(g) 4 CO 2(g) + 6 H 2 O(l) If I have 105 grams of C 2 H 6 and excess oxygen, how many grams of CO 2 will be produced? 1. Convert the given mass to given moles using molar mass 105 grams X 1 mole = 3. 5 moles C 2 H 6 1 30 grams 2. Change the given moles to moles wanted using ratios 3. 5 moles C 2 H 6 X 4 moles CO 2 = 7 moles CO 2 1 2 moles C 2 H 6 3. Change the moles wanted to grams wanted using the molar mass 7 moles CO 2 X 44 grams = 308 grams CO 2 1 1 mole CO 2