Balancing Chemical Equations Stoichiometry. Packet #24

Transcription

1 Balancing Chemical Equations Stoichiometry Packet #24

2 Introduction When baking cookies, there is a high probability that one would use a recipe. The recipe is used to indicate how much of each ingredient to use. If the individual, baking the cookies, needs to make a larger batch, he/she could double/triple the amount of ingredients.

3 Introduction II A balanced chemical equation provides a chemist with the same quantitative information that a recipe does. A balanced chemical equation is much like a recipe.

4 Introduction III Conservation Laws All chemical reactions MUST obey three conservation laws Law of Conservation of Matter (Mass) Law of Conservation of Energy Law of Conservation of Electric Charge This means that the total mass, energy content and electrical charge of the reactants MUST equal those of the products.

5 Introduction IV Chemists use balanced chemical equations as a basis to calculate how much reactant is needed OR how much product is formed in a chemical reaction. To do this, the chemist MUST know the quantities of ANY substance used or created in the chemical reaction. The quantity is measured typically in grams or moles.

6 Introduction V However, a balanced equation CAN BE INTERPRETED in terms of (different quantities of) number of Atoms Molecules Moles Mass Volume Any one of the above can be used for calculation purposes.

7 Introduction VI The calculation of quantities in chemical reactions is a subject of chemistry called stoichiometry.

8 Introduction VII To balance chemical equations (reactions), a coefficient number is useful This is the number that is placed before a substance to indicate how many units of the substance is needed for the chemical reaction. The number may be called the stiochiometric number.

9 Introduction VIII Students MUST be able to complete, what is shown in the picture, PLUS volume at STP, for all chemical reactions. The picture shows THREE of the FOUR stoichiometric calculations. Students should AIM to be able to complete those calculations for each balancing equation problem. Even of the problem does not ask one to do so.

10 Example #1 CH 4 (g)+o 2 (g) CO 2 + 2H 2 O If one counts the atoms on the left of the equation, they should equal to those on the right of the equation Remember, the reaction should ALWAYS obey the three conservation laws. Are all atoms balanced (equal)? If they are not balanced, what needs to be done to balance them? Does there need to be a change in any of the coefficient numbers?

11 Example #2 H 2 (g) + I 2 (g) HI(g) Are all atoms balanced (equal)? NO! On the left side of the equation, there are TWO hydrogen atoms and TWO iodine atoms. On the RIGHT side of the equation, there is ONE hydrogen and ONE iodine (The compound is HI). There are supposed to be TWO based on the left side of the equation. To balance the equation, a coefficient number of TWO is added in front of the compound HI to indicate that the chemist has TWO of the compounds and hence TWO hydrogen atoms and TWO iodine atoms. H 2 (g) + I 2 (g) 2HI(g)

12 Example #3 Hydrogen sulfide, which smells like rotten eggs, is found in volcanic gases. The balanced equation of hydrogen sulfide is: 2H 2 S(g) + 3O 2 (g) 2SO 2 (g) + 2H 2 O(g)

13 Example #3 Continued 2H 2 S(g) + 3O 2 (g) 2SO 2 (g) + 2H 2 O(g) Interpret the equation in terms of Numbers of representative particles and moles. Masses of reactants to products.

14 Example #3 Continued Analysis thus far The coefficients in the balanced equation give the relative number of molecules or moles of reactants and products. Remember, a balanced equation OBEYS the law of conservation of mass. NOW, solve the problem determine how many molecules, moles and grams are needed, within the equation, for each reactant and each product.

15 Example #4 Interpret the equation for the formation of water from its elements in terms of numbers of molecules and moles, and volume of gases at STP. 2H 2 (g) + O 2 (g) 2H 2 O(g)

16 Example #5 Balance the following equation. C 2 H 2 (g) + O 2 (g) CO 2 (g) + H 2 O(g) Interpret the balanced equation in terms of relative number of moles, volumes of gas at STP, and masses of reactants and products.

17 STEPS TO BALANCING CHEMICAL EQUATIONS

18 Steps to Balancing Chemical Equations Check for diatomic molecules. Balance the metals (not Hydrogen). Balance the nonmetals (not Oxygen). Balance oxygen. Balance hydrogen. The equation should now be balanced, but recount all atoms to be sure. Reduce coefficients (if needed).

19 Examples Set I Write an equation for each of the following reactions and balance it using smallest whole number coefficents: Nitrogen(g) + hydrogen(g) ammonia (g) Hydrogen(g) + oxygen(g) water(l) Copper (II) oxide(s) + carbon(s) copper(s) + carbon dioxide (g) Magnesium(s) + hydrogen sulfate(aq) hydrogen (g) + magnesium sulfate(aq) Ammonia(g) + oxygen(g) nitrogen monoxide(g) + water(l)

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