10 th Grade Chemistry: 2/23/14

Learning Target: Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties. 10th Grade Chemistry: 2/23/14 Must Do: Use a whiteboard and work on balancing the equation for the reaction between chlorine gas and sodium bromide, which produces bromine and sodium chloride Cl2 (g) + NaBr (s) Br2 (g) + NaCl (s) • You are not allowed to change the subscript numbers to balance the equation! Coefficients (multipliers) can only go in front of the elements or molecules (i.e, 2 CO2 is correct, C2O2 or C2O2 is not correct) • Agenda: • Must Do • WWK • Chemical Equation Balancing

WWD = Stoichiometry

Balancing Chemical Equations • Why does this matter? • The main principle is that atoms are “conserved” in the reaction • Same number of atoms must be found in the reactants and products – Conservation of Mass • An unbalanced chemical equation is analogous to an incorrectly completed math problem, claiming equality when it is clearly not the case. • They also help us to determine a quantitative relationship between reactants and products…how much of each reactant do we need (# of atoms, molecules, compounds) and how much product will we expect? Once we learn the equation balancing, we’ll learn how to relate these equations to quantitative measurements.

Formation of NaCl from sodium (Na) and chlorine (Cl) • Na(s) + Cl2(g) NaCl (s) • First step is to do some accounting. Make a table or drawing to represent each atom on the reactant and product side of the equation. • I prefer a table over a drawing as it is faster, and no colored pencils are needed. Use whatever method works best. Eventually, when you master balancing equations, you will get to a point where you only need the table to check your final answer • I recommend always tabulating your result to check your work)

Formation of NaCl from sodium (Na) and chlorine (Cl) • Na(s) + Cl2(g) NaCl (s) • We will need to multiply the NaCl by 2 (by writing a “2” in front of NaCl. This is called a stoichiometric coefficient

Formation of NaCl from sodium (Na) and chlorine (Cl) • Na(s) + Cl2(g) 2 NaCl (s) • Now we have an extra Na on the product side. What do we do to change this?

Formation of NaCl from sodium (Na) and chlorine (Cl) • 2 Na (s) + Cl2 (g) 2 NaCl (s) • Now we have the atoms matched on each side of the equation • It’s balanced!

This next equation looks a lot more complicated, but on analysis, you‘ll see that it’s fairly simple • Before we start this one, take a look at the reactants and products. Notice that the PO4 and NH4 are polyatomic anions that we’ve been memorizing. • What is the charge on PO4? On NH4? NH3 (g) + H3PO4 (l) → (NH4)3PO4 (s)

This next equation looks a lot more complicated, but on analysis, you‘ll see that it’s fairly simple • Before we start this one, take a look at the reactants and products. Notice that the PO4 and NH4 are polyatomic anions that we’ve been memorizing. • What is the charge on PO4? On NH4? PO43- NH4+ NH3 (g) + H3PO4 (l) → (NH4)3PO4 (s)

Tabulating the Reactant and Product atoms NH3 (g) + H3PO4 (l) → (NH4)3PO4 (s)

Tabulating the Reactant and Product atoms NH3 (g) + H3PO4 (l) → (NH4)3PO4 (s) What do we want to multiply first?

Tabulating the Reactant and Product atoms 3 NH3 (g) + H3PO4 (l) → (NH4)3PO4 (s) Balanced!

Here’s a good example when to use the “cross coefficient” method to balance even/odd subscripts P4 (s) + O2 (g)→ P2O5(s) P4 (s) + 5 O2 (g)→ 2 P2O5(s)

How to master RXN balancing • Practice! • Search for “chemical reaction balancing practice” in Google and you will find more practice sheets that you will ever need. • Many of these have answer checking built into the site. • ScienceGeek • Watch some videos on how other people approach the problems (link on Ms. Lockwood’s chemistry page)

10 th Grade Chemistry: 2/23/14