Chapter 7 Chemical reactions can be described in words, but it is common practice to represent chemical reactions with ‘equations’. Equations are simply a short hand way of describing a chemical reaction.

Equations (continued) Chemical(s) that exist before the chemical reaction are called reactants. Chemical(s) that exist after the reaction are called products. reactants -------> products The arrow points in the direction of where the reaction is moving. In a chemical reaction, atoms are re-arranged to form different substances.

Equations (cont) Even though atoms rearrange themselves in a chemical reaction, they are still present. In other words, they are conserved. This is referred to as the….Law of Conservation of Mass. To make the atoms of equal number on either side of the ----------> requires the use of what are called coefficients.

Equations (continued) Coefficients are used much the same in Chemistry as they are in Algebra. A coefficient is a number placed in front of an element or compound. Example: H2 (g)+ H2(g) + O2(g) ------> H2O(l) + H2O(l) ………..is not as good as……… 2H2 (g)+ O2(g) ------> 2 H2O(l)

Keys to Balancing Equations Be sure to correctly write the chemical formulas above all else. Don’t change the identity of any compound. Use the simplest ratio of coefficients when balancing equations. If the same polyatomic ion is found on both sides, balance them as if they were an element. If O2 is present in an equation, balance it last.

1) Correctly write formulas. Use the charges of ions (if present) to determine formulas if they are not already given. If you have the following reactants…. Potassium iodide + lead (II) nitrate, their formulas should be KI and Pb(NO3)2 …and the products are potassium nitrate and lead (II) iodide should have the formulas KNO3 and PbI2 .

2) No change of identity. In a chemical reaction, the compounds are very specific. These compounds cannot be altered just so that you can have a balanced equation. 2KI + Pb(NO3)2 -------> K2N2O6 + PbI2 (NO!!) 2KI + Pb(NO3)2 -------> 2KNO3 + PbI2 (correct)

3) Use the simplest ratio for coefficients. Just as was the case with chemical formulas, chemical equations should have the simplest ratio as well. 2H2 + O2 ------------> 2H2O is written correctly. 4H2 + 2O2 ------------> 4H2O is wrong….why???

4) Balancing polyatomic ions Most equations have a polyatomic ions and if the same one appears on both sides of the equation. It is much easier to balance a whole polyatomic ion that each individual element within it. Na2SO4 + Ca(NO3)2 ----> 2NaNO3 + CaSO4

5) Equations containing O2 In equations having a CH compound + an O2, balance C’s 1st, H’s 2nd, O’s last. C3H8 + O2 ----------> 3CO2 + H2O (C’s) C3H8 + O2 ----------> 3CO2 + 4H2O (H’s) C3H8 + 5O2 ----------> 3CO2 + 4H2O (O’s)

Word equations 2H2+ O2 ------> 2H2O The + can be represented as…. ‘plus’ or ‘reacts with’ and ------> can be represented as…. ‘yields‘ ‘to produce‘ or even ‘reacts to produce’