Describing Chemical Reactions

http://www.youtube.com/

watch?v=F54rqDh2mWA

Chemical Change v. Physical Change

Chemical Change Break or form chemical bonds Molecular formulas of the products are NOT

exactly the same as the reactants

Example:2NaCl (aq) + 2H2O (l)→ 2NaOH (aq) + Cl2(g) + H2(g)

Chemical Change v. Physical Change

Physical Change DO NOT break or form chemical bonds Molecular formulas of the products ARE exactly

the same as the reactants Change in state (phase) of matter Change in solution

Chemical Change v. Physical Change

Physical Change Examples: (s)→(l) or (l)→ (g) or (s)→ (aq)

Br2(l) → Br2 (g)

NaCl(s)→ NaCl(aq)H2O (l)→ H2O (s)

C6H12O6 (aq) → C6H12O6 (s)

Writing Chemical Equations used to convey as much info. as possible about

what happens in a chemical reaction

Word Equations write out what chemicals are reacting

hydrogen peroxide → water + oxygen hydrogen + oxygen → water

Chemical Equations show the chemical formulas of the chemicals

reacting H2O2 (aq) → H2O(l) + O2(g) H2(g) + O2(g) → H2O (l) (s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous solution

{see Table 11.1 in text} the above are often referred to as skeletal equations

because they are not mathematically balanced

a catalyst is a substance that speeds up the reaction but is not used up in the reaction

H2(g) + O2(g) H2O (l)MnO4

Balancing Chemical Equations each side of the equation must have the

same number of atoms of each elements bicycle example

frame + wheel + handlebar + pedal → bike frame + 2 wheels + handlebar + 2 pedals → bike

tricycle example frame + wheels + handlebar + bell → tricycle frame + 3 wheels + handlebar + bell → tricycle

this is NOT balanced

1. A solution of sodium iodide is added to a solution of potassium nitrate to make a potassium iodide precipitate and a sodium nitrate solution.

NaI (aq) + KNO3 (aq) KI (s) + NaNO3 (aq)

2. Magnesium metal burns in oxygen gas with a bright white light to make a white powder called magnesium oxide.

Mg (s) + O2 (g) MgO (s) + heat

2Mg (s) + O2 (g) 2MgO (s) + heat

3. Gaseous hydrogen (dihydride) and gaseous oxygen (dioxide) react explosively to form water vapor.

H2 (g) + O2 (g) H2O (g) + heat

2H2(g) + O2(g) 2H2O (g) + heat

?

How about these? Na2SiO3 + HF H2SiF6 + NaF + H2O Na2SiO3 + 8HF H2SiF6 + 2NaF + 3H2O

NaCl (aq) + AgNO3 (aq) AgCl (s) + NaNO3 (aq) balanced

Al(s) + O2(g) Al2O3(s) 4Al(s) + 3O2(g) 2Al2O3(s)