S1-L4: Chemical equations, The Mole & Molarity Flashcards
Define a “mole”
- one mole of any molecule contains Avogadro’s n. of molecules
- Is SI unit of amount of substance
- ->1 mole of any element contains same n. of atoms (Avogadro constant)
What is one mole equal to?
-6.022137 x 10^23
Outline what Avogadro’s number is
- (Na) defined as n. of atoms found in 12 g of C 12
- one mole of any molecule contains Avogadro’s n. of molecules
What is the mass of one mole of any substance?
-its relative atomic mass (elements) OR relative molecular mass (compound) in grams
Describe “molar mass” (figure 1)
- mass of one mole of substance
- ->unit is g/mol
- useful to calculate mass relationship in chemical reactions
What is “molecular mass”? Use the example of SO2
- 1S –> 32.07
- 2O –> (2x16.00)
- SO2 –> 64.07 so 1 molecule of SO2 is 64.07 amu–> 64.07
Outline what one mole of any gas occupies
-same volume at standard temp AND pressure
State the ideal gas law and what the different parts mean
-pV=nRT
-n= n. of moles/ R= a constant/ V= 22.4 L/ T= 273 K/
P= 100 kPa (IUPAC definition)
Example: What is the n. of moles of a molecule of 10g of NaCl?
- 10/(35.5 + 23) = 0.171
Define “molarity”
-solution conc expressed as moles of solute dissolved in 1 litre of solution
What is the molarity formula?
-molarity= n. of moles/ volume in litres
Example: What mass of glucose needed to prepare 500 mL of 1 M solution?
- 1 mol/L= n. of moles (0.5 L) n=0.5
- Glucose (C6H12O6): (6x12.01) + (12 x 1.008) + (6x16)= 180.16 g/mol
- ->0.5 moles weight 0.5 x 180.16 g/mol = 90.08 g
State what “1mL” and “1 mg” is
- 1 mL= 1/1000 L
- 1 mg = 1/1000 g
What do chemical reactions involve?
- compound identity changes
- energy changes
- breaking AND making chemical bonds
- rearrangement of valence e-‘s
How do chemical equations “describe” chemical reactions?
- symbols represent elements
- formulae describe composition of compounds
- equations show changes in compounds
Define a “chemical equation”
-symbolic representation of chemical reaction
Finish off the following chemical reaction equations:
1- 2Na + Cl2…
2- CH4 + 2O2…
3- 4Al + 3O2…
1 –>2NaCl
2 –> CO2 + 2H2O
3 –> 2Al2O3
What is the general structure of a chemical equation?
-Reactants–> Products
Outline what coefficients are in terms of chemical reactions
- number which may be included in reaction equation before formula
- if no number then “1” known to be in front of formula
What do coefficients represent in the chemical reactions?
-the n. of molecules of that compound/ atom needed in reaction
Why are coefficients important in balancing equations?
-can change coefficient to achieve mass balance BUT subscripts can never be changed to balance equation
Outline what the following equation means in terms of states and atoms/molecules reacting:
Zn(s) + 2HCl (aq)–> ZnCl2 (aq) + H2 (g)
-1 atom (65g) of solid Zn reacts with 2 molecules (73g) of aqueous HCl acid to form 1 molecules (136g) of aqueous ZnCl2 and 1 mole (2g) of gaseous H
What is the meaning of the “yield of reaction”?
-the % of predicted yield actually obtained
Example: Using the same reaction of Zinc reacting with Hydrochloric acid what would the yield of the reaction be if you isolate 115 g of ZnCl2 from the reaction?
-(115/136) x100 = 84.6%
Why is balancing equations important?
- nothing comes from nothing
- can’t create/lose matter from closed system
- ->so must balance equation for each side to represent equal quantity of each element
Example reaction: Na reacting with Cl to form NaCl
What would the reaction equation be?
- Na + Cl2 –> NaCl
- -> 2Na + Cl2 –> 2NaCl
Reaction: 2C2H6 + 7O2 –> 4CO2 + 6H2O
Na2SO4/ CuCl2
Outline the first step to balancing chemical equations
(figure 2)
-write correct formula(e) for reactants left side AND correct formula(e) for products right side of equation
- C2H6 + O2 –> CO2 + H2O
- Reminder: write symbols for elements AND ions: Na/ SO4/ Cu/ Cl
- write valencies above AND right of symbols
–>Na^1SO4^2 Cu^2^Cl^1
–>reverse valencies and write numbers below AND to right:
Na2SO4 AND CuCl2
What is the second step to balancing equations?-explain
- change coefficients on front of molecular formulae to make n. of atoms of each element same both sides of equation
- ->don’t change subscripts
- 2C2H6 –> 2 molecules of methane AND C4H12–> not exist
- subscripts indicate n. of bonded atoms in molecule
- ->changing subscripts changes compound
State step 3 in balancing equations
- balance most complicated molecule first
- ->this is the one which contains many different atoms
- then use smaller molecules to adjust for this major change
- balance elements last
What is the final step to balance equations? (figure 3) and state the final balanced equation
- check to make sure have same n. of each atom type both sides of equation
- final equation: 2C2H6 + 7O2 –> 4CO2 + 6H2O
Key points to balancing equations
- don’t separate complex ions which not changed in reaction such as
- -> SO4 2-/ NH4+/ CO3 2-/ HCO2/ NO3-
- ->can often be considered as unit- not individual elements
- sometimes (like thermochemistry) showing state of each component of reaction important
Summary of lecture
- mole–>molar mass/ molecular mass/ molarity of solution
- chemical reactions
- chemical equations–> writing chemical equations/ balancing equations
