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# Chem10 Topic 01 - Thermochemistry - PowerPoint PPT Presentation

Chem10 Topic 01 - Thermochemistry. Science 10 CT01D04. Stoichi what?. Stoikheion ( greek )= element Metron ( greek )= to measure Stoichiometry: the calculation of amounts of substances involved in chemical reactions This will be Topic 01 in IB Chemistry. Moles.

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### Chem10 Topic 01 - Thermochemistry

Science 10

CT01D04

• Stoikheion (greek)= element

• Metron (greek)= to measure

• Stoichiometry: the calculation of amounts of substances involved in chemical reactions

• This will be Topic 01 in IB Chemistry

• Moles (latin): heap or pile

• 1 Mole (mol) = 6.02 x 1023 representative particles (atoms or molecules)

• This is called Avogadro’s number

• The atomic mass of one mole of an element or molecule expressed in grams

• Round to 2 decimal places, your periodic table should already have this!

• Step One: Find the molar mass (atomic mass) of each element on the PT

• round to 2 decimal places

• Unit is grams (g) per mole (mol)

• Step Two: Multiply the number of atoms of the element by the molar mass

• Remember – the subscript tells you how many atoms you have of each element

• Step Three: Add up the masses of all elements in the compound

• H2O is 18.02 g/mol

• Stated as “grams per mol”

• Meaning 18.02 grams = 1 mole

• OR 1 mole = 18.02 grams

• Use like common conversions

• How many meters in 1.000 km?

• Use the conversion of 1000 m = 1.000 km

(Molar Volume of a gas 22.4)

Moles to Molecules

Moles to Mass

(use Molar Mass)

- Mole / Mass Conversions -

Use the Molar Mass of a substance to convert from Moles to Mass and Mass to Moles

80. g CuSO4

1 mol CuSO4

Mass to Moles 

= 0.50 mol CuSO4

159.5 g CuSO4

0.50 mol CuSO4

159.5 g CuSO4

Moles to Mass 

= 80. g CuSO4

1 mol CuSO4

- Mole / Molecule Conversions -

Use Avogadro’s Number : 6.022 x 1023 molecules (mc) in one mole of the substance

2 mol CuSO4

6.022x1023 (mc) CuSO4

Moles to (mc) 

= 1.2x1024 (mc) CuSO4

1 mol CuSO4

1.2x1024 (mc) CuSO4

1 mol CuSO4

(mc) to Moles 

= 2 mol CuSO4

6.022x1023 (mc) CuSO4

• You needed to calculate q (heat change) for NaOHand Na2S2O3. Lets use NaOH as our example:

• qNaOH cannot be found directly, but qH2O can be

• qNaOH = - (qH2O) (this is the conservation of energy)

• qH2O = msΔT

• m = 50.00 g

• s = 4.184 J/goc

• ΔT =20.00 oc

• qH2O = (50.00 g)(4.184 J/goc)(20.00oc)

• qH2O = 4,184 J

• qNaOH = -4,184 J

• Now, how many grams of NaOH were used to produce this amount of heat? (for my data it was 1.24 g NaOH)

First Law of Thermodynamics!

Energy lost = Energy gained

q = -q

• For a 7 on the Heat Change Lab you were asked to calculate the molar heat change

• In my reaction example, with 1.24 g NaOHI produced a value of qNaOH= -4,184 J

• So, lets write this in units of Joules per gram

• qNaOH =

• You need to find Joules per mole (molar heats of solution) so that you can relate to the heat production of the other salt.

• NaOH(s)  Na+(aq) + OH-(aq) qNaOH = -134 kJ/mol

• Na2S2O3(s)  2 Na+(aq) + S2O32-(aq) qNa2S2O3 = + or - ??

NaOH(s)

Is DH negative or positive?

System gives off heat

Exothermic

DH < 0

Na+(aq) + OH-(aq)

134 kJ are released for every 1 mole of sodium hydroxide that is dissolved into water.

• NaOH(s)  Na+(aq) + OH-(aq) qNaOH= -134 kJ/mol

• ΔH = -134 kJ/mol