Thermochemistry

Chapter 5Regular Chem book = Ch 11

Hmmmmmm…• When we eat calories

• where is the HEAT in food?

• What do we do with it (what do we convert it into) when we “burn” calories?

• Food’s heat is in its bonds- it’s energy is being used to hold the particles together

• We break the bonds and release that energy- then we convert it to movement and heat

Try this…

• Place a PEA-SIZED piece of anhydrous (aka dried out) CuSO4 in your hand

• Add a few drops of water• What do you feel?• Wash your hands

Put the THERMO in Chemistry

Mg + HCl MgCl2 + H2

But, that’s only part of the story because this reaction gives off lots of heat!

+ heat

That turns a chemical equation into a thermochemical equation

2 types:

1. Endothermic• Reaction or process that

__________ heat• ΔH is • Reactants have _____ heat than

products

absorbspositive

less

2 types:

2. Exothermic• Reaction or process that

__________ heat• ΔH is • Reactants have _____ heat than

products

releasesnegative

more

Exo and Endo (cont.)

• For every exothermic process, there’s an endothermic partner• What’s exo/endo in…

• Freezing ice?• Boiling water?• BBQing sausages? • You outside on a super cold day?

Thermo Equations

There are 3 different ways to write a thermo equation:

1. H2O(l) + heat H2O(g)

2. H2O(l) H2O(g) ΔH= -285kJ

3. H2O(g)

H2O(l)

Try it!

1 mole of ice needs to absorb 285kJ of heat in order to melt. Write the thermo equation 3 ways.

Try it!

The combustion of methane releases 890kJ of heat. Write the equation for this reaction 3 ways.

Wait, combustion releases heat but you have to add heat to make it happen.

What’s up with that?

Heat?

• Measured in• Calories

• 1 Cal = 1 kilocal or kcal = 1000 cal• Joules

• 4.184 J = 1 calorie• 1000 J = 1 kJ

Thermo Stoich Calcs

CaO + H2O Ca(OH)2 + 65.2kJ

Which means that 1 mole of CaO produces 65.2kJ of heat

…and that adds another arrow to the molar conversion map

• How much heat will 4 moles produce?• How much will 100g produce?

• What mass of CaO is needed to make 1000kJ?

Interesting little tidbit… this is the reaction when you make cement. You add little rocks

and stuff to CaO and then add water.

4 x 65.2 = 260.8 kJ

100/56 x 65.2 = 116.4 kJ

1000/65.2 x 56 = 858.9g

Calculations Practice

Fe2O3 + 3CO 2Fe + 3CO2 + 26.3kJ

• Is this exo or endothermic?• How much heat would be produced

from 1.2 moles of CO?• How about from 100L of CO (at STP)

Exothermic

1.2/3 x 26.3 = 78.9

100 / 22.4 / 3 x 26.3 = 39.1kJ

Specific Heat

So… you’re out walking barefoot on a 90o day and you have the choice to step on(a) the sidewalk(b) a manhole cover(c) a puddle

What would you do?

Specific Heat II

(Specific) Heat capacity = the amount of heat it takes to raise 1g of a substance by 1oC

-- Water is high, metal is low

-- What else is high & low? (p. 296)

-- What’s up with the oranges on p. 297?

Specific Heat III

q = mCΔT

q is the heat in cal or Jm is the mass in gC is the specific heat in J/goC or

cal/goCΔT is the temp change in oC

Try #1-3, 8-10 on page 299

Calorimetry

• Method of measuring temperature change in order to calculate heat change• Measure temp change of water*• Use q=mCDT to find q…

• q absorbed by water= q released by reaction

q=mCΔT

Example #1: 94.6g of Cu heats up 2540oC by absorbing 849J, what’s its specific heat?

0.387 J/goC

0.03 J/goC, Mercury

Example #2: 100g of an unknown metal cools

6020oC by absorbing 132 cal of heat. What metal is it?