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## PowerPoint Slideshow about ' Thermochemistry' - noam

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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:

- Endothermic
- Reaction or process that __________ heat
- ΔH is
- Reactants have _____ heat than products

absorbs

positive

less

2 types:

- Exothermic
- Reaction or process that __________ heat
- ΔH is
- Reactants have _____ heat than products

releases

negative

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?

- What’s exo/endo in…

Thermo Equations

There are 3 different ways to write a thermo equation:

- H2O(l) + heat H2O(g)
- H2O(l) H2O(g) ΔH= -285kJ
- 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

- Calories

Thermo Stoich Calcs

Interesting little tidbit… this is the reaction when you make cement. You add little rocks and stuff to CaO and then add water.

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?

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 J

m is the mass in g

C is the specific heat in J/goC or cal/goC

ΔT 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

Example #2:

100g of an unknown metal cools 6020oC by absorbing 132 cal of heat. What metal is it?

0.03 J/goC, Mercury

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