lawofconservationofmass-2nd time around.pptx

8/10/2019 lawofconservationofmass-2nd time around.pptx

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Physical Science-

5.2.8.B.1 , 5.2.8.B.2

Physical Science- 5.2.8.B.1 ,5.2.8.B.2


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10/8/14

Do Now: When water changes from solid to liquid to gas, is

the mass of the water ZERO? Could I gather it all back

together?

Objective: SWBAT understand the Law of Conservation of

Mass


3/23

Weve talked about changesin matter

The evaporation of a puddle of water

Rust forming on a metal fence


4/23

The Law of Conservation of

Mass

In the late eighteenth century, AntoineLavoisier, a French chemist, recognized

the importance of accurate

measurements. He extensively studied

and explained the nature of combustion.

He found out that combustion involvedreaction with oxygen.


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Law of Conservation of Mass

Total mass of reactants=

Total mass of productsAntoine Lavoisier

Mass is neithercreated nor

destroyed during

chemical or physicalreactions.


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How did scientists come up

with this law?

Just like we did with the vinegar and

baking soda in a balloon

We measured the mass of the reactants

(vinegar and baking soda) and then, after

the reaction took place, we measured the

mass of the products (reactants + carbon

dioxide)


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The Law of

Conservation of Matter So we know that during a chemical reaction, matter

cannot be created or destroyed.

Even though the matter may change from one state ofmatter to another(from liquid-solid, gas) (or just liquid togas), the same number of atoms exists before and after

the change takes place!


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Since the overall mass of the

matter does not change

The mass of the reactantsthe substances there

before the reaction occurs must equalthe mass of the

productthe substances there after the reaction takes

place.

Mass of

Reactants

Mass of

Products=


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10/23

10/9/14

Do Now: What is an example of a reactant and a

product? Think back to our lab

Objective: SWBATunderstand the Law of Conservation

of Mass
http://www.brainpop.com/science/matterandchemistry/conservationofmass/http://www.brainpop.com/science/matterandchemistry/conservationofmass/http://www.brainpop.com/science/matterandchemistry/conservationofmass/http://www.brainpop.com/science/matterandchemistry/conservationofmass/http://www.brainpop.com/science/matterandchemistry/conservationofmass/http://www.brainpop.com/science/matterandchemistry/conservationofmass/


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Fe Fe

Fe Fe

O2

O2 O2

Fe2O3 Fe2O3

TAKE A

LOOK AT

THE

NUMBERS

How many Ironmolecules?

How many

Oxygen

molecules?

What about

the PRODUCT?


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The same number of each kind of atom must be on the left side of

the arrow as are on the right side when an equation is balanced.

Al Al

Al Al

O O

O OO O

Al AlOO O

Al AlOO O


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Oxygen, as a product, must be multiplied by 2.A coefficient is placed in front of thecompounds that contains O.

Oxygenatoms are =

When a coefficient is added you must multiplyall atoms in the compound by this number.

Redo the inventory

After redoing the inventory, add coefficientsto balanced other atoms. With each additionalcoefficient the inventory must be updated.

Oxygenatoms are

Hydogenatoms are =

2 H atoms 2 H atoms

2 O atoms 1 O atom

Hydogenatoms are

Oxygenatoms are =

First: Inventory the atoms on both sides of theyield arrow

Balancing an equation:

H2 + O2 H2O

HH

HO O O

H

Second:Determine which atoms are not equal andadd coefficients to balance these atoms.

2

4 H atoms

2 O atoms

2

4 H atoms

2 O atoms

Hydogenatoms are =

This is now a balanced equation

l


14/23

So: Chemical equations are balanced, when the numbers andkinds of atoms on each side of the reaction arrow are equal.

HH

H

H O O

Reactants Products

HH

HH

OO

4 Hydrogen 4 Hydrogen

2 Oxygen 2 Oxygen

B t t


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Because atoms are conserve , t e massothe reactants in the chemical reaction is

equal to the mass of the products.

H2 + F2 2 HF

2g + 38g = 2( 20g)

40g = 40g

40 g 40 gReactants Products

H H F F H F H F


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If 27.0 g of mercuric oxide (the red solid compound) isheated and completely decomposes to give the elementsoxygen and mercury. 2.0 g of oxygen are produced. How

many grams of mercury are produced?

mercuric oxidemercury + oxygenHgO Hg + O


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Reactants Products

27.0 gHgO

? g Hg

2.0 g O

27 g HgO = 2 g O + ? Hg = 25 g Hg


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19/23

Which of the following chemical reactionsindicate that mass in conserved?

A. Mg + Cl22 MgCl

B. 2 Ca + O22 CaO

C. Zn + S 2 ZnS

D. C + O22 CO

The correct answer is B

CaCa

OO

OOCa

Ca

Because atoms must be equal on each sideof the equation:


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Determining the Mass of

Reactants and Products REMEMBER: The mass of the reactants is always equal to

the mass of the products.

Mass of

Reactants

Mass of

Products


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Example: Tin Fluoride

Tin + hydrogen fluoride -> tin fluoride +hydrogen

Whats the total mass of the reactants?

158.72 g

So what should the mass of the products be? 158.72 g

118.7g

40.02g


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Total Mass of Reactants:158.72 g

Tin + hydrogen fluoride -> tin fluoride + hydrogen

If we know the mass of the tin fluoride, can we figure out howmuch hydrogen was produced? Subtract the mass of one product from the total mass.

Suppose 100 g of iron metal becomes rusty We weigh the rust and find that the


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Suppose 100 g of iron metal, becomes rusty. We weigh the rust and find that therust has a mass of 143 g. What mass of oxygen reacted with the iron?

A. 243 g

B. 57 gC. 100 gD 43 g

Iron + Oxygen Rust 100 g + ?g 143g

What mass of

oxygen is needed?

43 g Oxygen Choice D

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