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CHEMICAL REACTIONS

More than just what scientists do, every single aspect of life is powered and made possible through chemical reactions
Being able to predict and control reactions has enabled us to make huge advances both in production and in our personal health

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CHEMICAL REACTIONS

THERE ARE SOME BASIC RULES THAT GOVERN HOW REACTIONS PROCEED

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LAW OF CONSERVATION OF MASS

States that in a CHEMICAL reaction the mass of the products always equals the mass of the reactants

Product – a term for the chemical/chemicals being made during a chemical reaction
Reactants – a term for the chemical/chemicals that are being made into a product

This can be changed to say that all the atoms that were in the reactants will also be in the products

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ACTIVATION ENERGY

ALL CHEMICAL REACTIONS REQUIRE AN AMOUNT OF ENERGY TO BEGIN THE REACTION THIS IS KNOWN AS ACTIVATION ENERGY, OTHERWISE THEY WOULD HAPPEN ENTIRELY ON THEIR OWN.

THE CONCEPT OF THINGS HAPPENING ON THEIR OWN IS KNOWN AS SPONTANEOUS

SOME CHEMICAL REACTIONS ACTIVATION ENERGY IS SO LOW THAT THE RANDOM ENERGY IN THE ENVIRONMENT IS ENOUGH TO START THEM. I.E., RUSTING

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CHANGE IN ENERGY

Some absorb energy

Others release energy, Exothermic

All chemical reactions will involve an overall change in energy near them

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RATE OF REACTION

CHEMICAL REACTIONS OCCUR AT DIFFERENT RATES

SOME ARE VERY SLOW; OXIDIZATION (RUST)

SOME ARE VERY FAST; LIKE COMBUSTION (BURNING)

SOME ARE INCREDIBLY FAST; LIKE THOSE THAT HAPPEN IN YOUR BODY AND EXPLOSIONS

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IDENTIFIABLE PROPERTIES

HOW TO KNOW WHAT YOU ARE LOOKING AT

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PHYSICAL AND CHEMICAL PROPERTIES

These describe the substance as it is (physical) or as it is being changed (chemical) into a new substance

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USING PROPERTIES TO IDENTIFY SUBSTANCES

By making several measurements or comparisons you can determine what a pure substance is

Density – “pure” substances have known values that you can compare to
Color/texture – many substances have signature colors/textures
Solubility – do they dissolve in water or in alcohol
Reactivity – take small samples and test to see if they react with water/acid/bases
Combustibility – Try to burn a small portion of it

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PROPERTIES AND CHEMICAL REACTIONS

Many times, the products of a chemical reaction will have properties similar to what they started with.

However, there will almost always be at least some changed properties and sometimes almost all the properties will change

Soft, silvery metal,

Reacts violently�with water

Gas nonmetal, with�a green color and

Extremely toxic

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SIGNS THAT A CHEMICAL REACTION HAS OCCURRED

It is important to be able to identify when a chemical reaction has occurred.

Now some of these signs can also signal a physical change, which can be misleading. Looking for multiple signs will help avoid mistaken identification.

5 main and easy signs that a chemical reaction has occurred

Change in color
Change in smell
Precipitate (solid) forms from mixing two liquids. (not at their melting point)
Bubbles/gases form from mixing substances (not boiling point)
Change in temperature (either increase or decrease)

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REACTION EXAMPLES

These could include any or all the 5 types from before, so you need to use almost all your senses. (Sorry no tasting today)

Poppers

Ammonia + HCl

Hand warmers

Salt Flames

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CHEMICAL EQUATIONS

HOW TO WRITE OUT THE CHEMICAL REACTIONS

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HOW TO READ AND WRITE MOLECULES

Here is Titin, the largest protein molecule we know of

C₁₆₉₇₂₃H₂₇₀₄₆₄N₄₅₆₈₈O₅₂₂₄₃S₉₁₂

That’s 539,030 atoms

This is done either using words (names for specific chemicals) or symbols.

Usually choosing which one is shorter

In chemistry we use the fastest shortest way to write out the molecules and chemical reactions

In large part because of the length that we can be faced with. Some of the largest molecules are made of tens of thousands or hundreds of thousands atoms… it would be like writing a book for a simple one step reaction

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HOW TO READ AND WRITE MOLECULES

Use element symbols (O, S, H, Na etc.) with subscripts (these are like exponents but instead are set lower than the numbers, i.e. O₂) and coefficients (2He)

The symbols tell you which elements are part of the molecule
Subscripts tell you how many of that specific element is in the molecule, H₂O
Coefficients tell you how many of the molecules there are, 3H₂O

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COMMON MOLECULES

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HOW TO WRITE CHEMICAL EQUATIONS

Every chemical equation must have

One or more reactants
One or more products
An arrow directing reactants to products
If there are more than one reactants or products, the chemical names/formulas are separated by a “+” sign (yes even on the product side)

Mg + 2HCl MgCl₂ + H₂

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THE MOST IMPORTANT CHEMICAL EQUATION IN 8^TH GRADE

and arguably for all living things

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PROPORTION IN CHEMICAL EQUATIONS

The coefficients in a Chemical equation tell us the proportion of the reactants to each other

For example Hydrogen and Oxygen react with each other in a 2:1 ratio.

2H₂ + O₂2 H₂O

You can also talk about proportions with the amount of reactants to the amount of products

For example it takes 6 sets of Water and Carbon Dioxide to make 1 molecule of Glucose
6H₂O + 6CO₂ C₆H₁₂O₆ + 6O₂

Proportion describes the relationship of one thing to another when it comes to things such as quantity.

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SCALE WITH ATOMS AND MOLECULES

This is all about comparing, the nice thing is that we don’t need to be completely specific.

For example you don’t need to know that a Fluorine atom is smaller than an Oxygen atom or that a Hydrogen atom is the smallest one possible
You just need to know that atoms in general are small

If we compare Atoms to Molecules, Molecules are larger.

Since a molecule is made up of two or more atoms, two is more than 1
Again think Lego, is a Lego set bigger than just a Lego piece?

Scale is a term we use to describe the relative size or extant of something