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What are the indicators of a chemical reaction

DIRECTIONS: Answer the questions below on the Webquest Worksheet.

5) Look at the Examples of chemical changes. In what way are the chemical changes different than the listed physical changes?

7) Balancing equations. What is the only thing that can be added to a chemical equation in order to balance it?

8) Write the balanced equation for nitrogen and hydrogen gas forming ammonia. Why is there a 2 placed in front of ammonia?

11) Balancing combustion (burning) equations. What can be difficult to balance in a combustion reaction? How is this problem solved?

12) Synthesis reactions. How can a synthesis reaction be recognized?

13) Decomposition reactions. How can this reaction be recognized?

14) Single replacement reactions. How can this reaction be recognized?

15) Double replacement reactions. How can this reaction be recognized?


A chemical reaction is material changing from a beginning mass to a resulting substance. The hallmark of a chemical reaction is that new material or materials are made. This does not mean that new elements have been made.

(In order to make new elements, the nuclear contents must change.There are

magnitudes of difference in the amounts of energy in ordinary chemical reactions

compared to nuclear reactions, the rearrangement of the nuclei of atoms to change to

new elements is enormous compared to the smaller energies of chemical changes.

This is why the ancient alchemists were never successful - they did not understand the

difference between nuclear and chemical changes.)

A chemical equa tion is a way to describe what goes on in a chemical reaction, the actual change in a material. Chemical equations are written with the symbols. There is an arrow pointing to the right that indicates the action of the reaction. The materials to the left of the arrow are the reactants. or materials that are going to react. The materials to the right of the arrow are the products. or materials that have been produced by the reaction.

The Law of Conservation of Mass states that in a chemical reaction no mass is lost or gained. The Law of Conservation of Mass applies to individual types of atom. One could say that for any element, there is no loss or gain of that element in a chemical reaction.

There are such things as reversible reactions, reactions in which the products reassemble to become the original products. Reversible reactions are symbolized in chemical equations by a double-headed arrow, but the standard remains to call the materials on the left the reactants and the materials on the right the products.


Indicators of chemical changes

Color changes

solid formation

bubbles of gas formation

increase or decrease in temperature

color disappearance


Chemical reactions (also called chemical changes) are not limited to happening in a chemistry lab. Here are some examples of chemical reactions with the corresponding chemical equations:

A silver spoon tarnishes. The silver reacts with sulfur in the air to make silver sulfide, the black material we call tarnish.

2 Ag + S

Ag 2 S

An iron bar rusts. The iron reacts with oxygen in the air to make rust.

4 Fe + 3 O 2

2 Fe 2 O 3

Methane burns. Methane combines with oxygen in the air to make carbon dioxide and water vapor.

CH 4 + 2 O 2

CO 2 + 2 H 2 O

An antacid (calcium hydroxide) neutralizes stomach acid (hydrochloric acid).

Ca(OH) 2 + 2 HCl

CaCl 2 + 2 H 2 O

Glucose (simple sugar) ferments to ethyl alcohol and carbon dioxide. The sugar in grapes or from grain ferments with yeast to make the alcohol and carbon dioxide. The carbon dioxide is the gas that bubbles out of beer or champaign.

C 6 H 12 O 6 ( glucose )

2 C 2 H 5 OH ( ethyl alcohol ) + 2 CO 2

Alcohol plus oxygen becomes vinegar and a molecule of water. As in the fermentation of glucose, this is a more complex reaction than it appears here because it is a biochemical reaction.

C 2 H 5 OH + O 2

HC 2 H 3 O 2 + H 2 O

As a general rule, biochemical happenings make poor examples of basic chemical

reactions because the actual reaction is carried on within living things and under enzyme control.


Here are some examples of changes that are NOT chemical reactions. In each case, the original material or materials may be reclaimed by physical processes.

Water boils out of a kettle or condenses on a cold glass.

An aluminum pot is put on a burner and gets hot.

Dry ice goes from a solid to a gaseous form of carbon dioxide (sublimation ).

Gold melts or solidifies.

Sand is mixed in with salt.

A piece of chalk is ground to dust.

Glass breaks.

An iron rod gets magnetized.

A lump of sugar dissolves in water.


In order to write the chemical equations, you must first know the formulas for the materials involved. The formulas must be written on the proper side of the arrow - - reactants on the left and products on the right. The order in which the reactants and products are written does not matter, just as long as every material is on the proper side.

Very often you will see the descriptions of the materials in the reaction in parentheses after the material. A gas is shown by (g ). A solid material is shown by (s ). A liquid is shown by (l ). A material dissolved in water (an aqueous solution) is shown by (aq ).

Now comes the fun part, balancing the reaction. The Law of Conservation of Mass states that in a chemical reaction there is no loss of mass. Each type of element will have the same amount before the reaction and after the reaction . You cannot change the subscripts but you can write an integer coefficient in front of each compound or element in the reaction to make sure every type of atom has the same number on each side of the yield sign.

Nitrogen gas plus hydrogen gas under pressure and at high temperature turn into ammonia. First write the formulas correctly. Nitrogen and hydrogen are diatomic gases. The nitrogen and hydrogen are the reactants, and the ammonia is the product. Leave room for the coefficients in front of the materials.

_ N 2 + _ H 2

_ NH 3

You can begin with either the nitrogen or the hydrogen. There are two nitrogen atoms on the left and only one on the right. In order to balance the nitrogen atoms, place a 2 in front of the ammonia.

_ N 2 + _ H 2

2 NH 3

There are two hydrogens on the left and six on the right. We balance the hydrogens by placing a 3 in front of the hydrogen gas.

_ N 2 + 3 H 2

2 NH 3

Now go back and check to make sure everything is balanced. There are two nitrogen and six hydrogens on both sides of the reaction. It is balanced. There is no coefficient shown in front of the nitrogen. There is no need to write ones as coefficients. The reaction equation is:

N 2 + 3 H 2

2 NH 3


Silver (I) nitrate and calcium chloride solutions combine to produce a precipitate of silver (I) chloride and a solution of calcium nitrate. This time we have polyatomic ions in the reaction.

_ Ag(NO 3 ) + _ CaCl 2

_ AgCl + _ Ca(NO 3 ) 2

Notice that from one side to the other there is no change in the nitrate ion. In this case you can count the nitrate ion as a whole rather than splitting it up into nitrogen and oxygen. Your thoughts might go this way: How many silvers on the right? One. How many silvers on the left? One. They are the same. How many nitrates on the left? One. How many nitrates on the right? Two. We need to put a coefficient of two in front of the silver nitrate.

2 AgNO 3 + _ CaCl 2

_ AgCl + _ Ca (NO 3 ) 2

This changes the balance of silvers, so we have to put a two in front of the silver chloride.

2 AgNO 3 + _ CaCl 2

2 AgCl + _ Ca (NO 3 ) 2

Now let's check again. Two silvers on each side. Two nitrates on each side. One calcium on each side and two chlorides on both sides. The balanced reaction is:

2 AgNO 3 + CaCl 2

2 AgCl + Ca (NO 3 ) 2

Sometimes counting the polyatomic ion as a whole (if it appears on both sides of the equation) rather than breaking it up into its individual ions makes balancing easier.


Sulfuric acid and potassium hydroxide neutralize each other to make water and potassium sulfate. This reaction is an acid-base neutralization. The products will always be a salt (Not necessarily common table salt.) and water.

_ H 2 SO 4 + _ K (OH)

_ K 2 (SO 4 )

+ _ H 2 O

The water is made from the hydrogen ion of the acid and the hydroxide ion of the base. Notice that it is a lot easier to understand how to balance the reaction if you write the water as if it were an ionic compound.

_ H 2 (SO 4 )+ _ K (OH)

_ K 2 (SO 4 ) + _ H (OH)

This is easier now because the hydrogen in the acid does not get confused with the hydrogen in the hydroxide of the base. Two hydrogens on each side. One sulfate on both sides. Two potassiums and two hydroxides on each side.

H 2 (SO 4 )+ 2 K(OH)

K 2 (SO 4 ) + 2 H(OH)

The reaction is now balanced.

Next is an example of having to go around the equation again. Phosphoric acid and calcium hydroxide react to make water and calcium phosphate.

_ H 3 PO 4 + _ Ca(OH) 2

_ H (OH) + _ Ca 3 (PO4) 2

First put a three on the water to balance the hydrogen in the phosphoric acid.

_ H 3 PO 4 + _ Ca(OH) 2

3 H(OH) + _ Ca 3 (PO4) 2

Now put a two on the phosphoric acid to balance the phosphate from the calcium phosphate.

2 H 3 PO 4 + _ Ca(OH) 2

3 H(OH) + Ca 3 (PO4) 2

We have changed the amount of hydrogen ion, so we will have to change it on the right again.

2 H 3 PO 4 + _ Ca(OH) 2

6 H(OH) + Ca 3 (PO4) 2

And change the coefficient in front of the Ca(OH) 2 to match the calcium on the right side.

2 H 3 PO 4 + 3 Ca(OH) 2

6 H(OH) + Ca 3 (PO4) 2

Only now does the rest of the equation balance with six hydrogens, six hydroxides, two phosphates, and three calciums on each side.

This could have been a very difficult equation to balance but using this TIP of breaking the water in H(OH) makes balancing a little easier.


Most burning reactions are the oxidation of a fuel material with oxygen gas. Complete burning produces carbon dioxide from all the carbon in the fuel, water from the hydrogen in the fuel, and sulfur dioxide from any sulfur in the fuel. Methane burns in air to make carbon dioxide and water.

_ CH 4 + _ O 2

_ H 2 O + _ CO 2

Easy. Put a two in front of the water to take care of all the hydrogens and a two in front of the oxygen.

CH 4 + _ O 2

2 H 2 O + CO 2

CH 4 + 2 O 2

2 H 2 O + CO 2

Sometimes it is difficult to get the Oxygens to balance. Let's consider the equation for the burning of butane, C 4 H 10.

_ C 4 H 10 + _ O 2

_ CO 2 + _ H 2 O

We balance the Carbon and Hydrogen by inserting the coefficients for carbon dioxide and water: 4 Carbons on both sides and 10 Hydrogens on both sides.

_ C 4 H 10 + _ O 2

4 CO 2 + 5 H 2 O

But we now have two oxygens on the left and thirteen oxygens on the right. The real problem is that we must write the oxygen as a diatomic gas. The trick is knowing that (like an algebraic equation) you can multiply both sides (except for oxygen) by the same thing and not change the equation. Multiply both sides by two to get the following.

2 C 4 H 10 +. O 2

8 CO 2 + 10 H 2 O

Now the oxygens are easy to balance. There are twenty-six oxygens on the right, so the coefficient for the oxygen gas on the left must be thirteen.

2 C 4 H 10 + 13 O 2

8 CO 2 + 10 H 2 O

Now it is correctly balanced. What if you finally balanced the same equation with:

4 C 4 H 10 + 26 O 2

16 CO 2 + 20 H 2 O

6 C 4 H 10 + 39 O 2

24 CO 2 + 30 H 2 O

Either equation is balanced, but not to the lowest integer. Algebraically you can divide these equations by two or three to get the lowest integer coefficients in front of all of the materials in the equation.

Now that we are complete pyromaniacs, let's try burning isopropyl alcohol, C 3 H 7 OH. (Ask Miss Segars why Miss Floyd had difficulty with this reaction last year in the lab!!)

_ C 3 H 7 OH + _ O 2

_ CO 2 + _ H 2 O

First take care of the carbon and hydrogen.

_ C 3 H 7 OH + _ O 2

3 CO 2 + 4 H 2 O

But again we come up with an oxygen problem. The same process works here. Multiply the whole equation ( except oxygen ) by two.

2 C 3 H 7 OH +. O 2

6 CO 2 + 8 H 2 O

Now the number nine fits in the oxygen coefficient. (Do you understand why?) The equation is balanced with six carbons, sixteen hydrogens, and twenty oxygens on each side.

2 C 3 H 7 OH + 9 O 2

6 CO 2 + 8 H 2 O

More Explosions here

The hallmark of a synthesis reaction is a single product. A synthesis reaction might be symbolized by:

A + B


Two materials, elements or compounds, come together to make a single product. Some examples of synthesis reactions are: Hydrogen gas and oxygen gas burn to produce water.

2 H 2 + O 2

2 H 2 O and

sulfur trioxide reacts with water to make sulfuric acid.

H 2 O + SO 3

H 2 SO 4

What would you see in a test tube if you were witness to a synthesis reaction? You would see two different materials combine. A single new material appears.

Magnesium Burning. Oxygen in the air has a strong affinity for extra electrons and magnesium atoms can readily give up electrons. The reaction gives Mg 2+ ions and O 2- ions which are strongly attracted to each other to form MgO, thus giving off lots of energy.

Salt Synthesis

A single reactant comes apart into t wo or more products. symbolized by:


X + Z

Some examples of decomposition reactions are: potassium chlorate when heated comes apart into oxygen gas and potassium chloride

Carbon Snake


Here is an example of a single replacement reaction: silver nitrate solution has a piece of copper placed into it. The solution begins to turn blue and the copper seems to disappear. Instead, a silvery-white material appears.

2 AgNO 3 + Cu

Cu(NO 3 ) 2 + 2 Ag

A solution of an ionic compound has available an element. The element replaces one of the ions in the solution and a new element appears from the ion in solution. This type of reaction is called a replacement because a free element replaces one of the ions in a compound. There are two types of single replacement reactions, anionic and cationic. A cationic single replacement is what happened in the case of the silver being replaced by the copper in the above reaction because both the silver and the copper are only likely to make cations. An anionic single replacement is also possible. Into a potassium iodide solution chlorine gas is bubbled. The chlorine is used up and the solution turns purple-brown from the iodine. This is an example of an anionic single replacement reaction.

2 KI + Cl 2

2 KCl + I 2

Could you start with copper II nitrate and silver metal and get silver nitrate and

copper metal, or could you start with potassium chloride and iodine and get

potassium iodide and chlorine? No. The reactions don't work that way.

You can arrange cations or anions in a list of which ion will replace the next.

This type of list is an activity series. The activity series of cation elements (metals)

shows that gold is the least active metal. That should not be surprising, because gold

does not tarnish.


AgNO 3 + KCl

AgCl (s ) + KNO 3

Dissolved silver nitrate becomes a solution of silver ions and nitrate ions. Potassium chloride ionizes the same way. When the two solutions with all their separated ions are mixed together, the silver ions and chloride ions find each other and become a solid precipitate. (They drop out of the solution, this time as a solid.) Since silver chloride is insoluble in water, the ions take each other out of the solution.

Ag + + (NO 3 ) - + K + +Cl -

AgCl + K + + (NO 3 ) -

Category: Forex

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