Reactions in Aqueous Solutions

By: Shivali Gupta, Michelle Sodipo, Sae’Von Springer , Timothee

Schlumburger, Anju Shilpa Suresh, Destiney Randolph

Predicting Whether a Reaction will

OccurFormation of a solidTransfer of electronsFormation of waterFormation of a gas

If any of these happen, a reaction is likely to take place.

Reactions in Which a Solid Forms

One kind of reaction which results in the formation of a solid is precipitation, which forms a precipitate.By mixing certain solutions of compounds, a chemical reaction occurs resulting in a solid.When ionic compounds dissolve in water, the ions separate and float freely in the solution.

We can test this by testing the electrical conductivity of the solutions”

Pure water, with no ions present cannot conduct electricity.

Water with ions present conducts electricity.This kind of solution is called a “strong electrolyte”

Ex. Barium Nitrate because each Ba(NO3)2 compound produces the ions Ba(2+), NO3(1-), and NO3(1-)

By finding these ions, or reactants, we can then proceed to make some educated guesses about the possible products that these ions can form.

To make an educated guess, you must first take each ion, and consider all the possible combinations.

Eliminate all the unlikely combinations, for example, all the combinations that have two cations or two anions.

However, to specify the formation of a solid product, you have to ensure that the combination has opposite charges.

￫ Na(1+) and Cl(1-) => solid￫ (vs.) Be(2+) and F(1-) => Not a solid

Describing Reactions in Aqueous Solutions

Molecular equation- a chemical equation showing the complete (undissociated) forms of all reactants and products.

Ex: K2CrO4(aq) + Ba(NO3)2(aq) BaCrO4(s) + 2KNO3(aq)

Complete Ionic Equation- a chemical equation for a reaction in solution representing all strong electrolytes as ions.

2K+(aq) + CrO4-2(aq) + Ba+2(aq) +

2NO3-(aq)

BaCrO4(s) + 2K+(aq) + 2NO3-(aq)

Spectator Ions- an ion present in solution that does not participate in a reaction

Net Ionic equation- a chemical equation for a reaction in solution showing only those components that are directly involved in the reaction. Strong electrolytes are represented as ions.

Ex: Ba+2(aq) + CrO4-2(aq) BaCrO4(s)

Acids!Originally associated with citrus fruits

Acid = Latin “Acidus” = Sour

Mineral Acids, such as sulfuric (H2SO4) & nitric acid (HNO3) found around 1300

Derived from from the treatment of minerals

In 1800s, Svante Arrhenius discovered nature of acids

Svante ArrheniusSwedish physics graduate

Experiment showed that HCL, HNO3, and H2SO4 behaved as strong electrolytes, because of ionization reactions.

Nearly every molecule of acids breaks down into ions when placed in water.

Acid = Substance that produces H+ ions (protons) id Acids when it is dissolved in water

Strong Acids = Acid that breaks apart to make H+ ions in solutions

BasesAqueous solution that exhibit basic behavior always contain hydroxide ions

Base = Substance that produces hydroxide ions (OH-) in water

Strong Base = Substance that breaks apart to form OH- ions in solutions

When strong acids and strong bases are mixed, H= and OH-, the chemical change forms water molecules

In Chapter 3, we talked about ionic compounds which are compounds formed in the reaction of a metal

and a nonmetal.

2Na(s) + Cl2(g) 2NaCl(s)

Sodium metal is composed of sodium atoms (net charge of zero) with seven protons & electrons

Similarly, the chlorine molecule consist of two uncharged chlorine atoms with seventeen protons & electrons

HOWEVER, in the product (sodium chloride), the sodium and chlorine is presented as Na+ and Cl- respectively

Na + Cl Na+ and Cl-

One electron is transferred from each sodium atom to each chlorine atom

This process coverts neutral atoms to become ions

Na + Cl Na+ and Cl-

An electron from sodium is transferred over to Chlorine. Now, sodium’s charge, which was originally zero because of the equal number of electrons and protons, is now positive because it gave away one of it’s electrons.

Similarly, Chlorine’s charge, which was also originally zero with a balance of seventeen protons and electrons, is now negative as it receives an extra electron from sodium.

In a nutshell

In order to form an ionic bond, it is necessary for the metal(cation) to transfer over one more electrons to the non-metal (anion)

This reaction of transferring over electrons is called OXIDATION-REDUCTION REACTION.

In the world around us and even in our bodies, literally

millions of chemical reactions are taking place.

What causes these chemical reactions?

“Driving Forces”

Chemical reactions have four major “driving forces”

Formation of a Solid

Formation of Water

Transfer of Electrons

Formation of a Gas

Formation of a Solid

Precipitation- formation of a solid when tow solutions are mixed

Precipitate- solid formed by mixing two solutions

K2CrO4 (aq) + Ba(NO3)2(aq) BaCrO4(s) +2KNO3 (aq)

NOTICE in this reaction that the two anions (NO3- and CrO42-) are exchanged.CrO42- was originally associated with K2 and NO3- was originally

associated with Ba2+ In the product, NO3- now associates with K2 and CrO42- now

associates with Ba2+In this reaction, associations are REVERSED

We call this DOUBLE-DISPLACEMENT REACTION.

AB + CD AD + CB

Formation of Water

strong acid + strong base formation of water

H+ (aq) + OH- (aq) H2O (l)We classify these reactions as ACID-BASE

REACTIONS

Involves an H+ ion that ends up in the product water

Transfer of Electrons

We see this driving force particularly in the “desire” of metal to donate electrons to non metals.

2Li(s) + F2(g) 2LiF(s)Each lithium atom loses one electron to form Li+ ion

Each fluorine atom gains one electron to form F- ion

Process of electron transfer is called OXIDATION-REDUCTION

Formation of a Gas

A reaction in aqueous solution that forms a gas (which escapes as bubbles) is pulled toward the products by this event.2H+(aq) + CO32-(aq) CO2(g) + H2O(l)

Note that it forms carbon dioxide gas as well as water which illustrates two driving forces.

Because this reaction involves H+ ion that ends up in the product water, what type of

reaction do we classify this as?

ACID-BASE REACTION

How about this one

Zn(s) + 2HCl(aq) H2(g) + ZnCl2(aq)NOTICE, in the reactants, zinc exists as uncharged atoms,

whereas in the product it exists as Zn2+. This can only mean that each Zn atom lost 2 ELECTRONS.

So what happened to the electrons? The electrons have been transferred to two H+ ions to

form H2What reaction has taken

place?

OXIDATION-REDUCTION REACTION!

BUT WAIT, THERE’S MORE!

Do you recall the double-displacement reaction where associations were reversed?

K2CrO4 (aq) + Ba(NO3)2(aq) BaCrO4(s) +2KNO3 (aq)

In double-displacement reaction, TWO anions (NO3- and CrO42-) have been exchanged.

So, what does this have to do with this chemical formula?

Zn(s) + 2HCl(aq) H2(g) + ZnCl2(aq)In this chemical formula, ONE anion (Cl-) is being exchanged between

H+ and Zn2+We call this SINGLE-REPLACEMENT

REACTION

A + BC B + AC

Other Ways to Classify Reactions

Combustion ReactionsCombustion reactions- a chemical reaction involving oxygen as one of the reactants that produce enough heat so that a flame results.

Ex. Natural Gas reacts with Oxygen

CH4 + 2O2 (g) CO2 (g) + 2H2O (g)

Combustion reactions, are a special class of oxidation-reduction reactants

Many combustion reactions may provide heat or electricity for homes or energy for transportation.

Ex. Combustion of Gasoline (used to power cars &trucks)

2C8H18 (l) +25O2 (g) 16CO2 (g) + 18H2O (g)

Synthesis (Combination) ReactionsSynthesis reactions (Combination reaction) – a chemical reaction in which a compound is formed from simpler materials.

Ex. Not involving oxygen 2Na(s) + Cl2 (g) 2NaCl(s)

Ex. Starting with elements 2H2 (g) + O2 (g) 2H2O (l)Synthesis reactions are another subclass of the oxidation – reduction class of reactions.

Decomposition Reactions

Decomposition reactions – a chemical reaction in which a compound is broken down into simpler compounds or to the component elements.

Ex. Heat

2HgO(s) 2Hg (l) + O2

A decomposition reaction, in which a compound is broken down into its elements, is just the opposite of synthesis (combination) reaction in which elements combine to form the compound