Week 4 - Reactions in aqueous solutions Flashcards
Electrolytes and non-electrolytes
Electrolytes are compounds whose aqueous solutions conduct electricity. Nonelectrolytes are substances whose aqueous solutions are nonconductors.
A conductivity apparatus shows the difference in conductivity of solutions.
It is the movement of ions that conduct electricity in water.
Acids, bases and salts are electrolytes because they produce ions in water when they dissolve.
Strong and weak electrolytes
Strong electrolytes are essentially 100% ionised in water. An example of a strong electrolyte is HCl.
Weak electrolytes are much less ionised. An example of a weak electrolyte is acetic acid, HC2H3O2.
Acids and bases that are weak electrolytes are called weak acids and weak bases. Acids and bases that are strong electrolytes are called strong acids and strong bases.
What is dissociation
Sodium chloride consists of an orderly arrangement of Na+ cations and Cl- anions. When dissolved in water, the ions separate. This is called dissociation. All salts dissociate into cations and anions when they dissolve in water.
NaCl(s) → Na+(aq) + Cl–(aq)
This forms hydrated sodium and chloride ions; each Na+ and Cl– ion become surrounded by water molecules.
The negative end of the water dipole is attracted to the Na+ ion, and the positive end is attracted to the Cl– ion.
What is precipitate
When solutions are mixed, sometimes we see a single solution forming, if all ions are soluble in the solvent, but sometimes a solid forms, called a precipitate. A precipitate is an insoluble product formed by a reaction in solution.
Pb(NO3)2 (aq) + 2 KI (aq) → PbI2 (s)+ 2 KNO3 (aq)
In the above reaction between lead nitrate and potassium iodide solution, two products are formed. The potassium nitrate is soluble and remains in the aqueous solution but lead iodide is insoluble in water and forms a precipitate
Saturated, unsaturated and supersaturated solutions
A saturated solution contains dissolved solution in equilibrium with undissolved solute.
An unsaturated contains less solute per unit of volume than does its corresponding saturated solution.
In some circumstances, a solution can be prepared that contains more solute than needed for a saturated solution at a particular temperature. These solutions are said to be supersaturated.
Net Ionic Equations and spectators ions
Ions which appear on both sides of the equation are spectator ions. They are present in the reaction but are not participating in it. They can therefore be removed from the ionic equation to allow us to concentrate on the species which are involved in the reaction.
This simplified equation is called a net ionic equation.
Using the example equation above, it can be seen that the potassium ions (K+) and the nitrate ions (NO3-) are present on both sides of the equation arrow; these are spectator ions and therefore do not need to be included in the net ionic equation. We are therefore left with the net ionic equation below:
Pb2+ (aq) + 2I- (aq) → PbI2 (s)
Steps to writing Net ionic equations
Strong electrolytes in solution are written in their ionic form.
NaOH(aq) is written Na+(aq) + OH–(aq)
2
Weak electrolytes and nonelectrolytes are written in their molecular form.
H2O(l), HC2H3O2(aq)
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Solids and gases are written in their molecular form.
CO2(g), AgCl(s)
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The net ionic equation does not include spectator ions.
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Equations must be balanced, both in atoms and in electrical charge.
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If none of the ions in an ionic equation is removed from a solution or changed in some way, then they are all spectator ions and a reaction does not occur
What are the 3 common definitions of acids and bases
Arrhenius Acids and Bases.
- An acid solution which contains an excess of H+ ions
- A base solution contains an excess of OH– ions.
Brønsted–Lowry Acids and Bases
- When an acid donates a H+, it forms the conjugate base of that acid.
- When a base accepts a H+, it forms the conjugate acid of that base.
Lewis Acid and Bases
- a base is an electron pair donor, and
- an acid is an electron pair acceptor.
Acid definition
Acids are substances that ionise in aqueous solutions to form hydrogen ions (H+); known simply as protons.
Molecules of different acids ionise to form different numbers of protons.
Base definition
Bases are substances that accept (react with) H+ ions. Bases produce hydroxide ions (OH-) which they dissolve in water. Ionic compounds like NaOH, KOH and Ca(OH)2 are some of the common bases we encounter.
Not all bases contain OH- ions. Ammonia (NH3) can accept a proton from water leaving OH- ions (see the Figure on the left).
Oxidation and reduction
Oxidation and reduction reactions are termed redox reactions and the most common redox reaction is the rusting or corrosion of metal.
When metal corrodes, the metal loses electrons and forms cations. These cations can combine with anions to form an ionic compound.
When the metals loses an electron, it becomes more positively charged or oxidised.
Oxidation number
The oxidation number of an atom represents the number of electrons lost, gained, or unequally shared by an atom.
Oxidation numbers can be zero, positive or negative.
Oxidation numbers have a variety of uses in chemistry — from writing formulas to predicting properties of compounds and assisting in the balancing of oxidation–reduction reactions in which electrons are transferred.
Steps to balancing oxidation-reduction equations in acidic solution (Steps 8-11 on balancing redox equations in basic solution)
Write as much of the overall unbalanced equation as possible and construct unbalanced oxidation and reduction half equations.
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Balance by inspection all the elements except hydrogen and oxygen.
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Balance O by adding H2O
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Balance H by adding H+
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Balance the change in each half reaction by adding electrons to either the reactants or the products.
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Balance the electron transfer by multiplying the balanced half equations by the appropriate integers
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Add the resulting half-reactions together and eliminate any common terms to obtain the balanced equation
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Take note of the number of H+ in the balanced equation and add the same number of OH- to each side of the equation
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Combine each pair of H+ and OH- to form H2O
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Cancel any water the occurs on both sides of the equation
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Check everything balances!
