Table of Common Acid-Base Indicators

Indicator	pH Range	Acid	Base
Thymol Blue	1.2-2.8	red	Yellow
2,4-Dinitrophenol	2.4-4.0	colorless	Yellow
Methyl yellow	2.9-4.0	red	Yellow
Methyl orange	3.1-4.4	red	Orange
Bromphenol blue	3.0-4.6	yellow	blue-violet
Tetrabromphenol blue	3.0-4.6	yellow	blue
Alizarin sodium sulfonate	3.7-5.2	yellow	violet
Bromcresol green	4.0-5.6	yellow	blue
Methyl red	4.4-6.2	red	yellow
Bromcresol purple	5.2-6.8	yellow	purple
Tropeolin OOO	7.6-8.9	yellow	rose-red
Thymol blue	8.0-9.6	yellow	blue
Phenolphthalein	8.0-10.0	colorless	red
α-Naphtholbenzein	9.0-11.0	yellow	blue
Thymolphthalein	9.4-10.6	colorless	blue
Nile blue	10.1-11.1	blue	red
Alizarin yellow	10.0-12.0	yellow	lilac
Salicyl yellow	10.0-12.0	yellow	orange-brown

Chemistry of Neutralization and Hydrolysis

When acids and bases react with each other, they can form a salt and (usually) water. This is called a neutralization reaction and takes the following form:

HA + BOH → BA + H₂O

 

Depending on the solubility of the salt, it may remain in ionized form in the solution or it may precipitate out of solution. Neutralization reactions usually proceed to completion.

Euilibrium constant is the ratio of the equilibrium concentrations of the productsraised to the power of their stoichiometric coefficients to the equilibrium concentrations of thereactants raised to the power of their stoichiometric coefficients.

For a reversible reaction:

aA + bB → cC + dD

 

The equilibrium constant, K, is equal to:

K = [C]^c·[D]^d/[A]^a·[B]^b

 

The equilibrium constant is calculated from the expression for chemical equilibrium. Learn what the equilibrium constant, how to calculate the equilibrium constant and what it means if the equilibrium constant is very large or very small.

For the following chemical reaction:

aA(g) + bB(g) ↔ cC(g) + dD(g)

 

The equilibrium constant K_c is:

K_c = [C]^c[D]^d / [A]^a[B]^b

where: [A], [B], [C], [D] etc. are the molar concentrations of A, B, C, D
a, b, c, d etc. are the coefficients in the balanced chemical equation

Significance of the Equilibrium Constant

For any given temperature, there is only one value for the equilibrium constant. K_c only changes if the temperature at which the reaction occurs changes.

If the value for K_c is very large, then the equilibrium favors the reaction to the right and there are more products than reactants. The reaction may be said to be "complete" or "quantitative."

If the value for the equilibrium constant is small, then the equilibrium favors the reaction to the left and there are more reactants than products. If the value of K_c approaches zero the reaction may be considered not to occur.

If the values for the equilibrium constant for the forward and reverse reaction are nearly the same then the reaction is about as likely to proceed in one direction and the other and the amounts of reactants and products will be nearly equal. This type of reaction is considered to be reversible.

The reverse of the neutralization reaction is called hydrolysis. In a hydrolysis reaction a salt reacts with water to yield the acid or base:

BA + H₂O → HA + BOH

More specifically, there are four combinations of strong and weak acids and bases:

· strong acid + strong base, e.g., HCl + NaOH → NaCl + H₂O

When strong acids and strong bases react, the products are salt and water. The acid and base neutralize each other, so the solution will be neutral (pH=7) and the ions that are formed will not reaction with the water.

· strong acid + weak base, e.g., HCl + NH₃ → NH₄Cl

The reaction between a strong acid and a weak base also produces a salt, but water is not usually formed because weak bases tend not to be hydroxides. In this case, the water solvent will react with the cation of the salt to reform the weak base. For example:

HCl (aq) + NH₃ (aq) ↔ NH₄⁺ (aq) + Cl^-

while
NH₄^- (aq) + H₂O ↔ NH₃ (aq) + H₃O⁺ (aq)

· weak acid + strong base, e.g., HClO + NaOH → NaClO + H₂O

 

When a weak acid reacts with a strong base the resulting solution will be basic. The salt will be hydrolyzed to form the acid, together with the formation of the hydroxide ion from the hydrolyzed water molecules.

· weak acid + weak base, e.g., HClO + NH₃ ↔ NH₄ClO

The pH of the solution formed from the reaction of a weak acid with a weak base depends on the relative strengths of the reactants. For example, if the acid HClO has a K_a of 3.4 x 10^-8 and the base NH₃ has a K_b = 1.6 x 10^-5, then the aqueous solution of HClO and NH₃will be basic because the K_a of HClO is less than the K_a of NH₃.

Hydrolysis Definition

Definition: Hydrolysis is a type of decomposition reaction where one reactant is water.

The general formula of a hydrolysis reaction is:

                       AB + H₂O → AH + BOH

 

Organic hydrolysis reactions involve the reaction of water and a ester. This reaction follows the general formula:

RCO-OR' + H₂0 → RCO-OH + R'-OH

the dash denotes the covalent bond that is broken during the reaction.

Examples:

Dissolving a salt of a weak acid or base in water is an example of a hydrolysis reaction.

Laws of Thermochemistry

Thermochemical equations are just like other balanced equations except they also specify the heat flow for the reaction. The heat flow is listed to the right of the equation using the symbol ΔH. The most common units are kilojoules, kJ. Here are two thermochemical equations:

H₂ (g) + ½ O₂ (g) → H₂O (l); ΔH = -285.8 kJ

HgO (s) → Hg (l) + ½ O₂ (g); ΔH = +90.7 kJ

 

When you write thermochemical equations, be sure to keep the following points in mind:

1. Coefficients refer to the number of moles. Thus, for the first equation, -282.8 kJ is the ΔH when 1 mol of H₂O (l) is formed from 1 mol H₂ (g) and ½ mol O₂.

2. Enthalpy changes for a phase change, so the enthalpy of a substance depends on whether is it is a solid, liquid, or gas. Be sure to specify the phase of the reactants and products using (s), (l), or (g) and be sure to look up the correct ΔH from heat of formation tables. The symbol (aq) is used for species in water (aqueous) solution.

3. The enthalpy of a substance depends upon temperature. Ideally, you should specify the temperature at which a reaction is carried out. When you look at a table of heats of formation, notice that the temperature of the ΔH is given. For homework problems, and unless otherwise specified, temperature is assumed to be 25°C. In the real world, temperature may different and thermochemical calculations can be more difficult.

Certain laws or rules apply when using thermochemical equations:

1. Δ H is directly proportional to the quantity of a substance that reacts or is produced by a reaction.

Enthalpy is directly proportional to mass. Therefore, if you double the coefficients in an equation, then the value of ΔH is multiplied by two. For example:

H₂ (g) + ½ O₂ (g) → H₂O (l); ΔH = -285.8 kJ

2 H₂ (g) + O₂ (g) → 2 H₂O (l); ΔH = -571.6 kJ

2. Δ H for a reaction is equal in magnitude but opposite in sign to Δ H for the reverse reaction.

For example:

HgO (s) → Hg (l) + ½ O₂ (g); ΔH = +90.7 kJ

Hg (l) + ½ O₂ (l) → HgO (s); ΔH = -90.7 kJ

This law is commonly applied to phase changes, although it is true when you reverse any thermochemical reaction.

3. Δ H is independent of the number of steps involved.

This rule is called Hess's Law. It states that ΔH for a reaction is the same whether it occurs in one step or in a series of steps. Another way to look at it is to remember that ΔH is a state property, so it must be independent of the path of a reaction.

If Reaction (1) + Reaction (2) = Reaction (3), then ΔH₃ = ΔH₁ + ΔH₂