Chemical Reactions in Solution
When the chemical reactions in solution takes place, the solvent is usually in so much excess that its concentration does not change appreciably as the reaction proceeds and is, therefore, not involved in the rate expression.
Definition of Solution
A solution consists of two or more substances dissolved in a liquid form. Not to get confused with a mixture, which means that atoms of the solute are evenly dispersed throughout the solvent (ex. water, ethanol). Think of it as comparing a cup of (dissolved) sugar water and a cup of water with lego blocks in it. The solute is the substance dissolved in the solution, and the solvent is the substance doing the dissolving.
How to Make a Solution
Solutions used in the laboratory are usually made from either solid solutes (often salts) or stock solutions. To make a solution from solid solutes, calculate how many moles of solute are in the desired solutions (using the molarity).
Calculate the amount of solid you need in grams using the required moles and the molar mass of the solute and weigh out the required amount.
Transfer the solute to a container (preferably a volumetric flask, which most accurately measures the volume of a solution labeled on the flask) and add a small amount of solvent. Mix thoroughly to dissolve the solute.
Once the solute has dissolved, add the remaining solvent to make the solution of the desired volume and mix thoroughly.
To make 0.5 Liters of 0.5 molar NaCl:
1. Multiply the concentration (0.5 mols/Liters) by the volume of solution you want (0.5 Liters) to find the moles of NaCl you need. 0.5 moles/Liter * 0.5 Liters = 0.25 moles of NaCl.
2. Multiply the moles of NaCl by its molar mass (58.44 g/mol) to find the grams of solute needed. (0.25 moles NaCl)*(58.44 grams/mole) = 14.61 grams of NaCl.
Making a solution of a certain concentration from a stock solution is called a dilution. When diluting a solution, keep in mind that adding a solvent to a solution changes the concentration of the solution, but not the amount of solute already present.
When a reaction follows the same mechanism in solution and the gaseous state, the kinetics remain the same in both. However, because of the increased interactions in condensed media, the mechanism is frequently changed and the kinetics correspondingly.
When a chemical reaction takes place in solution, the solvent is usually in so much excess that its concentration does not change appreciably as the reaction proceeds and is, therefore, not involved in the rate expression.
However, if solvent enters into chemical change and does not regenerate at the end of the process, the solvent would exert a chemical effect on the reaction. It will thus be involved in the rate expression.
Steps of Reactions Take Place in solution
The reaction in solution can be considered to take place in three steps
- Diffusion of reactant molecules towards each other.
- Chemical transformation.
- Diffusion of products away from each other.
The rates of many chemical reactions do not appear to depend on the solvent. This is because the activation energy for diffusion in a liquid is nearly 20 kJ mol–1, whereas it is quite large for chemical reactions.
Thus, Step (i) is usually not a rate-determining step in reactions in solutions. When the reaction takes place in solution,
Step (ii) determines the rate of a bimolecular reaction. This conclusion is supported by the fact that the rates of these reactions do not depend upon the solvent’s viscosity. The rate should be affected by the solvent if diffusion of reactant is the rate-determining step. S
ome processes occur in solutions, e.g., quenching of the fluorescence in solution, specific heterogeneous reactions, etc., in which the diffusion is the rate-controlling. These reactions occur very rapidly, e.g., ionic recombination.
Various Reactions in Solution
Here are some chemical reactions which take place in solution
1. Precipitation Reactions in Solution
Not all ionic compounds are soluble in water. If PbSO4 is added to water, none of the lead(II)sulfate will dissolve (the interaction between Pb2+ and SO42- is stronger than the attraction of the water to either the Pb2+ or SO42-.
Simple Rules for the Solubility of Salts in Water
- Most nitrate (NO3–) salts are soluble.
- The most salts containing the alkali metal ions (Li+, Na+, K+, Cs+, Rb+) and the ammonium ion (NH4+) are soluble.
- Most chloride, bromide, and iodide salts are soluble. Notable exceptions are salts containing the ions Ag+, Pb2+, and Hg22+.
- Most sulfate salts are soluble. Notable exceptions are BaSO4, PbSO4, HgSO4, and CaSO4.
- Most hydroxide salts are only slightly soluble. The important soluble hydroxides are NaOH and KOH. The compounds Ba(OH)2, Sr(OH)2, and Ca(OH)2 are marginally soluble.
- Most sulfide (S-2), carbonate (CO32-), chromate (CrO42-), and phosphate (PO43-) are only slightly soluble. If two solutions are mixed, it is possible that two ions could combine to form an insoluble ionic compound.
A solution of silver nitrate is combined with a solution of sodium chloride. The resulting solution contains Na+, Ag+, Cl+, and NO3– , but AgCl is not soluble in water. Since Ag+ is now in solution with Cl the two ions will combine to form AgCl, and the AgCl will precipitate from the solution.
The reaction can be described in a number of ways;
(1). A “molecular” equation could be used:
AgNO3(aq) + NaCl(aq) -> AgCl(s) + NaNO3(aq)
(2). A complete ionic equation could be used:
Ag+ (aq) + NO3– (aq)+ Na+ (aq) + Cl– – > (aq) AgCl (s) + Na+ (aq) + NO3– (aq)
(3). A net ionic equation could be used.
Ag+(aq) + Cl– – > (aq) AgCl (s)
Net ionic equations are found by writing the full equation and then eliminating the spectator ions; spectator ions do not participate in the reaction. In the example above, Na+ and NO3– are present as both products and reactants; they do not participate in the reaction.
Acid-Base Neutralization Reaction
First, we must recognize what acid is and what a base is. Then we must determine how they react with each other. For now, we will consider two definitions of the terms acid and base.
Arrhenius Acids and Bases—the first (historically) and simplest definition. An acid is a substance that donates (releases) protons (H+). A base is a substance which donates (releases) hydroxide ions(OH–).
Thus HCl is an acid because in water is ionized to form H+.
NaOH is a base because it ionizes in water to form OH–.
However, there are substances that are basic, but they do not contain an ionizable OH–.
For example– Ammonia is a base.
Another definition was created to deal with the observation that not all bases contain an OH–.
Bronsted-Lowry Acids and Bases
- An acid is a substance which donates (releases) protons (H+).
- A base is a substance which accepts protons (H+).
When ammonia is added to water, it accepts a proton from a water molecule.
NH3(g) + H2O(l) – > NH4+ (aq) + OH– (aq)
Actually, this reaction is like the acetic acid reaction. It does not proceed completely to the products, the reaction eventually comes to equilibrium, and the concentrations of NH3, NH4+, and OH– remain constant. Hydroxide (OH–) is a Bronsted base because it accepts H+ to make water.
OH–(aq) + H+(aq) – > H2O(l)
Ions in Aqueous Solution (theory developed by Arrhenius)
- Ionic solids (some, not all) dissolve in water, creating freely floating ions in the solution. The ions can move under an applied electric current.
- The positive ions move towards the negative electrode, and the negative ions move towards the positive electrode.