UNDER CONSTRUCTION
Ch. 08 Chemical reactions: What happens? Module 0804
Reactions as competitions
KEY IDEAS
Prof Bob has a view about chemical reactions that is not generally presented in other sources:
Reactions don't 'just go'.
Every chemical reaction involves a competition between the reactants for some species that is transferred between reactant molecules or ions.
There are ‘winners’ and ‘losers’.
The products of a reaction are the species remaining after the 'fight' has been won and lost.
Prof Bob has a view about chemical reactions that is not generally presented in other sources:
Reactions don't 'just go'.
Every chemical reaction involves a competition between the reactants for some species that is transferred between reactant molecules or ions.
There are ‘winners’ and ‘losers’.
The products of a reaction are the species remaining after the 'fight' has been won and lost.
Chemical reactions are not the only competitions in which different mobs compete for an object. Actually, this analogy is imperfect: in a chemical reaction mixture, the reactants compete for many 'footballs'. Who wins most of them? Perhaps it depends on the competitive abilities of the individuals, as well as how many there are of each?
Competition for what?
In every chemical reaction, some species is the object of competition between reactant atoms, molecules or ions. With reference to Module 0803 Categories of reaction in aqueous solution, the object of competition depends on the type of reaction:
No reactants ‘just react’ and lose electrons, protons, ligands, or ions in a crystal lattice without a fight.
In every chemical reaction, some species is the object of competition between reactant atoms, molecules or ions. With reference to Module 0803 Categories of reaction in aqueous solution, the object of competition depends on the type of reaction:
- In acid-base reactions, the objects of competition are protons (hydrogen ions).
- In oxidation-reduction reactions), the objects of competition are electrons.
- In complexation reactions, the objects of competition are ligands (small groups of atoms with a lone pair of electrons).
- In dissolving and precipitation processes, the objects of competition are ions.
No reactants ‘just react’ and lose electrons, protons, ligands, or ions in a crystal lattice without a fight.
It can be very useful when learning chemistry (or doing chemistry) to focus on the object of competition in any reaction - electrons, protons, ligands, or ions – rather than the reaction species themselves. To understand this, it is perhaps useful to familiarise yourself with this idea in Module 1101 Visualising dynamic chemical equilibrium - although it applies specifically to competition for H+ ions in acid-base reactions.
At last, so much for the preliminaries. Let's get to the main story ..........
1. Acid-base reactions as competition for protons
When ethanoic acid is dissolved in water, a reaction occurs in which some ethanoic acid molecules have protons taken from them (ethanoic acid is an acid), and the protons are ‘grabbed’ by water molecules (water is a base in this reaction). A chemical equation for this process is:
Can you visualise ethanoic acid molecules and water molecules bumping into each other (zillions of times per second), and at each collision the water molecules trying to ‘grab’ a proton from the ethanoic acid molecules (to form H3O+ ions)?
Well, some are successful, but the ethanoic acid molecules hang on to their protons pretty well: At any moment in a 0.01 M solution, only 4% of them have yielded their protons. [In the language of chemistry, 4% of them are ionised.]
As you would expect, the result of such competition depends upon which acid, and which base. For example, in 0.01 M solution of methanoic acid (HCOOH), about 12% of the molecules are ionised: they are not as competitive as ethanoic acid molecules in hanging on to their protons under “attack” from water molecules.
And hydrochloric acid (HCl)? Water molecules can grab the protons from all of the HCl molecules.
Well, some are successful, but the ethanoic acid molecules hang on to their protons pretty well: At any moment in a 0.01 M solution, only 4% of them have yielded their protons. [In the language of chemistry, 4% of them are ionised.]
As you would expect, the result of such competition depends upon which acid, and which base. For example, in 0.01 M solution of methanoic acid (HCOOH), about 12% of the molecules are ionised: they are not as competitive as ethanoic acid molecules in hanging on to their protons under “attack” from water molecules.
And hydrochloric acid (HCl)? Water molecules can grab the protons from all of the HCl molecules.
We have compared the abilities of molecules of three acids to resist having their protons taken by water molecules. What about if there were a more powerful “proton-grabber” in the solution – such as ammonia molecules. Well, 4% of ethanoic acid molecules are ionised in water, but if the solution also contains 0.01 M ammonia, the ammonia molecules manage to “grab” protons from 99% of the ethanoic acid molecules!
So yes, different species, different relative competing abilities for protons. We will see that these differences of competing ability for protons are quantitatively compared as ionisation constants of acids and bases.
A language issue
It is common parlance in most textbooks (except Prof Bob’s) to say that in an acid-base reaction, the acid molecules “lose” a proton. This suggests that the acid molecules don’t care very much about whether they have their protons, or not.
Rubbish! There are strong forces (called covalent bonds) by which the acid molecules hang on to their protons. Only a more greedy invader can take them away.
Perhaps even worse, it is common language to say that the acid is the proton “donor” in an acid-base reaction, and the base is the proton “acceptor”. Well, in everyday language, to “donate” is a voluntary process, and for two parties to “donate” and “accept” some object(s) sounds like an amicable win-win situation. For the same reasons as above, this is highly misleading.
Prof Bob tries not to fall back into the standard chemists’ language and prefers to describe the reactants in an acid-base reaction in the following ways:
It is common parlance in most textbooks (except Prof Bob’s) to say that in an acid-base reaction, the acid molecules “lose” a proton. This suggests that the acid molecules don’t care very much about whether they have their protons, or not.
Rubbish! There are strong forces (called covalent bonds) by which the acid molecules hang on to their protons. Only a more greedy invader can take them away.
Perhaps even worse, it is common language to say that the acid is the proton “donor” in an acid-base reaction, and the base is the proton “acceptor”. Well, in everyday language, to “donate” is a voluntary process, and for two parties to “donate” and “accept” some object(s) sounds like an amicable win-win situation. For the same reasons as above, this is highly misleading.
Prof Bob tries not to fall back into the standard chemists’ language and prefers to describe the reactants in an acid-base reaction in the following ways:
- Protons are taken away from the molecules of the acid.
- Molecules of the base “grab” protons from the acid molecules.
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2. Oxidation-reduction reactions as competition for electrons
Oxidation-reduction reactions are the result of competition between different reactant species to ‘grab’ (or to hold onto) electrons.
For example, when zinc metal reacts with hydrogen ions in hydrochloric acid solution, the aquated hydrogen ions 'rip off' electrons from the zinc atoms in the metallic zinc. The aquated hydrogen ions, H+(aq), win the competition for electrons.
Zinc atoms are transformed into aquated zinc ions, and the hydrogen ions become hydrogen gas.
Here is a chemical equation that represents this electron-transfer process:
Oxidation-reduction reactions are the result of competition between different reactant species to ‘grab’ (or to hold onto) electrons.
For example, when zinc metal reacts with hydrogen ions in hydrochloric acid solution, the aquated hydrogen ions 'rip off' electrons from the zinc atoms in the metallic zinc. The aquated hydrogen ions, H+(aq), win the competition for electrons.
Zinc atoms are transformed into aquated zinc ions, and the hydrogen ions become hydrogen gas.
Here is a chemical equation that represents this electron-transfer process:
An aside: If some copper metal is put into dilute hydrochloric acid solution, no reaction happens. Copper atoms in the metal are good competitors for electrons (much better than zinc atoms), and the aquated hydrogen ions are unable to 'rip off' the electrons: hydrogen ions 'lose' this competition.
3. Complexation reactions: Competiton for ligands
Complexation reactions are those in which there is competition between reactant species to “grab” (or to hold onto) atoms or groups of atoms that have at least one lone pair of electrons. These groups are called ligands.
For example:
Complexation reactions are those in which there is competition between reactant species to “grab” (or to hold onto) atoms or groups of atoms that have at least one lone pair of electrons. These groups are called ligands.
For example:
4. Dissolving and precipitation: Competition for anions and cations
Dissolving and precipitation of ionic compounds involve competition for cations and anions being attracted to the crystal lattice by the other anions and cations in the lattice, and also by the polar water molecules near the surface of the crystal, trying to 'grab' them and take them into solution (as aquated ions).
This is discussed in some detail in Module 0905 Dissolution of ionic salts in water: A competition.
The outcome of the competition decides whether or not an ionic compound dissolves in water (or, at least, how soluble it is), and whether or not it will be precipitated from solution.
Sodium chloride is quite soluble in water. The dissolving process can be represented by the following chemical equation:
The water molecules have 'won' the competition, and ‘grabbed’ the ions against the forces of attraction to nearby ions in the lattice.
For the same reason, if a solution containing sodium ions (such as a solution of sodium nitrate) and a solution containing chloride ions (such as a solution of potassium chloride) are mixed, no sodium chloride precipitate is formed. The water molecules surrounding the sodium and chloride ions, 'win out' over attraction between sodium ions and chloride ions.
But .... Put some solid silver chloride (AgCl) into water. Does it dissolve? Good try, water molecules, but not good enough! The ions in the crystal 'win', and hardly any silver chloride dissolves.
For the same reason, if a solution containing sodium ions (such as a solution of sodium nitrate) and a solution containing chloride ions (such as a solution of potassium chloride) are mixed, no sodium chloride precipitate is formed. The water molecules surrounding the sodium and chloride ions, 'win out' over attraction between sodium ions and chloride ions.
But .... Put some solid silver chloride (AgCl) into water. Does it dissolve? Good try, water molecules, but not good enough! The ions in the crystal 'win', and hardly any silver chloride dissolves.
Some explanatory notes
Reactions aren't usually so one-sided
In each of the cases presented above as examples, the competition is one-sided, and the reaction goes ‘to completion’. In most cases, the competition is more even, and as concentrations change during reaction, a point is reached when the competition is balanced. This is the state of dynamic chemical equilibrium - See Chapter 11 Dynamic chemical equilibrium.
In each of the cases presented above as examples, the competition is one-sided, and the reaction goes ‘to completion’. In most cases, the competition is more even, and as concentrations change during reaction, a point is reached when the competition is balanced. This is the state of dynamic chemical equilibrium - See Chapter 11 Dynamic chemical equilibrium.
Language issues
Acid-base reactions
It is common in most textbooks (except Prof Bob’s) to say that in an acid-base reaction, the acid molecules “lose” a proton. This suggests that the acid molecules don’t care very much about whether they hang on to their protons, or not. Rubbish! There are strong forces (called covalent bonds) by which the acid molecules hang on to their protons. Only a more greedy invader can take them away.
Perhaps even worse, in terms of what we visualise, it is common language to say that the acid is the proton “donor” in an acid-base reaction, and the base is the proton “acceptor”. Well, in everyday language, to “donate” is a voluntary process, and for two parties to “donate” and “accept” some object(s) sounds like an amicable win-win situation. For the same reasons as above, this is highly misleading.
Prof Bob tries not to fall back into the standard chemists’ language and prefers to describe the reactants in an acid-base reaction in the following ways:
It is common in most textbooks (except Prof Bob’s) to say that in an acid-base reaction, the acid molecules “lose” a proton. This suggests that the acid molecules don’t care very much about whether they hang on to their protons, or not. Rubbish! There are strong forces (called covalent bonds) by which the acid molecules hang on to their protons. Only a more greedy invader can take them away.
Perhaps even worse, in terms of what we visualise, it is common language to say that the acid is the proton “donor” in an acid-base reaction, and the base is the proton “acceptor”. Well, in everyday language, to “donate” is a voluntary process, and for two parties to “donate” and “accept” some object(s) sounds like an amicable win-win situation. For the same reasons as above, this is highly misleading.
Prof Bob tries not to fall back into the standard chemists’ language and prefers to describe the reactants in an acid-base reaction in the following ways:
- Molecules of the acid have a proton removed from them.
- Molecules of the base “grab” protons from the acid molecules.
Oxidation-reduction reactions
In the same way as discussed above in relation to transfer of protons, in the context of electron-transfer reactions (oxidation-reduction reactions) it is common chemistry-talk to say that the species oxidises “loses” electrons, or “donates” electrons. Similarly it is common to read in textbooks that the species reduces is the one that “accepts” or “receives” electrons. An amicable, voluntary process? Certainly not!
Here are some typical Prof Bob expressions of these reactions:
In the same way as discussed above in relation to transfer of protons, in the context of electron-transfer reactions (oxidation-reduction reactions) it is common chemistry-talk to say that the species oxidises “loses” electrons, or “donates” electrons. Similarly it is common to read in textbooks that the species reduces is the one that “accepts” or “receives” electrons. An amicable, voluntary process? Certainly not!
Here are some typical Prof Bob expressions of these reactions:
- Molecules or ions of the species oxidised have one or more electrons removed from them.
- Molecules or ions of the species reduced “grab” electrons from the species said to be oxidised.
Dissolving
No ionic salt just 'falls apart' when if is placed in water, as though the ions cannot stand each other! Indeed, their strong attraction for each other is only overcome by the attraction of water molecules for the ions.
And sometimes the attraction between ions is too competitive for the water molecules to grab them. We say that those substances insoluble: they do not dissolve.
No ionic salt just 'falls apart' when if is placed in water, as though the ions cannot stand each other! Indeed, their strong attraction for each other is only overcome by the attraction of water molecules for the ions.
And sometimes the attraction between ions is too competitive for the water molecules to grab them. We say that those substances insoluble: they do not dissolve.
Anthropomorphism
Prof Bob can be criticised for using anthropomorphism: that is, the suggestion that inanimate objects (like molecules) have emotions governing their behaviours – like 'wanting' to hang on to their protons, or having a strong 'desire' to 'grab' electrons.
His reply to that is “Too bad!” If anthropomorphism helps to imagine the forces that are being experienced by atoms, molecules and ions in a reaction mixture, then that is a good thing. Because everything that happens in chemistry is the result of different forces experienced by different species.
On the other hand, it is important to be conscious that this is a device: of course, atoms, molecules and ions do not have feelings, emotions, or thoughts.
Prof Bob can be criticised for using anthropomorphism: that is, the suggestion that inanimate objects (like molecules) have emotions governing their behaviours – like 'wanting' to hang on to their protons, or having a strong 'desire' to 'grab' electrons.
His reply to that is “Too bad!” If anthropomorphism helps to imagine the forces that are being experienced by atoms, molecules and ions in a reaction mixture, then that is a good thing. Because everything that happens in chemistry is the result of different forces experienced by different species.
On the other hand, it is important to be conscious that this is a device: of course, atoms, molecules and ions do not have feelings, emotions, or thoughts.
