Lab Cat

7 Sep 2007

Basic Concepts: Why Do Reactions Occur?

Filed under: Basic Concepts, Chemistry, Science — Cat @ 7:46 pm

I do not know why this thought popped into my head, but it seems that it is a fundamental question in chemistry. Why do certain molecules react and others don’t?

For example, acetic acid (vinegar) and sodium bicarbonate (baking soda) in the classic kitchen lab volcano experiment fizz nicely whereas when you add sucrose (table sugar) to a solution of sodium chloride (table salt) all that happens is that sucrose dissolves into the solution. In the former reaction the acetic acid and sodium bicarbonate reaction to form sodium acetate, carbon dioxide and water. In the latter reaction the molecules do not change, sucrose is no longer a solid but is dissolved into the solution*(1).

According to this website, reactions occur for two reasons:

1) The products have a low energy than the reactants

2) Products are more random (less ordered) than the reactants

Dealing with the first reason. Reactions are either exothermic or endothermic. Thermic means heat and exo means “outside” and endo means “within”. So reactions can occur either by giving off heat (exothermic) or by absorbing heat (endothermic). Endothermic reactions cannot occur unless heat is applied to the system. For example, water will not evaporate without added heat. Exothermic reactions are more likely to occur spontaneously. An example of an exothermic reaction is used in MRE kits by the military. When certain chemicals, e.g. lithium chloride, sodium hydroxide*(2), dissolve into water, they give off heat. Thus, their dissolution is exothermic. This is then used in the MRE to heat the provided food.

Unfortunately, we now cannot have any further discussions about why reactions occurring without meeting some thermodynamics. In particular, enthalpy and entropy. Rob Knop at Galactic Interactions has a good post about entropy from a physics perspective.

Entropy explains the second reason why reactions occur. Entropy is considered to be the disorder or randomness of a system and is related to the second law of thermodynamics. That is the law that most of us remember as order –> disorder. Or in my case an explanation as to why my bedroom was messy (neat –> messy as messy was the more disordered state). The more random or disordered the higher the entropy, which is, by the way, represented by S.

Enthalpy is a little tricker to understand. It is related to the heat energy of the system and is represented by H or ΔH where Δ is used to represent “a change in”. In the first reason for why reactions occur, enthalpy can be considered as an explanation. If:

ΔH = Hproducts – Hreactants

Then exothermic reactions have a negative enthalpy as the energy of the products is lower than the reactants and endothermic reactions have a positive enthalpy.

Entropy and enthalpy are linked, thus if the enthalpy of the system is such that the reaction appears to endothermic, the reaction can only occur if the entropy is increased. They are related by the equation for Gibb’s Free Energy:

ΔG = ΔH -TΔS

For a reaction to occur, ΔG needs to be negative.

So why do reactions occur:

1) The enthalphy of the products is lower than the enthalpy of the reactants

2) The entropy is higher in the final system than in the initial

3) ΔG of a system is negative

So that’s the best I can do. If I’ve got anything wrong*(3) or you need further explanation, please let me know in the comments.

*Notes

(1) Dissolution will have to be a topic of another blog. Janet at Adventures of Science and Ethics has a great post about phase change which is part of what is happening. For the pedantic chemists reading this post, dissolution can be exothermic or endothermic. I am discussing noisy and obvious reactions in this post.

(2) MRE heaters, known as flameless ration heaters are unlikely to contain sodium hydroxide as that is caustic soda. According to this website they contain iron, magnesium and sodium.

(3) I blame all errors on my dyslexia, which means exo and endo, positive and negative, left and right can, and do, have opposing definitions. I meant the other left (exo, positive) etc.

Further links

Why do reactions occur?

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5 Comments »

  1. […] Concepts: What are Solutions? In my last Basic Concept post when discussing why reactions occur, I left it rather vague as to what happens […]

    Pingback by Basic Concepts: What are Solutions? « Lab Cat — 18 Sep 2007 @ 10:24 pm

  2. I think that this explanation is very much perfect. Also i think, since exothermic reactions are energy releasing reactions, they are more liable to occur than endothermic reactions which require an energy input before it could take place.

    Comment by Alfred Addo-Yobo — 21 May 2008 @ 11:10 am

  3. Alfred

    Cool, thanks.

    Pats self on back. 🙂

    Comment by Cat — 21 May 2008 @ 11:16 am

  4. I feel that the tendency of a reaction to occur really depends on the products to be formed. The stability of the products is very necessary. When a body or substance contains so much heat,it fels very uneasy and thus very unstable. However, when the energy is released, it feels a bit cooler and stable. For this reason,When a reaction is energy releasing, the products formed are of less energy and thus very stable.Thus exothermic reactions are more liable to occur since it is energy releasing.

    Comment by Alfred Addo-Yobo ( Kwame Nkrumah University, Ghana ) — 4 Jan 2009 @ 1:59 pm

  5. While reaction happens for so many reasons like product stability with lower energy, higher entropy, lower Gibb’s free energy, it may be better explained by a unified concept of chemical energy concentration. In a reaction, the product will be such that its chemical energy concentration will be lower than that of its reactants. The chemical energy is a type of energy like thermal energy. It is represented by the term called the “Gibbs free energy”. Entropy is the measure of disorder and dispersion – a reduction of concentration.

    Comment by Soumen — 30 Aug 2011 @ 1:37 am


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