Strengths of acid and bases

Look at the two below diagrams showing the ionisation of a “strong” and “weak” acid and discuss what is meant by the strength/weakness of acids/bases compared to concentration/dilution and contrast the differences, if any.

About these ads

16 responses to “Strengths of acid and bases

  1. A strong acid will completely (or nearly completely, to be accurate – around 99.9% or so) deionise in water, releasing hydrogen ions (which react with water again to form hydronium ions – H3O+) and the anion (e.g.: for H2SO4, the anion would be HSO4-). This causes the logarithmic equation used to work out the pH of a solution that contains a strong acid to be relatively straight forward (with the example of a uniprotic acid such as HCl, simpler still) as all (or rather, around 99.9% or so) of the acid is deionised and affects the pH of the solution.
    However, when it comes to “weak” acids, only a small amount of the acid deionises, and depending on the relative strength of the acid (e.g.: acetic acid is a stronger acid than citric acid), the pH of the solution will be marginally higher for weaker acids, and lower for stronger. In this case, determining the strength of the acid is a little more complex, as the pKa of said acid must first be determined, and from this, the pH can be calculated (or if using an accurate pH probe, visa-versa). In addition to this, the ions formed from the weak acid (e.g.: hydrogen and acetate) will, at times, form back up to make the former acid molecule (acetic acid) and dissociate again into the ions.

    Chris Carson – 16521067

    • Chris, that’s great insight and analysis. How does the pKa (dissociation constant) value though, relate to the ionisation of the acid? Also, what other factor/s do you think could effect the pKa values?. In addition, how do pKa values change, depending on if its an exothermic or endothermic reaction.

      • The smaller the pKa value, the more the ionisation of the acid (larger Ka value). Some factors that affect the pKa values are ionic strength and temperature. The pKa values increase for an exothermic reaction while it decreases for an endothermic reaction.

      • Leena, you just earned yourself the acid-base merit badge and one year’s supply of free commercial grade ethanoic acid – aka vinegar!. However. others please feel free to add your comments.

      • There may not be an easy answer here since it may depend on the nature/type of radiation and the acids concerned. However, if we leave classroom chemistry and enter into the realm of biology, the answer gets more interesting. Some of the key building blocks of life are acids – amino acids. We know that they are affected by radiation depending on the strength, type and duration (gamma, x-ray, etc). In this case the pKa values which characterises these acid would likely change or it may cease to be an acid altogether. One of the most interesting example is DNA (deoxyribonucleic acid) which is know to be affected by radiation which may give rise to mutation. Should the radiation be intense and directed, the “acidic” property of DNA would likely change as well.

    • In an article by Garnett et al. (1995, p.82), a number of misconceptions by students in acid-base chemistry may have their roots in the transition from teaching the basic concepts in earlier schooling years using the Arrhenius model, to teaching more complex concepts using the Bronsted-Lowry model in later years.

      Arrhenius pioneered the theory of acid-base chemistry, and his model focuses on the disassociation of hydrogen ions from acids, and bases forming hydroxide ions (Miessler & Tarr, 1991). However, the Bronsted-Lowry model concentrates on the transfer of hydrogen ions from acids to bases and vice-versa, depending upon strength (Miessler & Tarr, 1991).

      One of the misconceptions held by students from their learning in the early years, that is challenged by the model taught in the later years is that “bases don’t contain hydrogen” Garnett et al. (1995, p.83).

      Miessler, G.L., Tarr, D.A. (1991). Inorganic Chemistry (2nd ed). Pearson Prentice-Hall.

      Garnett, P.J., Garnett, P.J. & Hackling, M.W. (1995). Student’s alternative conceptions in chemistry: A
      review of research and implications for teaching and learning. Studies in Science Education, 25, 69–95.

  2. Hi all,
    The research I done on the misconceptions about oxidation reduction reactions also provided some information on the Students’ alternative conceptions about Acids and Bases that I thought I would share.
    From the research there were 5 main alternative conceptions:
    1. More hydrogen gas is displaced from a strong acid because a strong acid contains more hydrogen bonds than a weak acid.
    2. A weak acid cannot perform as well as a strong acid.
    3. pH is a measure of acidity but not basicity
    4. Bases don’t contain hydrogen.
    5. When acids and bases are mixed they do not react; they form a physical mixture.
    (Garnett & Treagust, 1992; 1993)
    As we know and your presentation indicated this is not the case, but I thought it was interesting to mention that these misconceptions either indicated limited understanding or confused definitions of strong and weak, when applied to acids and bases or that students think of pH in terms of acidity rather than providing information about both the H+ and OH-concentrations – this highlights how simple misconceptions can lead to greater misunderstanding, so care needs to be taken to avoid these when teaching.
    - Alex Darmanin

  3. From Ray S

    I like your post:
    2. A weak Acid cannot perform as well as a strong acid.
    Perform? What do you think “perform” refers to?
    I think the alternative conception refers to students not bieng able to express the relative corrosive nature of a strong acid vs a weak acid? or perhaps the ability to neutralise a strong base.
    “perform” Can anyone provide input on this matter?
    Does the reference specify this?

  4. Thanks for letting us know about these misconceptions Alex and sharing your thoughts. Acids and bases are definitely a difficult topic for students to fully grasp because it requires deep understanding of prior knowledge regarding water, concentration in moles per L of solutions and hydrogen bonding. I think often when we write chemical equations, as Carole mentioned, the role of water is not given the emphasis it deserves. The chemical equations just put (aq) in brackets which doesn’t represent the central role of water in these chemical equilibria.

    Our research suggested similar misconceptions with an emphasis on students considering pH to be the main characteristic of a strong or weak acid with little understanding of ionisation and hydronium ion formation. Sometimes students also didn’t realise that a proton is a H ion and that in solution it doesn’t exist on its own but rather is donated to H2O to form the hydronium.

    Bases are generally less associated with pH by students because they don’t witness all the ‘metal and gas bubbling’ reactions associated with acid and metal reactions at the junior level. So you are completely right about how one basic (excuse the pun) misconception can fuel a chain of misconceptions.

  5. I think ‘perform’ may refer to many the many different properties emphasised in the junior science syllabus. For example, the reaction between a metal and acid yielding hydrogen gas and a soluble salt. ‘Perform’ may refer to the degree of bubbling observed in that case, which is a subset of ‘corrosion’ because corrosion also covers redox reactions like rusting which are not directly related to acids (ie pH and degrees of ionisation). It may also refer to how acids ‘perform’ when an indicator is used to test pH, so performance in terms of a pH test. This would be similar to the idea you put forward Ray that ‘it can neutralise a strong base’ but I think we should emphasise the distinction between pH, dilution, concentration and strength/ionisation because the students might think they have a very strong HCl acid but it might be very dilute (ie high pH) and not neutralise a concentrated sample of strong base (eg NaOH).

    I think ‘perform’ is not a scientific term so one teaching strategy might be to elicit prior knowledge and then getting students to explaing what they mean by ‘perform better’.

  6. “In a weak acid only a small amount of acid deionises in water, however a stronger acid will completely deionise in water”. Similar misconceptions were students emphasise on pH being the main characteristic of a strong or weak acid seems to be provoked by the relationship of the deionisation of acid and the affects of the pH solution, if there is only limited knowledge of a concept misunderstandings and confusion can be easily formed in a students mind.

    • When we think of concentration or dilution, its best to think in terms of molarity of the solute dissolved in an aqueous solution. For example, we could have a very concentrated sugar solution (high molarity of glucose or sucrose) which would show no acidic or basic property since it does not dissociate to form either the H+ or OH-. Contrast this with a concentrated acetic acid (high molarity) which dissociates, but not as much as H2SO4, say. We would expect its acidic property to be not as “strong” compared to another strong acid say HCL of the same molarity. This works the same for bases as well.

  7. Strong acids completely dissociate in water, forming H+ and an anion.

    A weak acid only partially dissociates in water to give H+ and the anion.
    Weak acids include:
    -Molecules that contain an ionizable proton. A molecule wih a formula starting with H usually is an acid.
    -Organic acids containing one or more carboxyl group, -COOH. The H is ionizable.
    -Anions with an ionizable proton. (e.g., HSO4- –> H+ + SO42-)
    -Cations •transition metal cations
    -Heavy metal cations with high charge
    -NH4+ dissociates into NH3 + H+

    Strong bases dissociate 100% into the cation and OH- (hydroxide ion). The hydroxides of the Group I and Group II metals usually are considered to be strong bases.

    -Most weak bases are anions of weak acids.
    -Weak bases do not furnish OH- ions by dissociation. Instead, they react with water to generate OH- ions.

    • This is a broad summary although its loaded with a lot of information which if not discussed in depth may raise more questions than what it purports to answer. Certainly good for memorising facts for the HSC exam.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s