05. Acids and bases

Outcomes:

> Be able to give a definition of an acid

> Give examples of strong and weak acids and bases

The concept of ions is largely credited to a Swedish chemist, Svante Arrhenius (1859-1927). While studying the conductivity of electrolytes for his doctorate, he discovered that acids, compounds that in pure form are covalently bonded polar molecules, dissociate into hydrogen ions and anions when they dissolve in water. The extent to which molecules dissociate is an indicator of the strength of an acid and its electrical conductivity.

Many foods contain weak acids and there is a direct relationship between our perception of sourness and the concentration of hydrogen ions (H⁺) they contain. Similarly, for Arrhenius, a base (alkali) was a compound that produced hydroxide ions (OH^-) and cations when it dissolved in water. Bases are used to make soaps, so you may know from experience that they have a bitter, alkaline taste. This has been attributed to the high concentration of hydroxide ions in their solutions.

ATL

Research skills: Organise information using a variety of models and/or graphic organisers

Activity: Use Canva, Piktochart or similar to create an infographic that explains what an 'acid' is and what the properties of an acid are.

Arrhenius's definition helped people recognise that acidity and alkalinity were associated with particular ions in aqueous solutions, but did not explain why substances such as ammonia (NH₃) were also bases. In 1923 two chemists independently developed a new definition. Danish Johannes Brønsted (1879-1947) and English Thomas Lowry (1874-1936) extended Arrhenius's concept by defining an acid as a 'proton donor' and a base as a 'proton acceptor'. A proton is another term for a hydrogen ion. Their theory explained how ammonia and hydrogencarbonate ions became weak bases when they reacted with water.

According to the Brønsted-Lowry theory, acids and bases interact in pairs. For example, when molecules dissociate in water, a water molecule accepts the hydrogen ion, and therefore acts as a base; and when ammonia reacts with water, the water molecule donates a proton and therefore acts as an acid. Molecules with duel acid-base properties involving proton transfer, like water, are described as amphiprotic. The prefix amphi is from the Greek word meaning 'both' and protic refers to the proton.

Brønsted and Lowery proposed H⁺(aq) should be shown as H₃O⁺(aq), arguing that showing the hydrogen ion bonded to a water molecule was a more accurate representation of how the ion would interact with water. The species, with symbol H₃O⁺(aq), is called a 'hydronium ion'.

ACTIVITY: Clarifying your understanding of acids and bases

ATL

Critical-thinking skills: Revise understanding based on new information and evidence.
1. Outline common properties of acids and bases listed in the Table. Suggest how you might define acids and bases from your interpretations of the pattern.
2. Predict which acids and bases in the Table will be the best electrolytes, assuming the same number of molecules is dissolved in water for comparison.
3. Explain, using a diagram, why a proton (H⁺) is another name for a hydrogen ion.
4. In the examples below, the gas is being bubbled through water.
5. a Identify the pairs of acids and bases in the two reactions.
6. i HCl(g) + H₂O(l) → Cl^-(aq) + H₃O⁺(aq) ii NH₃(g) + H₂O(l) → NH₄⁺(aq) + OH^-(aq) b Suggest what is happening to the water molecule in the two reactions.

Predict the result of the interactions between ions, if you mixed equal volumes and concentrations of each of these pairs of acids and bases. Complete (and balance) the equations below to find out! i HCl(aq) + NaOH(aq) → _____ + _____ ii HNO₃(aq) + KOH(aq) → _____ + _____ Iii H₂SO₄(aq) + Mg(OH)₂(aq) → _____ + _____

Assessment opportunities

In this activity you have practiced skills that are assessed using Criterion A: Knowing and understanding.

Citations

Termaat, Annie. Chemistry for the Ib Myp 4 & 5. Place of Publication Not Identified: Hodder Education, 2016. 108-09. Print.

Svante Arrhenius. Digital image. N.p., 12 Nov. 2010. Web. 2 May 2017. <https://commons.wikimedia.org/wiki/File:Svante_Arrhenius_01.jpg>.

Johannes Nicolaus Brønsted. Digital image. N.p., 23 Feb. 2011. Web. 2 May 2017. <https://commons.wikimedia.org/wiki/File:Johannes_Br%C3%B8nsted.jpg>.

Thomas Martin Lowry. Digital image. N.p., 10 Aug. 2011. Web. 2 May 2017. <https://commons.wikimedia.org/wiki/File:Thomas_Martin_Lowry2.jpg>.

Hydronium Ion. Digital image. N.p., 7 Nov. 2014. Web. 2 May 2017. <http://www.quirkyscience.com/introducing-hydronium-ion/>.

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