SELECTED PRINCIPLES: ACIDS & BASES (1) Certain definitions, which are intentionally narrow, will be useful in this introduction to acid-base chemistry. First, acids are substances which produce hydrogen ions when dissolved in water. Second, bases are substances which accept hydrogen ions (e.g., the hydroxides, oxides, sulfides, hydrogencarbonates, and carbonates of metals); those which are soluble in water are known as alkalis. Third, salts are ionic compounds which contain ions other than hydrogen, hydroxide, or oxide. And finally, neutralization is the reaction between an acid and a base.
When hydrogen chloride is dissolved in an organic solvent, the solution formed does not conduct an electric current - indicating the absence of free-moving ions. By contrast, when this gas is dissolved in water,
the aqueous solution of hydrochloric acid formed conducts an electric current strongly, because this 'strong' acid is completely dissociated;
When sodium hydroxide is dissolved in water,
the aqueous solution of sodium hydroxide formed conducts an electric current strongly, because this 'strong' base is completely dissociated;
The symbol equation for the neutralization reaction between aqueous solutions of nitric acid and sodium hydroxide is:
Because the reactants are completely dissociated, a better perspective of this reaction is gained by constructing an ionic equation;
Those ions that are not involved in the overall reaction, here aqueous sodium and nitrate, are referred to as 'spectator' ions; and, so as to focus on the change that actually occurs in this neutralization, a net ionic equation can be written; i.e.,
The measured heat energy change (DH) for this (exothermic) reaction is -56 kJ mol-¹.
1. The symbol equation for the neutralization reaction between aqueous solutions of sulfuric acid and potassium hydroxide is:
(a) State the spectator ions in this reaction. ________________________  (b) Construct the net ionic equation for this reaction. _______________ _______________________________________________________________________  2. In humans, both pancreatic juice and bile are rich in hydrogen- carbonate ions; these neutralize the acidity of the chyme flowing from the stomach, as well as provide the slightly alkaline pH required for optimal activity of the pancreatic enzymes. The symbol equation for the neutralization reaction, which occurs in the duodenum, is:
(a) Construct the ionic equation for this reaction. ___________________ _______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________  (b) Describe one other rôle of bile in digestion. _____________________ _______________________________________________________________________  3. Acidic gases are released into the atmosphere when fossil fuels are burnt. The condensation of atmospheric water vapour containing these gases, and others formed in the atmosphere, leads to the precipitation of 'acid rain' - which is a mixture of several acids. Depending on the weather conditions and location, this rain or snow can have a pH of between 2.2 and 5.5: by contrast, unpolluted rainwater has a pH of 5.6. (a) State the percentage increase in hydrogen ions of rainwater with a pH of 2.6 compared to that with a pH of 5.6. __________________________  (b) Hydrated metal ions usually enter living cells by diffusion across semi-permeable membranes. In unpolluted environments, the concentration of most toxic metal ions is very low because these ions are 'locked-up' in insoluble compounds [e.g., aluminium oxide, cadmium(II) carbonate, lead(II) sulfide, and mercury(II) sulfide]: but, such compounds react with the components of acid deposition - as a result, the concentration of toxic hydrated ions increases markedly in polluted environments. Contruct the symbol equation for the neutralization reaction between: Dilute nitric acid and aluminium oxide ________________________________ _______________________________________________________________________ Dilute nitric acid and lead(II) sulfide _______________________________ _______________________________________________________________________  (c) Acidified soils are often treated with either quicklime, CaO(s), or slaked lime, whereas acidified lakes are usually treated with powdered limestone. Construct a symbol equation for the neutralization reaction between: Dilute sulfuric acid and quicklime ____________________________________ _______________________________________________________________________ Dilute sulfuric acid and limestone ____________________________________ _______________________________________________________________________  (d) Saprotrophic organisms are vitally important in the recycling of chemical energy and nutrient ions within the biosphere. Suggest and explain one reason why the growth and reproductive rates of saprotrophs usually decrease in acidic environments. ______________________________ _______________________________________________________________________ _______________________________________________________________________ 
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