Organic acids


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Organic acids

  1. 1. Organic Acids<br />
  2. 2. Exam Objectives<br />describe the useful chemical and physical properties of organic acids, such as pH, acid behaviour and miscibility<br />
  3. 3. Organic Acids<br />You should be familiar with Ethanoic Acid – the main component of vinegar<br />This is the most common of the ORGANIC ACIDS which can also be called CARBOXYLIC acids<br />These are acids made from hydrocarbons – the mainly contain Carbon and Hydrogen<br />Other examples include:<br />Methanoic acid – aka formic acid, found in bee stings <br />Lactic acid – aka 2-hydroxypropanoic acid, found in milk and made during anaerobic respiration<br />Citric acid – aka 3-carboxy-3-hydroxypentanedioic acid, found in citrus fruits and sherbet<br />
  4. 4. Properties of Organic Acids<br />They are weak acids (NB: this is different from concentration, you can be a weak acid but very concentrated)<br />Like alcohols, the smaller organic acids (like the examples given earlier) are miscible in water.<br />Larger organic acids are only soluble in organic solvents like alcohols or oily compounds<br />The smaller acids are liquids at room temperature, while the larger ones are solid – the BP increases as the number of C atoms increases<br />
  5. 5. What is a weak acid?<br />pH measures how strong an acid is by how many H+ ions are dissolved in water.<br />A strong acid has a low pH and lots of H+ ions, while a weak acid has a higher pH (but still less than 7) and fewer H+ions.<br />So, hydrochloric acid is a strong acid, while organic acids are weak acids<br />In the picture, a strong acid has lots of free H+ ions, while in the weak acid more of the H+ ions are still attached to the negative ions. Unless they are free, the H+ ions won’t react, and so the acid is said to be weaker.<br />
  6. 6. Hydrogen Bonding<br />This is a brief explanation, but helps you understand the difference in properties of the alkanes compared to the carboxylic acids<br />Alcohols, acids and Esters all form hydrogen bonds.<br />These are stronger than the forces that alkanes have between the molecules.<br />This means they can be soluble in water, and have higher boiling points than similar alkanes.<br />The picture shows hydrogen bonds between water molecules because it contains O and H. Acids, alcohols and esters also contain O and H atoms and this means they also have hydrogen bonding.<br />
  7. 7. Hydrogen Bonding and Miscibility<br />When an acid is very long (more than 4 carbons), it cannot form enough hydrogen bonds to be stable – it becomes hydrophobic <br />This means its solubility in water decreases<br />However, with longer carbon chains it can form bonds with organic solvents which are also made up of lots of carbons.<br />It now becomes more soluble in organic solvents than water.<br />
  8. 8. Reactions of Organic Acids<br />Are the same as for the inorganic acids we already know<br />Acid + Alkali  Salt + water<br />Acid + Metal  Salt + hydrogen<br />Acid + Metal Carbonate  Salt + Carbon Dioxide + water<br />Acid + Metal Hydrogencarbonate  Salt + CO2 + water<br />The only thing that changes is the name:<br />Ethanoic acid + sodium hydroxide  sodium ethanoate + water<br />Methanoic acid + potassium carbonate  potassium methanoate + carbon dioxide + water <br />
  9. 9. Questions to practice<br />Imagine you are carrying out the following tests. Write down what your observations would be and explain them. It might be easier to put this in a table:<br />A Test a little ethanoic acid with universal indicator. What is the pH?<br />B Fill a tube about one-third full with dilute ethanoic acid. Add a piece of magnesium. Note your<br />observations and test the gas that is given off.<br />C Fill a tube about one-third full with dilute ethanoic acid. Add a spatula of sodium hydrogen<br />carbonate. Note your observations and test the gas that is given off.<br />D Repeat step C using sodium carbonate.<br />
  10. 10. RESULTS TABLE<br />Finish off by writing a word and balanced chemical equation for the reactions in B, C and D<br />