Dr Hall's Chemistry Blog (copy)

\(\require{mhchem}\) In English we have lots of names for a number of items: a pair is 2, a dozen is 12, a score is 20, and a gross is 144. In chemistry the mole is also a number of items, but a much larger number. The mole is a convenient way for chemists to count particles (atoms, molecules, ions, or electrons) so that we can think about them in manageable numbers. If I say that 4 g of He gas contains 602 214 07 000 000 000 000 000 atoms of helium, that becomes painful to think about and is hard to write down. If I write the number in standard form, as \(6.022 \times 10^{23}\), it becomes easier to write but is still tricky to imagine. If instead I give that amount of substance a short and friendly name (“mole”), chemical reactions become easier to talk about. One mole of oxygen reacts with two moles of hydrogen to make two moles of water. Mole really just means “a standard number of particles”. $$\ce{hydrogen + oxygen -> water}$$ $$\begin{equation} \ce{2H2(g) + O2(g) -> 2H2...

\(\require{mhchem}\) On 4 August 2020 a massive explosion devastated the city of Beirut, Lebanon. It is thought a fire amongst a shipment of fireworks ignited a warehouse containing 2,750 tons of ammonium nitrate that had been stored at the docks. More than 130 people died and 4,000 were injured. Ammonium nitrate is used as a fertilizer and in explosives for mining and construction. In its pure form it is not highly explosive, but when mixed with something to act as a fuel, explosive. The mixture of 94% ammonium nitrate and 6% number 2 fuel oil is commonly used as an explosive and this is given the name ANFO. (Number 2 fuel oil comes from fractional distillation of crude oil; this fraction is used as heating oil and its carbon chains are 14 to 20 long.) In Beirut, the ammonium nitrate fertilizer had been impounded from a shipment in 2014 and stored since then. Ammonium nitrate is made industrially by reacting concentrated nitric acid with ammonia (an \(\ce{acid-base}\) reaction...

\(\require{mhchem}\) The tests for negative ions we need to know about for GCSE are: halides (chloride \(\ce{Cl-}\), bromide \(\ce{Br-}\), iodide \(\ce{I-}\)), carbonate \(\ce{CO_3^2-}\), and sulfate \(\ce{SO_4^2-}\). The simplest test is for carbonate so let's start with that. Carbonate ions The basis for this test is that: $$\ce{acid + carbonate -> salt + water + carbon\ dioxide}$$ If we add acid to a substance and find that it produces \(\ce{CO2}\), we know we have a carbonate. We test for \(\ce{CO2}\) by bubbling the gas through our old friend limewater; if the limewater turns milky, the gas was \(\ce{CO2}\). So, by putting acid in our solution to be identified, and testing any gas produced with limewater, we can tell whether carbonate ions are present. (We can also use this reaction to test for an acid, by adding some carbonate and seeing if \(\ce{CO2}\) is produced. Examiners sometimes ask the question that way round.) If we used hydrochloric acid and potassium ca...