Acid Bases And Salts Past Papers

[Vespasiano Jilg]

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For students planning to take part in the Chemistry Olympiad, trying out past papers gives them the opportunity to hone key skills and boost their confidence ahead of the competition. This collection brings together past papers from 2003 onwards, each accompanied by a summary of the topics covered as well as a mark scheme with answers.

Past papers can be used flexibly by teachers and students, with varying degrees of independence. Browse the summary of papers and topics below to find a particular question, or select a paper to work through from beginning to end.

Topics include reactions that produce phosphine; carbon capture by calcium looping; synthesis and reactions of levulinic acid; Newman projections; synthesis of hydroxychloroquine; and kinetics of the formation of xenon difluoride.

Topics include the reactions and structure of calcium carbide; the thermodynamics of hydrogen as a fuel; the structure of UV-absorbing chemicals; structures of silicon oxides; kinetics in colourful compounds; and synthesising [5]-ladderanoic acid.

Topics include carbon dioxide in fizzy drinks; electrolysis of precious metals and NMR spectra of platinum complexes; the kinetics of nerve agent treatments; synthesising pesticides; and calculations on biodegradable polymers.

Topics include reactions within lithium-ion batteries; sustainable methods of producing ammonia; isotopes and reactions of Uranium; the synthesis of dextromethorphan; and compounds of helium and sodium.

Topics include the reactions of lanthanum carbonate; ionisation energies of sodium; the synthesis of tazarotene; analysis of chemicals that bombardier beetles use to defend themselves; and the structure and reactions of methane hydrates.

Topics include synthesising Ambrox; analysing a copper complex using titrations; thermodynamics of halogen fluorides; calculations on salty solutions; using osmium compounds in organic reactions; and the structure of gold.

Topics include methods of producing pure silicon; the kinetics of vitamin D production in mushrooms; thermodynamics and structure of mercury fulminate; reactions and analysis of aluminium compounds; mass spectrometry of polypeptides; and the synthesis of fexofenadine.

Topics include the reactions and thermodynamics of rocket fuels; structures of phosphorus allotropes; analysing phosphate levels in blood; spectroscopic analysis of flame retardants; the synthesis of Tamiflu; and reactions of chlorine dioxide.

Topics include reactions of the ingredients in sherbet lemons; reactions in vehicle exhausts; structures of acyl chloride compounds; thermal decomposition of copper(II) sulfate; producing oxygen in emergencies; the synthesis of sildenafil; and mass spectrometry and NMR of haloalkanes.

Topics include the properties of carbon oxides; reactions of diiodine pentoxide; calculations with methanoic acid; NMR spectra of NanoPutians; estimating blood alcohol levels; and the synthesis of rimonabant.

Topics include redox reactions; reactions of pollutants that erode monuments; calculating dissolved oxygen in water; the structure of agent orange; the thermodynamics of white and grey tin; electronic transitions in hydrogen; and structures of sulfur-containing compounds.

Acids and bases are an important class of chemicals in chemistry. They can come in many different forms, from small ionic molecules to larger and more complex organic compounds. They can also be strong or weak. The strength of an acid or a base will have a strong influence on the way it behaves in a reaction. The strength of an acid or a base (acidity or basicity) is measured using pH scale.

We can also define the term alkali. Alkali are bases that are soluble. When a soluble base is dissolved, the resulting solution is referred to as being alkaline.

The more \textH^+ ions that a substance produces in solution, the more acidic the substance. The more \textOH^- ions a substance produces, the more basic (or alkaline) it is.

Common acidic substances include vinegar, citric acid, and even milk (though only very mildly in the last instance). Bases are much more commonly found in cleaning supplies, for example bleach and ammonia are both strongly basic compounds.

The pH Scale is the most commonly used method to measure the acidity or basicity of a substance. The scale is a measure of the amount of \textH^+ ions that a substance will produce in solution (i.e. the concentration of \textH^+ ions, represented by \left[\textH^+\right]). The lower the value of its \textpH, the higher the concentration of \textH^+ ions.

The \textpH of a substance can be measured using a universal indicator. Universal indicators are chemical mixtures that will react with substances across the whole pH range. When added to acids or bases (which are typically colourless), these reactions will produce solutions with colours corresponding to a specific \textpH value in the scale. An example of the \textpH colour scale is shown above.

When dissolved in aqueous solutions, acids will dissociate. Dissociation is the process where by a chemical compound separates to form aqueous ions. When acids dissociate, they do so into \textH^+ ions and anions (e.g. \textCl- for \textHCl or \textSO_4^2- in \textH_2\textSO_4).

As acidity is governed by the concentration of \textH^+ ions in solution, the extent of dissociation will determine how strong or weak an acid is. Strong acids are those that completely dissociate in solution. This means that every molecule of the acid has separated out into its constituent ions. In these cases, the concentration of \textH^+ ions will be be high, leading to a low \textpH.

In weak acids there is only partial dissociation. This means only a small proportion of the molecules in the solution will dissociate into solution. This dissociation is said to be reversible and so an equilibrium is set up:

As only a small amount of the acid actually dissociates, the equilibrium is taken to lie far to the left. This means that \left[\textH^+\right] will be much lower than in a solution of strong acid and so the \textpH will be higher.

As the concentration of \textH^+ ions, and therefore the strength of an acid, increases its \textpH will decrease. The \textpH will decrease by 1 for every factor of 10 increase in the concentration of \textH^+ ions. This can be expressed by the general formula:

It is important to distinguish between acids that are strong and acids that are concentrated. A solution of a weak acid may have a relatively low \textpH if it is highly concentrated as the concentration of \textH^+ ions will be relatively high. Similarly, a solution of a strong acid may have a relatively high \textpH if its fairly dilute. In this case the \textH^+ concentration will be relatively low, despite the acid being a strong acid.

Salts are typically insoluble in water. As a result of this, salts will often precipitate from a solution when they are formed. When a compound precipitates, tiny crystals are formed in solution. These crystals are more dense than the solvent and so fall to the bottom of the reaction vessel. The accumulation of these crystals at the bottom of the reaction vessel is called a precipitate. As salts are often insoluble, it is possible to separate them from solutions using filtration.

Titrations are a type of experiment in chemistry that can be used to determine the volumes and concentrations of different chemicals required for a chemical reaction. The most common use of titrations it to find out how much of a strong base is needed to neutralise a strong acid or vise versa. This information can then be used to calculate the concentration of the acid/ base.

Once the burette has been rinsed, we partially fill it with the titrant. The tap of the burette should be opened and some of the titrant allowed to run though. This ensure that the jet space of the burette is filled with titrant. We then fill the burette so that the meniscus (the U shape formed between the surface of the titrant and the glass walls of the burette) lines up with the 0 mark of the burette.

To run the titration, we open the tap of the burette and allow the titrant to run into the analyte. When the indicator changes colour, we close the tap and read off the volume of titrant added, this volume is known as the titrant.

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