Chemistry-C5

Question	Answer
Chemistry-C5 Weakest Topics	Chemistry-C5 Weakest Topics
Precipitation reaction of Barium Chloride and Sodium Sulfate...	Image: image (image/jpg)
How can you tell it is a precipitation reaction?	Because you start off with two solutions but end up with a solid. The solid is a precipitate and will turn the water cloudy.
What is a spectator ion?	An ion that doesn't change in a reaction. In this reaction, the lector ions are the sodium and chloride ions. They we dissolved in solution before and they're still dissolved afterwards.
What is the ionic equation for the precipitation reaction of Barium Chloride and Sodium Sulfate?...	The reaction can also be shown by an ionic equation: Cu2+(aq) + 2OH–(aq) → Cu(OH)2(s) This only shows the reaction between the ions that produce the precipitate.
Test for sulfate ions...	1) Add dilute HCl, followed by barium chloride 2) A white precipitate of barium sulfate means the original compound was a sulfate For example, adding HCl and barium chloride to potassium sulfate or magnesium sulfate will produce a white precipitate.
Test for halides...	To test for chloride, bromide or iodide ions, add dilute nitric acid followed by lead nitrate. A chloride gives a white precipitate of lead chloride A bromide gives a cream precipitate of lead bromide An iodide gives a yellow precipitate of lead iodide
What makes water hard?	The presence of calcium ions and magnesium ions in the water.
Why is hard water difficult to lather?	The dissolved calcium ions and magnesium ions in hard water react with the soap to form scum, so more soap is needed.
What is scum?	An insoluble precipitate which forms with soap and hard water.
How can you avoid scum with hard water?	Use a soapless detergent.
What determines how hard of soft the water will be?	The types of rocks found in different regions.
Why is some water hard and some soft?	The water in some parts of the country is soft because it has low levels of dissolved calcium and magnesium compounds, while the water in other parts of the country is hard because it has higher levels of dissolved calcium and magnesium compounds.
Where is water hard and soft?	There are varying degrees of hardness in water - from slightly hard to very hard. Image: hard_water.gif (image/gif)
How can you measure hardness in water?	One way is to perform a titration with soap solution.
Explain the process of this titration...	A known volume of water is put into a conical flask. Soap solution is added to it from a burette or pipette. The mixture is swirled to help it form lather. The volume of soap solution that needs to be added to form permanent lather is recorded. The harder the water, the greater the volume of soap solution needed.
Types of hardness...	Temporary hard water can be softened by boiling it. Permanent hard water stays hard, even when it is boiled.
What do these results show? Distilled water 0.1 A 6.4 A after boiling 1.6 B 3.8 B after boiling 3.8	Water B contained only permanent hardness because boiling made no difference to the volume of soap solution needed to form permanent lather. Water A contained both types of hardness. Less soap solution was needed after boiling, but more was needed to form permanent lather than was needed for the distilled water control.
Explain temporary hardness...	Temporary hard water contains dissolved hydrogen carbonate ions, HCO3–. When heated, these ions decompose (break down) to form carbonate ions, CO32–. The carbonate ions in the boiled water react with dissolved calcium and magnesium ions to form insoluble precipitates (calcium carbonate and magnesium carbonate).
Explain permanent hardness...	Permanent hard water contains dissolved sulfate ions, SO42–. These do not decompose when heated. They remain dissolved and do not react with calcium and magnesium ions - so the water stays hard even when boiled.
What are the benefits of hard water?	Hard water has some benefits compared to soft water. For example, the dissolved calcium compounds in hard water: can improve the taste of the water are good for the development and maintenance of bones and teeth can help to reduce heart disease
What are the disadvantages of hard water?	hard water also has some drawbacks compared to soft water. For example: More soap is needed to produce lather, which increases costs. This happens with temporary or permanent hardness. The scum produced is unsightly - spoiling the appearance of baths and shower screens, for example. Temporary hardness can reduce the efficiency of kettles and heating systems. This is because limescale (a solid containing calcium carbonate) is produced when the water is heated. It coats the heating element in kettles, and the inside of boilers and hot water pipes. This means more energy is needed to heat the water, again increasing costs. Pipes may become blocked by limescale - causing the heating system to break down.
How can you soften hard water?	The damaging effect that hard water can have means that it may be beneficial to soften the water. Methods for softening hard water involve the removal of calcium ions and magnesium ions from the water. There are two methods for softening hard water: adding sodium carbonate to the water using ion exchange columns
How does adding sodium carbonate soften water?	Sodium carbonate, Na2CO3, is also known as washing soda. It can remove temporary and permanent hardness from water. Sodium carbonate is soluble but calcium carbonate and magnesium carbonate are insoluble. The carbonate ions from sodium carbonate react with the calcium and magnesium ions in the water to produce insoluble precipitates. For example: calcium ions + sodium carbonate → calcium carbonate + sodium ions Ca2+(aq) + Na2CO3(aq) → CaCO3(s) + 2Na+(aq) The water is softened because it no longer contains dissolved calcium ions and magnesium ions. It will form lather more easily with soap. However, the calcium carbonate and magnesium carbonate precipitates to form limescale. As well as being unsightly on your taps, it can also clog up pipes in heating systems (causing them to break down). This makes treatment with sodium carbonate suitable for softening water only in certain circumstances - such as softening water for hand washing clothes
How does ion exchange soften water?	Commercial water softeners often use ion exchange resins. These substances are usually made into beads, which are packed into cylinders called ion exchange columns. These can be built into machines, such as dishwashers, or plumbed into water systems to continuously soften the water. The resin beads have sodium ions attached to them. As the hard water passes through the column, the calcium and magnesium ions swap places with the sodium ions. The calcium and magnesium ions are left attached to the beads, while the water leaving the column contains more sodium ions. The hard water is softened because it no longer contains calcium or magnesium ions. Some ion exchange resins use hydrogen ions instead of sodium ions.
Reaction of magnesium with acids...	Magnesium reacts with acids to produce a magnesium salt and hydrogen: magnesium + hydrochloric acid → magnesium chloride + hydrogen magnesium + ethanoic acid → magnesium ethanoate + hydrogen The rate of reaction is determined by measuring the volume of hydrogen produced as the reaction goes on.
Reaction of calcium carbonate with acids...	Calcium carbonate reacts with acids to produce a calcium salt, water and carbon dioxide: calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide calcium carbonate + ethanoic acid → calcium ethanoate + water + carbon dioxide The rate of reaction is determined by measuring the volume of carbon dioxide produced as the reaction goes on.
How can you determine the amount of gas produced?	If the same concentration and volume of acid is used in these reactions, the same volume of gas is produced whether hydrochloric acid or ethanoic acid is used. This is because both acids contain the same amount of acid reactant (same number of moles). The strength of the acid does not matter here.
Explain why a weak acid reacts with calcium carbonate slower than a strong acid does, but still produces the same amount of gas under the same conditions...	If the same concentration and volume of acid is used in these reactions, ethanoic acid will react more slowly than hydrochloric acid. This is because the ethanoic acid will contain fewer hydrogen ions in a solution (it will have a lower concentration of hydrogen ions). There will be fewer collisions between hydrogen ions and particles of the magnesium or calcium carbonate, so the rate of reaction will be lower than for hydrochloric acid.
Number of moles=	mass in grams/ Molecular mass (of element of compound)
Relative Molecular Mass (Mr)	Sum of relative atomic mass (Ar) of all atoms in a molecule
Relative atomic mass (Ar)	Mass of one atom of the element/ Mass of 1/12 of an atom of carbon 12
Percentage Composition by Mass	Relative atomic mass x number of atoms of that element/ Molecular mass of whole compound x 100
Concentration	number of moles / volume
Concentration triangle	Image: no.ofmoles (image/jpg)
Converting moles per dm3 to grams per dm3	Image: molestograms (image/png)
Gas Volumes	One mole of gas always occupies 24dm3 at room temperature and pressure. Image: volumemoles (image/png)
Strong acid... (eg, hydrochloric acid)	In aqueous solution, a strong acid is one that completely dissociates (splits up) into ions. HCl(g) →H+(aq) + Cl-(aq)
Weak acid... (eg, ethanoic acid)	In aqueous solution, a weak acid is one that only partially dissociates into ions. As a result, a state of equilibrium is produced. CH3COOH(aq)<---> H+(aq) + CH3COO-(aq)

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