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GCSE: Aqueous Chemistry
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The rates of aqueous reactions
- 1 The rate of reaction can be increased by increasing the concentration of the solution. This will mean there are more molecules in the same amount of space, so there will be more collisions.
- 2 The rate of reaction can be increased by increasing the temperature of the solution. This will give the molecules more energy, making them move faster and collide more. It will also mean they stand a better chance of having the activation energy.
- 3 The rate of reaction can be increased by increasing the pressure of the solution. This will mean there are more molecules in the same amount of space, so there will be more collisions.
- 4 Adding a catalyst to the solution will increase the rate of reaction. This is because the catalyst lowers the activation energy needed for the solution to react.
The definition for rate of reaction is “change in concentration of product or reactant over time”.
It has the units mol dm-3 s-1
How to calculate the number of moles in a solution
The two most important equations to learn are:
moles = mass / Mr and moles = volume x concentration
If you know the moles of one chemical in your balanced equation, you can find out the moles of anything else by looking at the “big number” ratios. For example:
2NaOH + H2SO4 = Na2SO4 + 2H2O
If you had 10 moles of H2SO4, because there is a 2:1 ratio, you would have 20 moles of NaOH.
- 3 Your volume MUST be converted into dm3 before you use it in your equation. To convert cm3 into dm3 divide your number by 1000.
- 4 Do not forget to round your answer to a sensible number of significant figures (usually the least amount of significant figures that the question itself goes to).
- 5 Your Mr can be found by looking at the mass number on the periodic table (this is the bigger of the two numbers- the smaller one is called the proton number
Top tips for aqueous reactions
- 1 Anything that is dissolved in an aqueous solution will have the state symbols (aq). For Na+(aq)
- 2 If your reaction is dissolved in water, then water will have the state symbol (l), for “liquid”.
- 3 If the question says that your reaction is done under standard conditions, then it means at 1 atmosphere of pressure, at 25'C.
- 4 When constructing balanced reactions, do not forget to balance your charges when making salts. For example: HCl + Mg = MgCl + 0.5H2 would be wrong. The correct answer would be 2HCl + Mg = MgCl2 + H2.
- 5 The most important equation reaction to remember is acid + base = salt + water. This crops up all of the time in exams!
- Marked by Teachers essays 36
- Peer Reviewed essays 19
You can now read off the volume of base you've added. From this you can workout the concentration of the acid. As I know all the concentrations of the acids and alkalis given I can do another experiment, which is to measure the heat of neutralisation otherwise known as the enthalpy of neutralisation. Enthalpy is the measure of energy usually heat energy that a substance has. You can't measure enthalpy directly, but you can measure the change in enthalpy when a reaction happens.
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In any state, the particles present will have a very wide range of energies. For gases, this can be shown on a graph called the Maxwell-Boltzmann Distribution which is a plot of the number of particles having a particular energy. This graph below only applies to gases, but the conclusions that we can draw from it can also be applied to reactions involving the liquids. The area under the curve is a measure of the total number of particles present.
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Pour 5cm cm� of special indicator containing starch into the 10 cm� measuring cylinder. 7. Pour all the liquids into the 100 cm� beaker and start the stop clock. 8. Stir the solution until it turns a blue/black colour and the cross below is not visible. 9. Record the time it took for the solution to change colour. 10. Wash your apparatus. 11. Pour 30 cm� of potassium iodide and 40 cm� of sulphuric acid into the 100 cm� measuring cylinder and also 5 cm� special of indicator.
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Investigate the effect of changing the concentration of sodium hydroxide (alkali) on the volume of hydrochloric acid needed to neutralize a fixed volume of alkali by measuring the temperature and noting colour changes of the solution mixture.4 star(s)
Those with a pH of 8-14 are alkaline. PH 7 represents a neutral solution. Indicators are dyes which change colour to indicate weather a substance is acidic, alkaline or neutral. Some indicators such as the universal indicator are made of mixtures of dyes. Acidic substances turn the indicator red and those that are alkaline change the indicators colour to blue. Substances that are neutral change the indicator to green. In this experiment the most suitable indicator that should be used would be the universal indicator. This is because at the point of neutralization the indicator will give a specific colour change to show that the mixture has been neutralized.
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Linking prediction to theory: The collision theory describes how the rate of reaction (the speed in which a reaction takes place) will increase as the concentration of hydrochloric acid increases. This is due to the knowledge that particles are closer together in a concentrated solution. The closer the particles are, the more often the particles collide. With more collisions comes a greater chance of the reactants reacting. This also explains why I can predict that the greatest rate of reaction will be found as the reactants have come into contact, as they are both at their highest concentrations, and the rate of reaction decreases as the reaction continues as the concentration of the reactants decreases.
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These bond with the hydrogen ions and become hydrogen atoms. These atoms bond in pair to form molecules which effervesce and escape as gas. Rates of Reaction There are three factors that influence the speed of which magnesium reacts with sulphuric acid. I will look at each of these in detail. * The concentration of sulphuric acid. If the concentration of sulphuric acid is increased then the speed of the reaction will be increased. This is because with an increase in concentration then there will be more hydrogen ions present.
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This therefore allows them to act as catalysts. As there are more spaces for electrons to be lost and gained the reaction can take place faster and better. The general equation for the experiment is: CATALYST 2H2O2 O2+2H20 From the equation it can be seen that Oxygen is produced in the reaction and this is what is being collected and measured in the gas syringe. Prediction: The transition metal oxides MnO2 (Manganese oxide), ZnO (Zinc Oxide) and CuO (Copper Oxide) will be compared with SiO2 (Silicon Oxide), A12O3 (Aluminium Oxide)
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For my investigation I am going to investigate the reaction rate of sodium thiosulphate and hydrochloric acid. The chemical reaction between sodium thiosulphate and hydrochloric acid is: Sodium + Hydrochloric Sodium + water + sulphur + sulphur Thiosulphate acid chloride dioxide 2HCl(aq) + Na2S2O3(aq) 2NaCl(aq) + SO2(g) + S(s) + H2O(l) When sodium thiosulphate reacts with hydrochloric acid a cloudy precipitate is formed, this is the sulphur held in a suspension of water and sodium chloride. A precipitate is a solid formed from a reaction between two solutions.
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The activation energy is the height of the potential barrier (sometimes called the energy barrier) separating two minima of potential energy (of the reactants and of the products of reaction). This can be interpreted through the diagram to the left: This graph also illustrates what happens to the reactants within the reaction, as you can see from the graph the reactants remain stationary, it then increases as you can see a dramatic rise of the line; though successful collisions only take place where the activation energy is shown, the deterioration of the line shows the reaction is thus coming to an end and there is a lowering of the collision frequency making successful collisions less frequent.
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How long the magnesium piece is left in - Temperature of solution and surroundings I will have to keep these the same so to keep the whole experiment as fair as possible. As any of these variables would have an unwanted affect, e.g. if the surface area was different for each magnesium piece, we would assume that the rate of reaction would occur greater in the magnesium piece with the larger surface area, resulting in the whole experiment becoming inequitable.
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Only a certain fraction of the total collisions cause chemical change; these are called successful collisions. The successful collisions have sufficient energy (activation energy) at the moment of impact to break the existing bonds and form new bonds, resulting in the products of the reaction. Increasing the concentration of the reactants and raising the temperature bring about more collisions and therefore more successful collisions, increasing the rate of reaction. This is called 'Collision Theory'. The same argument applies whether the reaction involves collision between two different particles or two of the same particle.
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Density: The density of a substance is the mass of the substance divided by the volume of the substance. Mathematically it can be expressed as, ? = _m_ V where ? is the density of the substance, m is its mass and v its volume. The density of an element depends on its relative atomic mass, as for an element of a certain volume, a certain number of atoms will be present, and as each atom will have a different mass to those of other element, the overall mass relative to volume will change.
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In this experiment I am finding out how much sulphuric acid is present in sodium carbonate solution.
Now I have worked out the volume of anhydrous sodium carbonate needed is 0.25dm3. Working out the number of moles of sodium carbonate needed We can use the formula n=cv to work out the number of moles of sodium carbonate needed. N: Represents Number of moles (mol) C: Represents concentration (mol dm-3) V: Represents Volume (dm3) I need to rearrange the equation to work out the number of moles of sodium carbonate needed. This is the rearranged equation Moles = concentration (mol-1 dm-3) x volume (dm3) Therefore, 0.1 x 0.25 = 0.025 mol 0.025 mol of sodium carbonate is needed.
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The clinker is allowed to cool and then ground to a fine powder to produce Portland cement. A small amount of gypsum (calcium sulphate) is added to the resulting clinker to slow down the setting of the cement. CONCRETE: Concrete is made by mixing cement, sand and gravel with water. The cement and water form a paste, which surrounds and binds the aggregate particles together. A chemical reaction (called hydration) takes place between the cement and the water. This causes the cement paste to stiffen (or set) in a about 2 hours and then gain strength (or harden)
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In order to find out the exact concentration of sulphuric acid, I will have to make up a standard solution of sodium carbonate, which later on can be used to titrate the acid. Titrations
Acid A + Base B � conjugate acid of base B + conjugate base of acid A. This particular titration is going to involve me finding out a precise concentration of an acid solution. Sodium Carbonate Sodium carbonate is a sodium salt of carbonic acid. Its formula is Na2CO3. Sulphuric Acid Sulphuric acid is a strong mineral acid which is soluble in water in all concentrations and has a formula of H2SO4. Anhydrous sulphuric acid is an extremely polar liquid, which is fully capable of dissociating itself by protonating. This property of sulphuric acid allows it to be an excellent solvent for many reactions, because protons are very mobile in H2SO4.
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The speed at which a reaction takes place differs because some can take place instantly while some can take months before a product is formed. The speed of a reaction is not the same as the how much a reaction takes place. The amount of the taking place depends on how much chemicals is used. For example, a reaction between magnesium and hydrochloric acid gives off hydrogen gas. The amount of gas produced depends on how much magnesium and hydrochloric acid used 1. If a lot of hydrochloric acid and magnesium are used, the hydrogen gas produced is much 2.
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Azo dyes are also used as pigments and in color photography. In the UK, companies like James Robinsons Limited make about thirty thousand tones of azo dyes each year. 2) Commercial And Laboratory Synthesis The synthesis of azo dyes involves the reaction of diazotation and coupling and at the end of the reaction, separation of the azo dye from the mixture that is going to be produced. 1) Diazotisation This is the reaction in which produces diazonium salt as well as a -N diazonium ion. This is very unstable when it exceeds temperatures above 10�C.
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To investigate the factors affecting the volume of carbon dioxide produced when a carbonate reacts with an acid.
to acid (HCl) is 1:2. Mass of CaCO3 = 0.42 RFM of CaCO3 = 100 So, Moles = Mass/RFM Moles = 0.42/100 = approx. 1/240 The ratio of HCl:CaCO3 is 2:1, so we divide this by 2 to give 1/120 This is in dm3, so we multiply it by 1000 to give it in cm3. 1000 x 1/120 = Amount (mol) We want the volume, so we divide the amount by the concentration (as shown in a previous general equation): Volume = (1000 x 1/120)
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Calculations: I shall be using 30.00 cm� of limewater solution of approximately 1.00g dm��. Due to this, the mass of Ca(OH)2 in 1g dm�� of the limewater solution is 1.00g. to work out the molarity of the calcium hydroxide in the solution I need to firstly work out what 1 mole of Ca(OH)2is in grams: Below I have displayed the RMM of calcium, oxygen, and hydrogen. Ca = 40 O = 16 H = 1 Due to this I can then work out the weight of 1 mole of calcium hydroxide: Ca(OH)2 = 40 + (16 + 1)
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2Cr2O72 -(aq) + 3C2H5OH(l) + 16H+(aq) 4Cr3+(aq) + 3CH3CO2H(aq) + 11H2O(l) Tertiary alcohols resist oxidation. To confirm presence of an alcohol the addition of sodium will cause effervescence. 2R-OH + 2Na 2RO-Na+ + H2 Carbonyl compounds react with 2,4 Dinitrophenylhydrazine to form coloured compounds called hydrazones (orange/ yellow precipitate). A positive test of an unknown with 2,4 dinitrophenylhydrazine confirms presence of an aldehyde or ketone. This reaction is a condensation reaction. Oxidation of secondary alcohols produces ketones, e.g. propanone and contains a C=O bond, where the carbon atom is also bonded to a CH3 group.
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To use the term "neutral point" in this context would be misleading. The equivalence point is often determined by visual indicators are available for titration based on acid-base neutralization, complexation, redox reactions and is determined by some type of indicator that is also present in the solution. For acid-base titration, indicators are available that change color when the pH changes. When all of the analyte is neutralized, further addition of the titrant causes the pH of the solution to change causing the color of the indicator to change.
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at regular intervals is plotted along one axis and the number of moles of added acid or base at these intervals along the other axis; such a plot is called a titration curve and is usually sigmoid (S-shaped), with the inflection point, where the curve changes direction, corresponding to the equivalence point. From the pH at the equivalence point, the dissociation constant of the acidic or basic group can be determined (see chemical equilibrium). If a compound contains several different acidic or basic groups, the titration curve will show several sigmoid-shaped curves like steps and the dissociation constant of each group can be obtained from the pH at its corresponding equivalence point.
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Indicators or are weak acids or bases that undergo dissociation in a known pH range. It is in this range that the acid (or base) is a different color from its conjugate base (or acid). There are many different kinds of chemical indicators. The classic example is litmus paper. I will use methyl orange for my experiment because it is the most suitable indicator for a weak alkali and strong acid. This indicator will change color from red (at pH 3.1) to orange-yellow (at pH 4.4) when acid is neutralised. Equation of the Titration I will be doing: Acid + Base Salt + Water Sodium carbonate + sulphuric acid ===> sodium sulphate + water + carbon dioxide Na2CO3(aq)
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Investigating the Effects of Increasing Copper Sulphate Solution Concentrations on the Germination of Cress Seeds
I will also prepare one of purely water, so that I can compare with a batch that had no copper sulphate whatsoever in the solution. I will use pure distilled water to add to each solution. I am using distilled water in the solution because it means I can control the levels of other macronutrients and micronutrients available. If I remove the other nutrients, it will mean that it is only the levels of copper sulphate available that is changing.
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Recrystallization - choose the most appropriate solvent to obtain a successful recrystallization of benzoic acid.
Decolorizing charcoal is added if colored impurities are present. 6. The hot solution is filtered through the flute filter paper into the heated flask. The original flask and the filter paper are rinsed with little hot water. 7. The solution of benzoic acid is removed from the hot plate and allows it to cool, undisturbed at room temperature. 8. After several minutes, some crystal is growing in the solution. After 15 minutes, the flask is cooled in the ice bath.
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