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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!
* Always be prepared for a spill. * Make sure that the burette is closed when not in use. * Do not fill the burette right to the top. * Make sure the clamp stand is held tightly by your partner. * Make sure the burette is held tightly by the clamp stand. * Make sure the solution are put in a safe place and is not put on the edge of the table. * Make sure every solution, liquid, etc, solutions are put in a safe place and is not put on the edge of the table. * MAKE SURE YOU HAVE YOUR SAFETY GLASSES ON DURING THE WHOLE EXPERIMENT.
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Reaction Rates Tom Etra Aim the Aim of the Experiment Is To Investigate the Effect of Changing the Temperature of Hydrochloric Acid On the Speed of the Reaction Between Acid and Magnesium.
Disconnect the conical flask, wash out, refill with another acid and heat to a different temperature and repeat exp. The acid I am using is 2 molar. When I measure out the acid I will have to make it as accurate as I can so I will use a measuring cylinder and a beaker of acid so I can get the measurement as close to 25cm(. The Measuring of the magnesium has to be as accurate to the nearest millimeter so I will use a ruler. By keeping the length of mg, concentration and volume of acid the same throughout the experiment and just varying the temperature of the acid makes my experiment a fair test.
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The eye level readings have to be exactly the meniscus mark to deliver the most accurate measurement. The end point in the titration process means the two solutions have the same number of moles at that point. The methyl orange indicator was used in the experiment because it matches the solution and the color is easy to see. 0.2653 was used in the calculation because it's only 1/10 of the whole Na2CO3 in petty pan Concentration of a solution is the number of moles of solute per litre of solution The volumetric flasks were used in the experiment because it can deliver more accurate volume due to its long neck.
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Use a measuring cylinder to measure 100ml of water] 2. Use a measuring scale to measure the mass of each alcohol (excluding the cap provided with the bottle) 3. Make sure each alcohol is experimented 3 times so that an average can be calculated for a more meaningful result 4. Follow the order of alcohol according to the list (the order of the structures of the alcohols) 5. Make sure the aluminum foil is covering the flame of the fuel as much as it can without being burnt 6.
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Add aqueous sodium hydroxide until no further change is apparent. Warm the mixture gently and test the gas evolved. OBSERVATION INFERENCE Addition of NaOH (aq) immediately forms a bluey/green gelatinous precipitate which is yellowy/brown on the side of the test tube. The gas evolved when mixture was warmed had pH 10. Turned red litmus paper blue. Fe2+ usually forms this sort of green precipitate when reacted with sodium hydroxide to give the following: � Fe2+ + 2OH- --> Fe(OH)2 The brown precipitate is likely to be the oxidation of the Fe(OH)2 (s)
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I have also found that this is due to the collision theory. When two substances collide with enough energy they react. By increasing the temperature, the activation energy of the atoms increases, and so there is more chance of a successful collision and a reaction between them. This means that there will be more successful collisions in a shorter period of time, causing the reaction rate to increase. Scientific information Reactivity series. The Reactivity Series is an arrangement of metals in an order that shows how well they react with other substances. Metals vary widely in their ability to react.
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Where acid rain comes from Sulphur dioxide and Nitrogen oxides are the primary causes. When these gases react in the atmosphere with water, oxygen and other chemicals to form various compounds. Sunlight increases the rate of these reactions. The result is a mild solution of Sulphuric acid and Nitric acid. Prevailing winds are what blow the compounds of wet and dry deposition across state and national borders and sometimes hundreds of miles. One of the main causes of acid rain is sulphur dioxide.
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Now I will be ready to pour the sodium thiosulphate into the conical flask and start the stop clock. Shortly after the chemicals have started to react the concentration should become cloudy, so then I will not be able to see the X anymore, then I will stop the clock and mark down the time. Then I shall pour the concentration away and wash out the flask to make sure that all the chemicals are gone and will not interfere with the next concentration to make sure the result is clear.
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Another factor would be the surface area of the metal, if there was a larger amount of surface area of the metal in contact with the acid then the reaction rate would increase. Preliminary Experiment I used the preliminary experiment to make the following decisions. Which Metal ? * Magnesium Filings * Magnesium Ribbon * Zinc * Iron Filings What concerntrations of acid to use? To stir or not to stir For safety in our preliminary we are using dilute sulphuric acid which is an irritant so we will be using eye protection.
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Fill the conical flask with 10cm� sulphuric acid using the pipette. Add 5 drops of methyl orange indicator. Set up apparatus as in diagram. To obtain a rough idea of how much sodium carbonate will be needed to neutralise the sulphuric acid, perform one rough titration. Note down initial burette reading. Add sodium carbonate solution to the conical flask in 1cm� or 2cm� intervals, stirring after each addition. When the methyl orange indicator changes from red to yellow-orange in colour. Note the final burette reading, and the difference (the titre). Now perform 5 accurate titration trials - add sodium carbonate whilst stirring, but stop 5 cm� before the previous reading.
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The Rate of Reaction between Hydrochloric Acid and Magnesium when the Concentration of Acid is changed.
The collision theory states that for a chemical reaction to take place, the reacting particles must collide with eachother, with enough energy to break, or form new bonds between other particles - a successful collision. If you increase the temperature that the reaction takes place, (heating up the reactants) it will speed up the reaction. For most reactions a 1o degrees increase will double the speed of the reaction. The particles collide more frequently and have more energy. If you increase the concentration of the reactants the solution will have more particles of the reactants moving around in it, meaning it is more likely they will collide and cause a reaction.
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+ H2O(l) I have decided to investigate the relationship between concentration of acid and the rate of reaction. The other two variables I am going to keep constant so that the experiment will be a fair test. The surface area will be constant by choosing lumps of calcium carbonate that are relatively the sane size. Apparatus For this experiment I will be using: * Marble chips * Hydrochloric acid * Water * Weighing machine * Test tube * Measuring cylinder * Bowl * Retort stand Fair Test I shall keep the variables of temperature and mass of calcium carbonate constant throughout the experiment.
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Neutralisation is used in everyday life, for example in the treatment of wasp stings. Wasp stings contain bases, such as histamine, so they are treated with anti histamines which are acidic substances. Another example is the making of fertilisers, in which the acid is neutralised with another substance, for instance Ammonium Sulphate ((NH4)2SO4) is made by neutralising Sulphuric acid with Ammonium. The concentration of a solution is shown in Moles. The equation for working out the molarity of a solution from the volume of acid and base used is Na = Ma x Va Nb Mb x Vb In this
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I will also make sure my experiment is safe in all the possible ways I can. I will make sure we are safe by wearing rubber gloves just in case we spill some boiling water on our hands. Prediction: I predict that the bigger the beaker the faster it will cool, and the smaller the beaker the longer it will take to cool. I know this because of the "Food Theory", the theory tells me that if there was a plate with a pile of hot food on it, it will take quite long for it to cool down, where as if you were to break that pile of hot food into smaller pieces of food
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To decide how much gas to collect I knew that the chemical reaction would have to be slow enough to accurately stop the stopwatch when the amount had been collected. I also knew I needed not to collect too much gas otherwise the 1M acid reaction would take a very long time to collect the amount of gas. I did the experiment with 1M acid and 4M acid, the 4M acid slowed down at around 20cm� of gas, the 1M acid also didn't take too long to reach that amount so I decided I would collect 20cm� of gas.
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Sulphuric acid = 0.001 moles in 11.00cm3 From this we can work the concentration of the sulphuric acid to be: 11.00/1000= 0.011 (changing cm3 to dm3) 0.001/0.011 = 0.09 mol/dm3 (number of moles/volume) The results and calculations clearly show what I believe to be an accurate concentration of sulphuric acid. I carried out the experiment as stated in my method 3 times after doing a rough test. This rough test was not included when I worked out my average titre.
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The same weight of Marble chips of smaller size will increase the reaction speed. Prediction The variable in the experiment that I am going to change is acid Strength. As the strength increases the amount of particles increase, giving more acid particles to react with the marble chips. I predict that as the concentration of the acid increases the speed of the reaction will increase, also increasing the amount of gas produced. Hypothesis My Hypothesis is that the higher the concentration of acid the faster the reaction will take place, this is because in a higher concentration there will more hydrochloric acid molecules per set volume.
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Investigating the factors that affect the rate of reaction between hydrochloric acid and calcium carbonate
Equipment Needed * Hydrochloric acid * Calcium carbonate * Conical Flask * Gas tube/syringe * Stand and clamp * Stopwatch * Distilled water Here is a diagram of how I will set up my equipment: Safety * I will wear goggles to protect my eyes in case anything explodes * I will work in an area out of danger of being knocked over or spilling over Fair Test To ensure that this experiment is a fair test I will make sure I keep the other factors that can affect the reaction rate the same.
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concentration of the sodium hydroxide I would have to use the equation: Concentration NaOH = Volume of HCl x Conc of HCl Volume of NaOH In order to investigate how much hydrochloric acid is needed to neutralise the sodium hydroxide I would have to use an indicator to tall me when the solution is neutral. The indicator which I will use is called phenolphthalein. I am going to have to conduct a pre-test to investigate the colour ranges of when the solution is acid, alkali and neutral.
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Another way I could have done the experiment would have been to tip the marble chips into the conical flask then push in the bung and put a rubber tube into the bung and inject the hydrochloric acid into the flask but doing this would be difficult change over the tubes so the burette could collect the gas. I will wait until 20cm3 of volume has been collected in the burette and I will stop the clock and record my results in a table.
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(The information on the Collision Theory was from CGP Double Science Higher Tier Revision Guide) Fair Test For my experiment to be a fair test I am going to make sure that the following factors are kept the same: * I am going to use Calcium Carbonate in a powder form, as it is far more accurate to measure out so the test will be more accurate. * I am going to increase the amount of Calcium Carbonate by 2 grams each time I repeat the experiment. * I am going to use 50ml of Hydrochloric Acid in each experiment.
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Investigating how varying the concentration of hydrochloric acid affects the rate its reaction in a given time.
I will be using small marble chips therefore having a small surface area. As I said at the beginning, the acid particles being added will collide with the marble chips, causing a reaction. The rate of reaction is found out by looking at the number of collisions in a given time. Not all of the particles that collide react, for example if only 10 particles are added, on their first collision only 7 will react. So instead of putting 10 particles in we add 100 but only 70 of them collide, already we have a faster rate of reaction.
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Altering the pressure is simply not possible, as we do not posses the equipment to do this. Changing the temperature of the experiment is not a good variable as in the classroom environment; it is very hard to make the experiment a fair test with the equipment available to us. Catalysts are not available for use in this experiment. It is possible to alter the surface area of the chips but the problem lies in the fact that all of the chips vary in size and so you could never measure the differences in surface areas.
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In water they are floating around freely and in ice they are "stationary", they only vibrate slowly. This is because they do not have enough energy to break their intermolecular bonds. So in order to melt ice you need to add energy to give the molecules kinetic energy to move around, becoming less stationary. If enough energy is given to the molecules they break the intermolecular bonds and the ice melts to ice. 1. Why is ice more effective for cooling a drink than cold water The cooling of a drink with a cube of ice is more effective than using cold water because ice can absorbs a lot more heat.
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