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# Investigating the effect of changing the concentration of an acid on the rate of diffusion using agar jelly

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Introduction

Investigating the effect of changing the concentration of an acid on the rate of diffusion using agar jelly - Plan Aim * To find out how changing the concentration of an acid will quantitatively affect the rate of diffusion in agar Introduction Background Theory The reaction in the experiment will be between two reactants- sodium hydroxide (NaOH) mixed into the agar jelly, and hydrochloric acid (HCl) in which the agar segments will be immersed, in each experiment. The acid and alkali react to form a metal salt- sodium chloride (NaCl), as well as water and hydrogen gas, which will be seen as effervescence (bubbles forming and given off). The word and symbol equations below show the reaction more clearly: sodium hydroxide + hydrochloric acid --> sodium chloride + water + hydrogen The reaction is a neutralisation reaction, because the alkaline agar is being neutralised (i.e. its pH is being brought down to 7- the pH of pure water). An indicator has been incorporated into the agar- phenolphthalein, so I can track the neutralisation (and therefore diffusion) process; phenolphthalein is pink in alkalis, and turns colourless when neutral or acidic. In the experiment I will time how long it takes for the agar segment to turn colourless i.e. to be neutralised. I will then calculate the rate of diffusion from this. The reaction throughout the agar segments will depend on the collisions between the reactant molecules. The reacting molecules in every reaction must collide with enough energy to react, in order for anything to actually happen. If they do hit each other with enough energy to break their current bonds and form new ones, this is called a successful (or fruitful) collision, and the reaction will continue. If they do not collide with enough energy, this is called an unsuccessful or unfruitful collision and the different molecules just bounce off each other. The image to the right (from www.tiscali.co.uk/reference) ...read more.

Middle

0.8 5 Large straw 19.5 232 230 228 n/a 230 0.00435 1.0 5 Large straw 19.5 188 135 208 176, 192 185 0.00540 NB: Results in red are anomalies, and were not used when the average time was calculated- the repeated results were used instead. Accuracy It would have been better if I had timed the time the diffusion took to 1 decimal place, as it would have made my rate of reaction more accurate, but by the time I had realised this, I had recorded around half of my results, so I would either have to repeat them all to get times to 1 decimal place, which would take a very long time, or fabricate my results, which would not be scientific at all. Detecting the end point To detect the end point of the diffusion, when it turned colourless, I held the test tube up to a piece of white paper in relatively bright light to see when all the phenolphthalein had turned colourless. Analysis Statement about the graph The graph shows that the rate of diffusion has increased with the concentration of the hydrochloric acid, so a good summary statement for this would be: "As the concentration is increased, the rate of reaction increases" - OR - "The higher the concentration, the faster the diffusion/the higher the rate of diffusion." Gradient of graph= My Conclusion The graph has shown that my initial hypothesis is correct. My prediction was "as the concentration of the acid is doubled, the rate of diffusion will double." When the concentration of the acid was doubled from 0.2moldm-3 to 0.4moldm-3, the rate of diffusion doubled exactly from 0.0013s-1 to 0.0026s-1. Also, when the concentration of the acid was doubled again from 0.4moldm-3 to 0.8moldm-3, the rate of diffusion doubled from 0.0026s-1 to 0.0054s-1. With this evidence I can keep my prediction, and use it as my conclusion. ...read more.

Conclusion

If the temperature increased (and not due to heat given off by reaction) as the room warmed up, the reaction would occur faster, so the time taken would decrease, so the rate of diffusion would be higher. I may have detected the end point too late- that is I did not notice that the whole agar section was colourless until a long time after the neutralisation had been completed. This would increase the time, and decrease the rate of diffusion. The anomalies were excluded when the averages were calculated, so they would be correct. The rate of diffusion for the 0.8moldm-3 hydrochloric acid was abnormally low at 0.0052s-1, and it was very close to the point for the 0.6moldm-3 acid, so I am calling it a consistently anomalous result. I think it was caused by a combination of the sources of error in each repeat. Other experiments Another experiment, similar to this one, could be conducted to see if my conclusion (and also prediction) is applicable to other substances, or over a wider range of data points (concentrations), but in the same type of experiment. In this second experiment I could have 5 aerosol cans, each containing a gas of different concentration- ranging from 0.2moldm-3 to 1.0moldm-3, increasing in 0.2moldm-3 increments (like in the last experiment). In each experiment, a person would be standing 2 metres away from the spray can, when the gas inside it is sprayed, for 1 second exactly. At that moment the stopwatch will be started, and someone will time how long it takes for the "sniffer" to smell the gas that has been sprayed. When the sniffer smells the gas, they will give a quick signal, and the stopwatch will be stopped accordingly. The same will be done for each concentration of gas, at least 3 times for each one, to maintain reliability. The conclusion should be the same, or very, very similar to the one for the previous experiment. ?? ?? ?? ?? JA Biology Coursework - 1 - ...read more.

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