Find out the effect of concentration of acid, in thereaction between dilute hydrochloric caid and magnesium ribbon.

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Aim

The aim of this investigation is to find out the effect of concentration of acid, in the

reaction between dilute hydrochloric caid and magnesium ribbon. The rate of a chemical

reaction is a measure of how fast the reaction takes place. It is important to remember

that a rapid reaction is completed in a short period of time. Some reactions are very fast,

e.g (the formation of silver chloride precipitate when silver nitrate and hydrochloric acid

solutions are mixed.In this investigation we will test different concentrations of acid

reacting with magnesium.

Prediction

I predict that as the concentration of the hydrochloric acid increases, the time taken for

the magnesium to disappear decreases. I predict that when the concentration of the

hydrochloric acid doubles, the rate of the reaction doubles.

Linking prediction to theory

Reaction rate and concentration.

The collision theory describes how the rate of reaction increase (the time taken for the

magnesium ribbon to disappear when it is reacted with hydrochloric acid) when the

concentration of HCL increases. The theory states that if, the more concentrated the

reactants, the greater the number of collisions between particles increase. This also

explains why the greatest rate of reaction is usually as soon as the reactants have been

mixed,i.e they are both at thier highest concentrations. As the reaction continues,the

concentration of the reacting substances decrease and so does the rate of reaction. We

must consider what happens when a reaction takes place. First of all the particles of the

reacting substances must collide with each other, and secondly a fixed amount of energy

called activation energy(Ea) must be reached if the reaction is to take place. If the

particles can produce the right amount of energy(i.e if they collide fast enough and in the

right direction) a reaction will take place. The reaction is speeded up if the number of

collisions is increased. In this investigation we must consider the topic of variables.

Clearly, the time taken for the magnesium to disappear when it is placed in different

concentrations of HCL, is related in some way. The higher the concentration of HCL you use, the less time it takes for the magnesium to disappear and so the rate reaction increases. The concentraton of HCL you use is the independant variable because it will vary, and the time taken for the rate of reaction to take place(i.e the magnesium to disappear) is the dependant variable because it depends upon the concentration of hydrochloric acid. Other variables throughout the investigation, which will vary are the volume of water used , and the volume of hydrochloric acid. The variables which will remain unchanged are the temperature(room temperature) will stay the same in order for it to be a fair test, because if the temperature changes it will effect the the rate of reaction  between the reactants, either by speeding it up if the temperature rises because the particles move faster and travel a greater distance in a given time and so will be involved in more collisions. Or the temperature may slow the reaction down due to particles moving slower. The amount of magnesium used will stay the same (2cm long), so that it is a fair test. The time it takes for the magnesium to disappear will be measured accurately using a stop clock as soon as the HCL is poured into beaker with the magnesium in it. This way most results will be accurate. It is important to keep the reactants separate while setting up the apparatus so that the starting time of the reaction will be measured accurately. Factors that may not be easy to control are, how well the solution is mixed when it is diluted, to get the correct concentration. A way to come around could be(stirring the test tube twice , or shaking the testube twice). This factor is quite important because it determines exactly what the concentration of the solution is each time it is mixed , if the stirring factor is not carefully controlled it could lead to inaccurate results if the concentration of the acid solution is not what we calculated it to be. During a chemical reaction the particles have to collide with enough energy to first break the bonds and then to form the new bonds and the rearranged electrons, so it´s safe to say that some of the particles don't have enough energy to react when they collide. The minimum amount of energy that is required to break the bonds is called activation energy(Ea). If the activation energy is high only a small amount of particles will have enough energy to react so the reaction rate would be very small, however the activation energy is very low the number of particles with that amount of energy would be so high, so start, so the reaction rate would be higher. An example of low (Ea) would be in explosives when they only need a small input of energy to start their exceedly exothermic reactions. A change in concentration is a change in the number of particles in a given volume. If we increase the volume a) the particles are more crowded so they collide more often.

b)even though the average amount of energy possessed by a particle does not change,there are more particles with each amount of energy - more particles with the activation energy.

a) is a major effect which effects the rate but b) is a minor effect which effects the rate only slightly. In this experiment we are not concerned with whether the reaction is exothermic or endothermic because we are interested with the activation needed to start and continue the reaction.

Apparatus/method

Hydrochloric acid(3 mole)

Water(to dilute acid)

10ml testube

magnesium(2cm long)

stop clock(sensitivity 1/10s)

beaker

Method

To get the amount of magnesium and the amount of hydrochloric acid to use in the situation, we have to use an excess of acid so that all of the magnesium disappears.

An equation for the reaction:

Magnesium + hydrochloric acid >magnesium chloride + hydrogen

Mg(s) + 2HCL(aq)>Mgcl2(aq) + H2(g)

1mole 2moles 1mole 1mole

So we can say that one mole of magnesium reacts with 2 moles of hydrochloric acid.We know that the concentration of hydrochloric acid that we start with is 3 moles.Throughout the experiment, we diluted the acid solution using water, as we did this the concentration of the acid decreased. We put the acid(3 moles) into the 10ml testube and then poured the solution into the beaker with the magnesium ribbon init(2cm long) and then measured the time taken for the magnesium ribbon to disappear, using the stop clock. Next, we diluted the acid by placing 9ml of acid and 1ml of water into the testube and then poured it into the beaker with the magnesium and measured the time taken for the magnesium ribbon to disappear using the stop clock. Then 8ml of acid and 2ml of water solution on the magnesium, measured time taken for magnesium to disappear using stop clock . Then 7ml of acid and 3ml of acid, repeated as above . Then 6ml of acid and 4ml of water, repeated as above. We repeated the experiment 3 times to get a fair test by proving that all results are accurate, if they were inaccurate we repeated the experiment again, or ignored wrong results.We tabulated the results in a clear table as soon as each experiment was completed ,in case we lost them and to check for possible errors or gaps in our results and it helped us to spot patterns in the results. We calculated the different,concentrations of the acid throughout the experiment using the method below as in the table.When we did the experiment, we made sure that we started the stop clock, at exactly the

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same time as the acid was poured onto the magnesium, to measure the start of the reaction to the finish of the reaction where the equivalent care was taken to stop the clock at the point the magnesium disappeared which was at the end of the reaction. While we set up the apparatus we kept the reactants separate so that the starting time of the reactants could be measured accurately and for it to be a fair test. We wore safety goggles when we did the experiment, to protect our eyes from the very harmful acid. We considered the factors ...

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