Hi and welcome back to freesciencelessons.co.uk. By the end of this video you should be able to describe how to carry out a practical investigation to explore the effect of concentration on the rate of a reaction. I'm going to show you two different methods to carry out this practical and you need to learn both of them. Now before we start looking at the practical, We need to look at a key idea in science, which is a hypothesis.
A hypothesis is a proposal that could explain a fact or an observation. And in science, a hypothesis must be testable. Okay, so we're going to start by looking at an experiment called the disappearing cross reaction. We're going to use this to explore the effect of concentration on the rate of a reaction.
If we react sodium thiosulfate solution with hydrochloric acid, then one of the products is sulfur. And sulfur is a solid. The sulfur makes the solution go cloudy.
Scientists call this cloudiness turbidity. We can use this to see how long the reaction takes to finish. So let's take a look at the method.
First we use a measuring cylinder to put 10cm3 of sodium thiosulfate solution into a conical flask. We then place the conical flask onto a printed black cross. Next we add 10cm3 of hydrochloric acid into the conical flask. Now we swirl the solution and start a stopwatch. We now look down through the top of the flask.
After a certain time, the solution will turn cloudy. We stop the clock when we can no longer see the cross. We then carry out the experiment again using lower concentrations of sodium thiosulfate solution. Finally, we repeat the whole experiment and calculate mean values for each concentration of sodium thiosulfate solution. Remember that we do not include any anomalous results when calculating a mean.
Now there is one important idea linked to this practical which you need to learn and that is reproducibility. A measurement is reproducible if it can be repeated by another person or using a different technique or equipment and still get the same result. Now the problem with the disappearing cross experiment is that different people have different eye sights.
That means that some people can see the cross for longer than others, so they may not get the same results. However, because all the students use the same size printed cross, hopefully this problem won't be too great. Okay, we're going to look now at the second method for this required practical. We're going to measure the volume of a gas produced by a reaction.
Take a look at this reaction. Here we're reacting magnesium with hydrochloric acid. This reaction produces hydrogen gas and This allows us to measure the volume of hydrogen gas produced. So let's look at the method. First we use a measuring cylinder to place 50 centimetres cubed of hydrochloric acid into a conical flask.
We then attach the conical flask to a bung and the delivery tube like this. Now we place the delivery tube into a container filled with water. We then place an upturned measuring cylinder also filled with water over the delivery tube.
We then add a 3 cm strip of magnesium to the hydrochloric acid and we start a stopwatch. The reaction produces hydrogen gas and that's trapped in the measuring cylinder. Every 10 seconds we measure the volume of hydrogen gas in the measuring cylinder and we continue until no more hydrogen is given off.
We now repeat the experiment using different concentrations of hydrochloric acid. Now both of these experiments show us that the greater the concentration of a chemical in a reaction, the faster the reaction takes place. So because this result is shown by two different experiments, we can say that this finding is reproducible. Remember you'll find plenty of questions on this required practical in my revision workbook, you can get that by clicking on the link above.
Okay so hopefully now you should be able to describe how to carry out a practical investigation to explore the effect of concentration on the rate of a reaction.