The Lost Cross and Mandatory Lab 6.1

Mandatory Lab 6.1 refers to ‘Investigation of rates of reaction experimentally and evaluation of the results’.

Here, in the UK, I have been introduced to a new way of carrying out the ‘lost cross’ experiment with respect to changing the temperature.

The experiment is a classic. A solution of sodium thiosulfate is added to hydrochloric acid. Over time, a precipitate of sulfur is produced:

2HCl_(aq) + Na₂S₂O_3(aq)  –> 2NaCl_(aq) + SO_2(g) + S_(s) + H₂O_(l)

The initial rate of reaction can be measured by timing how long it takes for a cross to be obscured by the precipitate.

It is usually carried out with regards to concentration – the sodium thiosulfate has its concentration changed by having water added to it and the experiment gives really good results.

So when I read about this lab with respect to changing the temperature I was intrigued. I have always found the temperature / rate labs the hardest to carry out as my school does not have water baths.

This has always meant that when I have used it with students I have usually been restricted to something like hydrochloric acid and magnesium.

The temperature has been altered either by directly heating the hydrochloric acid – I’m not a fan of this for safety reasons or by putting a boiling tube of magnesium and hydrochloric acid into a water bath.

The problem with this water bath method was that even with a good insulator (ie, a styrofoam / polystyrene cup) it would take too long for the temperature of the acid to rise sufficiently to produce good data.

The ‘new’ method involves purchasing some ‘take away food and lids’ plastic trays (from somewhere like Amazon or ebay) – they are cheap and are also good insulators.

It’s a mini scale (not microscale) lab so it cuts down on reagents and waste as well. Let’s say that five of these containers can be used and water (at, say, five different temperatures) can be added to each container. The containers are fairly good insulators but towards the end of the lab they may need topping up with hot water from a kettle.

You can see from the photo that each ‘water bath’ holds five reaction vessels – so each tray could be used by five different groups / students.

In the ‘reaction’ hole, there is a cross drawn on the bottom of the container.

The pink solution is a ‘stop bath’ – it is a solution sodium carbonate and phenolphthalein. The reaction mixture is added to this at the end of the experiment to neutralise the sulfur dioxide gas being produced. Once the colour changes the stop bath is used up.

I believe, this method was developed by the AQA examination board due to health and safety concerns about the amounts of SO₂ being released into classrooms. If I have got this wrong and you would like this changed or amended, please leave a message below and I will do so.

The reaction works well and it generates good results – so why not give it a go?

Do you have any novel or different ways to carry out labs? If you do, I would love to hear about them so please reply below!