# An experiment on determining the standard enthalpy and entropy of the dissolution of borax

The salt A2B5 partly dissolves in solution. There are also pointers to other sources of information. Two common enthalpy change measurements Enthalpy changes involving solutions The experiments There are a whole range of different enthalpy changes that can be measured by reacting solutions or a solution plus a solid in a simple expanded polystyrene cup.

A common example would be the measurement of the enthalpy change of neutralisation of, say, hydrochloric acid and sodium hydroxide solution. The polystyrene cup serves to insulate the reaction mixture, and slows heat losses from the side and bottom. Heat is still lost from the surface of the liquid mixture, of course, and that can be reduced by using a polystyrene lid with a hole for a thermometer.

You can allow for heat losses during the reaction by plotting a cooling curve. All of this is described in some detail at the beginning of Chapter 5 of my chemistry calculations bookand I can't repeat it on Chemguide.

There are other sources of error in these experiments - in particular in the accuracy of the thermometer used. Since the temperature rise probably won't be very great, you would need to use the most accurate thermometer possible in order to keep the percentage error low.

The calculations The heat evolved or absorbed during a reaction is given by the expression: So for water the value you are most likely to come acrossthe specific heat is 4.

That means that it takes 4. In experiments of this sort, you will quite commonly have measured out a volume of solution rather than a mass, and it is very common to assume that the density of the solution is exactly the same as water 1 g cm That means that 25 cm3 of solution would have a mass of 25 g.

That is an approximation, though, and it does introduce some error. The assumption is also made that the specific heat of a solution is the same as the specific heat of water.

That's another approximation, and it also introduces errors into the answer. What is generally true is that these errors are relatively small compared with errors caused by heat losses, and so on. To do the calculation, you would normally just work out the amount of heat evolved or absorbed in your particular reaction, and then scale it up to give an enthalpy change per mole.

You will find examples of this in Chapter 5 of my chemistry calculations book. Enthalpy changes of combustion As normally measured in a lab at this level, these are far less accurate than the simple solution reactions above.

It is impossible to eliminate heat losses, or to be sure that you have complete combustion. You will find these experiments discussed in my chemistry calculations bookbut you will also find that a Google search will throw up endless versions with practical details, and possibly sample calculations.

The examples in my book include a fairly simple way of getting around the heat loss problem. I'm afraid that is as far as I am prepared to go on this topic without risking sales of my book, or breaching my contract with my publishers.

Where would you like to go now?The molar solubility of a substance is the number of moles that dissolve per liter of solution.

## Chemistry Help | lausannecongress2018.com

For very soluble substances (like sodium nitrate, NaNO 3), this value can be quite high, exceeding moles per liter of solution in some cases. The standard enthalpy of formation,, is the enthalpy change accompanying the formation of 1 mole of a substance from the elements in their most stable states at 1 bar (standard state).

Many of the processes are carried out at K. enthalpy and entropy of a borax solution revised 4/28/15 1 enthalpy and entropy of a borax solution report instructions format your eln as you did in week 2 for the rest of the experiments in chem 1lc.

Experiment #8: The Entropy and Enthalpy of the Dissolution of Borax Abstract In this experiment, the change in enthalpy and entropy of tetraborate and sodium ions from borax was investigated.

## Introduction

It was concluded that the change in enthalpy for the dissolution of borax was kJ/mol and that the change in entropy was J/ (mol K). The standard enthalpy change of the overall reaction is therefore equal to: (ii) the sum of the standard enthalpies of formation of all the products plus (i) the sum of the negatives of the standard enthalpies of formation of the reactants.

Find Enthalpy lesson plans and worksheets. They calculate enthalpy and entropy changes for different reactions and describe signs of change in different situations. This Get Free Access See Review students calculate the standard enthalpy change for the different combustion reactions. This worksheet has 3 problems to solve.

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