Chemguide: Support for CIE A level Chemistry ``` ``` Learning outcome 5.1.7 Measuring enthalpy changes experimentally ``` ``` This statement looks at how enthalpy changes can be measured experimentally. Before you go on, you should find and read the statements in your copy of the syllabus. ``` ``` You will find an introduction to this on the Chemguide page measuring enthalpy changes. The amount of information is limited because this is covered in some detail in my chemistry calculations book. it isn't difficult to find examples of these experiments and calculations on the web or in textbooks. There is one clarification of the syllabus that is needed. Statement 5.1.7 gives you two very similar versions of an equation. q = mcΔT and ΔH = -mcΔT In the first equation, q is the amount of heat evolved or absorbed that you have actually measured during the experiment and will be measured in joules. This is the amount of heat absorbed or evolved when the quantities you have used in the experiment react. ΔT is the temperature change in the reaction. In the second equation, ΔH is the enthalpy change for the reaction after you have converted it into kilojoules per mole. The negative sign in the second equation needs explanation. If you have an exothermic reaction, this will have a negative enthalpy change of reaction. The temperature in the reaction will have increased, and so ΔT (the temperature change) will be positive. The negative sign converts that into a negative enthalpy change of reaction. If you have an endothermic reaction, this will have a positive enthalpy change of reaction. The temperature in the reaction will have decreased, and so ΔT will be negative. The negative sign converts that into a positive enthalpy change of reaction. Interestingly, this is the first time that the first equation has appeared in the CIE syllabus. The almost identical statement in the previous syllabus just quoted the second equation. ``` ``` Go to the Section 5 Menu . . . To return to the list of learning outcomes in Section 5 Go to the CIE Main Menu . . . To return to the list of all the CIE sections Go to Chemguide Main Menu . . . This will take you to the main part of Chemguide. © Jim Clark 2019