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Aim
Aim
To calculate the molar enthalpy change for two reactions and use Hess’s law to determine the enthalpy change for the reactions
To calculate the molar enthalpy change for two reactions and use Hess’s law to determine the enthalpy change for the reactions
A. Report & Analyse of Results
A. Report & Analyse of Results
1. Record your results in a suitable table
Learning Tip
Learning Tip
Tabulate data
Include headings
Include units
record to same number of dps each time
Things to consider CPAC 4b
Things to consider CPAC 4b
CPAC 4b - Obtains accurate, precise and sufficient data for experimental and investigative procedures and records this methodically using appropriate units and conventions.
· Record temperature changes accurately to 0.5oC
· Record masses accurately to 2dps
· Record data in table or suitably with units
The equations for the reactions occurring are:
reaction 1: K2CO3(s) + 2HCl(aq) → 2KCl(aq) + CO2(g) + H2O(l)
reaction 2: KHCO3(s) + HCl(aq) → KCl(aq) + CO2(g) + H2O(l)
2. Calculate the energy change for each reaction in J. The specific heat capacity of water is 4.2 J g–1 °C–1 .
3. Calculate the enthalpy change, ∆H, for each reaction in kJ mol–1 . Assume that the hydrochloric acid is in excess.
4. Use your results to calculate the enthalpy change for the thermal decomposition of potassium hydrogen carbonate: reaction 3: 2KHCO3(s) → K2CO3(s) + CO2(g) + H2O(l)
Learning Tip
Learning Tip
The heat capacity of the final solution can be assumed to be the same as the heat capacity of water. The volume of water made in the reaction is so small it can be ignored.
For exothermic reactions the enthalpy change, ∆H, is negative.
Beware of the incorrect use of the equals sign. It is very easy to end up stating that a negative number equals a positive number
Math that you will require
Math that you will require
Understand and use the symbols: =, <, <<, >>, >, ∝⇌., ~
Substitute numerical values into algebraic equations using appropriate units for physical quantities.
Solve algebraic equations
B. Evaluate your report by answering the questions
B. Evaluate your report by answering the questions
1.Why is it not possible to measure the enthalpy change for the decomposition of potassium hydrogencarbonate directly?
2. Show that the hydrochloric acid is in excess in both reactions.
3. Draw energy level diagrams for reactions 1, 2 and 3.
4. Explain why the reactions are conducted in a polystyrene cup rather than a glass beaker.
C. Evaluate your Practical by Completing the Form - so you know where you should aim to make improvements in future
C. Evaluate your Practical by Completing the Form - so you know where you should aim to make improvements in future
Required Resources
Required Resources
Core Practical 8 Student Evaluation.docxAS-and-A-level-Chemistry-Core-Practical-8-hess-law-Student Worksheets.docxConsult CLEAPSS Hazcards® 47A, 95A. Perform a risk assessment using up-to-date information before this practical is carried out
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