3.1.2Physical Chemistry
Calorimetry — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 12 parts Lesson 3 of 19
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5What calorimetry measures
Measuring energy through temperature
- Measuring energy through temperature Calorimetry finds how much energy a reaction transfers by measuring the temperature change of a known mass of water or solution.
- A rise in temperature shows an exothermic reaction and a fall shows an endothermic one.
Two experiments to know
- Two experiments to know You need to know the method for an enthalpy change of reaction in solution,
- and the method for an enthalpy change of combustion of a fuel.
The key equation
- The key equation Q = m × c × ΔT,
- where Q is the energy transferred in joules, m is the mass of liquid heated in grams, c is the specific heat capacity in J/g/°C and ΔT is the temperature change in °C.
Part 2 of 5Reactions in solution
1
Measure a fixed volume of the first solution into an insulated container, such as a polystyrene cup with a lid, and record its starting temperature.
2
Add the second reagent, using an excess where appropriate, and stir continuously.
3
Record the highest temperature reached (or the lowest if the temperature falls).
4
Work out ΔT by subtracting the start temperature from the final temperature.
5
Calculate Q using m × c × ΔT, taking the mass from the volume of solution.
Part 3 of 5Burning a fuel to heat water
1
Pour a fixed volume of water into a copper can and clamp it above a spirit burner.
2
Weigh the spirit burner with its fuel and record the starting temperature of the water.
3
Light the burner, stir the water and let it heat until the temperature has risen by about 20 °C.
4
Put out the flame, record the highest water temperature and re-weigh the burner.
5
The mass of fuel burned is the difference between the two readings; the energy gained by the water comes from Q = m × c × ΔT.
Part 4 of 5Assumptions made in the calculations
| Assumption | Why it is made |
|---|---|
| The solution has the specific heat capacity of water, 4.18 J/g/°C | Dilute solutions are mostly water, so this is close enough |
| The density is 1 g/cm3 | Lets you treat each cm3 of solution as 1 g |
| The heat capacity of the container is ignored | It is small compared with the water |
| The reaction goes to completion | All reactant is used up, so the full energy change is measured |
| Heat losses are negligible | Insulation and a lid keep losses small |
Part 5 of 5Sources of error and improvements
Why results are not perfect
Heat loss
- Heat loss Some energy escapes to the surroundings, so the measured temperature rise is too small.
- A lid, draught shield and extra insulation reduce this.
Energy absorbed by the can
- Energy absorbed by the can The metal can warms as well as the water, so part of the energy is not counted.
Incomplete combustion
- Incomplete combustion If the fuel burns in too little oxygen, less energy is released and soot may form on the can.
- Keep the can close above the flame.
Practical tips
- Practical tips Stir all the time so the temperature reading is even, and use a thermometer with good resolution.
Key terms
| Term | Meaning |
|---|---|
| Calorimeter | A container, often insulated, used to measure the energy transferred in a reaction. |
| Specific heat capacity | The energy needed to raise the temperature of 1 g of a substance by 1 °C; 4.18 J/g/°C for water. |
| Exothermic | A reaction that transfers energy to the surroundings, so the temperature rises. |
| Endothermic | A reaction that takes in energy from the surroundings, so the temperature falls. |
| Enthalpy change | The energy change in a reaction measured at constant pressure. |
Avoid these mistakes
1
Using the mass of the fuel or the reagent as m. In Q = m × c × ΔT, m is the mass of the water or solution being heated.
2
Forgetting that ΔT is final temperature minus starting temperature, not the final temperature alone.
3
Giving Q in kJ without converting. The calculation gives joules, and you divide by 1000 for kilojoules.
4
Blaming 'human error' for poor results. Name a specific cause such as heat loss or incomplete combustion.
5
Saying the lid is there 'to keep the water clean'. It reduces heat loss to the surroundings.
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