Calorimetry
Calorimetry measures heat by watching a temperature change. The heat absorbed is q = mcΔT, where m is mass, c is specific heat capacity and ΔT is the temperature change. Water has c = 4.184 J/(g·°C).
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Analogy
A kettle: how much the water warms tells you how much energy went in.
Example
m = 50.0 g, ΔT = 10.0 °Cq = 50.0 × 4.184 × 10.0q = 2092 J ≈ 2.09 kJ
In practice
Food labels list Calories measured by burning food in a calorimeter and recording how much it warms the water around it.
Practice
Flashcards
- Formula for heat from a temperature change?q = mcΔT.
Interview questions
- Describe how a coffee-cup calorimeter finds the enthalpy of a neutralisation reaction.
- Measure the temperature change of the solution
- Use q = mcΔT
- Heat gained by water = heat lost by reaction
- Divide by moles reacted to get ΔH per mole
Questions people ask
What is specific heat capacity?
Specific heat capacity is the heat needed to raise the temperature of 1 g of a substance by 1 °C. Water's value, 4.184 J/(g·°C), is unusually high. This is why water warms slowly and cools slowly, and why oceans moderate coastal climates.
Why does a reaction in a coffee-cup calorimeter give ΔH?
A coffee cup is open to the air, so the reaction runs at constant pressure. The heat that the reaction gives out or takes in changes the water's temperature. Heat gained by the water equals heat lost by the reaction, so ΔH = −q(water), per mole of reaction.