ThermochemistryThermochemistry

Study of heat in chemical reactions. Heat will travel between a system and

its surroundings Heat is measured in Joules (J) Metric English: BTU (British

Thermal Unit) 1 BTU = 1.06 kJ

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Calculating HeatCalculating Heat

The heat that transfers between substances can be calculated with the following equation:

q = (s) (m) (∆T)

Calculating HeatCalculating Heat

q : amount of heat (energy) transferred

s: specific heat m: mass (use density to change

from mL to g!) ∆T: change in temperature

Calculating HeatCalculating Heat

Specific Heat is the quantity of heat needed to raise one gram of a substance 1.0 ⁰C.

Each specific substance has its own specific heat. (pg. 394)

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Round to 4.18

Calculating HeatCalculating Heat

Example: How much heat is needed to

warm a 200.0 mL cup of coffee (H2O) from 20.0 ⁰C to 75.0 ⁰C?

Calculating HeatCalculating Heat

45980 J, but only 3 significant digits so….

4.60 x 104 J or 46.0 kJ

C))(55.0C g

J g)(4.18 (200.0 q o

Calculating HeatCalculating Heat

What is the specific heat of lead?

100.0 °C

20.0 °C

28.8 °C

1st Step - Calculate “q” for the H2O

C)20.0 - C)(28.8 C g

J g)(4.18 (50.0 q oo

q = 1840 J

20.0 °C

2nd Step - Calculate “s” for Lead

100.0 °C

q = (150.0 g)(s)(71.2oC)1840 J

Energy that water absorbed in step one was supplied

by the metal!

s = 0.172 J/(g x C) for Lead