Thermochemistry The study of heat changes that occur

Thermochemistry: The study of heat changes that occur during chemical reactions and physical changes of state

Energy • Heat (q) = – Form of energy – always flows from a warmer object to a cooler object • Law of Conservation of Energy – Energy is neither created nor destroyed

Heat (q) • The SI unit is the joule • 1 calorie = 4. 184 J • 1 Calorie (food) = 1000 cal (science) = 1 kcal

Specific Heat (C) • Specific Heat (C) – the amount of heat required to raise 1 gram of a substance by 1 C – Specific heat is an intensive property, – Every substance has its own specific heat • Ex. Water = 4. 184 J/(g ∙ ºC) Glass = 0. 50 J/(g ∙ ºC)

Specific Heat • Units for C = J / g ● ºC (joules per gram degree Celsius) • Equation for Specific Heat: C = q / (m Δ T) • This equation can be rearranged to solve for heat (q) q= CmΔT • C = specific heat; • q = heat; • m = mass and • ΔT = change in temperature (Tf – Ti)

Specific Heat • A 10. 0 g sample of iron changes temperature from 25. 0 C to 50. 4 C while releasing 114 joules of heat. Calculate the specific heat of iron.

Example • C= q/ (m∆T) • C=114 J/ (10. 0 g x 25. 4°C) • c = 0. 449 J/g C

Another example • If the temperature of 34. 4 g of ethanol increases from 25. 0 C to 78. 8 C how much heat will be absorbed if the specific heat of the ethanol is 2. 44 J/g C

Another example • First, rearrange the specific heat formula to solve for heat • q = Cm T • q = (2. 44 J/g°C)(34. 4 g)(78. 8°C – 25. 0°C) • q = 4520 J

Yet another example • 4. 50 g of a gold nugget absorbs 276 J of heat. What is the final temperature of the gold if the initial temperature was 25. 0 C & the specific heat of the gold is 0. 129 J/g C

Yet another example • • C= q/ (m∆T); rearrange to find ∆T = q / (C x m) ∆T = 276 J / (. 129 J/g°C x 4. 50 g) T = 475 C • T = Tf-Ti • 475 = Tf - 25 • Tf = 500 C