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Energy Changes in
Reactions
Learning objectives
 Perform simple energy calculations using
different energy units
 Apply specific heat concept to heat
calculations
 Distinguish between endothermic and
exothermic reactions
 Calculate specific heat from calorimetry
 Calculate heat changes in calorimetry
Energy is capacity to do work
 Energy comes in different forms
 Kinetic energy is energy due to motion
 Potential energy is energy due to position or
state
Energy comes in many forms
 Kinetic energy is energy due to motion
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Heat
Rotation
Vibration
Translation
 Potential energy is energy due to position or state
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Height
Chemical
Electrical
Coiled spring
Energy is interchangeable
 Processes convert energy from one form to another
 Chemical reaction
• (potential → heat)
 Turbine
• (heat → mechanical)
 Generator
• (mechanical → electrical)
 Light bulb
• (electrical → light)
 Photosynthesis
• (light → chemical)
 Some energy is always wasted during conversion
Measuring energy: calories, Calories
and joules
 calorie is energy required to raise temperature of 1
g of water 1 degree C
 Calorie is the food version = 1,000 cal
 Raises temperature of 1 pint of water 3.8ºF
 Joule is SI unit derived from mechanical work:
work done when force of 1 Newton is applied for 1
meter
1 cal = 4.18 J
Specific heat
 Specific heat is energy required to raise
temperature of 1 g of substance by 1ºC
 Units are J/gºC or cal/gºC
Champagne from a styrofoam cup:
Measuring heat change
 Calorimetry is process used
to measure heat change of
reaction
 Heat change in calorimeter
is:
Q = mH2O x ΔTH2O x SHH2O
Weigh
Measure
Know
Principle of Calorimetry
 Conservation of energy:
 Exothermic:
 Temperature in calorimeter increases
 Heat lost by process (system) = heat gained by
H2O (surroundings)
 Endothermic
 Temperature in calorimeter decreases
 Heat gained by process = heat lost by H2O
Exo-thermic and endo-thermic
 H2 + O2 gives out
energy – exothermic
 The system (chemical
bonds) lose potential
energy)
 N2 + O2 absorbs
energy – endothermic
 The system gains
potential energy
Calorimetry example: Calculating
specific heat
 What is specific heat of lead if 57.0 J are required
to raise temperature of 35.6 g Pb by 12.5ºC?
Q = ms x ΔTs x SHs
Specific heat metal example
 35.2 g metal at 100ºC are placed in calorimeter containing
42.5 g H2O at 19.2ºC. Final temperature is 29.5ºC. What is
SH of metal? SH H2O = 4.18 J/g ºC
 Heat lost by metal = heat gained by H2O
 Qmetal = massmetal x ΔTmetal x SHmetal
 Qwater = masswater x ΔTwater x SHwater
Heat of reaction
 Heat of reaction is the enthalpy change
when reactants are converted into products
 Note sign:
 Exothermic (energy out) is –
 Endothermic (energy in) is +
Nutrients and energy
 Macronutrients supply energy
 Fats (9 Cal/g) (Main form of energy storage)
 Carbohydrates (4 Cal/g)
 Proteins (4 Cal/g)