1. Chapter 17 – Thermochemistry
Understanding the role of heat
in chemical reactions and phase changes

2. What is heat?
Energy (q) that transfers between two objects of different temperature.
Heat always travels from the warmer object to the cooler object. (high to low temperature)
Important to identify the system and the surroundings.

3. Energy transfers
Exothermic process: the system releases heat to the surroundings (temp of surroundings ). Negative q value.
Endothermic process: the system gains energy from the surroundings (temp of surroundings ).
Positive q value.

4. Specific Heat Capacity
When placed in a cup of boiling water, a metal spoon will heat up faster than a wooden spoon.
Beach sand heats up quickly, but the ocean water needs all summer to raise its temp just a few degrees.
These statements explain the role of a substance’s specific heat capacity (C) .
The amount of heat it takes to raise the temperature of 1g of the substance 1˚C.

5. When energy is added to a substance, its temperature rises except during a phase change!!!
Not all substances convert that energy gain, into a temperature change, equally.
Water and wood have high specific heats – they require a lot of energy to change temp.
Sand and metals have low specific heats – they require little energy to change temp.

6. Units for measuring heat flow
calorie - the quantity of heat needed to raise the temperature of 1g of pure water 1˚C.
1 dietary Calorie = 1 kilocalorie = calories
Joule – SI unit of energy – raises the temp of 1g of pure H2O by ˚C
1 J = cal or 4.184J =1 cal

7. Calculating energy flow
850 J was absorbed

8. When 2 materials touch…
Let’s say a hot piece of metal is immersed in a cool bath of water. Describe their temperature changes.
Discuss the energy flow between the 2 materials.
Calorimeter instrument.
Calorimetry - the precise measurement of heat flow into or out of a system for chemical and physical processes

9. Calorimetry Constant-pressure Constant-volume
Example: styrofoam cups (simple)
Constant-volume
Enthalpy (H)– is the same as the heat content of a system at constant pressure
∆H = q = the heat released or absorbed by a reaction.
Can be calculated b/c we know the specific heat of water

10. Calorimetry Dissolve reacting chemicals in known volumes of water
Measure initial temp of each solution
Mix the solutions in calorimeter
Measure the final temp of the mixed sol. When the rxn is complete
qsurr=mc∆T
qsurr= but opposite sign of qsystem
qsys= ∆H=-qsurr=(-mc∆T)

11. Thermochemical Equations
Chemical reactions absorb or release heat.
This energy can be included in the balanced chemical equation.
Example: the complete combustion of ethane gas produces 1560 kJ of energy per mole of ethane.