Energy Conservation & The System
Understanding the boundaries of a chemical reaction and the law of conservation of energy.
System vs. Surroundings
In thermodynamics, we must strictly define where the reaction happens.
The System
The chemical reactants and products themselves.
Example: The actual Magnesium atoms and Hydrogen ions reacting.
The Surroundings
Everything else outside the system.
Example: The water solvent, the beaker, the thermometer, the air, and YOU.
The Temperature Trap
This is the #1 mistake in Energetics.
We CANNOT measure the temperature of the System (the bonds breaking/forming). We can only measure the temperature of the Surroundings (the water).
- If the water gets HOT (), the System LOST energy ().
- If the water gets COLD (), the System ABSORBED energy ().
Heat vs. Temperature
| Property | Heat () | Temperature () |
|---|---|---|
| Definition | The total energy transferred between objects due to a difference in temperature. | A measure of the average kinetic energy of the particles. |
| Units | Joules (J) or Kilojoules (kJ) | Kelvin (K) or Celsius (°C) |
| Dependence on Mass | Extensive Property More substance = More heat energy stored. | Intensive Property A bucket of boiling water is the same temp as a cup, but has far more heat. |
Putting it into Practice
Identifying the System
Paper 2 StyleA student adds solid sodium hydroxide () to a beaker of water. The temperature of the water increases.
Identify the System and the Surroundings in this experiment.
1. Define the System: The system is the chemical reaction itself. In this case, it is the dissolving of the solid NaOH.
2. Define the Surroundings: The surroundings are everything else that can exchange energy with the system. This includes the water solvent, the glass beaker, the thermometer, and the air.
Practice: Temperature Changes
A chemical reaction occurs in a test tube and the test tube feels cold to the touch.
Explain whether the reaction gained or lost heat energy from the system's perspective.