System and surrounding

Where does the energy go?

Energetics is the study of energy changes during a chemical reaction. These changes come from bonds breaking and bonds forming: breaking a bond needs energy, and forming a bond gives energy out.

To follow that energy, chemists split the world in two. The system is the change you are studying: the reactants, products, catalyst, solvent and anything else that matters to the reaction. Everything else is the surrounding.

Boiling water in a beaker: the water molecules are the system; the beaker, the burner and the room around them are the surrounding.

Energy flowing from the surrounding into the system is an endothermic change, with a positive sign; the surroundings get cooler. Energy flowing from the system out to the surrounding is exothermic, with a negative sign; the surroundings get warmer.

The sign is always from the system's point of view: energy in is plus, energy out is minus.

Key terms

System

Deciding what gains or loses energy.

Surrounding

Where released energy goes, or absorbed energy comes from.

Endothermic

Changes that cool the surroundings.

Exothermic

Changes that warm the surroundings.

Short questions with model answers

1. Q1. Water is boiled in a beaker. Is this endothermic or exothermic, and which form of energy moves? (Exercise, p. 69)

   • System: the water
   • Heat flows burner → water
   • Energy in: sign +

   Endothermic; heat energy is transferred into the water.

2. Q2. A reaction in a test tube makes the tube feel warm. Which way did energy flow, and what is the sign? (Fig 5.1, p. 69)

   • Surrounding got warmer
   • Energy: system → surrounding

   Out of the system: exothermic, negative sign.

Common mistakes

• ✗ The beaker is part of the system.

  ✓ The beaker holds the change but is not the change: it is the surrounding.

• ✗ Exothermic has a positive sign because heat is produced.

  ✓ The system loses that heat, so the sign is negative.

• ✗ If the surroundings get cooler, the reaction gave out cold.

  ✓ The system took heat from the surroundings: endothermic.

MCQs

1. 1. In a reaction, which of these is part of the system?

   1. (a) The catalyst
   2. (b) The burner
   3. (c) The laboratory air
   4. (d) The table
   Show answer

   (a) p. 68: the system includes reactants, products, catalyst and solvent.

2. 2. Energy moves from the surrounding into the system. The change is:

   1. (a) Exothermic, negative
   2. (b) Endothermic, positive
   3. (c) Exothermic, positive
   4. (d) Endothermic, negative
   Show answer

   (b) Energy in is endothermic with a positive sign (p. 68).

3. 3. In an endothermic change, the surroundings:

   1. (a) Get warmer
   2. (b) Get cooler
   3. (c) Stay exactly the same
   4. (d) Become part of the system
   Show answer

   (b) Fig 5.1: endothermic, surroundings get cooler.

4. 4. Forming a chemical bond:

   1. (a) Absorbs energy
   2. (b) Releases energy
   3. (c) Has no energy change
   4. (d) Needs a catalyst
   Show answer

   (b) p. 67: energy is released when a bond is formed and absorbed when it is broken.

5. 5. Boiling water in a beaker: the water molecules are the

   1. (a) System
   2. (b) Surrounding
   3. (c) Catalyst
   4. (d) Container
   Show answer

   (a) p. 68 uses exactly this example.

Quick revision

• Energetics follows the energy taken in or given out when bonds break and form.
• The system is the change under study; everything else is the surrounding.
• Energy in: endothermic, positive. Energy out: exothermic, negative.