1.1.1—Energy transfer
- Syllabus
- First assessment 2025
- Objective
- 1.1.1
- Level
- HL
Energy Transfer in Reactions
A chemical reaction transfers energy between the system and surroundings. Total energy is conserved. Heat is energy transferred because of a temperature difference; temperature describes the thermal state.
State which part is the system, which part is the surroundings, and the direction of energy transfer before interpreting a temperature change.
For an exothermic hand-warmer reaction, define the reacting chemicals as the system: energy leaves that system and enters your hand and the air as surroundings. A temperature rise is evidence about the surroundings, while heat names the energy crossing the boundary; neither quantity is 'stored temperature'.
Describing Energy Transfer
Assessment in practice
Representative question
Question 1
[Maximum number: 1]
Consider a reaction mixture that is in thermal equilibrium with the surroundings. When a reaction takes place, the temperature of the mixture decreases.
Which row correctly shows the changes in energy when the new thermal equilibrium is established?
Energy of system
Energy of surroundings
decreases
decreases
increases
increases
decreases
increases
increases
decreases
D
Measuring Enthalpy Summary
Retrieve the route: define system/surroundings transfer, classify endothermic or exothermic, read stability from energy profiles, then calculate Q and ΔH with the correct sign.
Check the energy direction, surroundings temperature, reactant/product levels, moles, units, and ΔH sign.