Exothermic, endothermic and enthalpy notation
≈ 48 minExothermic, endothermic and enthalpy notation
An energy-change description must say whose energy changes and in which direction. In an exothermic reaction, energy is transferred from the reacting system to the surroundings; products lie lower on a reaction profile and ΔH is negative. In an endothermic reaction, the system absorbs energy from the surroundings; products lie higher and ΔH is positive. The temperature of a surrounding solution can provide evidence, but it is an observation with limits: heat can escape, the container can absorb energy and mixing may complicate the measurement. Use the term enthalpy change for the energy difference between reactants and products under stated conditions.
Work it through
A dissolving process cools a beaker from 25 °C to 20 °C. The observed surroundings become cooler, so energy is being taken in by the system; the process is endothermic. Do not write that 'cold is endothermic' without identifying the system and surroundings. In an energy diagram, show a positive ΔH from lower-energy reactants to higher-energy products.
Mastery target
Interpret the sign of ΔH, relate it to a reaction profile and explain a temperature observation using a named system and surroundings.
Which sign of ΔH describes an exothermic reaction?
Name the key chemistry term from Exothermic, endothermic and enthalpy notation that best fits the explanation and visual model.
A beaker becomes cooler while a process occurs. What is the best conclusion?
Which statement corrects a common misunderstanding in Exothermic, endothermic and enthalpy notation?