How do you tell whether a reaction absorbs or releases energy?
Endothermic and exothermic reactions: distinguish endothermic and exothermic processes by the direction of energy flow and the sign of the enthalpy change.
A focused Virginia SOL Chemistry answer on reaction energy under CH.6: the difference between endothermic and exothermic reactions, the direction of energy flow, the sign of the enthalpy change, and how temperature change signals each type.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this topic is asking
Standard CH.6 opens with the energy of a reaction. Virginia expects you to distinguish endothermic and exothermic processes by the direction of energy flow, to read the sign of the enthalpy change, and to connect each type to a temperature change you could observe. Energy change is a core idea for potential energy diagrams and for predicting how reactions respond to heating.
Energy flow in a reaction
Every reaction either takes in or gives out energy, because the bonds in the products store a different amount of energy than the bonds in the reactants. Breaking bonds requires energy; forming bonds releases energy. The net result, the enthalpy change, decides whether the reaction warms or cools its surroundings.
Exothermic reactions
Examples include burning fuel (combustion), the neutralization of an acid by a base, and the reaction in a hand warmer. Because energy is given off, you feel heat leaving the system. "Exo" is a useful reminder that energy exits.
Endothermic reactions
Examples include the decomposition of many compounds (which needs heat) and the reaction inside an instant cold pack, which pulls heat from its surroundings and feels cold. Because the reaction takes energy in, the surroundings lose heat.
Try this
Q1. A reaction warms its surroundings. Is it endothermic or exothermic? [1 point]
- Cue. Exothermic; it releases energy, so the surroundings warm.
Q2. State the sign of for an endothermic reaction. [1 point]
- Cue. Positive (), because the products store more energy than the reactants.
Exam-style practice questions
Practice questions written in the style of VDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SOL (multiple choice)1 marksIn an exothermic reaction, energy is (A) absorbed and the surroundings cool (B) released and the surroundings warm (C) absorbed and the surroundings warm (D) released and the surroundings coolShow worked answer →
The answer is (B) released and the surroundings warm.
An exothermic reaction releases energy to the surroundings, so the surroundings warm up (you feel heat coming out). The products have less stored chemical energy than the reactants, so the enthalpy change is negative. An endothermic reaction is the opposite: it absorbs energy and the surroundings cool.
The trap is confusing the two; "exo" means energy exits (released), and the surroundings get warmer.
SOL (tech-enhanced, fill in the blank)2 marksA reaction is written as . (a) Classify the reaction as endothermic or exothermic. (b) State the sign of the enthalpy change.Show worked answer →
A 2-point item on reading an energy term.
(a) Exothermic (1 point): energy appears as a product, meaning energy is released.
(b) Negative (1 point): an exothermic reaction has a negative enthalpy change (), because the products store less energy than the reactants.
Markers reward linking "energy as a product" to exothermic and to a negative enthalpy change. If energy appeared on the reactant side, the reaction would be endothermic with a positive enthalpy change.
Related dot points
- Potential energy diagrams and activation energy: interpret a potential energy diagram, identify the activation energy and the energy change, and explain the effect of a catalyst.
A focused Virginia SOL Chemistry answer on energy diagrams under CH.6: reading a potential energy diagram, identifying the activation energy, the energy of the products versus reactants, and how a catalyst lowers the activation energy.
- Reaction rates and collision theory: explain reaction rate using collision theory, including effective collisions, orientation and the activation energy.
A focused Virginia SOL Chemistry answer on collision theory under CH.6: what reaction rate measures, why particles must collide with enough energy and the correct orientation, the role of activation energy, and the meaning of an effective collision.
- Factors affecting reaction rate: describe how concentration, temperature, surface area, a catalyst and the nature of the reactants change the rate of a reaction.
A focused Virginia SOL Chemistry answer on rate factors under CH.6: how concentration, temperature, surface area, catalysts and the nature of the reactants change reaction rate, each explained with collision theory.
- Chemical equilibrium and Le Chatelier's principle: describe dynamic equilibrium in a reversible reaction and predict the shift when concentration, temperature or pressure changes.
A focused Virginia SOL Chemistry answer on equilibrium under CH.6: reversible reactions and dynamic equilibrium, and using Le Chatelier's principle to predict how an equilibrium shifts when concentration, temperature or pressure changes.
- Phase changes and heating curves: name the phase changes and their energy changes, and interpret a heating or cooling curve including the plateaus.
A focused Virginia SOL Chemistry answer on phase changes under CH.4: the names and energy direction of melting, freezing, vaporization, condensation and sublimation, and how to read a heating curve, including why temperature stays constant during a phase change.
Sources & how we know this
- 2018 Science Standards of Learning - Chemistry — Virginia Department of Education (2018)
- Chemistry Curriculum Framework — Virginia Department of Education (2018)