One of the obstacles that occur in Chemistry learning is low student motivation due to monotonous media. Another thing that currently becomes an obstacle is the COVID-19 pandemic which limits face-to-face learning. However, Chemistry practicum learning in the laboratory is difficult to carry out at students' homes. The purpose of the study is to develop a virtual laboratory, the validity and effectiveness of which are measured, to increase students' understanding and motivation in Chemistry, the topic of exothermic and endothermic reactions. The 4D model is used in the development of learning media in the form of a virtual laboratory. The validity of virtual laboratory products is confirmed by media experts and material experts in their fields. Product effectiveness was measured using a posttest-only control group design with two groups of students involving 63 students. Data collection was carried out through questionnaires, tests, and interviews, with descriptive analysis for the results of validation of the learning media and pilot test and t-tests for testing the effectiveness of virtual laboratories. The results showed that the virtual laboratory is easy to use and has feedback at the end of the practicum simulation so that the virtual laboratory can be applied as a valid and effective learning media in increasing student understanding and motivation.