This study examined the use of the 5E instructional model in teaching redox reactions and its association with high school students’ problem-solving competence (PSC). A quasi-experimental design was employed involving an experimental group (n = 48) and a control group (n = 50) at a Vietnamese high school. Data were collected through achievement tests, a PSC rubric, classroom observations, and student self-assessment questionnaires. The results showed that the experimental group obtained higher post-test scores and demonstrated a more favorable score distribution compared to the control group. However, the experimental group also exhibited higher baseline performance in the pre-test, indicating that the groups were not fully equivalent at the outset. The calculated effect size (Cohen’s d = 0.89) suggests a substantial between-group difference in post-test performance, although this should be interpreted cautiously due to baseline differences. In addition, the experimental group showed higher scores across key dimensions of PSC, including analysis, planning, implementation, evaluation, and reflection, as well as a higher overall PSC composite score. A significant positive correlation was found between teacher evaluations and student self-assessments (r = 0.71, p < 0.001). Overall, the findings indicate that the 5E instructional model was associated with more favorable learning outcomes in this context, but no causal inference can be made.