Adding hydrogen or iodine to a hydrogen iodide equilibrium increases the rate of:

1. Rate depends on the concentration of the species being consumed. 2. If the system is stressed by adding reactant, either hydrogen or iodine, the resulting increase in concentration causes the rate of the forward reaction to increase. 3. That forward rate then exceeds the rate of the reverse reaction. 4. The system responds by making more product until the two rates match again. _Source: OpenStax Chemistry 2e (CC BY 4.0), Ch 13 'Fundamental Equilibrium Concepts', sections 13.1-13.4_