Under which conditions will the rate of a chemical reaction always decrease? select one: a. the concentration of the reactants increases, and the temperature decreases. b. the concentration of the reactants decreases, and the temperature increases. c. the concentration of the reactants increases, and the temperature increases. d. the concentration of the reactants decreases, and the temperature decreases.

Option D : The concentration of the reactants decreases, and the temperature decreases.

Rate of the reaction depends on both, the concentration of reactant and the temperature. A chemical reaction effectively take place due to the collision between the particles of reactant. If the concentration of reactant decreases, the contact between the particles also decreases and reaction rate eventually becomes zero. The increase in concentration of reactant, increases the collision rate and the rate of the reaction.

Increase in temperature, increases the average kinetic energy of reactant particles thus, particles move with faster rate and collide more frequently this will increase the rate of the reaction.  Thus, rate of the reaction is directly proportional to concentration of reactant and temperature.  

Hence, the rate of a chemical reaction always decreases with decrease in the concentration of the reactants and decrease in the temperature.

substituting a hydrogen atom with a halogen in a hydrocarbon:

- the single bond remains single. a is wrong.

- the bond with hydrogen is as strong as with a halogen. c is wrong

- a hydrocarbon is saturated if there is no double bond. we do not know that so d is wrong.

- carbon will always have four valence electrons. e is wrong.

by elimination, the correct answer is b. the boiling point of the new compound increases.