Question

The minus sign in rate $ = - \dfrac{{d\left[ A \right]}}{{dt}}$ indicates the ______ in concentration of the ______ with time. The rate of a reaction is always ______ quantity. The rate of reaction increases with _______ in concentration of reactants. The blanks in the question corresponds to:
A. decrease, products, positive, increase
B. increase, reactants, negative, decrease
C. decrease, reactants, positive, increase
D. increase, products, positive, increase

Answer 1

Hint: According to the kinetic theory, reactants get consumed in a reaction. Thus, the rate equation is written in terms of the disappearance of the reactants. Thus, there is a directly proportional relationship between the rate and concentration of the reactants.

Complete step by step answer:
Let us first consider a reaction:
$A \to P$
The reactant here is $A$ and the product is $P$. According to the kinetic theory, we can write the rate of this reaction in terms of the change in concentration of the reactant over time.
rate $ = - \dfrac{{d\left[ A \right]}}{{dt}}$
Since the reactants are getting used up, we indicate this by putting a negative sign in front of the rate equation. Thus, the minus sign in rate $ = - \dfrac{{d\left[ A \right]}}{{dt}}$ indicates the decrease in concentration of the reactants with time.
As we know, the rate is something which describes how fast a reaction is happening. Thus, the rate of a reaction is always a positive quantity.
And moving on to the last part of the question, as we can see from the rate equation, the rate is directly proportional to the concentration of the reactant. Thus, the rate of reaction increases with increases in concentration of reactants.
Hence, the correct words to be filled in each blank are: decrease, reactants, positive, increase.

So, the correct answer is Option C

Note: The rate of a reaction can also be expressed in terms of the formation of the products, in which case the negative sign is omitted. Note that the concentration is usually expressed in molarity for solid and liquid components, while partial pressures can be used to express concentration if the reaction is happening in the gaseous phase.