【People's Education Press】High School Chemistry: Elective Compulsory Course 1: Macroscopic Representation and Quantitative Calculation of Chemical Reaction Rates

If chemical equilibrium determines the 'extent' of a reaction, thenchemical reaction rateit determines the 'speed' of the reaction. From a macroscopic perspective, rate serves as a bridge between experimental observation and theoretical modeling. Under specific conditions, the speed of a reaction is typically expressed as the decrease in reactant concentration or the increase in product concentration per unit time.

Core Mathematical Expressions

For a general reaction $mA + nB = pC + qD$, the rate expression follows stoichiometric ratios:

Basic Formula: $v = \frac{\Delta c}{\Delta t}$.
Relationship Between Reactants and Products: $v(A) = -\frac{\Delta c(A)}{\Delta t}$, $v(C) = \frac{\Delta c(C)}{\Delta t}$.
Proportionality Rule: $\frac{v(A)}{m} = \frac{v(B)}{n} = \frac{v(C)}{p} = \frac{v(D)}{q}$.

Industrial and Organic Reaction Examples

Precise measurement of reaction rates is crucial in complex industrial processes or organic syntheses:

Ammonia Synthesis: $N_2 + 3H_2 \rightleftharpoons 2NH_3$. If the concentration of $N_2$ decreases from 0.8 to 0.7 mol/L within 5 minutes, its rate is $v(N_2) = 0.02 \text{ mol}/(\text{L} \cdot \text{min})$.
Organic Transformation: Dehydration reaction converting $\gamma$-hydroxybutyric acid to $\gamma$-butyrolactone: $HOCH_2CH_2CH_2COOH \xrightarrow{H^+/\Delta} \text{Lactone} + H_2O$.
Environmental Remediation: $2\text{NO}(\text{g}) + 2\text{CO}(\text{g}) = \text{N}_2(\text{g}) + 2\text{CO}_2(\text{g})$, with rate inferred from monitoring changes in cylinder pressure.

Thermodynamic Warning

Although this section focuses on how 'fast' a reaction proceeds, do not overlook thermodynamic constraints: when $\Delta H > 0$ and $\Delta S < 0$, the reaction cannot proceed spontaneously at any temperature, making the study of its rate meaningless in practice.

QUESTION 1

If a 4 mol/L $H_2SO_4$ solution is used instead of a 1 mol/L $H_2SO_4$ solution in a reaction with zinc granules, how will the time required to produce the same volume of hydrogen gas change? Explain your reasoning.

Time increases because the excessive acidity inhibits the reaction.

Time decreases because increased concentration leads to more activated molecules per unit volume, increasing the frequency of effective collisions.

Time remains unchanged because reaction rate depends only on the surface area of zinc.

QUESTION 2

In a 2 L sealed container, $N_2$ and $H_2$ are introduced to undergo ammonia synthesis. If the rate expressed using $H_2$ is $0.12 \text{ mol}/(\text{L} \cdot \text{s})$ over 10 seconds, what is the amount of $N_2$ consumed during this period?

0.4 mol

0.8 mol

1.2 mol

2.4 mol

QUESTION 3

Which of the following measures significantly increases the chemical reaction rate?

Increasing the amount of water when sodium reacts with water

Using concentrated sulfuric acid instead of dilute sulfuric acid when aluminum reacts to produce $H_2$

Converting block-shaped marble into powdered marble when reacting with hydrochloric acid to produce $CO_2$

QUESTION 4

Given the reaction: $4NH_3 + 5O_2 = 4NO + 6H_2O$. Which of the following rate relationships is correct?

$4v(NH_3) = 5v(O_2)$

$5v(NH_3) = 4v(O_2)$

$v(O_2) = v(H_2O)$

QUESTION 5

What scientific methods were used in the experiment investigating the reaction rate between zinc and sulfuric acid?

Controlled variable method, comparative experiment method, quantitative observation method

Analogical reasoning method, model construction method