Rate of Reactions

Question 1

What are three methods to measure rate of reaction

Answer 1

Measuring time taken for completion of reaction
Measuring volume of gas evolved at regular time intervals
Measuring mass of reaction mixture at regular time intervals

Question 2

Why is measuring mass of reaction mixture of regular time intervals not suitable for a reaction that produces hydrogen gas

Answer 2

The molar mass of hydrogen is very low and changes in mass of reaction due to loss of hydrogen gas would be insufficient

Question 3

Why would the reaction between hydrochloric acid and magnesium be slow at the start

Answer 3

There could be magnesium oxide on the surface of the magnesium which hinders the reaction between magnesium and acid. Magnesium oxide dissolves in acid later and Mg underneath will be exposed to acid

Question 4

What must reacting particles have in order for chemical reaction to occur

Answer 4

Reacting particles must collide with one another, be orientated properly and must possess a minimum energy, known as activation energy, before reaction can take place

Question 5

Define activation energy

Answer 5

The minimum amount of energy that reacting particles must possess in order for chemical reaction to occur

Question 6

Explain why rate of reaction increases when particle size of solid reactant decreases

Answer 6

When particles size of reactant decreases, total surface area of reactant increases. There is larger total surface area exposed for other reacting particles to collide with at any one time. Frequency of effective collisions between reacting particles increases, and rate of reaction increases

Question 7

Explain effect of increasing concentration of solution on rate of reaction

Answer 7

When concentration of reactant is increased, number of reacting particles per unit volume increases. Frequency of effective collisions between reacting particles increases. Hence rate of reaction increases

Question 8

Why does volume of carbon dioxide remain the same during reaction between calcium carbonate and hydrochloric acid when hydrochloric acid is added (CaCO3 is limiting)

Answer 8

Before and after HCl was added, limiting reactant is calcium carbonate. This means for the same number of moles of CaCO3 used in both experiments, the same number of moles of CO2 will be produced. Hence, volume of carbon dioxide produced will be the same

Question 9

When acid is limiting reactant, describe volume of gas produced when concentration of acid doubles
Experiment 1 - 1.00mol/dm^3
Experiment 2 - 2.00 mol/dm^3

Answer 9

Concentration of acid used in Experiment 2 is twice that in experiment 1. For the same volume of acid, number of moles of acid in Experiment 2 will be twice of experiment 1. Number of moles of gas produced in experiment 2 will also be twice of experiment 1.

Question 10

Describe why rate of reaction increases for increased basicity of acid (comparing sulfuric acid and hydrochloric acid)

Answer 10

Sulfuric acid is dibasic acid and hydrochloric acid is monobasic acid. For same concentration of acid used in both experiments, concentration of hydrogen ions in sulfuric acid is twice of hydrochloric acid, hence reaction is faster for experiment using sulfuric acid

Question 11

Describe when acid is limiting reactant, volume of gas increases when basicity increases(comparing sulfuric acid and hydrochloric acid)

Answer 11

Hydrochloric acid is monobasic and sulfuric acid is dibasic. For same number of moles of acids used, number of moles of hydrogen ions in sulfuric acid is twice that of hydrochloric acid. Hence, number of moles and volume of gas produced will be doubled.

Question 12

Describe why solution becomes more opaque when reaction produces solid product

Answer 12

As reaction progresses, amount of _ solid increases, reaction mixture becomes cloudy.

Question 13

Effect of pressure for gaseous reactants on rate of reaction

Answer 13

Pressure of reaction mixture involving gaseous reactants is increased, the number of reacting particles per unit volume increases. Frequency of effective collisions between reacting particles increases, rate of reaction increases

Question 14

Effect of temperature on rate of reaction

Answer 14

At higher temperature, reacting particles will gain kinetic energy and move faster, there will be an increase in number of reacting particles with energy equal to or more than activation energy. Both ways lead to increase in frequency of effective collisions between reacting particles, rate of reaction increases.

Question 15

Define catalyst

Answer 15

A substance that increases the rate of chemical reactions without being chemically changed at the end of reaction

Question 16

How does catalyst increase rate of reaction

Answer 16

A catalyst provides an alternative pathway with lower activation energy for reaction to occur. Number of reacting particles having energy equal to or more than the activation energy increases. Hence, frequency of effective collisions between reacting particles increases and rate of reaction increases