Energy and energy changes

Energy is the ability to do work. All matter possesses energy in one form or another. There are some substances that have energy as a result of particles in motion which are known as kinetic energy while some substances have energy reserved within them because of the position of its particles and the reserved energy is known as potential energy. The study of energy is known as energetics.

Heat content and Heat of reaction

Heat content is also known as enthalpy. An enthalpy change is the heat that would be exchanged with the surroundings, if the temperature and pressure of the system were the same before and after the reaction, i.e enthalpy change = heat content = heat content (heat of reaction) of products of reactants. 

Heat of reaction is the amount of heat evolved or absorbed when a chemical reaction occurs between molar quantities of the substances as represented in the equation of reaction under standard conditions.

Exothermic and Endothermic reactions

A chemical reaction in which heat is given off to the surroundings is Known as exothermic reactions.

Endothermic reactions is that in which heat is absorbed from the surroundings.

Note that it is not only applied in chemical changes but also in physical changes.

Note: the basic unit for measuring heat change is the kilocalorie (Kcal) but it has been replaced by the joule by the international unit of energy so 1calories = 4.2 joules.

*Heat change during chemical reaction

Source: www.commons.wikimedia.org 

Standard conditions for energy changes

 The conditions are as follows:

i. A temperature of 298k (or 25°C).

ii. The pressure must be at 1.01×10⁵Nm^-2 (1 atm at 760mmHg).

iii. The solution must have concentrations of 1mol dm^-3.

iv. The substances involved must be in their normal physical states at the temperature chosen.

Types of heat change in chemical reactions

1. Heat of formation: the standard heat of formation of a substance, ∆H^o, is the heat change when one mole of that substance is formed from its element under standard conditions. 

2. Heat of neutralization: the standard heat of neutralization, ∆H^o_n, is the heat change when one mole of hydrogen ion, H⁺, from an acid, reacts with one mole of hydroxide ion, OH^–, from an alkali to form one mole of water, under standard conditions.

3. Heat of combustion: the standard heat of combustion of a substance, ∆H^o_c, is the heat evolved when one mole of the substance is burned completely in oxygen under standard condition.

Note: bomb calorimeter is used to determine accurate heat of combustion of a substance.

*A bomb calorimeter

Source: www.google.com 

*Determination of the heat of combustion

Source: www.commons.wikimedia.org 

4. Heat of solution: the standard heat of solution, ∆H^o_s, is the heat change when one mole of a substance is dissolved in so much water at standard conditions that further dilution results in no detectable heat change. 

5. Heat of vapourisation: the standard heat of vapourisation, _∆H^ovap, is the heat change when one mole of a liquid is vapourized. The standard heat of vapourisation is always positive, i.e Endothermic.

6. Heat of fusion: this is the heat change when one mole of a solid is melted.

7. Heat of hydrogenation: this is the heat change when one mole of an unsaturated organic compound is converted to the corresponding saturated organic compound by reaction with hydrogen at standard conditions of temperature and pressure.

Note: number of moles in a specified solution= 

concentration in mol dm³ × volume in cm³ 

                           1000

Spontaneous reaction

A spontaneous reaction is that which has the potential to occur on its own without the assistant of any external agent. The following are spontaneous reaction:

1. Entropy: is a measure of the degree of disorder or randomness of a system. It is represented by letter “S” and a change in it is given as ∆S.

∆S^o= S^o_product S^o_reactant

The S.I unit for entropy is JK^-1 mol^-1

2. Entropy change in reversible process: the influence of entropy on a process is given by the second law of thermodynamic which states that a spontaneous process occurs only, if there is an increase in the entropy of the system and its surroundings. ∆S=∆H

                                          T

3. Gibb’s free energy: the free energy, G, of a chemical system is the energy which is available for doing work. It is the driving force that brings about a chemical change.

∆G^o=G^o_product _ G^o_reactant

Past questions

1. In a chemical reaction, when the energy of the colliding reactant particles is less than the activation energy, the reaction will? (Jamb 2016)

1. Be spontaneous
2. Not occur
3. Occur
4. Be slow

Answer: B

2. A chemical reaction with entropy change of 20JK^-1 and free energy change + 55000J occured at -23°C. Calculate the enthalpy change? (Jamb 2015)

1. 55,460J
2. 44,540J
3. 50,000J
4. 60,000J

Answer: D

Solution: ∆G=∆H-T∆S

                 55000=∆H-(-23+273) (20)

                 55000=∆H-(250)(20)

                 55000=∆H-5000

                 ∆H=55000+5000

                 ∆H=60000J

3. In which of the following is the entropy change positive? (Jamb 1992)

1. H₂O_(I) > H₂O_(g)
2. Cu²⁺_(aq) + Fe_(s) > Fe²⁺_(aq)+ Cu²⁺_(aq)
3. N_2(g)+3H_2(g)>2NH_3(g)
4. 2HCl_(s)>H_2(g)+Cl_2(g)

Answer: D

4. Fe₂O_3(s) + 2Al_(s) > Al₂O₃ + 2Fe_(s). If the heats of formation of Al₂O and Fe₂O₃ are -1670KJ mol^-1 and -822 kJ mol^-1 respectively, the enthalpy change in kJ for the reaction is ? ( Jamb 1989)

1. +2492
2. +848
3. -848
4. -2492

Answer: C

Solution: ∆H = H_p – H_R = -1670-(-822)= (-1670+822) kJ = -848kJ

5. If the value of ∆H is positive for a reaction, it means that the reaction is? (Wassce 2019)

1. Exothermic
2. Endothermic
3. Spontaneous
4. Slow

Answer: B

6. Define the term enthalpy of neutralization. (Wassce 2018)

Answer: 

the standard heat of neutralization, ∆H^o_n, is the heat change when one mole of hydrogen ion, H⁺, from an acid, reacts with one mole of hydroxide ion, OH^–, from an alkali to form one mole of water, under standard conditions.

7. A reaction is Endothermic if the? ( Wassce 2018)

1. Reaction vessel feels cool during the reaction 
2. Enthalpy change is negative
3. Bond forming energy exceeds bond breaking energy
4. Heat of formation of reactants exceeds heat of formation of products.

Answer: A

8. Define enthalpy of combustion. (Wassce 2017)

Answer: the standard heat of combustion of a substance, ∆H^o_c, is the heat evolved when one mole of the substance is burned completely in oxygen under standard condition.