Heat Exchanges in Chemical Reactions

8/12/2019 Heat Exchanges in Chemical Reactions

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Specific heat capacity

Water plays an important role that water plays, particular the oceans, in moderating temperatures. This is because

of the relatively large quantities of heat absorbed or released when the temperature of water changes.

This demonstrates the fact that water, unlike other substances such as metals or concretes, does not increase in

temperature as fast when heated.

For example a bucket of water when exposed to same amount of radiant energy from the Sun as the surrounding

environment does not increase in temperature to the same extent as the surroundings.

The explanation for this that water has a specific heat capacity than the surroundings.

Specific heat capacity of various substances

SubstanceSpecific Heat Capacity

(kJ kg1K

1or J g

1K

1)

SubstanceSpecific Heat Capacity

(kJ kg1K

1or J g

1K

1)

water 4.18 aluminium 0.90pentane 1.66 chloroform 0.55

ethanol 1.41 carbon tetrachloride 0.54

toluene (methylbenzene) 1.13 glass 0.50

phenol (hydroxybenzene) 1.11 iron 0.45

benzene 1.05 copper 0.39

nitrogen gas 1.04 silver 0.23

oxygen gas 0.92 mercury 0.14

Calculating heat absorbed or released

The specific heat capacity may be used to determine the energy absorbed or released, when a temperature of a known

mass or substance changes.

Calorimetry

In order to measure heat changes during a chemical reaction, we use a calorimeter.

If two objects are brought into contact, heat will flow from the hot object to the cold object until the temperature

of the two objects are equal.

Two objects can be at same temperature but contain different amounts of heat

The specificheatcapacity(C), also called specific heat, is the amount of energy required to change the

temperature of 1 gram of substance of a substance by 1 Kelvin (or Celsius).

Q = mCT

Q = mCT

Calculating the amount of heat absorbed or released:

where Q is the amount of heat released or absorbed in kilojoules, m is mass in kilograms, C is the specific heat

capacity,

and T is the temperature change in Kelvin (or Celsius)

where Q is the amount of heat released or absorbed in joules, m is mass in grams, C is the specific heat capacity,and T is the temperature change in Kelvin (or Celsius)

Express the final answer in a positive number. Use the words released or absorbed to specify whether the heat

The heat released by the hot body is equal to the heat gained by the cold body.


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Enthalpy

The release of energy in chemical reactions occurs when the reactants have a higher chemical energy than the

products. The chemical energy of a substance is a type of potential energy stored within the substance. This stored

chemical potential energy is called the heat content or enthalpyof the substance is given the symbol H.

Calculating change in enthalpy

Exothermic reactions

If the enthalpy in the system decreases during a chemical reaction, a corresponding amount of energy (Q) must be

released to the surroundings, i.e. the enthalpy of the products is less than the enthalpy of the reactants.

The enthalpy difference between the reactants and the products is equal to the energy released to the

surroundings.

A reaction is which heat energy (Q) is released to the surroundings is called an exothermicreaction. Examples of

exothermic reactions include synthesis reactions.

Since exothermic reactions release energy into the surroundings, the result of this type or reaction is that the

surroundings heats up.

Endothermic reactions

If the enthalpy in the system increases during a chemical reaction, a corresponding amount of energy (Q) must

have been absorbed from the surroundings i.e. the enthalpy of the products is greater than the enthalpy of the

reactants.

The enthalpy difference between the reactants and the products is equal to the energy absorbed from the

surroundings.

A reaction is which heat energy (Q) is absorbed from the surroundings is called an endothermicreaction.

Since endothermic reactions absorb energy from the surroundings, the result of this type of reactions is that the

surroundings are cooled down.

Decomposition reactions are endothermic reactions, as often the energy input required for the reaction is

absorbed from the surroundings.

A change in the enthalpy of a substance is given the symbol H.

It is the difference between the total enthalpy of products and the enthalpy of the reactants, i.e. the change inthe enthalpy of a system. It is equal to the following expression: H = H products H reactants

Calculating the change in enthalpy:

H = mCT

where H is changein enthalpy, m is mass in g, C is the specific heat capacity of the substance being cooled or

heated, and T is change in temperature in Kelvin (or degrees Celsius)

Important things to remember: Always convert the final answer to kilojoules, and where specified: kilojoules per mole, which are the units

usually used for H

Always pay careful attention to the sign. Exothermic reactions should be a negative answer, while endothermic

reactions should be a positive answer.


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Heat changes when substances dissolve

When ionic substances dissolve in water, there is a noticeable change in temperature. This means the reaction is

endothermic (absorbing heat) or exothermic (releasing heat). For example:

i. When sodium hydroxide NaOH dissolves in water, the solution heats up. The dissolution process releases heat

which warms up the solution. The dissolution of NaOH is said to be exothermic.

ii. When potassium nitrate KNO3dissolves in water, the solution cools. It requires an input of energy which is taken

from normal thermal energy of the water and the solid substance. The dissolution of KNO3is endothermic.

Energy is needed to break the ionic bonds in the crystal lattice of the solute, and energy is also needed to break the

intermolecular forces (i.e. the hydrogen bonding) between water molecules. But energy is released when the separated

ions form bonds with water molecules. Determining these factors will help determine the whether the dissolution is

exothermic or endothermic.

Molar heat of solution

If Hsolnis positive, then the reaction is endothermic, but if Hsolnis negative, then the reaction is exothermic.

The molar heat of solution Hsolnof a substance is the heat absorbed when one mole of the substance dissolves in a

large excess of water

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Heat Exchanges in Chemical Reactions