Heat Exchanges in Chemical Reactions

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

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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

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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

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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.

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The specific heat capacity (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).

Specific heat capacity of various substances Substance water pentane ethanol toluene (methylbenzene) phenol (hydroxybenzene) benzene nitrogen gas oxygen gas

Specific Heat Capacity –1 –1 –1 –1 (kJ kg K or J g K ) 4.18 1.66 1.41 1.13 1.11 1.05 1.04 0.92

Substance aluminium chloroform carbon tetrachloride glass iron copper silver mercury

Specific Heat Capacity –1 –1 –1 –1 (kJ kg K or J g K ) 0.90 0.55 0.54 0.50 0.45 0.39 0.23 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. Calculating the amount of heat absorbed or released: Q = mC∆T where Q is the amount of heat released or absorbed in kilojoules, m is mass in kilograms, C is the specific heat capacity, and ΔT is the temperature change in Kelvin (or Celsius) Q = mC∆T where Q is the amount of heat released or absorbed in joules, m is mass in grams, C is the specific heat capacity, and ΔT 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

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Make sure that the final answer in in kilojoules not joules, i.e. for the second formula, remember to divide by

Calorimetry 

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

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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.

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Two objects can be at same temperature but contain different amounts of heat The heat released by the hot body is equal to the heat gained by the cold body.

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 enthalpy of the substance is given the symbol H. 

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

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

Calculating change in enthalpy Calculating the change in enthalpy: ∆H = −mC∆T where ΔH is change in enthalpy, m is mass in g, C is the specific heat capacity of the substance being cooled or heated, and ΔT 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 ΔH

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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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If kilojoules per mole are required, use relevant formula to convert mass to moles and then divide through by

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.

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

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A reaction is which heat energy (Q) is released to the surroundings is called an exothermic reaction. Examples of exothermic reactions include synthesis reactions.

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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.

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

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

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Since endothermic reactions absorb energy from the surroundings, the result of this type of reactions is that the surroundings are cooled down.

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Decomposition reactions are endothermic reactions, as often the energy input required for the reaction is absorbed from the surroundings.

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.

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When potassium nitrate KNO3 dissolves 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 KNO 3 is 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 The molar heat of solution ΔHsoln of a substance is the heat absorbed when one mole of the substance dissolves in a large excess of water If ΔHsoln is positive, then the reaction is endothermic, but if ΔHsoln is negative, then the reaction is exothermic.