Chemical Reactions Unit Learning Goal 5: Examine the Law of Conservation of Energy

Chemical Reactions Unit

Learning Goal 5: Examine the Law of Conservation of Energy

Copyright © Houghton Mifflin Company

10-2

Figure 10.2: Equal masses of hot and cold water.


10-3

Figure 10.3: H2O molecules in hot and cold water.


10-4

Figure 10.4: H2O molecules in same temperature water.

The Nature of Energy

Energy The ability to do work or produce heat. Potential Energy

Stored energy Kinetic Energy

Energy due to the motion of an object.

Law of Conservation of Energy

States that energy can be converted from one form to another but cannot be created nor destroyed. That is the energy in the universe is constant.

Temperature vs. Heat

Temperature A measurement of the random motions of the

components of a substance. Heat

The flow of energy due to a temperature difference.

Enthalpy (Heat Change)

Exothermic Energy is released from the reaction.

Feels hot.


10-9

Figure 10.5: The energy changes accompanying the burning of a match.

Enthalpy (Heat Change)

Endothermic Energy is absorbed from the environment.

Feels cold.

Hess’s Law Going from a particular set of reactants to a

particular set of products, the change in enthalpy is the same whether the reaction takes place in one step or two.

N2(g) + 2O2(g) 2NO (g) H = 180 kj

2NO + O2(g) 2NO2(g) H = -112 kj

N2(g) + 2O2(g) 2NO2(g) H = 68 kj

Characteristics of Enthalpy Changes

1. If the reaction is reversed, the sign of H is also reversed.

2. The magnitude of H is directly proportional to the quantities of reactants and products in a reaction. If the coefficients in a balanced reaction are multiplied by an integer, the value of H is multiplied by the same integer.

Thermodynamics

First Law of Thermodynamics The energy of the universe is constant.

E = q + w(delta) means a change in the function that

follows.

E = energy

q = heat

w = work

Units for Measuring HeatUnits for Measuring Heat

The Joule is the SI system unit for measuring heat.

The calorie is the heat required to raise the temperature of 1 gram of water by 1 Celsius degree

1 cal = 4.184 j

Specific Heat Capacity

The amount of energy required to change the temperature of one gram of a substance by one Celsius degree.

Q = s x m x T Q = energy (heat required s = specific heat capacity m = mass of the sample (g) T = change of temperature (oC)

ENTROPY A measure of disorder or randomness. As randomness increases entropy (S)

increases. The entropy of the universe is always

increasing.


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Figure 10.10: Comparing the entropies of ice and steam.

ELECTROCHEMICAL CELL


Many oxidation-reduction reactions occur spontaneously, giving off energy. An example involves the spontaneous reaction that occurs when zinc metal is placed in a solution of copper ions as described by the net ionic equation shown below.

Cu+2 (aq) + Zn (s) -------> Cu(s) + Zn+2 (aq)



The zinc metal slowly "dissolves" as its oxidation produces zinc ions which enter into solution. At the same time, the copper ions gain electrons and are converted into copper atoms which coats the zinc metal or sediments to the bottom of the container. The energy produced in this reaction is quickly dissipated as heat, but it can be made to do useful work by a device called, an electrochemical cell. A common example of an electrochemical cell is a standard 1.5-volt "battery".

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Chemical Reactions Unit Learning Goal 5: Examine the Law of Conservation of Energy