Excelsior Chemistry Unit 7

The study of energy changes that occur during chemical reactions and changes in state.
Energy that transfers from one object to another because of a temperature difference between them. Heat always flows from a warmer object to a cooler object.
Endothermic vs Exothermic
Endothermic processes absorbs heat from its surroundings and exothermic processes release heat into its surroundings.
Specific Heat
The quantity of heat required to change the temperature of 1 gram of a substance 1 degree Celsius. It is measured in calories per gram-degree Celsius.
The specific heat of Aluminum is 0.90 J/g•degrees C. How many joules of energy are needed to warm 7.89 g of aluminum from 72 degrees Celsius to 78.5 degrees Celsius?
The answer is

Given a specific number of grams of a substance and the specific heat of that substance, determine the amount of heat required to raise the temperature a specific number of degrees Celsius.

0.90 = J / 7.89 • (78.5-72)
0.90 = J / 7.89 • (6.5)
0.90 = J / 51.285
0.90 • 51.285 = J
46.16 = J
The heat content of a system at constant pressure.
Thermochemical equations
In a chemical equation, the enthalpy change for the reaction can be written as either a reactant or a product. These include an enthalpy change is equal to the heat flow at constant pressure.
Heat in changes of state
The changes in heat cause a change of state instead of a change in temperature during state changes. It is only after the state change that the substance begins to change temperature. Whenever a change of state occurs by a gain or loss of heat, the temperature of the substance undergoing the change remains constant.
You completely melt an ice cube at 273K and measure that it required 24.0 kJ of heat to transform it to a liquid. What is the mass of ice that was melted? The heat of fusion of ice is 6.0 kJ/mol.
The answer is 72 g

The molar mass of water is 18.0 g
24.0 kJ • 18 g / 6.0 kJ =
72.0 g
Molar heat of vaporization
The amount of heat necessary to vaporize one mole of a given liquid. It is measured in kJ per mol or kJ/mol. The quantity of heat absorbed by a vaporizing liquid is exactly the same as the quantity of heat released when the vapor condenses.
A substance releases 684 kJ of heat as 3.70 moles condense from a gas into a liquid. What is the molar heat of vaporization of the substance?
The answer is 185 kJ/mol

To determine this, remember that molar heat of vaporization is measured in kJ/mol. Then divide the kJ give by the moles given.

684 kJ / 3.70 moles = 184.86 kJ/mol
Heat of solution
During the formation of a solution, heat is either released or absorbed. The enthalpy change caused by dissolution of one mole of substance, or the heat absorbed or released when a solid dissolves is the heat of solution.
Activation energy
The minimum energy that colliding particles must have in order to react or an energy barrier between reactants and products.
Transition State
Another term for an activated complex or an unstable arrangement of atoms that forms momentarily at the peak of the activation-energy barrier where the atoms have the highest energy.
A substance that increases the rate of a reaction by lowering the activation energy without being used up during the reaction. The catalyst does not change during the reaction.
Chemical Equilibrium
At chemical equilibrium, no net change occurs in the actual amounts of the components of the system. When the rates of the forward and reverse reactions are equal, the reaction has reached a state of balance known as equilibrium. Changing the concentration of the reactants or products changes the shift in the chemical equation.
Le Chatelier's Principle
If a stress is applied to a system in dynamic equilibrium, the system changes in a way that relieves that stress. These stresses could be changes in the concentration of reactants or products, changes in temperature, or changes in pressure.
Le Chatelier's Principle and Pressure
When you compress a gas, or decrease the volume, you increase the pressure. For gaseous systems, that changes the equilibrium. When you decrease the volume of the gases, the reaction shifts towards the creation of more products. When you increase the volume of the gases and reduce the pressure, the reaction shifts towards the creation of more reactants.
Equilibrium Constant
The ratio of product concentrations to reactant concentrations at equilibrium, with each concentration raised to a power equal to the number of moles of that substance in the balanced chemical equation. The numerator is the products with the exponent as the moles of each product. The denominator is the reactants with the exponent as the moles of each reactant. Check the highlighted info found on page 556 of your textbook. If the constant is greater than 1, products are favored. If it is less than 1, the reactants are favored.
Heat of combustion
The heat of reaction for the complete burning of one mole of a substance.
Properties of Acids
Acids taste sour, will change the color of an acid-base indicator, and can be strong or weak electrolytes in aqueous solutions.
Properties of Bases
Bases taste bitter, feel slippery, will change the color of an acid-base indicator, and can be strong or weak electrolytes in aqueous solution. Bases are also called alkaline.
Arrhenius Acids and Bases
Arrhenius said that acids are hydrogen-containing compounds that ionize to yield hydrogen ions (H+) in aqueous solutions and that bases are compounds that ionize to yield hydroxide ions (OH-) in aqueous solutions. Acids with 1 hydrogen are monoprotic acids. Acids with 2 hydrogens are diprotic acids. Acids with 3 hydrogens are triprotic acids.
The reaction in which water molecules produce ions is called self-ionization of water. The reaction looks like liquid water which changes into Hydrogen ions (H+) in an aqueous solution + Hydroxide ions (OH-) in an aqueous solution.
Acidic/Neutral/Basic solutions
Neutral solutions have a pH of 7 and their H+ is equal to 1 x 10^-7 M. Acidic solutions have a pH < 7 and their H+ is greater than 1 x 10^-7M. Basic solutions have a pH of > 7 and their H+ is less than 1 x 10^-7M.
When looking at the exponents of the scientific notation for the moles per liter of Hydrogen ion (H+), the larger the exponent, the more acidic the solution and the smaller the exponent, the more basic. So an exponent of -2 would be more acidic than an exponent of -13.
When looking at the exponents of the scientific notation for the moles per liter of Hydroxide ions (OH-), the smaller the exponent, the more acidic the solution and the larger the exponent, the more basic. So an exponent of -2 would be more basic than the exponent of -13.
Bronsted-Lowry Acids and Bases
The Bronsted-Lowry theory defines an acid as a hydrogen-ion donor, and a base as a hydrogen-ion acceptor. A substance that can either accept or donate ions is amphoteric. Water is an example of an amphoteric substance (or both an acid and a base).
Acids and Bases
When acids react with base compounds containing hydroxide ions, they form water and salt.
Lewis Acids and Bases
Lewis proposed that an acid accepts a pair of electrons during a reaction, while a base donates a pair of electrons. According to Lewis, the H+ of an acid is equal to the OH- of the base.
Strong and Weak Acids and Bases
Weak acids have small acid dissociation constants (Ka) values. The stronger an acid, the larger the Ka value. A larger value of Ka means the dissociation or ionization of the acid is more complete or the degree of ionization is higher or complete in water.
The pH of a solution is the negative logarithm of the hydrogen-ion concentration. In a neutral solution, the pH is 7. Acids have pH's less than 7 and bases have pH's greater than 7. The pH of a solution is the same as the exponent of H+ ions. For example, a pH of 4 means the H+ ion concentration is 10^-4. The pH of 12 would mean the H+ ion concentration is 10^-12.
Find the pH of a solution with a hydrogen ion concentration of 4.2 x 10^-10.
The answer is 9.38

To find the pH of a solution with a hydrogen ion concentration of 4.2x10^-10, you would use the equation pH=-log[H+]. Find the log of 4.2x10^-10 and then change the sign. The log of 4.2x10^-10 is -9.38, so the answer is 9.38.
Using pH to find H+
Use the formula pH=-log[H+]. Substitute the pH they gave you, in the case of the example on page 600, it is 6.35 so -log[H+]=6.35. Change the signs on both sides so log[H+]=-6.35. Use the antilog of -6.35 which is 4.5x10^-7.

Flickr Creative Commons Images

Some images used in this set are licensed under the Creative Commons through Flickr.com.
Click to see the original works with their full license.