Energy and States of Matter
Energy - Is needed to “do work.”
Work is done when :
Potential Energy -Is energy that is stored for use at a later time. Examples are:
Kinetic Energy - Is the energy of motion. Examples are:
Learning CheckIdentify the energy as 1) potential or 2) kinetic
- Roller blading.
- A peanut butter and jelly sandwich.
- Mowing the lawn.
- Gasoline in the gas tank.
Forms of Energy
Energy has many forms:
Heat
Heat energy flows from a warmer object to a colder object.
The colder object gains kinetic energy when it is heated.
During heat flow, the loss of heat by a warmer object is equal to the heat gained by the colder object.
Some Equalities for Heat measured in calories or joules.
1 kilocalorie (kcal) = 1000 calories (cal)
1 calorie = 4.18 Joules (J)
1 kJ = 1000 J
Energy in Chemical Reactions
Reaction Conditions
- A chemical reaction occurs when the reacting molecules collide.
- Collisions between molecules must have sufficient energy to break the bonds in the reactants.
- Once the bonds between atoms of the reactants are broken, new bonds can form to give the product.
Chemical Reactions
In the reaction H2 + I2 2 HI, the bonds of H2 and I2 must break, and bonds for HI must form.
H2 + I2 collision bonds break 2HI
Activation Energy
The activation energy is the minimum energy needed for a reaction to take place.
When a collision has the energy that is equal to or greater than the activation energy, reaction can occur.
Exothermic Reactions
The heat of reaction is the difference in the energy of the reactants and the products.
An exothermic reaction releases heat because the energy of the products is less that the reactants.
Endothermic Reactions
In an endothermic reaction, heat is absorbed because the energy of the products is greater than that of the reactants.
Learning Check
Identify each reaction as exothermic or endothermic
A. N2 + 3H2 →2NH3 + 22 kcal
B. CaCO3 + 133 kcal →CaO + CO2
C. 2SO2 + O2 →2SO3 + heat
Specific Heat
Specific heat is the amount of heat (calories or Joules) that raises the temperature of 1 g of a substance by 1°C.
Learning Check
- A substance with a large specific heat1) heats up quickly2) heats up slowly
- When ocean water cools, the surrounding air 1) cools 2) warms3) stays the same
- Sand in the desert is hot in the day and cool at night.
Sand must have a1) high specific heat 2) low specific heat
Learning Check
When 200 g of water are heated, the water temperature rises from 10°C to 18°C.
If 400 g of water at 10°C are heated with the same amount of heat, the final temperature would be
1) 10 °C2) 14°C 3) 18°C
Calculation with Specific Heat
To calculate the amount of heat lost or gained by a substance, we need the
grams of substance,
temperature change T, and the
specific heat of the substance.
Heat =
Sample Calculation for Heat
A hot-water bottle contains 750 g of water at 65°C. If the water cools to body temperature (37°C), how many calories of heat could be transferred to sore muscles?
The temperature change is 65°C - 37°C = 28°C. heat (cal) = g x T x Sp. Heat. (H2O)
Learning Check
How many kcal are needed to raise the temperature of 120 g of water from 15°C to 75°C?
Energy and Nutrition
On nutrition and food labels, the nutritional Calorie, written with a capital C, is used.
1 Cal is actually 1000 calories.
1 Calorie = 1 kcal1 Cal = 1000 cal
Caloric Food Values
The caloric values for foods indicate the number of kcal provided by 1 g of each type of food.
Calories in Some Foods
Energy Requirements- The amount of energy needed each day depends on age, sex, and physical activity.
Loss and Gain of Weight- If food intake exceeds energy use, a person gains weight. If food intake is less than energy use, a person loses weight.
Learning Check
A cup of whole milk contains 12 g of carbohydrates, 9.0 g of fat, and 9.0 g of protein. How many kcal (Cal) does a cup of milk contain?
States of Matter
Solids
Solids have
- A definite shape.
- A definite volume.
- Particles that are close together in a fixed arrangement.
- Particles that move very slowly.
Liquids
Liquids have
- An indefinite shape, but a definite volume.
- The same shape as their container.
- Particles that are close together, but mobile.
- Particles that move slowly.
Gases
Gases have
- An indefinite shape.
- An indefinite volume.
- The same shape and volume as their container.
- Particles that are far apart.
- Particles that move fast.
Learning Check
Identify each as: solid liquid or gas.
A. It has a definite volume, but takes the shape of the container.
B. Its particles are moving rapidly.
C. It fills the volume of a container.
D. It has particles in a fixed arrangement.
E. It has particles close together that are mobile.
Attractive Forces between Particles
- In ionic compounds, ionic bonds are strong attractive forces that hold positive and negative ions together.
- In covalent compounds, polar molecules exert attractive forces called dipole-dipole attractions.
- Hydrogen bonds are strong dipole attractions between hydrogen atoms and atoms of F, O, or N, which are very electronegative.
- Nonpolar molecules form liquids or solids through weak attractions called dispersion forces.
- Dispersion forces are caused by temporary dipoles that develop when electrons are not distributed equally.
Melting Points and Attractive Forces
- Ionic compounds require large amounts of energy to break apart ionic bonds. Thus, they have high melting points.
- Hydrogen bonds are the strongest type of dipole-dipole attractions. They require more energy to break than other dipole attractions.
- Dispersion forces are weak interactions and very little energy is needed to change state.
Melting Points and Attractive Forces of Some Typical Substances
Learning Check
Identify the type of attractive forces for each:
1) ionic 2) dipole-dipole3) hydrogen bonds 4) dispersion
NCl3
H2O
Br-Br
KCl
NH3
Changes of State
Melting and Freezing
A substance is melting while it changes from a solid to a liquid.
A substance is freezing while it changes from a liquid to a solid.
The freezing (melting) point of water is 0°C.
Calculations Using Heat of Fusion
The heat of fusion is the amount of heat released when 1 gram of liquid freezes at its freezing point.
The heat of fusion is the amount of heat needed to melt 1 gram of a solid at its melting point.
For water the heat of fusion (at 0°C) is
Calculation Using Heat of Fusion
The heat involved in the freezing (or melting) a specific mass of water (or ice) is calculated using the heat of fusion.
Heat = g water x 80. cal
g water
Problem: How much heat in calories is needed to melt 15.0 g of water?
15.0 g water x 80. cal = 1200 cal
1 g water
Learning Check
How many calories are needed to melt 5.0 g of ice of 0°C?
1) 80. cal2) 400 cal 3) 0 cal
How many calories are released when 25 g of water at 0°C freezes?
1) 80. cal2) 0 cal 3) 2000 cal
Boiling & Condensation
Water evaporates when molecules on the surface gain enough energy to form a gas.
At boiling, all the water molecules acquire enough energy to form a gas.
Heat of Vaporization
- Is the amount of heat needed to change 1 g of liquid to gas at the boiling point.
- Is the amount of heat released when 1 g of a gas changes to liquid at the boiling point.
Boiling (Condensing) Point of Water =
Heat of Vaporization (water) =
Learning Check
How many kilocalories (kcal) are released when 50.0 g of steam in a volcano condenses at 100°C?
Heating and Cooling Curves
A heating curve illustrates the changes of state as a solid is heated.
Sloped lines indicate an increase in temperature.
Plateaus (flat lines) indicate a change of state.
Learning Check
A. A flat line on a heating curve represents
1) a temperature change
2) a constant temperature
3) a change of state
B. A sloped line on a heating curve represents
1) a temperature change
2) a constant temperature
3) a change of state
Cooling Curve
- A cooling curve illustrates the changes of state as a gas is cooled.
- Sloped lines indicate a decrease in temperature.
- This cooling curve for water begins at 140°C and ends at -30°C.
Learning CheckUse the cooling curve for water to answer each.
A. Water condenses at a temperature of 1) 0°C2) 50°C3) 100°C
B. At a temperature of 0°C, water 1) freezes2) melts3) changes to a gas
C. At 40 °C, water is a 1) solid 2) liquid3) gas
D. When water freezes, heat is 1) removed2) added
Combined Heat Calculations
To reduce a fever, an infant is packed in 250 g of ice. If the ice at 0°C melts and warms to body temperature (37.0°C), how many calories are removed from the body?
Step 1: Diagram the changes.
Step 2: Calculate the heat to melt ice (fusion)
Step 3: Calculate the heat to warm the water from
Total: Step 2 + Step 3
Final answer= 29 000 cal
Learning Check
150 g of steam at 100°C is released from a boiler. How many kilocalories are lost when the steam condenses and cools to 15°C?