Unit 09. Thermochemistry. Unit 9 Vocabulary. Thermochemistry - concerned with heat changes that occur during chemical reactions Energy - capacity of an object to do work or produce heat Kinetic energy - the energy an object has due to its motion Potential energy - stored energy
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Heat is transferred from one object to another.
Law of Conservation of energy – energy can neither be created nor destroyed
Heat flow is from the warmer to the cooler object
Hot day 50°C = 122°F
Energy transferred from fire to beans. Change in energy for beans would be positive.
Beans lost energy, it was transferred to its surroundings. This change of energy would be negative change = ∆
Exothermic reactions – releases heat, ∆E is negative, the surroundings heat up
Describing Energy Changes
calorie – called “little calorie” – (cal)
kilocalorie – called “Big calorie”- (CAL)
1000 calories – same as a food calorie
Joule – the SI unit of heat - (J)
1 cal = 4.184 joules
1 cal 4.184 J
4.184 J 1 cal
1. Convert 32.7 cal to joules.
2. Convert 6.24 J to cal.
Amount of heat it takes to raise 1g of a substance by 1°C
Metals have a low specific heat
Water has a high specific heat
1. On a sunny day, the concrete deck around an outdoor swimming pool becomes hot, while the water stays cool. This is because…
A. The deck has a higher specific heat than the water
B. The deck has a lower specific heat then the water
C. Both objects have the same specific heat
2. Two objects are sitting next to each other in the sunlight. Object A gets hotter than object B.
A. Object A has a lower specific heat than object B
B. Object A has a higher specific heat than object B
C. Both objects have the same specific heat
4.18 J/g ×°C
1 cal/g ×°C
Matches definition: Amount of heat it takes to raise 1g of a substance by 1°C
q = amount of heat – measured in cal or J
m = mass – measured in g
C = specific heat – units are J/g x °C or cal/g x °C
ΔT = change in temp – measured in °C
q = mc ΔT
1. Aluminum has a specific heat of 0.9020 J/g × °C. How much heat is lost when a piece of aluminum with a mass of 23.984 g cools from a temperature of 415.0°C to a temperature of 22.0°C?
2. 850 calories of heat are applied to a 250 g sample of liquid water with an initial temperature of 13.0 °C.
Find: a) the change in temperature and
b) the final temperature.
(remember, the specific heat of liquid water, in calories, is 1.00 cal/g x °C.)
4. When 34,700 J of heat are applied to a 350 g sample of an unknown material the temperature rises from 22.0°C to 173.0°C. What must be the specific heat of this material?Specific Heat Problems:
When 34,700 J of heat are applied to a 350 g sample of an unknown material the temperature rises from 22.0°C to 173.0°C. What must be the specific heat of this material?
You do not have to write this
When energy is added to a solid substance, its temperature increases until its melting point is reached. Upon the addition of more energy, the substance begins to melt.
The temperature remains the same until all the substance has melted. ****
Heating Curve: ( For Water)
۞The temperature remains the same until all the substance has boiled.***Energy and Change of State
Energy required to vaporize 1 gram of a substance at its boiling point
When a substance is being heated, use the following formula: (inclined line)
q = (m)(c)(DT)
When a substance is melting or boiling, use the following formula: (flat line)
q = (m) (DHfus or DHvap)
(C) = Specific Heat:
- of ice = 2.06 J/g x °C
- of water = 4.18 J/g x °C
- of steam = 2.02 J/g x °C
Enthalpy Values for Water:
DHfus = 334 J/g
DHvap = 2260 J/g
How much heat is required to melt 5.67g of ice?
Q = (m)(ΔH fus)
Q = (5.67g)(334 J/g)
How much energy is required to boil 85g of water?
How much energy is required to raise the temperature of 25g of water from 15°C to 75°C ?
How much energy is required to convert 40g of ice at -10°C to steam at 150°C ?
Use Both Formulas
Draw a graph
What is the final temperature when 1250 J of energy is added to 15g of water at 25°C ?
How much energy is required to boil 50 grams of water?
What is the final temperature when 2500 J of heat is added to 35 grams of water at 15 degrees Celsius?
How much energy is required to convert 75 grams of ice at -20 degrees Celsius to steam at 125 degrees Celsius? (Hint: This problem has 5 steps!!)