It flows from hot to cold objects and is known by the letter q
heat
Slide 5
The study of energy changes that occur during chemical
reactions thermochemistry
Slide 6
What is a calorie or joule? Unit of heat (q)
Slide 7
Defined as the amount of heat needed to increase the
temperature of an object by 1 o C. Heat capacity
Slide 8
Defined as the amount of heat needed to increase 1 gram of an
object by 1 o C. Specific heat or specific heat capacity
Slide 9
The formula for q? q = C x m x T
Slide 10
Solve the previous equation for the other three variables. C =
q/m T m = q/C T T = q/Cm
Slide 11
A balance bar has 200 Calories. How many kilojoules is this?
How many joules is this? 836.8 kj 836,800 joules
Slide 12
True or False. cal/ o C is an acceptable unit for specific
heat. False, cal/g o C
Slide 13
True or False. Metals generally have a higher specific heat
capacity than water. False
Slide 14
True or False. Metals generally have a higher specific heat
capacity than molecular compounds. False
Slide 15
You measure 1200 joules of heat during a 30 o C temperature
change with a substance that weighs 100 g. What is the specific
heat of the substance. 0.40 J/g o C
Slide 16
During a phase change, the temperature of a substance ________.
Remains constant
Slide 17
A calorimeter can effectively measure the heat of another
substance because of the ____________? Law of Conservation of
Energy
Slide 18
H of fusion involves which phase change? Melting
Slide 19
H of solidification involves which more commonly known phase
change? Freezing
Slide 20
If the percent mass of a solution weighing 300 g is 6%, what is
the mass of the solute?.06 = x/300g x = 18 g
Slide 21
The heat content of a system at a constant pressure is known as
the ________ of that system. enthalpy
Slide 22
What is the enthalpy change in a chemical reaction known as?
Heat of reaction, H
Slide 23
If the H of a reaction is negative then the reaction is
_______. exothermic
Slide 24
The heat of the reaction for the complete burning of one mole
of a substance. Heat of combustion
Slide 25
The enthalpy change when a mole of solute is dissolved in a
solvent. Heat of solution
Slide 26
True or False. The quantity of heat absorbed when a solid melts
is the same as the quantity released when the substance freezes.
True. H fus = - H solid.
Slide 27
What makes Hesss law useful? It allows you to determine heats
of reactions indirectly
Slide 28
The change in enthalpy that involves the formation of one mole
of a compound from its elements (at 25 o C) is known as? Standard
heat of formation, ( H f o )
Slide 29
True or False. The standard heat of a reaction can be
calculated by the following equation H o = H f o (products) - H f o
(reactants) True
Slide 30
How to solve a phase change problem You just multiply H of
fusion/vaporization by mass Water is vaporizing H vap = 2260 J/g of
H 2 O q = m x H vap Water is melting H fus = 334 J/g of H 2 O q = m
x H fus (J)
Slide 31
Heating/Cooling Curve: No Phase Changes What happens at A, C,
and E?? (J) We use our old formula q = m x C x T q = m x C liq x T
q = m x C solid x T q = m x C gas x T
Slide 32
Heating/Cooling Curve: Putting it all Together (J) You should
be able to calculate the total heat going all the way from heating
a substance from its solid to its gas q = m x C liq x T q = m x C
solid x T q = m x C gas x T q = m x H fus q = m x H vap
Slide 33
Sample Problem You have a 4.30 grams of ice at -13.2 o C. You
heat it until it completely vaporizes. How much heat was needed to
complete this process? Here are some numbers you might need. (C ice
= 2.10 J/g o C) (C water = 4.18 J/g o C) (C steam = 1.70 J/g o C) (
H fus = 334 J/g) ( H vap = 2260 J/g) q = m x C ice x T
(4.30)(2.10)(0.00 - - 13.2)= 119.2 J q = m x H fus (4.30)(334)=
1436.2 J q = m x C wat x T (4.30)(4.18)(100. - 0.00)= 1797.4 J q =
m x H vap = (4.30)(2260)= 9718 J To get answer you simply add these
4 numbers together: 13071 J or 13.1 kJ (3 sig. figs)
