energy and causes of change -...
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Energy and Causes of Change Honors chemistry – Semester 2
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Energy is Everywhere
Everything around us contains energy
List some types of energy you might encounter
• Heat
• Light
• Sound
• Chemical
• Electrical
• Kinetic
• Potential
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Energy is Constant
Can’t create or destroy energy
Energy moves between system and surroundings
Example:
Measuring temperature of a beaker of water
• What are the system and surrounding?
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Energy is Constant
Beaker of water is the system What are the surroundings?
Energy
H2O
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Energy and Change
Chemical reactions driven by changes in energy
Greek letter Delta (Δ) signifies a change to system
• +Δ = energy added (endothermic)
• -Δ = energy released (exothermic)
Changes in energy predict how/if reaction runs
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Energy and Change
Energy of a system = enthalpy (H) in Joules (J)
• Energy in bonds and motion of molecules
Calculate ΔH
• How much energy can I get out of the reaction?
• How much energy do I need to make it go?
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Heat and Temperature
Temperature measures how hot or cold
• Intensive
• Average kinetic energy of particles
• Celsius or Kelvin
• 0° C = 273.15 K, ΔT = 1° C = 1 K
Heat is a type of energy
• Extensive
• Moves from hot to cold
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Heat and Temperature
Same temperature Larger fire gives off more heat
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Molar Heat Capacity, C
= amount of heat to raise temperature of 1 mol of a substance by 1 K (J/mol∙K)
Depends on # atoms in sample, not mass or volume
• See Table 1 (pg. 343)
Convert to moles to use molar heat capacity!
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Molar Heat Capacity
Energy to heat a sample
q = nCΔT
q = amount of heat energy
n = number of moles of the substance
C = molar heat capacity
ΔT = temperature change in K
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Example
How much heat energy is needed to raise 10.0 mol Hg by 7.5 K? CHg = 27.8 J/mol∙K
q = nCΔT
𝒒 = 𝟏𝟎. 𝟎 𝒎𝒐𝒍 ×𝟐𝟕. 𝟖 𝑱
𝒎𝒐𝒍 ⋅ 𝑲× 𝟕. 𝟓 𝑲
= 𝟐𝟏𝟎𝟎 𝑱 (𝒕𝒐 𝒄𝒐𝒓𝒓𝒆𝒄𝒕 𝒔𝒊𝒈 𝒇𝒊𝒈)
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Practice
q = nCΔT
Molar heat capacity of tungsten (W) is CW = 24.2 J/mol-K. How much heat is needed to raise 0.40 mol W by 10.0 K?
• 97 J
A sample of NaCl increases in temperature by 2.5 K when it absorbs 1.7 x 102 J of heat energy. How many moles of NaCl are present if CNaCl is 50.5 J/mol-K?
• 1.3 mol
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Specific Heat, cp
Heat energy needed to raise one gram of substance by 1 K (J/g∙K)
Specific heat and molar heat capacity related by
𝑴𝒐𝒍𝒂𝒓 𝒎𝒂𝒔𝒔 𝒈
𝒎𝒐𝒍× 𝒄𝒑
𝑱
𝒈 ⋅ 𝑲= 𝑪
𝑱
𝒎𝒐𝒍 ⋅ 𝑲
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Example
If CAg = 25.3 J/mol∙K, what is its specific heat?
𝑴𝒐𝒍𝒂𝒓 𝒎𝒂𝒔𝒔 × 𝒄𝒑 = 𝑪
= 𝟎. 𝟐𝟑𝟒𝑱
𝒈 ⋅ 𝑲
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Practice
If cp CsI is 0.199 J/g∙K, what is CCsI?
𝟓𝟏. 𝟕𝑱
𝒎𝒐𝒍 ⋅ 𝑲
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Warm Up (Literally!) 2 beakers of water at room temp Two pieces of metal (Al and Cu) with same volume and both at 100° C are added to the beakers (one per beaker) What happens to temperature of the beakers? Why?
H2O
H2O