Energy and Causes of Change Honors chemistry – Semester 2

Energy is Everywhere

Everything around us contains energy

List some types of energy you might encounter

• Heat

• Light

• Sound

• Chemical

• Electrical

• Kinetic

• Potential

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?

Energy is Constant

Beaker of water is the system What are the surroundings?

Energy

H2O

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

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?

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

Heat and Temperature

Same temperature Larger fire gives off more heat

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!

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

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)

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

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

Example

If CAg = 25.3 J/mol∙K, what is its specific heat?

𝑴𝒐𝒍𝒂𝒓 𝒎𝒂𝒔𝒔 × 𝒄𝒑 = 𝑪

= 𝟎. 𝟐𝟑𝟒𝑱

𝒈 ⋅ 𝑲

Practice

If cp CsI is 0.199 J/g∙K, what is CCsI?

𝟓𝟏. 𝟕𝑱

𝒎𝒐𝒍 ⋅ 𝑲

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