Energy and Chemical Reactions

Themes: Conservation & EquilibriumChapter 10 Sections 1 & 4

By the end of…you should be able to:

• Class:– Identify how energy flows.– Determine whether a reaction is endergonic or

exergonic.– Distinguish between: entropy, enthalpy, energy, and

heat.

• Upcoming:– Solve problems using the equations q=mcΔT and

q=nΔH– Use a calorimeter to collect experimental data and

determine the specific heat of a metal.

If energy is not destroyed…

• Quality vs. Quantity– Quality energy allows us to do work

• Heat Death– When no usable energy remains

• Entropy– State of disorder

Flow of energy: Quality?

Entropy

Why do ionic compounds dissolve?

Does energy always spread?• Energy spread vs. matter spread• Think about dissolving water…what happens to

the energy there?

Two types of reactions

• Endothermic• Exothermic

Free “useful” energy• Enthalpy: energy

that flows as heat (ΔH)

• What if?– Temperature

increases?– Temperature

decreases?– Entropy increases (as

in a living system)– Entropy decreases

So how do we measure energy (heat=q) changes?

• We look at the system and its affect on the surroundings (amount of work done by a system, thus q not ΔH).

• If we run a reaction in an isolated system, we can use qsystem = -qsurroundings

What do we need to know to measure energy changes?

• 1 calorie = 4.184 Joules• Calorie: Amount of heat required to raise the

temp. of water 1 degree Celsius– Problem: How many joules is 15 Calories?

– FYI: the term Calorie in nutrition is misleading. Its actually a kilocalorie. What does that mean?

Not all substances absorb transfer energy at the same rate

• Q=smΔT(note sometimes s is symbolized as C)

s: specific heat: unique to the substanceM: mass in gΔT: change in temperature

*Your online activity will derive and use this equation to understand the energy change in a closed system.

Enthalpy change in a reaction is related to the moles of the

reactant.• Combustion of Glucose:– C6H12O6 (s) + 6 O2 (g) ---> 6 CO2 (g) + 6 H2O (l)

– ΔH = -2801 kJ

If 12 moles of oxygen gas and excess glucose are reacted, how much energy is released?

If 10 g of glucose is combusted, how many kJ of energy are released?

Theme: Equilibrium drives the heat transfer

Exit Ticket (Day 1):• Are heat and temperature the same? Explain.

• What term describes the tendency of matter to move towards a less orderly state?

• CaCl2 (rock salt) melts ice. What type of energy change occurs when the salt dissolves in water (endergonic or exergonic)? Explain and include an understanding of conservation or equilibrium in your answer.

Exit ticket (Day 2)

• When you are cooking dinner, you can often touch the tomato sauce in the pot on a stove. But once, you left a silver spoon in the sauce and went to take it out and “ouch”! Your hand was burned. Explain in terms of what you learned about metals and specific heat.

• It takes 487.5 J to heat 25 g of copper from 25°C to 75°C . What is the specific heat of copper in Joules/g°C ?