Chemistry Exam 2: Energy

Internal Energy

100

How many Joules are in 1 cal?

What is the formula for change in energy?

1 cal = 4.184 J

ΔU = q + W

ΔU = U_final - U_initial

200

Since ΔU _{isolated system} = 0, ΔU _system = -ΔU _surroundings and energy is conserved.

What law of thermodynamics is applied?

First law.

300

Classify each example as either endothermic or exothermic.

a) An explosion

b) Frost forming on a window

c) Melting ice cubes

a) Exothermic

b) Exothermic

c) Endothermic

400

A sample of methanol gas is ignited in the presence of oxygen inside a closed film canister.

As the reaction occurs, 140 J of heat are lost to the surroundings.

The expanding gas does 15 J of work to pop the top off the canister.

What is the change in internal energy, ΔE, for the system?

155 J

Enthaply

100

Work in a system is negative for exothermic reactions, such as explosions.

True or False?

True.

- Work = exothermic

+ Work = endothermic

200

What points represent the transition from liquid to gas? At what point does the substance fully change into a liquid?

Transition: D-E

Liquid: C

300

How much heat is generated when 3.21 moles of O2 are burned?

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)

change heat degrees= -802 kJ/mol

-2.57x10³ kJ

400

Hydrogen peroxide decomposes according to the following thermochemical reaction:

H2O2(l) → H2O(l) + 1/2 O2(g); ΔH = -98.2 kJ

Calculate the change in enthalpy, ΔH, when 1.00 g of hydrogen peroxide decomposes (in kJ).

-2.89 kJ

500

                                                                       

Substance A, with a weight of 15.4g and a specific heat of 2.75 J/g°C with an initial temperature of 21.2°C, has been mixed with substance B, the temperature of substance A has increased to 40.0°C. What is the heat energy (in J) substance A obtains from substance B?

796.18 J