Chapter 5 - Work and Energy

Question 1

State the equation for kinetic energy

Answer 1

Ek = 1/2 mv^2

energy is scalar, capable of producing work

Question 2

What is the work energy theorum of net force

Answer 2

Net force = sum of interior and exterior forces

W = △Ek

Question 3

List some (four) of the equations for potential energy

Answer 3

Ep = mgh
Ep = k (q1q1/r)
Ep = G (m1m2/r)
Ep = kx^2/2

Question 4

What is the equation for mechanical energy?

Answer 4

Et = sum of kinetic = potential energy
Et = Ek + Ep
where an isolated system will have a constant Et since not being acted upon by an exterior force

Question 5

A 6.8 x 10^3kg frictionless car at rest 30m above ground. What is the speed at 20m above and at ground level?

Answer 5

Et = Ep + Ek
     = mgh + 1/2 mv^2
     = (30) (10) = 0
     = 300mj
V = 0 at res, g approx 10m/s so heght is 30
for 20m
Et = 300 = gh + 1/2v^2
Need V so multiply each side by 2
V = square root of 2(300) - 2(10)20 = square root 2(100) = 14
for h = 0
Et = 300 = 1/2v^2 + 0 (meters cancel out, multiply by 2 for V)
V = square root 600 = 24m/s

Question 6

What is the relation of gravity and friction to conservative forces

Answer 6

Energy of a particle returns to its original value, the work done after a round trip is zero, the work done depends on initial and final positions.
Friction disobeys this so it is not conservative force
Force of an ideal spring is frictionless therefore Fs = -kx and is conservative
Gravity is also a conservative force as it will return to the same KE it had

Question 7

What is the equation for Power

Answer 7

P = work / time
Power is the rate of doing work
P = force x velocity also
Watts = joules/sec

Question 8

Boiling point elevation Kb is 0.52C/m for water what is the Kb for 0.1mol C2H6O2 and 0.1mol Ch3OH in 18g of H2O

Answer 8

n = M/Mr = 18g / 18 = 1 mol and 0.52 x 1 = 5.2
or
0.2mol/0.018kg = 0.52 x 10 = 5.2

△H fusion of water is a constant

Question 9

Define molarity and molality

Answer 9

Molarity = number of moles per litre
Molality = number of moles per 1000g of solvent
when the volume of a solvent in cm^3 is equal to its mass in grams molarity and molality will be equal

Question 10

Why is the boiling point elevation of sucrose (C12H22O11) lower than that of glucose (C6H12O6)?

Answer 10

The molecular mass of glucose is clearly smaller hence there are more moles of it when equal quantity with sucrose. The molality of glucose is higher hence the boiling point elevation is also higher

Question 11

What would be the freezing point (in kelvin) of a solution 0.5 molal in sucrose and 0.5 molal in acetic acid Kf H2O = 2 mol

Answer 11

273 - 2 = 271

Question 12

2.0 M of sucrose freezes at -3.72 degrees. What is the freezing point after hydrolysis (when hydrolysed becomes glucose and fructose)

Answer 12

Sucrose = 1 molecule = -3.72

afterwards is 2 molecules so -3.72 x 2 = -7.33 degrees

Question 13

Define Ksp and its relation to solubility

Answer 13

Ksp is the solubility constant

↑ Ksp = ↑ solubility

Question 14

Which of the follow is LEAST soluble?

a. 2.5 x 10^-9
b. 2.1 x 10^-11
c. 8 x 10^-25

Answer 14

lowest Ksp = lowest solubility

therefore c is the answer