Homework Problems - What to do?

Question 1

A chemical reaction occurs in an aqueous in a flask. What is the system and the surroundings?

Answer 1

The system is the reaction inside the flask

surroundings is everything outside the flask

Question 2

Convert between energy units

231 cal to J

2.88 x 10^4 J to kcal

Answer 2

1 cal = 4.184 joules

231 cal x (4.184J) = 967J

1 Cal = 1kcal

2.88x10^4 J x 1/4.184 x 1kcal/1000 = 6.88kcal

Question 3

A system absorbs 196kJ of heat and the surroundings do 117kJ of work on the system. What is the change in internal energy of the system?

Answer 3

we know q = 196kJ

w = 117Kj

Find E = internal energy of system

use equation Esystem = q + w

(196kJ) + (117kJ) = 313kJ

Question 4

How much heat is required to warm 1.50 kg of sand from 25 degrees Celsius to 100 degrees Celsius? Cs for sand = 0.84

Answer 4

Given

mass = 1.50kg ==> (must be in grams bc of Cs)

Cs = 0.84 J/gC

100.0 - 25.0 = 75.0 = deltaTemp

Equation q = m x Cs x deltaT

(1.50kg x 1000) x (0.84 J/gC) x (75C) = 94500 J of heat required

Question 5

How much work is required to expand the volume of a pump from 0.0L to 2.5L against an internal pressure of 1.1 atm?

Answer 5

Given

2. 5L - 0.0L = 2.5L = deltaVolume
3. 1 atm = pressure

Equation

The gas does work and the volume increases (deltaV > 0) so the work done is negative

w = -P x deltaV

-(1.1 atm) x (2.5L) = -2.75 L x atm

negative liters-atmopsheres makes no sense so make it positive

need final units to be in joules

-2.75 L x atm x (