The temperature of a sample of liquid water changes from 50°c to 30°c. which statement best explains the change that must happen at the molecular level?

a. the molecules move more quickly and their average kinetic energy increases.
b. the molecules move more slowly and their average kinetic energy decreases.
c. the molecules move more quickly and their average kinetic energy decreases.
d. the molecules move more slowly and their average kinetic energy increases

B. The molecules move more slowly and their average kinetic energy decreases.

Explanation: Kinetic energy is the energy possessed by a body by virtue of its motion.

Average kinetic energy is defined as the average of the kinetic energies of all the particles present in a system. It is determined by the equation:

R = gas constant

T = temperature of system

From above, it is visible that kinetic energy is directly related to the temperature of the system. So when the temperature is decreased from to ,the average kinetic energy of the system will also decrease and the molecules will start moving more slowly.

when 25.0 ml of 0.700 mol/l naoh was mixed in a calorimeter with 25.0 ml of 0.700 mol/l hcl, both initially at 20.0 °c, the temperature increased to 22.1 °c. the heat capacity of the calorimeter is 279 j/°c.

the equation for the reaction is:

naoh + hcl → nacl + h₂o

moles of hcl = 0.0250 l hcl ×

0.700

mol hcl

1 l hcl = 0.0175 mol hcl

volume of solution = (25.0 + 25.0) ml = 50.0 ml

mass of solution = 50.0 ml soln ×

1.00

ml soln

= 50.0 g soln

δt=t2-t1

= (22.1 – 20.0) °c = 2.1 °c

the heats involved are

heat from neutralization + heat to warm solution + heat to warm calorimeter = 0

q1+q2+q3=0

nδh+mcδt+cδt=0

0.0175 mol ×

δh

+ 50.0 g × 4.184 j·g⁻¹°c⁻¹ × 2.1 °c + 279 j°c⁻¹ × 2.1 °c = 0

0.0175 mol ×

δh

+ 439.32 j + 585.9 j = 0

0.0175 mol ×

δh

= -1025.22 j

δh=1025.22j0.0175 mol= -58 600 j/mol = -58.6 kj/mol