Enthalpy/Entropy/Hess's Law

Hess' Law makes use of which principle to calculate the enthalpy change of a reaction?

• The law of conservation of energy

• The law of conservation of matter

• The law that you will always find a lost item in the last place you look for it

• Murphy's law

Tags

NGSS.HS-PS1-4

NGSS.HS-PS3-1

The following equations show the oxidation of carbon and carbon monoxide to carbon dioxide.

C_(s) +O_2(g) → CO_2(g) ΔH = –x kJ mol^–1

CO_(g) + O_2(g) → CO_2(g) ΔH = –y kJ mol^–1

What is the enthalpy change, in kJ mol^–1, for the oxidation of carbon to carbon monoxide?

C_(s) + O_2(g) → CO_(g)

• x + y

• -x - y

• y - x

• x - y

Consider the following equations.

Mg_(s) + O_2(g) → MgO_(s) ∆H = –602 kJ

H_2(g) + O_2(g) → H₂O_(g) ∆H = –242 kJ

What is the ∆H value (in kJ) for the following reaction?

MgO_(s) + H_2(g) → Mg_(s) + H₂O_(g)

• -844

• -360

• +360

• +844

Tags

NGSS.HS-PS1-4

Using the equations below:

C_(s) + O_2(g) → CO_2(g) ∆H = –390 kJ

Mn_(s) + O_2(g) → MnO_2(s) ∆H = –520 kJ

what is ∆H (in kJ) for the following reaction?

MnO_2(s) + C_(s) → Mn_(s) + CO_2(g)

• 910

• 130

• -130

• -910

Tags

NGSS.HS-PS1-4

NGSS.HS-PS3-1

What is true of an endothermic reaction?

• The substances in the reaction will release energy

• The substances in the reaction will absorb energy

When substances change from solid, to liquid, to gas, entropy increases. Why?

• The molecules become more randomly arranged and move more chaotically

• The molecules spread out and take up more space

• The molecules become more reactive

• The molecules become more compact and organized

• decrease

• increase

• remains the same

What is true of an endothermic reaction?

• The substances in the reaction will release energy

• The substances in the reaction will absorb energy

This is a model of the combustion of propane. Which is the best description of what will happen?

(Choose TWO correct answers)

• When the propane burns, the propane will release energy and the surroundings will absorb energy

• When the propane burns, the propane will absorb energy and the surroundings will release energy

• When the propane burns, the entropy (disorder) of the surroundings will increase

• When the propane burns, the entropy (disorder) of the surroundings will decrease

Imagine you are conducting a chemical experiment in a lab. If the reaction you are observing is ENDOTHERMIC, what would happen to the temperature on your thermometer?

• The temperature will stay the SAME because of equilibrium

• The temperature will DECREASE because the reaction will absorb energy away from the thermometer and the particles of the thermometer will slow down.

• The temperature will DECREASE because the reaction will release energy and chemical particles will collide more with the thermometer. Therefore the particles of the thermometer will speed up.

• The temperature will INCREASE because the reaction will release energy and chemical particles will collide more with the thermometer. Therefore the particles of the thermometer will speed up.