Thermochemistry pt 1

Multiple Choice

How do endothermic and exothermic reactions differ in terms of energy change? (pg 268)

• In an endothermic reaction, more energy is absorbed to break bonds than is released forming new bonds. In an exothermic reaction, more energy is released forming bonds than is absorbed breaking bonds.

• Both reactions release the same amount of energy.

• Endothermic reactions release energy; exothermic reactions absorb energy.

• Energy changes only occur in exothermic reactions.

Multiple Choice

Under what conditions would a collision NOT result in a reaction? (pg 368)

• If the temperature is too high

• If the reaction is endothermic.

• If particles lack enough kinetic energy to reach activation energy or collide in the wrong orientation

• If the products are more stable than reactants.

Multiple Choice

In chemistry, energy is conserved—this means it cannot be created or destroyed, only transferred. Given this, where is energy conserved: in the system alone, or in the system and its surroundings?"(pg 369/speak with instructor)

• System only (energy disappears when released as heat)

• System and surroundings (energy transfers between them, but the total stays the same)

• Surroundings only (systems lose all energy)

Multiple Choice

If a clay pot were to heat up just as quickly as an iron pot with the same heat energy, what must be true about their masses?" (pg 370/ speak with instructor)

• The clay pot has more mass.

• Specific heat doesn’t affect heating rate.

• The clay pot has less mass—about half of the iron pot’s mass.

• Their masses are equal

Multiple Choice

According to page 371 of Texas Experience Chemistry, what is a change in enthalpy (ΔH)?

• The work done when a gas expands

• The total kinetic energy of all particles in a reaction.

• The energy required to initiate a reaction.

• The heat absorbed or released by a system at constant pressure.

Multiple Choice

1. How much energy is needed to break/form the H–H bond?

2. What does the positive vs. negative sign indicate?

• 432 kJ/mol. Positive means energy absorbed to break bonds; negative means energy released when bonds form.

• 216 kJ/mol. Signs show bond strength.

• 432 kJ/mol. Signs indicate temperature change

Multiple Choice

On page 374, what is activation energy (Eₐ) in terms of collision theory?

• The average energy of all collisions at a given temperature

• The minimum kinetic energy colliding molecules must have to react

• The total potential energy difference between reactants and products

Multiple Choice

Based on the reaction profile diagram on page 374, why is the activation energy not as large as the change in enthalpy for the reaction?

• Not all of the bonds have to break completely for the activation complex to form.

• Activation energy excludes the energy released during bond formation

• The diagram scales enthalpy and activation energy differently.

Multiple Choice

The diagram shows ΔH = –65.2 kJ. What does this mean? Is this reaction endothermic or exothermic? (pg 375)

• 65.2 kJ is absorbed (endothermic), breaking bonds

• 65.2 kJ of energy is released (exothermic), as new bonds are formed.

• No energy change occurs; ΔH is zero

Multiple Choice

Compare the two representations of the same reaction:

1. CaO(s) + H₂O(l) → Ca(OH)₂(s) + 65.2 kJ

2. CaO(s) + H₂O(l) → Ca(OH)₂(s)  ΔH = –65.2 kJ

How do these equations differ in showing energy change? (pg 375)

• The first includes energy as a product; the second shows ΔH separately with a sign.

• The first is endothermic; the second is exothermic.

• The second equation is unbalanced

• They represent different reactions

Multiple Choice

How would the equation differ for an endothermic reaction?(pg 375)

• The ΔH sign would flip halfway through the reaction.

• Energy would be a reactant (e.g., '+ 25 kJ') or ΔH = +25 kJ.

• No energy term would appear

Multiple Choice

The graph on p.378 shows temperature changes when hot metal is added to cooler water in a calorimeter. Where do the metal and water reach thermal equilibrium, and what does this mean?

• When both lines plateau separately—their temperatures stabilize but remain different.

• When the temperature lines converge—their temperatures become equal, and heat transfer stops.

• At the highest point of the water's temperature line—the metal loses all its energy