Energetics

Question 1

What is meant by the term “exothermic”?

Answer 1

In an exothermic reaction:

• Heat leaves or ‘exits’ the reaction
• The reaction releases heat to the surroundings
• The surroundings will get warmer and the products will have less energy than the reactants
• The enthalphy change ΔH is negative

Question 2

What is meant by the term “endothermic”?

Answer 2

In an endothermic reaction:

• Heat is absorbed or ‘enters’ the reaction
• The surroundings will get cooler and the products will have more energy than the reactants
• The enthalphy change ΔH is positive

Question 3

How is an exothermic reaction shown on an energy level diagram?

Answer 3

The products are BELOW the reactants;

-The products have less energy than the reactants

Question 4

How is an endothermic reaction shown on an energy level diagram?

Answer 4

The products are ABOVE the reactants;

-The products have more energy than the reactants

Question 5

Where is the activation energy on an energy level diagram?

Answer 5

Ea: the difference b/t the reactant line and the top of the peak

Question 6

Where is the overall enthalpy change in an energy level diagram?

Answer 6

ΔH: the difference b/t the reactant and product lines

Question 7

How do you calculate the energy change of a reaction?

Answer 7

[Sum of bonds BROKEN] - [Sum of bonds FORMED]

Question 8

What is meant by the term “bond energy”?

Answer 8

The amount of energy needed to break a bond

Question 9

What are some limitations to using bond energy data?

Answer 9

• They are average values; caluclations using these values are not specific to the molecule used;
• Not accurate
• You can only use bond energy values to calculate enthalpy change if the reactions take place entirely in the gaseous states

Question 10

What is the equation for energy change?

Answer 10

q = mcΔT
q, energy change: (J)
m, Mass of object heated (g)
c, specific heat capacity (4.18 J g-1 C-1)
ΔT, change in temp (kelvin or celcius)

Question 11

What is the equation for enthalpy change?

Answer 11

ΔH=q/n
ΔH, enthalpy change (KJ mol-1)
q, energy change (KJ)
n, moles (mol)

Question 12

What are some sources of error with calorimetry in combustion experiments?

Answer 12

• Incomplete combustion
• Heat loss to the surroundings
• Heat is lost to the calorimeter