Endothermic & Exothermic Reactions

Question 1

Describe an endothermic reaction

Answer 1

Energy is absorbed from the surroundings → bond breaking

Question 2

Describe an exothermic reaction

Answer 2

Energy is released into the surroundings → bond making

Question 3

What do Ea stand for and what does it mean?

Answer 3

Ea = activation energy

Activation energy is the minimum energy required for a reaction to take place

Question 4

If a reaction goes from solid → powder → gas, what reaction is it?

Answer 4

Endothermic

Question 5

If a reaction goes from gas → powder → solid, what reaction is this?

Answer 5

Exothermic

Question 6

What does H stand for and what does it mean?

Answer 6

H = enthalpy

Enthalpy is the energy change that occurs after a reaction takes place

Question 7

What does ΔH stand for and what does it mean?

Answer 7

ΔH = change in enthalpy

The enthalpy change is the energy change of a reaction

Question 8

How can you find ΔH?

Answer 8

ΔH = H(products) - H(reactants)

Question 9

What should an endothermic reaction graph look like?

Answer 9

y axis = potential energy (kJ)
x axis = reaction pathway

• Lowest energy at the start (reactants), highest at the end (products)
• ΔH will be positive
• Mark ΔH between the product line and reactant line
• Mark the activation energy from the reactant line to the top of the graph

Question 10

What should an exothermic reaction graph look like?

Answer 10

y axis = potential energy (kJ)
x axis = reaction pathway

• Highest energy at the start (reactants), lowest energy at the end (products)
• ΔH will be negative
• Mark ΔH between the product line and reactant line
• Mark the activation energy from the reactant line to the top of the graph

Question 11

How do you calculate ΔrH?

Answer 11

ΔrH = ΣE bonds broken - ΣE bonds made

ΔrH = Σreactants - Σproducts

Question 12

How do you calculate the enthalpy change in n=m/M questions?

Answer 12

1. Write down all the known values for n, m and M
2. Figure out any of the unknown values
3. Put the atom and energy in a ratio (eg, C6H12O6 : energy)
4. Put the number of moles in a ratio to the given ΔrH value (eg, 1 : -2820)
5. Put the mole value (from step 2) in a ratio to x (eg, 0.278 : x)
6. Cross multiply the last two ratios and solve for x

Question 13

If the sign of ΔrH is positive, which reaction is it?

Answer 13

Endothermic

Question 14

If the sign of ΔrH is negative, which reaction is it?

Answer 14

Exothermic

Question 15

In an endothermic reaction are there more products or reactants?

Answer 15

Products

Question 16

In an exothermic reaction are there more products or reactants?

Answer 16

Reactants