Energy Changes in Chemical Reactions Worksheet
Completion requirements
# Energy Changes in Chemical Reactions Worksheet
## Part 1: Bond Energies and Energy Change
**Learning Objective:** Calculate the energy change in a chemical reaction using bond energies.
When a chemical reaction occurs, energy is required to break bonds in the reactants, and energy is released when new bonds form in the products. The overall energy change can be calculated using the formula:
$$ \Delta H = \sum \text{(bonds broken)} - \sum \text{(bonds formed)} $$
Where:
- Breaking bonds is endothermic (requires energy): positive values
- Forming bonds is exothermic (releases energy): negative values
### Example:
For the reaction: $\text{H}_2 + \text{Cl}_2 \to 2\text{HCl}$
Bond energies:
- H−H: 436 kJ/mol
- Cl−Cl: 242 kJ/mol
- H−Cl: 431 kJ/mol
$$ \Delta H = [436 + 242] - [2 \times 431] = 678 - 862 = -184 \text{ kJ/mol} $$
The negative value means the reaction is exothermic.
### Practice Problems:
1. Calculate the energy change for the combustion of methane:
$\text{CH}_4 + 2\text{O}_2 \to \text{CO}_2 + 2\text{H}_2\text{O}$
Bond energies:
- C−H: 413 kJ/mol
- O=O: 495 kJ/mol
- C=O: 799 kJ/mol
- O−H: 463 kJ/mol
2. Calculate the energy change for the formation of ammonia:
$\text{N}_2 + 3\text{H}_2 \to 2\text{NH}_3$
Bond energies:
- N≡N: 945 kJ/mol
- H−H: 436 kJ/mol
- N−H: 391 kJ/mol
## Part 2: Effect of Reaction Conditions
**Learning Objective:** Analyze how changes in reaction conditions affect energy changes.
### Investigation Questions:
1. How would increasing the temperature affect the rate of an exothermic reaction? How would it affect the total energy released?
2. For the reaction $\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3 + \text{heat}$:
- What would happen to the position of equilibrium if:
a) The temperature is increased?
b) The pressure is increased?
c) A catalyst is added?
- Explain your answers in terms of energy changes.
3. Why do some endothermic reactions still occur spontaneously despite requiring energy input?
## Part 3: Experimental Design
**Learning Objective:** Design a simple experiment to measure energy changes.
### Design Task: Coffee Cup Calorimeter
Design an experiment to measure the heat of neutralization when a strong acid reacts with a strong base.
1. Materials needed (list at least 5):
2. Step-by-step procedure (minimum 6 steps):
3. What measurements will you take?
4. How will you calculate the energy change using the formula:
$$ q = mc\Delta T $$
Where:
- q = heat energy (J)
- m = mass of solution (g)
- c = specific heat capacity (J/g°C)
- ΔT = temperature change (°C)
5. What sources of error might affect your results? How could you minimize them?
## Part 4: Energy Diagrams
**Learning Objective:** Construct energy diagrams for chemical reactions.
### Energy Diagram Components:
An energy diagram should show:
- Energy level of reactants
- Energy level of products
- Activation energy (energy barrier)
- Overall energy change (ΔH)
- Reaction pathway (progress of reaction)
### Drawing Tasks:
1. Draw an energy diagram for an exothermic reaction with a high activation energy.
2. Draw an energy diagram for the same reaction with a catalyst present. Label how the diagram changes.
3. Draw an energy diagram for an endothermic reaction. Label all components.
4. The decomposition of hydrogen peroxide (H₂O₂) to water and oxygen is highly exothermic but occurs very slowly without a catalyst. Draw two energy diagrams to represent this reaction:
a) Without catalyst
b) With catalyst (such as manganese dioxide)
Explain the difference between the diagrams.
### Analysis Question:
For a reaction with ΔH = -45 kJ/mol and activation energy Ea = 65 kJ/mol:
- Calculate the activation energy for the reverse reaction
- Explain which direction (forward or reverse) is more likely to occur at room temperature and why
Last modified: Tuesday, 25 February 2025, 9:55 AM