Energy Changes During Chemical Change Notes

Signs of Chemical Change and Thermal Energy

Identifying whether a chemical reaction has occurred involves observing specific indicators. A primary sign of chemical change is a noticeable change in temperature.

Thermal Energy Release: Some reactions release energy into the environment. A common example is burning charcoal in a barbecue. In this process, charcoal reacts with the oxygen in the atmosphere, releasing a massive amount of thermal energy used for cooking food. This results in a feeling of warmth or heat.
Thermal Energy Absorption: Not all chemical reactions produce heat. Some reactions absorb thermal energy from their environment. This absorption leads to a cooling effect, where the temperature of the surroundings decreases.

The Molecular Basis of Energy Exchange

Chemical reactions involve the transformation of substances through the manipulation of atomic bonds. Understanding why energy is absorbed or released requires looking at the process in stages:

Bond Breaking: Energy is taken in (absorbed) from the surroundings to break the existing bonds between atoms in the reactants.
Atomic Rearrangement: Once bonds are broken, the atoms rearrange themselves into new configurations.
Bond Formation: New bonds form to create the resulting products. Energy is released (given out) to the surroundings during the formation of these new bonds.
Net Energy Change: The overall energy change of the reaction depends on the balance between the energy required to break bonds and the energy released when new bonds are formed.

Defining the Surroundings

In a laboratory setting, the term "surroundings" refers to everything outside the immediate chemical reactants. For a reaction taking place within a flask, the surroundings include:

The water in which the chemical substances are dissolved (the solvent).
The glass material of the flask itself.
The air immediately surrounding the flask.

Temperature Indicators in the Surroundings:

If heat energy is absorbed from the surroundings, the temperature of the water, the glass, and the air will decrease.
If heat energy is released to the surroundings, the temperature of the water, the glass, and the air will increase.

Classification of Reactions: Exothermic and Endothermic

Chemical reactions are categorized based on their net energy exchange with the environment.

Exothermic Reactions: These occur when more energy is given out during the formation of bonds than the energy taken in to break bonds.
- Energy Balance: \text{Energy released (forming)} > \text{Energy absorbed (breaking)}
- Observation: The temperature of the surroundings increases.
Endothermic Reactions: These occur when more energy is taken in to break bonds than the energy given out during the formation of new bonds.
- Energy Balance: \text{Energy absorbed (breaking)} > \text{Energy released (forming)}
- Observation: The temperature of the surroundings decreases.

Experimental Case Study: Magnesium and Hydrochloric Acid

The reaction between magnesium and hydrochloric acid is a classic example of an exothermic process. It is represented by the following word equation:

$magnesium + hydrochloric acid \rightarrow magnesium chloride + hydrogen$

Experiment Aim

To determine through observation and measurement if the reaction between magnesium and hydrochloric acid is endothermic or exothermic.

Method and Procedure

Safety Precautions: Participants must wear safety goggles and lab aprons. The acid must be handled with care, and gloves should be used.
Preparation of Reactants: Measure a $2\,cm$ length of magnesium ribbon.
Measurement of Acid: Measure $50\,cm^3$ of hydrochloric acid and pour it into a polystyrene cup.
- Note: A polystyrene cup is used as an insulated vessel to minimize heat loss to the surrounding air, ensuring more accurate temperature readings.
Initial Temperature: Place a thermometer in the acid and record the starting temperature.
Reaction Initiation: Add the $2\,cm$ magnesium ribbon to the acid.
Monitoring: Quickly stir the solution and record temperature readings every $15\,s$ for a duration of three minutes ( $180\,s$ ).

Data Collection and Results

The following table tracks the temperature of the solution over time:

Time ( $s$ )	Temperature ( $^\circ C$ )
$0$	$20.0$
$15$	$23.5$
$30$	$27.0$
$45$	$30.0$
$60$	$32.5$
$75$	$34.5$
$90$	$35.5$
$105$	$35.8$
$120$	$35.7$
$135$	$35.3$
$150$	$34.9$
$165$	$34.5$

Note: The highest temperature reached was $35.8\,^\circ C$ at the $105\,s$ mark.

Analysis of Results and Conclusion

To determine the energy change, the maximum temperature reached is compared against the initial starting temperature using the following formula:

$\text{Temperature Change} = \text{Final Temperature} - \text{Start Temperature}$

Calculation for this experiment: $\text{Temperature Change} = 35.8\,^\circ C - 20.0\,^\circ C = 15.8\,^\circ C$

Interpretation:

Because the temperature change is a positive value ( $15.8\,^\circ C$ ), it indicates the temperature increased. Therefore, the reaction is exothermic.
If the result of the calculation had been a negative number (indicating a decrease in temperature), the reaction would have been classified as endothermic.