What causes the energy changes in chemical reactions? (ESBQK)

When a chemical reaction occurs, bonds in the reactants break, while new bonds form in the product. The following example explains this. Hydrogen reacts with oxygen to form water, according to the following equation:

In this reaction, the bond between the two hydrogen atoms in the \(\text{H}_{2}\) molecule will break, as will the bond between the oxygen atoms in the \(\text{O}_{2}\) molecule. New bonds will form between the two hydrogen atoms and the single oxygen atom in the water molecule that is formed as the product.

For bonds to break, energy must be absorbed. When new bonds form, energy is released. The energy that is needed to break a bond is called the bond energy or bond dissociation energy. Bond energies are measured in units of \(\text{kJ·mol$^{-1}$}\).

Bond energy

Bond energy is a measure of bond strength in a chemical bond. It is the amount of energy (in \(\text{kJ·mol$^{-1}$}\)) that is needed to break the chemical bond between two atoms.

Remember when we discussed bonding (chapter 3) we used the following energy diagram:

We can use this diagram to understand why bond breaking requires energy and bond making releases energy. Point X on the diagram is at the lowest energy. When a bond breaks, the atoms move apart and the distance between them increases (i.e. the atom moves to the right on the \(x\)-axis or from point X to point A). Looking at the diagram we see that when this happens, the energy increases (i.e. the energy at point A is greater than the energy at point X). So when a bond breaks energy is needed.

When a bond forms the atoms move closer together and the distance between them decreases (i.e. the atom moves to the left on the \(x\)-axis or from point A to point X). Looking at the diagram we see that when this happens, the energy decreases (i.e. the energy at point X is less than the energy at point A). So when a bond forms energy is released.

Looking at the example of hydrogen reacting with oxygen to form water:

We see that energy is needed to break the bonds in the hydrogen molecule and to break the bonds in the oxygen molecule. And we also see that energy is released when hydrogen and oxygen bond to form water. When we look at the entire reaction and consider both bond breaking and bond forming we need to look at the enthalpy of the system.

Enthalpy

Enthalpy is a measure of the total energy of a chemical system for a given pressure, and is given the symbol H.

As we learn about exothermic and endothermic reactions we will see more on the concept of enthalpy.

Exothermic and endothermic reactions (ESBQM)

In some reactions, the energy that must be absorbed to break the bonds in the reactants, is less than the energy that is released when the new bonds of the products are formed. This means that in the overall reaction, energy is released as either heat or light. This type of reaction is called an exothermic reaction.

Exothermic reaction

An exothermic reaction is one that releases energy in the form of heat or light.

Another way of describing an exothermic reaction is that it is one in which the energy of the products is less than the energy of the reactants, because energy has been released during the reaction. We can represent this using the following general formula:

In other reactions, the energy that must be absorbed to break the bonds in the reactants, is more than the energy that is released when the new bonds in the products are formed. This means that in the overall reaction, energy must be absorbed from the surroundings. This type of reaction is known as an endothermic reaction.

Endothermic reaction

An endothermic reaction is one that absorbs energy in the form of heat or light.

Another way of describing an endothermic reaction is that it is one in which the energy of the products is greater than the energy of the reactants, because energy has been absorbed during the reaction. This can be represented by the following general formula:

The difference in energy (E) between the reactants and the products is known as the heat of the reaction. It is also sometimes referred to as the enthalpy change of the system. This is represented using \(\Delta \text{H}\)

Endothermic and exothermic reactions - part 1

Apparatus and materials

You will need:

• citric acid
• sodium bicarbonate
• a polystyrene cup
• a lid for the cup
• thermometer
• glass stirring rod
• scissors

You can get polystyrene cups with lids from coffee shops or fast food stores. Cardboard cups will also work fine. Some of the lids will have a hole for a straw, which is useful for this experiment.

Note that citric acid is found in citrus fruits such as lemons. Sodium bicarbonate is actually bicarbonate of soda (baking soda), the baking ingredient that helps cakes to rise.

Method

1. If your lid does not have a hole for a straw, then cut a small hole into the lid.

2. Pour some citric acid \((\text{C}_{6}\text{H}_{8}\text{O}_{7})\) into the polystyrene cup, cover the cup with its lid and record the temperature of the solution.

3. Stir in the sodium bicarbonate \((\text{NaHCO}_{3})\), then cover the cup again.

4. Immediately record the temperature, and then take a temperature reading every two minutes after that. Record your results.

The equation for the reaction that takes place is:

\[\text{C}_{6}\text{H}_{8}\text{O}_{7}\text{(aq)} + 3\text{NaHCO}_{3}\text{(s)} → 3\text{CO}_{2}\text{(g)} + 3\text{H}_{2}\text{O(}ℓ\text{)} + \text{Na}_{3}\text{C}_{6}\text{H}_{5}\text{O}_{7}\text{(aq)}\]

Results

Time (\(\text{mins}\))	0	2	4	6
Temperature (\(\text{℃}\))

Plot your temperature results on a graph of time (\(x\)-axis) against temperature (\(y\)-axis).

Discussion and conclusion

• What happens to the temperature during this reaction?

• Is this an exothermic or an endothermic reaction? (Was energy taken in or given out? Did the temperature increase or decrease?)

• Why was it important to keep the cup covered with a lid?

Endothermic and exothermic reactions - part 2

Apparatus and materials

• Vinegar
• steel wool
• thermometer
• polystyrene cup and plastic lid (from previous experiment)

Method

1. Put the thermometer through the plastic lid, cover the cup and record the temperature in the empty cup. You will need to leave the thermometer in the cup for about 5 minutes in order to get an accurate reading.

2. Soak a piece of steel wool in vinegar for about a minute. The vinegar removes the protective coating from the steel wool so that the metal is exposed to oxygen.

3. Take the thermometer out of the cup. Keep the thermometer through the hole of the lid.

4. After the steel wool has been in the vinegar, remove it and squeeze out any vinegar that is still on the wool. Wrap the steel wool around the thermometer and place it (still wrapped round the thermometer) back into the cup. The cup is automatically sealed when you do this because the thermometer is through the top of the lid.

5. Leave the steel wool in the cup for about 5 minutes and then record the temperature. Record your observations.

Results

You should notice that the temperature increases when the steel wool is wrapped around the thermometer.

Conclusion

The reaction between oxygen and the exposed metal in the steel wool is exothermic, which means that energy is released and the temperature increases.

Examples of endothermic and exothermic reactions (ESBQN)

There are many examples of endothermic and exothermic reactions that occur around us all the time. The following are just a few examples.

1. Endothermic reactions

   • Photosynthesis

     Photosynthesis is the chemical reaction that takes place in green plants, which uses energy from the sun to change carbon dioxide and water into food that the plant needs to survive, and which other organisms (such as humans and other animals) can eat so that they too can survive. The equation for this reaction is:

     \[6\text{CO}_{2}\text{(g)} + 6\text{H}_{2}\text{O(ℓ)} + \text{energy} → \text{C}_{6}\text{H}_{12}\text{O}_{6}\text{(s)} + 6\text{O}_{2}\text{(g)}\]

     Photosynthesis is an endothermic reaction. Energy in the form of sunlight is absorbed during the reaction.

   • The thermal decomposition of limestone

     In industry, the breakdown of limestone into quicklime and carbon dioxide is very important. Quicklime can be used to make steel from iron and also to neutralise soils that are too acid. However, the limestone must be heated in a kiln (oven) at a temperature of over \(\text{900}\) \(\text{℃}\) before the decomposition reaction will take place. The equation for the reaction is shown below:

     \[\text{CaCO}_{3}\text{(s)} → \text{CaO(s)} + \text{CO}_{2}\text{(g)}\]

2. Exothermic reactions

   • Combustion reactions

     The burning of fuel is an example of a combustion reaction, and we as humans rely heavily on this process for our energy requirements. The following equations describe the combustion of a hydrocarbon such as petrol \((\text{C}_{8}\text{H}_{18})\):

     fuel + oxygen \(→\) heat + water + carbon dioxide

     \[2\text{C}_{8}\text{H}_{18}\text{(ℓ)} + 25\text{O}_{2}\text{(g)} → 16\text{CO}_{2}\text{(g)} + 18\text{H}_{2}\text{O(g)} + \text{heat}\]

     This is why we burn fuels (such as paraffin, coal, propane and butane) for energy, because the chemical changes that take place during the reaction release huge amounts of energy, which we then use for things like power and electricity. You should also note that carbon dioxide is produced during this reaction. The chemical reaction that takes place when fuels burn has both positive and negative consequences. Although we benefit from heat, power and electricity the carbon dioxide that is produced has a negative impact on the environment.

   • Respiration

     Respiration is the chemical reaction that happens in our bodies to produce energy for our cells. The equation below describes what happens during this reaction:

     \[\text{C}_{6}\text{H}_{12}\text{O}_{6}\text{(s)} + 6\text{O}_{2}\text{(g)} → 6\text{CO}_{2}\text{(g)} + 6\text{H}_{2}\text{O}\text{(ℓ)} + \text{energy}\]

     In the reaction above, glucose (a type of carbohydrate in the food we eat) reacts with oxygen from the air that we breathe in, to form carbon dioxide (which we breathe out), water and energy. The energy that is produced allows the cell to carry out its functions efficiently. Can you see now why you must eat food to get energy? It is not the food itself that provides you with energy, but the exothermic reaction that takes place when compounds within the food react with the oxygen you have breathed in!

Lightsticks or glowsticks are used by divers, campers, and for decoration and fun. A lightstick is a plastic tube with a glass vial inside it. To activate a lightstick, you bend the plastic stick, which breaks the glass vial. This allows the chemicals that are inside the glass to mix with the chemicals in the plastic tube. These two chemicals react and release energy. Another part of a lightstick is a fluorescent dye which changes this energy into light, causing the lightstick to glow! This is known as phosphorescence or chemiluminescence.