12.1 Energy changes in chemical reactions | Energy and chemical change (2022)

You have probably seen a fire burning or burnt fuel for warmth or cooking or light. A fire burning is one of the most noticeable examples of a chemical reaction that produces a lot of energy.

All chemical reactions involve energy changes. In some reactions, we are able to observe these energy changes as either an increase or a decrease in the overall energy of the system. In some reactions we see this as a change in the temperature. In other reactions we can observe this change when a reaction starts to give off light or when a reaction will only work after light is shone on it.

The study of energy changes (particularly heat) in chemical reactions is known as chemical thermodynamics. This is also sometimes called thermochemistry.

12.1 Energy changes in chemical reactions (ESBQJ)

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:

\[2\text{H}_{2}\text{(g)} + \text{O}_{2}\text{(g)} → 2\text{H}_{2}\text{O}\text{(g)}\]

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:

12.1 Energy changes in chemical reactions | Energy and chemical change (1)

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:

\[2\text{H}_{2}\text{(g)} + \text{O}_{2}\text{(g)} → 2\text{H}_{2}\text{O}\text{(g)}\]

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.

A chemical system is a closed system that contains only the reactants and products involved in the reaction.

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:

\[\text{Reactants} → \text{Products} + \text{Energy}\]

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:

(Video) Exothermic Energy Diagram: Activation Energy, Transition States and Enthalpy Change - TUTOR HOTLINE

\[\text{Reactants} + \text{Energy} → \text{Products}\]

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 \(\Delta \text{H}\)

\(\Delta\) is read as delta and means a change in. You may recall this symbol from physics.

A recommended project for formal assessment is included. In this experiment learners will investigate an exothermic reaction and an endothermic reaction. This experiment is split into two parts. The first part looks at an endothermic reaction, while the second part looks at an exothermic reaction. You will need polystyrene or cardboard cups, plastic lids, thermometers, vinegar, steel wool, citric acid, sodium bicarbonate and stirring rods. There is also a further investigation on exothermic and endothermic reactions that learners can complete. All these experiments can be combined into one project in which learners investigate several different reactions and classify these reactions as exothermic or endothermic.

This series of experiments starts with an endothermic reaction between citric acid and sodium bicarbonate. The second experiment in the series looks at the exothermic reaction between steel and oxygen in the air. The final part of the series is given as an investigation into various exothermic and endothermic reactions.

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

12.1 Energy changes in chemical reactions | Energy and chemical change (2)
  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 (\(\text{mins}\))

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?

    (Video) 12 1 to 12 3 Rates of Chemical Reactions

  • 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

12.1 Energy changes in chemical reactions | Energy and chemical change (3)
  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.

Note that we are only discussing chemical changes (recall from grade 10 about physical and chemical changes). Physical changes can also be classified as exothermic or endothermic. When we are referring to physical change then we talk about exothermic or endothermic processes. Evaporation is an endothermic process while condensation is an exothermic process.

  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 \((\text{C}_{8}\text{H}_{18})\):

      (Video) Chapter 10 Energy Changes in Chemical Reactions

      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.

12.1 Energy changes in chemical reactions | Energy and chemical change (4)

Exothermic and endothermic reactions 1

Textbook Exercise 12.1

A bond is formed between hydrogen and fluorine to form a molecule of hydrogen fluoride.

This is bond forming and so energy is released.

(Video) 12 1 Acivation Energy

A molecule of nitrogen (\(\text{N}_{2}\)) is formed.

A bond is formed and so energy is released.

A molecule of carbon monoxide breaks apart.

A bond is broken and so energy is absorbed.

Reactants react to give products and energy.

Exothermic

The energy that must be absorbed to break the bonds in the reactants is greater than the energy that is released when the products form.

Endothermic

The energy of the products is found to be greater than the energy of the reactants for this type of reaction.

Exothermic

Heat or light must be absorbed from the surroundings before this type of reaction takes place.

Endothermic

FAQs

What are the energy changes in chemical reactions? ›

The energy change in a chemical reaction is due to the difference in the amounts of stored chemical energy between the products and the reactants. This stored chemical energy, or heat content, of the system is known as its enthalpy.

How does the amount of energy taken by chemical reactions compare with that taken in by physical changes? ›

Physical Change: There is no addition or deduction of energy during the physical change, but the energy required for completion of change is released when the change is reversed. Chemical Change: Energy like light, pressure, heat energy is required for chemical changes.

What causes energy to be released in a chemical reaction? ›

A chemical reaction involves the breaking of bonds in the reactants and the forming of bonds in the products. It takes energy to break bonds. Energy is released when bonds are formed.

How do you calculate energy change? ›

To calculate an energy change for a reaction: add together the bond energies for all the bonds in the reactants- this is the 'energy in' add together the bond energies for all the bonds in the products - this is the 'energy out' energy change = energy in - energy out.

What causes an energy change? ›

1 Answer. The energy change that occurs as the result of a reaction depends on the difference between the energy of the reactants and the energy of the products. For reactions like combustion, the fuel (for example wood) has energy stored in the chemical bonds between the atoms that make it up.

What type of reaction is ch3oh o2 → co2 2h2o heat? ›

Type of Reaction for CH3OH + O2 = CO2 + H2O - YouTube

Which form of energy undergoes change in each situation what are the forms of energy produced? ›

Answer: Chemical energy is the potential of a chemical substance to undergo a chemical reaction to transform into other substances. Examples include batteries, food, gasoline, and etc.

What are the different types of energy needed for different kinds of chemical changes? ›

Different type of energy which is needed for different kind of chemical changes ​include Kinetic Energy and Potential Energy.

What type of chemical reaction absorbs energy and requires energy for the reaction to occur? ›

Chemical reactions that absorb (or use) energy overall are called endothermic. In endothermic reactions, more energy is absorbed when the bonds in the reactants are broken than is released when new bonds are formed in the products.

What is energy stored called? ›

Potential energy is stored energy and the energy of position. Chemical energy is energy stored in the bonds of atoms and molecules.

Which type of reaction requires energy input? ›

Reactions that require energy input to proceed are called endergonic reactions.

What is the formula for energy? ›

Overview of Work, Energy and Power
What is Work, Energy and Power?
Work
Energy
DefinitionEnergy is defined as the capacity to do work.
FormulaThe energy stored in an object due to its position and height is known as potential energy and is given by the formula: P.E. = mgh
8 more rows

How do you calculate free energy change? ›

In chemical reactions involving the changes in thermodynamic quantities, a variation on this equation is often encountered:
  1. ΔGchange in free energy=ΔHchange in enthalpy−TΔS(temperature) change in entropy.
  2. aA+bB→cC+dD.
  3. ΔrGo=cΔfGo(C)+dΔfGo(D)−aΔfGo(A)−bΔfGo(B)
  4. ΔfG0=∑vΔfG0(products)−∑vΔfG0(reactants)
  5. ΔGo=ΔHo−TΔSo.
18 Sept 2022

How do you calculate the change in a reaction? ›

Use the formula ∆H = m x s x ∆T to solve.

Once you have m, the mass of your reactants, s, the specific heat of your product, and ∆T, the temperature change from your reaction, you are prepared to find the enthalpy of reaction. Simply plug your values into the formula ∆H = m x s x ∆T and multiply to solve.

What is it called when energy changes from one form to another? ›

Energy conversion, also termed as energy transformation, is the process of changing one form of energy into another.

When heat energy is released from the system what physical change occurs at D? ›

A chemical reaction or physical change is exothermic if heat is released by the system into the surroundings. Because the surroundings are gaining heat from the system, the temperature of the surroundings increases. The sign of q for an exothermic process is negative because the system is losing heat.

When bonds are broken energy is? ›

Breaking and making bonds

Energy is absorbed to break bonds. Bond-breaking is an endothermic process. Energy is released when new bonds form. Bond-making is an exothermic process.

Is combustion endothermic or exothermic? ›

Combustion is a high-temperature exothermic (heat releasing) redox (oxygen adding) chemical reaction between a fuel and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke.

What is the correct way to balance this equation h2 o2 → h2o? ›

How to Balance H2 + O2 = H2O - YouTube

Which type of reaction occurs without being started by some outside force? ›

A spontaneous process occurs without the need for a continual input of energy from some external source, while a nonspontaneous process requires such.

What is an example of gravitational potential to kinetic to electrical current? ›

The water behind a hydroelectric dam stores gravitational potential energy since it is at a higher level than the water on the other side of the dam. As the water falls, this potential energy is converted into kinetic energy, which turns turbines to generate electricity.

What do you mean by transformation of energy and explain with one example of transformation of energy in nature? ›

An energy transformation is the change of energy from one form to another. Energy transformations occur everywhere every second of the day. There are many different forms of energy such as electrical, thermal, nuclear, mechanical, electromagnetic, sound, and chemical.

Which of the following objects when used shows that electrical energy is transformed to heat? ›

Correct answer:

An oven converts electrical energy into heat energy, a toaster concerts electrical into heat energy, and an alarm clock has an output of light and sound energy that is converted from electrical energy.

Can energy cause a change? ›

When something has energy, it can do work and bring about change. Energy produces light and movement. Energy is required to produce heat, to generate electricity, and to overcome forces such as friction.

Why does temperature increase when energy is released? ›

The process of dissolving is exothermic when more energy is released when water molecules “bond” to the solute than is used to pull the solute apart. Because more energy is released than is used, the molecules of the solution move faster, making the temperature increase. Project the image Endothermic Dissolving.

What kind of chemical reaction produces heat? ›

Exothermic reactions are chemical reactions that produce heat.

What is the source of the heat light sound and or electricity that can be produced during a chemical reaction? ›

Chemical reaction caused by heat, light, and electricity - YouTube

What is the role of energy in forming bonds? ›

The Formation of Bonds Releases Energy and the Cleavage of Bonds Requires Energy. Bonds form because favorable interactions between orbitals and the electrons in those orbitals allow the system to become more stable. As a result, the formation of a bond is accompanied by the release of energy, usually as heat.

What is a process that leads to the transformation of one set of chemical substance to another? ›

A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another.

What are energy changes? ›

The conversion of one form of energy into another, or the movement of energy from one place to another. energy transformation. noun. An energy transformation is the change of energy from one form to another.

What is the energy change in an exothermic reaction? ›

In exothermic reactions, more energy is released when the bonds are formed in the products than is used to break the bonds in the reactants. Exothermic reactions are accompanied by an increase in temperature of the reaction mixture. Chemical reactions that absorb (or use) energy overall are called endothermic.

What is heat change in chemical reaction? ›

The Heat of Reaction (also known and Enthalpy of Reaction) is the change in the enthalpy of a chemical reaction that occurs at a constant pressure. It is a thermodynamic unit of measurement useful for calculating the amount of energy per mole either released or produced in a reaction.

What happens to energy in exothermic and endothermic reactions? ›

An exothermic process releases heat, causing the temperature of the immediate surroundings to rise. An endothermic process absorbs heat and cools the surroundings.”

What are 10 examples of energy transfer? ›

Energy transfers
  • A swinging pirate ship ride at a theme park. Kinetic energy is transferred into gravitational potential energy.
  • A boat being accelerated by the force of the engine. The boat pushes through the water as chemical energy is transferred into kinetic energy.
  • Bringing water to the boil in an electric kettle.

How does heat flow from one object to another? ›

There are three different ways for heat to flow from one object to another. They are conduction, convection, and radiation.

What kind of energy can be transferred? ›

This is Expert Verified Answer. Energy can be transferred from one form to another like kinetic energy to potential energy, light energy to heat energy, kinetic energy to electrical energy, light energy to chemical energy .

In what type of reaction or process does heat flow into the system? ›

A reaction or process in which heat is transferred to a system from its surroundings is endothermic. The first law of thermodynamics states that the energy of the universe is constant. The change in the internal energy of a system is the sum of the heat transferred and the work done.

Why is bond making exothermic? ›

Bond forming is an exothermic process, because it releases energy. In reaction profile diagram, the energy change in a reaction, is the difference between the reactants and products. In an endothermic reaction, energy is taken in from the surroundings. The temperature of the surroundings therefore decreases.

What kind of process is bond making? ›

Bond-making is an exothermic process. Whether a reaction is endothermic or exothermic depends on the difference between the energy needed to break bonds and the energy released when new bonds form.

What are the types of heat of reaction? ›

Answer: Five types of the heat of reaction are given below:
  • The heat of Combustion or Enthalpy of combustion.
  • The heat of Formation or Enthalpy of Formation.
  • Bond Dissociation Energy.
  • The heat of Hydration or enthalpy of Hydration.
  • The heat of Solution or Enthalpy of the solution.

Is a Light a chemical? ›

Something comprised entirely of energy would not be matter. This, it would not be a chemical. Light, for example, has apparent mass, but it doesn't take up space. You can see and sometimes feel energy, so the senses sight and touch aren't reliable ways to distinguish better matter and energy or to identify a chemical.

What is the source of the heat light sound and or electricity that can be produced during a chemical reaction? ›

Chemical reaction caused by heat, light, and electricity - YouTube

What units are used to measure heat flow? ›

Heat flow is measured in one of two common units: the calorie and the joule. The joule (J) is the SI unit of energy.

Why are enthalpies involved in forming bonds are always negative while enthalpies involved in breaking bonds are always positive? ›

Bond enthalpies are always positive because it takes energy to break bonds.

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