Suggested languages for you:

Americas

Europe

|
|

# Energy Changes

Lerne mit deinen Freunden und bleibe auf dem richtigen Kurs mit deinen persönlichen Lernstatistiken

Nie wieder prokastinieren mit unseren Lernerinnerungen.

Let’s say we are cooking an egg, as well as seeing a change in colour we can feel some heat coming from the pan, so where does this heat come from? One law of energy is that it cannot be created or destroyed, but it can change to different types of energy. In this case, we see it in the form of heat and it is known as an exothermic reaction.

• We will be exploring two types of energy change: exothermic and endothermic.
• Then we will explore what formulas we can use to determine the different energy changes.
• Finally, we will explore what a battery is.

## Types of energy change

There are so many different chemical reactions that can take place in so many different places. Even in our own bodies, we have a variety of reactions happening all at the same time. All of these reactions are categorised as either exothermic or endothermic. So let us explore what they are and some examples that we see in our day-to-day life.

### Exothermic

Exothermic reactions are those in which energy is transferred from the reaction to the surrounding. This means energy is released and a lot of the time it can take place in the form of heat and so the temperature increases. As energy is released it generally means the products of said reactants will have less energy than the reactants at the start.

Wood Fire, Wikimedia Commons

When we burn methane gas, which is a combustion reaction. Energy is released as heat and why we can feel the heat when something is burning.

### Endothermic

Endothermic reactions are completely opposite to exothermic reactions. Energy is transferred from the surroundings to the reaction. This means energy is absorbed and so the temperature decreases. In endothermic reactions, the products will have more energy than the reactants.

A key example is a thermal decomposition, where we take some calcium carbonate and heat it, to form carbon dioxide and calcium oxide. Energy is absorbed and heat is required for the reaction to take place.

### Reaction profiles

So now we have explored that there are two different types of energy changes we can use reaction profiles to present both of these and explore the change in energy is the reactants and products.

The first diagram shows the reaction profile of an exothermic reaction. The reactants A + B are higher than the product C. This shows that energy has been released into the surroundings and as discussed before it is usually in the form of heat energy.

Fig. 2: Endothermic Reaction, Wikimedia Commons

The second diagram shows the reaction profiles of an endothermic reaction. The reactants A + B is lower than the product C. This shows that energy has been absorbed and the temperature decreases.

This is the basis of energy reaction profiles. We will be exploring these diagrams further in a later article.

## Change in energy formula

We need to be able to calculate the overall energy change. To do this we need to know the different bond energies.

Bond energy: The energy needs to break the bond between two atoms. It is measured in kJ/mol.

To calculate the change in energy we need 2 things:

1. The energy required to break bonds from the reactants.
2. The energy released when bonds are formed to produce products.

Together we can form this equation to calculate the overall energy change:

$$Energy\ change=total\ bond\ energy\ of\ the\ reactant\ -\ total\ bond\ energy\ of\ the\ products$$

### Calculating energy change

We now know the equation to calculate energy, so let’s go through an example

 Bond Bond Energy (kJ/mol) N≡N 940 N-H 400 H-H 440

#### Energy changes examples:

We will be using a table of bond energies to calculate our answer. These are made up values to go through an example.

For this example, lets us use the Haber Process, where nitrogen and hydrogen react to form ammonia. This reaction can go forwards and backwards, we will be calculating the forwards reaction.

Chemical equation:

$$N_{2(g)} + 3H_{2(g)}\rightleftharpoons 2NH_{3(g)}$$

1. First we need calculate the bond energy between the reactants, so for this reaction it is the one mole of nitrogen to nitrogen three moles of hydrogen to hydrogen single bonds.

1 x NN (940) = 820

3 x H-H (440) = 1600

So the total bond breaking energy is: 940 + 1320 = 2260 kJ/mol

2. Now we need to calculate the bond energy between the products. For this reaction it is two moles of nitrogen to hydrogen single bonds. For this bond, each nitrogen is bond to three hydrogens so it is 6 bonds.

6 · N-H (400) = 2400

3. Finally, we will calculate the energy change.

Energy change = 2260 - 2400 = -140 kJ/mol

Overall energy change = -140 kJ/mol

## Cells and batteries

All devices have batteries, we usually see these are small metal blocks that can be removable or permanent. But how do they provide energy to our devices? This is done using reactive metals that produce energy for them to work. We can use the order of reactivity of metals, we can use two to form an electrical cell. These cells allow the flow of electrons to form an electric current which powers the device.

Fig. 3: Electrical Cell, Wikimedia Commons

If we look at this diagram we can see that zinc (Zn) donates electrons to copper (Cu). The electrons travel from one end to another which forms an electric current and this reaction takes place within our devices to power them.

The energy current can be recorded using a voltmeter, which detects the difference in reactivity between the two metals and forms a voltage.

The bigger the difference, the higher the voltage.

Another type of cell is a fuel cell. This is done through oxidising hydrogen, whereby hydrogen and oxygen react to produce water and is presented in the following equation:

$$2H_{2} + O_{2}\rightarrow 2H_{2}O$$

Two moles of hydrogen react with one mole of oxygen to form two moles of water.

Some cars run using this type of fuel cell, however, a constant supply of hydrogen is required and is therefore not commonly used.

## Energy Changes - Key takeaways

• All chemical reactions have an energy change.
• There are two types of energy changes: exothermic and endothermic.
• In exothermic reactions, energy is released and the products have less energy than the reactants.
• In endothermic reactions, energy is absorbed and the products have more energy than the reactant.
• We can explore the different of energy in exothermic and endothermic reactions using reaction profiles.
• Energy change can be calculated by: total bond energy of the reactant – total bond energy of the products
• Reactive metals are used to produce batter.
• Two metals react so electrons can be transferred which produces an electric current.

Energy changed occur by either the release of absorption of energy.

Energy change = total bond energy of the reactant – total bond energy of the products

Energy can be transformed to another type of energy. For example kinetic energy to heat energy

Yes, energy change happens whenever there is a chemical change

There are two; endothermic and exothermic

## Energy Changes Quiz - Teste dein Wissen

Question

What is activation energy?

The minimum energy required for a reaction to start

Show question

Question

What does endothermic mean?

A reaction where energy is transferred from the reaction to the surrounding

Show question

Question

What does exothermic mean?

The minimum energy required for a reaction to start

Show question

Question

In an exothermic reaction profile, does the products have a higher or lower amount of energy compared to the reactants?

Lower

Show question

Question

In an endothermic reaction profile, does the products have a higher or lower amount of energy compared to the reactants?

Lower

Show question

Question

Where can we see activation energy on a reaction profile?

The peak of the energy curve

Show question

Question

Is bond breaking exothermic or endothermic?

Endothermic

Show question

Question

Is bond forming exothermic or endothermic?

Exothermic

Show question

Question

Does bond breaking absorb or release energy?

Absorb

Show question

Question

Does bond forming absorb or release energy?

Release

Show question

Question

What does oxidised mean?

A reaction where the reactants lose electrons

Show question

Question

What does reduced mean?

A reaction where the reactants gain electrons

Show question

Question

What does redox mean?

A reaction where oxidation and reduction both take place

Show question

Question

What is a cathode?

Electrode where reduction takes place

Show question

Question

What is an anode?

Electrode where oxidation takes place

Show question

Question

What is a primary cell?

Cell that cannot be recharged

Show question

Question

What type of elements are used in cells?

Reactive elements

Show question

Question

What can apparatus can be used to measure an electrical current?

Voltmeter

Show question

Question

How do re-chargeable cells work?

By plugging it into a power source, like a charger the reaction of the cell is reversed, so now the reactants can go back to their original form and can start reacting again. Forming a flow of electrons, and therefore, an electrical current.

Show question

Question

What is the voltage?

The difference in reactivity between the two elements in a cell

Show question

Question

What is an exothermic reaction?

A reaction where energy is transferred from the reaction to the surrounding

Show question

Question

What is an endothermic reaction?

A reaction where energy is absorbed from the surroundings to the reaction

Show question

Question

What is bond energy?

The energy needs to break the bond between two atoms

Show question

Question

What is the units of bond energy?

kJ/mol

Show question

Question

Is making bonds endothermic or exothermic?

Endothermic

Show question

Question

Is forming bonds endothermic or exothermic?

Exothermic

Show question

Question

Does making bonds absorb or release energy?

Absorb energy

Show question

Question

Does forming bonds absorb or release energy?

Release energy

Show question

Question

How do we calculate energy change?

Energy change = total bond energy of the reactant – total bond energy of the products

Show question

Question

What are two things that are required to calculate change in energy?

The energy required to break bonds from the reactants

Show question

Question

Do fuel cells need a constant supply of fuel?

Yes

Show question

Question

Do batteries need a constant supply of fuel?

No

Show question

Question

Which statement is an advantage for hydrogen fuel cells?

They do not produce carbon emissions

Show question

Question

Which statement is an advantage for hydrogen fuel cells?

They can be built in various sizes

Show question

Question

Which statement is an advantage for hydrogen fuel cells?

They do no need to be electrically recharged

Show question

Question

Which statement is a disadvantage for hydrogen fuel cells?

Hydrogen is hard to store

Show question

Question

Which statement is a disadvantage for hydrogen fuel cells?

Hydrogen is highly flammable

Show question

Question

Which statement is an advantage for hydrogen fuel cells?

May use non-renewable sources to get hydrogen supply

Show question

Question

In a hydrogen fuel cells, does oxygen react with hydroxide ions or mobile electrons?

Mobile electrons

Show question

Question

In a hydrogen fuel cells, does hydrogen react with hydroxide ions or mobile electrons?

Hydroxide ions

Show question

Question

Does a hydrogen fuel cell produce any waste?

Yes

Show question

Question

Which of these compounds is a waste product of hydrogen cells?

H2O

Show question

Question

Is there an energy change during all chemical reactions?

Yes

Show question

Question

What are the two types of chemical reaction in terms of energy changes?

Exothermic and Endothermic

Show question

Question

Which one of the following is an exothermic reaction?

Gives off energy to surroundings

Show question

Question

Which one of the following is an endothermic reaction?

Takes in energy

Show question

Question

Which type of reaction is described by the following equation: reactants => products + energy

exothermic

Show question

Question

Give the general reaction equation for an endothermic reaction.

reactants + energy => products

Show question

Question

Under what conditions do the products have more energy than the reactants?

Endothermic

Show question

Question

In what reaction type do the products have less energy than the products

Exothermic

Show question

60%

of the users don't pass the Energy Changes quiz! Will you pass the quiz?

Start Quiz

## Study Plan

Be perfectly prepared on time with an individual plan.

## Quizzes

Test your knowledge with gamified quizzes.

## Flashcards

Create and find flashcards in record time.

## Notes

Create beautiful notes faster than ever before.

## Study Sets

Have all your study materials in one place.

## Documents

Upload unlimited documents and save them online.

## Study Analytics

Identify your study strength and weaknesses.

## Weekly Goals

Set individual study goals and earn points reaching them.

## Smart Reminders

Stop procrastinating with our study reminders.

## Rewards

Earn points, unlock badges and level up while studying.

## Magic Marker

Create flashcards in notes completely automatically.

## Smart Formatting

Create the most beautiful study materials using our templates.