Energy Changes

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Energy Changes

Exothermic and endothermic reactions

Chemical reactions can take in or give out energy when bonds are broken and formed.

Exothermic reactions

• Energy is released to the surroundings shown by an increase in temperature. e.g. combustion, neutralisation, respiration.
• Bond making is an exothermic process.
• The energy change (∆H) in an exothermic reaction is negative (as energy is lost to the surroundings).

Endothermic reactions

• Energy is taken in from the surroundings shown by a decrease in e.g. thermal decomposition of calcium carbonate, photosynthesis.
• Bond breaking is an endothermic process.
• The energy change (∆H) in an endothermic reaction is positive (as energy is gained from the surroundings).

ΔH is also known as the enthalpy change.

 

Activation energy

The minimum amount of energy needed to start a reaction is called the activation energy.

 

Colliding particles

• When chemicals react their particles have energy and are constantly moving.
• In order for a reaction to take place the particles must collide.
• Not all particles will result in a successful collision.
• The reactant particles must collide with this minimum amount of energy in order for a reaction to take place.

Image: Pixabay

 

Catalysts

A catalyst is a substance that allows an alternative pathway for the reaction to take place without being used up in the reaction. Reactions will take place faster when a catalyst is involved.

Measuring temperature changes

The temperature change can be measured in a reacting solution such as acid plus metal, acid plus carbonates, neutralisation, displacement of metals using a polystyrene cup, lid and thermometer.

Method

1. Place the first reactant (e.g. acid) into the polystyrene cup.
2. Stand the cup inside a beaker. This will make it more stable.
3. Use the thermometer to measure the temperature of the reactant. Record the result in a table.
4. Measure out the second reactant using a measuring cylinder.
5. Pour the second reactant into the polystyrene cup.
6. Place the lid on the cup and gently stir the solution with the thermometer through the hole in the lid.
7. When the reading on the thermometer stops changing, record the highest temperature reached.
8. Repeat steps 4–7 to add further amounts of the second reactant to the cup, recording the temperature reading each time.
9. Repeat until a maximum of reactant two has been added.
10. Repeat the experiment. Calculate the average temperature change.

 

Reaction Profiles

An energy level diagram can be used to show how a reaction progresses. This type of diagram is called a reaction profile.

Endothermic profile

Image: BBC Bitesize

Exothermic profile

Image: BBC Bitesize

 

Reaction profiles need to show:

• The relative energy of the reactants compared to the products.
• The activation energy (the difference between the reactants line and the peak).
• The overall energy change (the difference between the reactants and product line).

Bond making and bond breaking

• Bond breaking is
• Bonds making is
• The difference between the energy required to break the reactant bonds and the energy released when new bonds in the products are formed determines the type of reaction.

 

• Exothermic reaction: MORE heat energy is released in bond making than is taken in during bond breaking (positive energy change).
• Endothermic reaction: LESS heat energy is released in bond making than is taken in during bond breaking (Negative energy change).

 

Bond energy: The amount of energy needed to break one mole of a particular covalent bond.

 To calculate the energy change for a reaction:

1. Add together the bond energies for all the bonds broken – ‘energy in’.
2. Add together the bond energies for all the bonds made – ‘energy out’.
3. Overall energy change = energy in – energy out.
4. Overall energy change = bonds broken – bonds made.

 

Bonds broken:

(2 x H-H) + ( 1x O=O)

(2 x 436) + 498 = 1370 KJ/mol

Bonds made:

4 x (O-H)

4x 464= 1856 KJ/mol

Overall energy change = energy in – energy out.

= bonds broken – bonds made

= 1370-1856

= -486 KJ/mol

The reaction is exothermic as the answer has a negative value.

 

Chemical Cells

 A chemical cell is made up of two DIFFERENT metals in contact with an electrolyte.  The voltage produced by the cell depends on the type of metals and electrolyte.

 

The cell has two metals of different reactivity. If the two metals are put into an electrolyte-the movement of ions due to the difference in reactivity (displacement) will cause a flow of current. This also produces a voltage.

The bigger the difference in the reactivity of the two metals used, the greater the voltage produced.

There are two types of cells:

Non-rechargeable

The chemical reactions stop when one of the reactants has been used up.  Alkaline batteries are non-rechargeable.

Rechargeable

The chemical reactions are reversed using an external electric current. Mobile phones use rechargeable batteries.

Fuel Cells

Fuel cells use the reaction between hydrogen and oxygen to produce an electric current.

• The hydrogen atom loses an electron (oxidation)
• They then pass through the electrolyte.
• The oxygen atoms gain two electrons each (reduction).

At the cathode the oxygen then reacts with water:

O₂ + 4e^– + 2H₂O  —–>   4OH^–

At the anode, hydrogen combines with the hydroxide ions to make water and electrons: 

 2H₂ + 4OH^–   —–>    4H₂O + 4e^–

Overall reaction:  hydrogen + oxygen  water.

2H₂ + O₂  —–>   2H₂O

As the hydrogen is oxidised electrochemically and not combusted, the reaction can take place at a much lower temperature.

 

Questions:

 

1. The reaction of hydrogen with oxygen is exothermic. Draw an energy level diagram for this reaction.
2. What is the activation energy?
3. Why can a rechargeable cell be recharged?
4. Why would a hydrogen fuel cell considered to be good for the environment?

 

Answers:

1. Products lower than reactants. Reaction curve correctly drawn with activation energy labelled.
2. The minimum amount of energy needed to start a reaction is called the activation energy.
3. The reaction is reversible.
4. The product of the reaction is water which is non-polluting.