Energy Changes

What is the conservation of energy principle?

Energy is conserved in chemical reactions → energy cannot be created or destroyed → the total amount of energy in the universe remains the same before and after a reaction.

What is an exothermic reaction?

A reaction that transfers energy to the surroundings → the temperature of the surroundings increases.

What are examples of exothermic reactions?

Combustion, many oxidation reactions and neutralisation reactions.

What happens to the temperature of the surroundings during an exothermic reaction?

The temperature of the surroundings increases because energy is transferred from the reaction to the surroundings.

What is an endothermic reaction?

A reaction that takes in energy from the surroundings → the temperature of the surroundings decreases.

What are examples of endothermic reactions?

Thermal decomposition and the reaction between citric acid and sodium hydrogencarbonate.

What happens to the temperature of the surroundings during an endothermic reaction?

The temperature of the surroundings decreases because energy is transferred from the surroundings into the reaction.

What is the sign of the energy change for an exothermic reaction?

Negative.

What is the sign of the energy change for an endothermic reaction?

Positive.

How can you distinguish between an exothermic and endothermic reaction experimentally?

Measure the temperature change of the surroundings → an increase in temperature indicates an exothermic reaction → a decrease indicates an endothermic reaction.

What happens to the energy of the products compared with the reactants in an exothermic reaction?

The products have less energy than the reactants → the difference in energy is transferred to the surroundings.

What happens to the energy of the products compared with the reactants in an endothermic reaction?

The products have more energy than the reactants → energy is taken in from the surroundings.

What are some everyday uses of exothermic reactions?

Self-heating cans and hand warmers.

Why can exothermic reactions be used in hand warmers?

The reaction transfers energy to the surroundings → thermal energy is released → the hand warmer becomes warm.

Why can endothermic reactions be used in sports injury packs?

The reaction takes in thermal energy from the surroundings → the surroundings cool down → this provides a cooling effect.

What is activation energy?

The minimum amount of energy that reacting particles need to react.

Why is activation energy needed?

Reacting particles must collide with sufficient energy → if they do not have enough energy, the collision does not result in a reaction.

What is a reaction profile?

A graph showing the relative energies of the reactants and products, the activation energy and the overall energy change of a reaction.

What does the y-axis of a reaction profile represent?

Energy.

What does the x-axis of a reaction profile represent?

The progress of the reaction.

How is activation energy shown on a reaction profile?

It is the energy difference between the reactants and the highest point of the curve.

How is the overall energy change shown on a reaction profile?

It is the energy difference between the reactants and products.

How does a reaction profile show an exothermic reaction?

The products are at a lower energy level than the reactants → energy has been transferred to the surroundings.

How does a reaction profile show an endothermic reaction?

The products are at a higher energy level than the reactants → energy has been taken in from the surroundings.

Why is the peak of a reaction profile higher than the reactants?

Reacting particles need to gain the activation energy before the reaction can occur.

What does the height of the curve above the reactants represent?

The activation energy.

What happens to activation energy during a reaction?

Energy must be supplied to overcome the activation energy barrier before particles can react.

What happens to the energy of particles as they move towards the peak of a reaction profile?

Their energy increases until the activation energy is reached.

What happens after the activation energy has been reached?

The reaction can proceed and the energy changes as products are formed.

What happens to the energy of particles when new bonds are formed?

Energy is released.

What happens to the energy of particles when bonds are broken?

Energy is supplied.

HIGHER TIER: Why does breaking a chemical bond require energy?

Energy must be supplied to overcome the forces holding the atoms together → energy is absorbed when the bond is broken.

HIGHER TIER: Why does forming a chemical bond release energy?

Atoms move into a more stable bonded state → energy is released when the new bond forms.

HIGHER TIER: What happens to bonds during a chemical reaction?

Bonds in the reactants are broken → new bonds are formed in the products.

HIGHER TIER: What is the energy change in terms of bonds broken and bonds formed?

Energy is supplied to break bonds in the reactants → energy is released when bonds in the products are formed.

HIGHER TIER: What happens during an exothermic reaction in terms of bond energies?

The energy released when new bonds form is greater than the energy needed to break the existing bonds → there is a net transfer of energy to the surroundings.

HIGHER TIER: What happens during an endothermic reaction in terms of bond energies?

The energy needed to break existing bonds is greater than the energy released when new bonds form → there is a net transfer of energy from the surroundings to the reaction.

HIGHER TIER: What is the equation for calculating the energy change of a reaction using bond energies?

Energy change = sum of energy needed to break bonds − sum of energy released when bonds are formed.

HIGHER TIER: How can bond energies be used to determine whether a reaction is exothermic or endothermic?

Calculate the energy needed to break the bonds → calculate the energy released when the new bonds form → compare the two values → greater energy released means exothermic → greater energy needed means endothermic.

HIGHER TIER: What does a negative energy change indicate?

More energy is released when bonds form than is needed to break bonds → the reaction is exothermic.

HIGHER TIER: What does a positive energy change indicate?

More energy is needed to break bonds than is released when bonds form → the reaction is endothermic.

What is a cell?

A device containing chemicals that react to produce electricity.

What is a simple cell made from?

Two different metals connected through an electrolyte.

What is an electrolyte?

A substance containing ions that allows electrical conduction when molten or dissolved.

What happens in a cell?

A chemical reaction occurs → chemical energy is transferred into electrical energy → a potential difference is produced.

What determines the voltage produced by a cell?

The types of electrodes and the identity and concentration of the electrolyte.

Why can different metals produce different voltages in a cell?

Different metals have different tendencies to lose electrons → this produces different potential differences between the electrodes.

What is a battery?

Two or more cells connected together in series.

Why are cells connected in series in a battery?

The voltages of the cells are combined → a greater overall voltage is produced.

What is a non-rechargeable cell?

A cell in which the reactants are eventually used up and the chemical reactions cannot be reversed by supplying an external electrical current.

What happens when a non-rechargeable cell stops working?

One or more reactants have been used up → the chemical reaction stops → electricity is no longer produced.

Give an example of a non-rechargeable battery.

An alkaline battery.

What is a rechargeable cell?

A cell in which the chemical reactions can be reversed by supplying an external electrical current.

How can a rechargeable cell be reused?

An external electrical current is supplied → the chemical reactions are reversed → the reactants are regenerated → the cell can produce electricity again.

What is one advantage of cells and batteries?

They are a convenient source of electrical energy.

What is another advantage of cells and batteries?

Some cells and batteries are rechargeable.

What is a disadvantage of cells and batteries?

They can contain harmful chemicals.

What is a fuel cell?

A cell supplied by an external source of fuel and oxygen or air → the fuel is oxidised electrochemically → electricity is produced.

What fuel is commonly used in a hydrogen fuel cell?

Hydrogen.

What does a hydrogen fuel cell require?

Hydrogen and oxygen or air.

What happens to hydrogen in a hydrogen fuel cell?

Hydrogen is oxidised electrochemically → electrons are transferred → electrical energy is produced.

What is the overall reaction in a hydrogen fuel cell?

2H₂ + O₂ → 2H₂O.

What is the product of the overall reaction in a hydrogen fuel cell?

Water.

HIGHER TIER: What is the half-equation at the hydrogen electrode in a hydrogen fuel cell?

2H₂ → 4H⁺ + 4e⁻.

HIGHER TIER: What is the half-equation at the oxygen electrode in a hydrogen fuel cell?

O₂ + 4H⁺ + 4e⁻ → 2H₂O.

HIGHER TIER: How do the half-equations combine to give the overall hydrogen fuel cell reaction?

Hydrogen is oxidised and loses electrons → oxygen is reduced and gains electrons → the electrons cancel → 2H₂ + O₂ → 2H₂O.

What is one advantage of hydrogen fuel cells?

They produce no pollutants at the point of use because the overall product is water.

What is another advantage of hydrogen fuel cells compared with rechargeable cells?

They do not need to be recharged in the same way because they are supplied continuously with fuel.

What is one disadvantage of hydrogen fuel cells?

Hydrogen is highly flammable.

Why is hydrogen difficult to store?

Hydrogen is a small, low-density gas → it requires specialised storage systems and high pressures or low temperatures.

Why can hydrogen fuel cells still contribute to pollution indirectly?

Hydrogen may be produced using fossil fuels → this can result in carbon dioxide emissions.

Why can producing hydrogen by electrolysis be expensive?

Electrolysis requires electrical energy → producing and storing hydrogen can be costly.

Why does the environmental impact of a hydrogen fuel cell depend on how the hydrogen is produced?

The fuel cell itself produces water → however, producing hydrogen using fossil fuels can release carbon dioxide → hydrogen produced using low-carbon electricity can reduce this impact.

How can hydrogen fuel cells be compared with rechargeable batteries?

Fuel cells can be supplied continuously with hydrogen and produce water at the point of use → rechargeable batteries can be recharged using an external electrical current → both can provide electrical energy but have different storage, production and environmental considerations.

REQUIRED PRACTICAL 4: What is investigated in the Energy Changes required practical?

The variables that affect temperature changes in reacting solutions.

REQUIRED PRACTICAL 4: What types of reactions can be investigated in the Energy Changes required practical?

Acid + metal, acid + carbonate, neutralisation and displacement reactions.

REQUIRED PRACTICAL 4: What is measured in the Energy Changes practical?

The temperature change of the reacting solution.

REQUIRED PRACTICAL 4: What is the independent variable?

The variable deliberately changed in the investigation, such as the reactants or their concentrations/volumes depending on the investigation.

REQUIRED PRACTICAL 4: What is the dependent variable?

The temperature change of the reacting solution.

REQUIRED PRACTICAL 4: Why should the initial temperature be measured?

It provides a starting temperature → the temperature change can then be determined accurately.

REQUIRED PRACTICAL 4: Why should the maximum or minimum temperature be recorded?

It gives the largest temperature change produced by the reaction.

REQUIRED PRACTICAL 4: Why can heat loss affect the results?

Energy can be transferred from the reacting solution to the surroundings → the measured temperature change is smaller than the true temperature change.

REQUIRED PRACTICAL 4: How can heat loss be reduced?

Use an insulated container such as a polystyrene cup and a lid → less energy is transferred to the surroundings → the measured temperature change is more representative of the actual change.

REQUIRED PRACTICAL 4: Why should the same apparatus and conditions be used when comparing reactions?

It controls other variables → differences in temperature change are more likely to be caused by the independent variable.

REQUIRED PRACTICAL 4: Why should measurements be repeated?

Repeats allow anomalous results to be identified → a mean can be calculated from reliable results → reliability is improved.

REQUIRED PRACTICAL 4: What should be done with an anomalous result?

Check the result and experimental procedure → repeat the measurement if possible → do not include a genuine anomaly when calculating the mean.

Why does an exothermic reaction cause a temperature increase?

Energy is transferred from the reaction to the surroundings → the surroundings gain thermal energy → their temperature increases.

Why does an endothermic reaction cause a temperature decrease?

Energy is transferred from the surroundings into the reaction → the surroundings lose thermal energy → their temperature decreases.

Why are exothermic reactions associated with a negative energy change?

The products have less energy than the reactants → energy has been transferred to the surroundings → the energy change is negative.

Why are endothermic reactions associated with a positive energy change?

The products have more energy than the reactants → energy has been taken in from the surroundings → the energy change is positive.

Why must particles collide with sufficient energy for a reaction to occur?

Particles need enough energy to overcome the activation energy barrier → collisions below the activation energy do not result in reaction.

Why does a reaction profile have a curved peak?

The reacting particles must gain activation energy before the reaction can occur → the energy rises to a maximum → then falls as products form.

Why are the products lower in energy than the reactants in an exothermic reaction?

More energy is released when new bonds form than was required to break the original bonds → the products have a lower overall energy.

Why are the products higher in energy than the reactants in an endothermic reaction?

More energy is required to break the original bonds than is released when new bonds form → the products have a higher overall energy.

Why can cells produce electricity from chemical reactions?

Chemical reactions involve electron transfer → electrons can flow through an external circuit → electrical energy is produced.

Why does a rechargeable cell need an external electrical current?

The external current supplies energy → this reverses the chemical reactions → the original reactants are regenerated.

Why does a battery produce a greater voltage than a single cell?

A battery contains two or more cells connected in series → the cell voltages combine → a greater total voltage is produced.

Why does changing the electrode affect the voltage of a cell?

Different metals have different reactivities and tendencies to lose electrons → changing the electrode changes the potential difference between the electrodes.

Why does changing the electrolyte affect the voltage of a cell?

The identity and concentration of ions in the electrolyte affect the chemical reactions at the electrodes → this changes the potential difference produced.

Why can a hydrogen fuel cell produce electricity continuously?

Hydrogen and oxygen are continuously supplied → electrochemical reactions continue → electrons flow through the external circuit → electricity is produced.