Chemistry 5. Energy Changes

What is the fundamental law regarding energy in chemical reactions?

Energy is conserved; it can neither be created nor destroyed.

Define an exothermic reaction in terms of energy transfer.

A reaction that transfers energy to the surroundings, usually as heat.

How does the temperature of the surroundings change during an exothermic reaction?

The temperature of the surroundings increases.

Define an endothermic reaction in terms of energy transfer.

A reaction that takes in energy from the surroundings.

How does the temperature of the surroundings change during an endothermic reaction?

The temperature of the surroundings decreases.

Name three examples of exothermic reactions.

Combustion, many oxidation reactions, and neutralisation.

Name two examples of endothermic reactions.

Thermal decomposition and the reaction of citric acid with sodium hydrogencarbonate.

What is an everyday use of an exothermic reaction?

Self-heating cans or hand warmers.

What is an everyday use of an endothermic reaction?

Sports injury cold packs.

In an exothermic reaction, how does the energy of the products compare to the reactants?

The products have less energy than the reactants.

In an endothermic reaction, how does the energy of the products compare to the reactants?

The products have more energy than the reactants.

What is the definition of activation energy ($E_{a}$)?

The minimum amount of energy that particles must have to react.

What do reaction profiles show?

The relative energies of reactants and products, and the energy change during a reaction.

On a reaction profile, where does the arrow for activation energy start and end?

It starts at the energy level of the reactants and goes to the peak of the curve.

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

The vertical difference between the energy levels of the reactants and the products.

Process: Bond Breaking

Energy is supplied to break bonds, making it an endothermic process.

Process: Bond Making

Energy is released when bonds are formed, making it an exothermic process.

How is the overall energy change of a reaction calculated using bond energies (Higher Tier)?

Energy Change = Total energy needed to break bonds $-$ Total energy released by making bonds.

In terms of bond energies, what defines an exothermic reaction (Higher Tier)?

The energy released from forming new bonds is greater than the energy needed to break existing bonds.

In terms of bond energies, what defines an endothermic reaction (Higher Tier)?

The energy needed to break existing bonds is greater than the energy released from forming new bonds.

If the calculated overall energy change is negative, what type of reaction is it?

Exothermic.

If the calculated overall energy change is positive, what type of reaction is it?

Endothermic.

What are the basic components needed to make a simple chemical cell?

Two different metals connected in an electrolyte.

What factors determine the voltage produced by a chemical cell?

The type of electrodes and the electrolyte used.

How does the reactivity of metals affect the voltage in a simple cell?

The bigger the difference in reactivity between the two metals, the higher the voltage.

What is a battery in the context of chemical cells?

Two or more cells connected in series to provide a greater voltage.

Why do non-rechargeable batteries eventually stop working?

The chemical reaction stops when one of the reactants is used up.

Give an example of a non-rechargeable battery.

Alkaline batteries.

How are rechargeable cells and batteries recharged?

The chemical reactions are reversed by an external electrical current.

What are fuel cells supplied by?

An external source of fuel (e.g., hydrogen) and oxygen or air.

How do fuel cells produce a potential difference?

The fuel is oxidised electrochemically within the cell.

What is the overall reaction in a hydrogen fuel cell?

The oxidation of hydrogen to produce water ($2H{2} + O{2} \rightarrow 2H_{2}O$).

What is the only waste product of a hydrogen fuel cell?

Water.

Write the half equation for the reaction at the negative electrode in a hydrogen fuel cell (Higher Tier).

$2H_{2} \rightarrow 4H^{+} + 4e^{-}$

Write the half equation for the reaction at the positive electrode in a hydrogen fuel cell (Higher Tier).

$O{2} + 4H^{+} + 4e^{-} \rightarrow 2H{2}O$

Name one advantage of hydrogen fuel cells over lithium-ion batteries.

No toxic chemicals to dispose of (or higher range/water is the only byproduct).

Name one disadvantage of hydrogen fuel cells.

Hydrogen is flammable and difficult to store or transport.

What is the goal of Required Practical 4 (Temperature Changes)?

To investigate the variables that affect temperature changes in reacting solutions.

In Required Practical 4, why is a polystyrene cup used instead of a glass beaker?

Polystyrene is a better insulator, which reduces heat loss to the surroundings.

In Required Practical 4, what is the purpose of placing the polystyrene cup inside a glass beaker?

For stability and further insulation.

What improvement can be made to the apparatus in Required Practical 4 to reduce heat loss?

Use a lid on the polystyrene cup.

In the temperature change practical between $HCl$ and $NaOH$, what is the independent variable?

The volume of the reactant being added (e.g., sodium hydroxide).

In the temperature change practical between $HCl$ and $NaOH$, what is the dependent variable?

The maximum temperature reached.

What should be controlled in the temperature change practical to ensure a fair test?

The starting temperatures of the solutions and the concentrations of the reactants.

How can you determine the volume needed for neutralisation from a temperature change graph?

It is the point where the two lines of best fit intersect.

In Required Practical 4, why does the temperature start to fall after a certain volume of alkali is added?

The acid has all reacted (it's the limiting reactant), so adding more cool liquid just lowers the overall temperature.

What is the 'Error Carried Forward' (ecf) rule in GCSE Chemistry exams?

If a student makes a mistake in one step, they can still get marks for subsequent steps if they use their incorrect value correctly.

Why is it important to stir the solution in Required Practical 4?

To ensure the reactants are well mixed and the temperature is uniform throughout.

How should bond energy calculations be set up to avoid errors (Exam Tip)?

Draw out the molecules, list all bonds broken vs. bonds made in columns, then subtract 'made' from 'broken'.

What type of energy change is the thermal decomposition of metal carbonates?

Endothermic.

What does a 'curved line' represent on a reaction profile?

The energy of the system as the reaction proceeds.

How does a catalyst affect a reaction profile?

It provides an alternative pathway with a lower activation energy, shown by a lower peak.

In a simple cell, which electrode will be the negative electrode?

The more reactive metal.

Formula: Overall Energy Change (HT)

$\Delta H = \Sigma(\text{bond breaking energy}) - \Sigma(\text{bond making energy})$

The minimum energy needed for a reaction to occur is called _.

Activation Energy

When magnesium reacts with copper sulfate, the temperature rises. What does this indicate?

The reaction is exothermic.

Identify the type of reaction: $H^{+}(aq) + OH^{-}(aq) \rightarrow H_{2}O(l)$ with a temperature increase.

Exothermic (Neutralisation).

Which greenhouse gas is often produced when fossil fuels are burned to provide energy for endothermic processes?

Carbon dioxide ($CO_{2}$).

In Required Practical 4, if the volume of acid used is $30\text{ cm}^{3}$, what equipment should be used to measure it accurately?

A $50\text{ cm}^{3}$ measuring cylinder.

Why is it important to record the initial temperature in Required Practical 4?

To calculate the temperature change ($ΔT$) from the maximum temperature reached.

Explain the significance of a negative overall energy change value in a bond energy calculation.

It means more energy was released making bonds than was required to break them, so the reaction is exothermic.

What is the purpose of 'cryolite' in the electrolysis of aluminium oxide?

It lowers the melting point of the electrolyte, reducing the energy needed.

A student adds $5\text{ cm}^{3}$ of $NaOH$ to $HCl$ eight times. How many total $cm^{3}$ of $NaOH$ have been added at the end?

$40\text{ cm}^{3}$.

In a reaction profile for an endothermic reaction, which level is higher: reactants or products?

Products.

How does the size of the molecules in poly(ethene) affect its intermolecular forces compared to methane?

Poly(ethene) has much larger molecules, so it has much stronger intermolecular forces.

Why is methane a gas but poly(ethene) a solid at room temperature?

Methane has weak intermolecular forces that require little energy to overcome, whereas poly(ethene) has stronger forces requiring more energy.

In a hydrogen fuel cell, what happens to the hydrogen at the negative electrode?

It is oxidised (loses electrons).

What does a flat line at the start of a temperature investigation graph represent?

The initial temperature of the reactants before they are mixed.

What is the unit for energy change typically used in bond energy tables?

$kJ/mol$.

True or False: All oxidation reactions are endothermic.

False (many oxidation reactions are exothermic, like combustion).

Describe the energy transfer in a sports injury cold pack.

Energy is transferred from the surroundings (the injured area) to the chemicals in the pack.

How can the reliability of Required Practical 4 be checked?

By repeating the experiment and comparing the results to see if they are consistent.

What determines the direction of the energy change arrow ($ΔH$) in an exothermic reaction profile?

It points downwards from the reactant level to the product level.

In a simple cell using Zinc and Copper, if Zinc is replaced with Magnesium, what happens to the voltage?

The voltage increases (because Magnesium is more reactive than Zinc, increasing the reactivity difference with Copper).

What is meant by an 'anaerobic' process in ethanol production?

A process that occurs without oxygen.

In Required Practical 4, why should the thermometer not touch the bottom of the polystyrene cup?

To ensure it measures the temperature of the solution, not the container.

When a gas escapes from a non-enclosed system, what happens to the measured mass?

The mass appears to decrease.

What happens to the relative amount of products at equilibrium in an exothermic reaction if the temperature is increased (HT)?

The relative amount of products decreases (equilibrium shifts left).

What happens to the relative amount of products at equilibrium in an endothermic reaction if the temperature is increased (HT)?

The relative amount of products increases (equilibrium shifts right).

In the Haber process, why is $450^{\circ}C$ considered a 'compromise' temperature?

It is low enough to give a reasonable yield (since the forward reaction is exothermic) but high enough to maintain a fast rate.

How does a catalyst reduce the energy costs of an industrial process like the Haber process?

It allows the reaction to occur at a lower temperature while still maintaining a fast rate.

What is the effect of a catalyst on the position of equilibrium (HT)?

It has no effect on the position of equilibrium.

On an energy level diagram, what does the 'peak' of the curve represent?

The transition state where old bonds are breaking and new bonds are starting to form.

Why is the overall energy change for a reaction negative if it is exothermic?

Because the system has lost energy to the surroundings.