AQA Chemistry 8462 - Energy Changes

A set of 65 vocabulary flashcards covering exothermic and endothermic reactions, Required Practical 4, reaction profiles, bond energy calculations, chemical cells, and hydrogen fuel cells.

Exothermic reaction

A reaction that transfers energy to the surroundings, usually causing the surroundings to warm up.

Temperature change in an exothermic reaction

The temperature of the surroundings usually increases.

Examples of exothermic reactions

Combustion, many oxidation reactions, and neutralisation.

Endothermic reaction

A reaction that takes in energy from the surroundings, usually causing the surroundings to cool down.

Temperature change in an endothermic reaction

The temperature of the surroundings usually decreases.

Examples of endothermic processes

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

Overall energy transfer in a chemical reaction

Energy is transferred between the reacting chemicals and the surroundings.

Everyday use of exothermic reactions

Self-heating cans or hand warmers.

Everyday use of endothermic reactions

Instant cold packs for sports injuries.

Method for investigating reaction energy transfer

Measure the temperature before and after reactants are mixed while controlling other variables.

Required Practical 4 focus

The variables that affect temperature changes in reacting solutions, such as acid plus metal, acid plus carbonate, neutralisation, or displacement.

Dependent variable in Required Practical 4

The temperature change of the reaction mixture.

Purpose of measuring initial temperature in RP4

It provides a starting value so the temperature change can be calculated.

Temperature change calculation (RP4)

Final or maximum/minimum temperature minus initial temperature, using the sign or magnitude appropriately.

Polystyrene cup in RP4

Reduces energy transfer between the reaction mixture and the surroundings.

Purpose of a lid in RP4

Reduces heat loss to the surroundings.

Purpose of stirring in RP4

Distributes thermal energy evenly and makes the temperature measurement more representative.

Purpose of regular temperature recording in RP4

Identifies the highest or lowest temperature reached and improves the estimate of the true temperature change.

Control variable in temperature-change investigation

A factor kept the same so the effect of the independent variable can be compared fairly.

Discrepancy in experimental vs theoretical temperature change

Occurs because some energy is transferred to or from the surroundings and apparatus.

Reaction profile

A diagram showing the energy of reactants and products as a reaction progresses.

Activation energy

The minimum amount of energy particles must have for a reaction to occur.

Location of activation energy on a reaction profile

Shown as the energy difference from the reactants' energy level to the top of the energy barrier.

Exothermic reaction profile representation

The products are at a lower energy level than the reactants.

Endothermic reaction profile representation

The products are at a higher energy level than the reactants.

Overall energy change on a reaction profile

Shown by the energy difference between the reactants and products.

Energy levels in an exothermic reaction

The reactants have more energy than the products.

Energy levels in an endothermic reaction

The products have more energy than the reactants.

Chemical bond changes during a reaction

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

Energy change of breaking bonds

Requires energy.

Energy change of forming bonds

Releases energy.

Exothermic reaction (bond energies definition)

A reaction where more energy is released forming new bonds than is required to break bonds.

Endothermic reaction (bond energies definition)

A reaction where more energy is required to break bonds than is released when new bonds form.

Estimation formula for overall energy change

$$\text{Energy change} = \text{total energy required to break bonds} - \text{total energy released when bonds form}$$

Expected sign for exothermic reaction energy change

A negative value.

Expected sign for endothermic reaction energy change

A positive value.

Prerequisite counting step for bond energy calculations

Every bond broken in the reactants and every bond formed in the products must be counted.

Reason bond-energy calculations are estimates

Bond energies are average values and can vary depending on the molecule.

Exothermic bond energy calculation example

$1200 - 1500 = -300\,kJ/mol$, indicating an exothermic reaction.

Endothermic bond energy calculation example

$1800 - 1500 = +300\,kJ/mol$, indicating an endothermic reaction.

Chemical cell

A device in which chemical reactions produce a potential difference between two electrodes.

Components of a simple chemical cell

Two different electrodes in contact with an electrolyte.

Electrolyte in a chemical cell

A liquid or solution containing ions that can move and carry charge.

Factors affecting chemical cell voltage

The choice of electrode materials and the electrolyte.

Reason electrodes in a chemical cell must be different

A difference in their chemical reactivity allows a potential difference to be produced.

Voltage polarity in a chemical cell

Reversing which materials act as the two electrodes reverses the polarity.

Battery

Two or more cells connected together in series to provide a greater potential difference.

Non-rechargeable cell behavior

Chemical reactions continue until one of the reactants is used up, so the cell eventually stops producing a voltage.

Rechargeable cell mechanism

An external electrical current reverses the chemical reactions so the reactants are regenerated.

Fuel cell

A cell supplied continuously with fuel and oxygen or air that produces a potential difference by chemical reaction.

Focus fuel for AQA fuel cells

Hydrogen.

Overall reactants in a hydrogen fuel cell

Hydrogen and oxygen.

Overall product of a hydrogen fuel cell

Water.

Overall equation for a hydrogen fuel cell

$$2H_2 + O_2 \rightarrow 2H_2O$$

Advantage of hydrogen fuel cells at point of use

They produce water rather than carbon dioxide.

Reason hydrogen fuel cells operate continuously

Fuel and oxygen are supplied continuously from outside the cell.

Practical issues with hydrogen fuel cells

Producing, storing, and transporting hydrogen, cost, safety, and the source of energy used to make the hydrogen.

Environmental considerations of hydrogen fuel cells

The environmental impact depends partly on how the hydrogen is produced and supplied.

Comparison factors between batteries and fuel cells

Refuelling/recharging, lifetime, waste, cost, and environmental impact.

Electrode processes in a fuel cell

Oxidation occurs at one electrode and reduction occurs at the other, involving electron transfer.

Conservation in fuel-cell half equations

Must conserve both atoms and electrical charge.

Key exam distinction between exothermic and endothermic

Exothermic transfers energy to the surroundings; endothermic takes energy in from the surroundings.

Key exam distinction between breaking and making bonds

Breaking bonds requires energy; making bonds releases energy.

Interpretation checks for a reaction profile

Reactant level, product level, activation-energy barrier, and overall energy difference.

Checks required after a bond-energy calculation

Check that all bonds were counted correctly, units are included, and the sign matches exothermic or endothermic behaviour.