C7 Energy Changes (Chemistry AQA Seperate)
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107 Terms
What is an exothermic reaction?
A reaction that transfers energy to the surroundings.
What is an endothermic reaction?
A reaction that takes in energy from the surroundings.
What happens to the temperature of the surroundings in an exothermic reaction?
It increases.
What happens to the temperature of the surroundings in an endothermic reaction?
It decreases.
What is meant by energy transfer?
Energy moving from one place or substance to another.
What type of energy is transferred in chemical reactions?
Thermal (heat) energy.
What is the law of conservation of energy?
Energy cannot be created or destroyed, only transferred.
What happens to the surroundings during an exothermic reaction?
They gain energy.
What happens to the surroundings during an endothermic reaction?
They lose energy.
Give three examples of exothermic reactions.
Combustion, neutralisation and oxidation.
Give three examples of endothermic reactions.
Thermal decomposition, citric acid with sodium hydrogencarbonate, and photosynthesis.
What is combustion?
The reaction of a substance with oxygen, releasing energy.
Why are combustion reactions exothermic?
More energy is released making new bonds than is needed to break old bonds.
What is oxidation?
The gain of oxygen or loss of electrons.
Why are oxidation reactions usually exothermic?
Energy released when new bonds form is greater than the energy needed to break bonds.
What is thermal decomposition?
Breaking down a compound using heat.
Why are thermal decomposition reactions endothermic?
They require energy to break chemical bonds.
What is photosynthesis?
The process by which plants use light energy to make glucose.
Why is photosynthesis endothermic?
It absorbs energy from sunlight.
What happens to the energy stores during an exothermic reaction?
Chemical energy is transferred to the surroundings.
What happens to the energy stores during an endothermic reaction?
Energy is taken from the surroundings and stored chemically.
What is the difference between activation energy and overall energy change?
Activation energy starts the reaction; overall energy change is the difference between reactants and products.
What is activation energy?
The minimum energy needed for a reaction to start.
Why do all reactions require activation energy?
Bonds in reactants must be broken first.
What provides activation energy in many reactions?
Heat, light or electricity.
What happens if particles do not have enough activation energy?
The reaction will not occur.
What is shown on a reaction profile?
The energy changes during a chemical reaction.
What does the peak on a reaction profile represent?
The activation energy.
How do the products compare with the reactants on an exothermic reaction profile?
The products have lower energy.
How do the products compare with the reactants on an endothermic reaction profile?
The products have higher energy.
Why are products lower in energy in exothermic reactions?
More energy is released than absorbed.
Why are products higher in energy in endothermic reactions?
More energy is absorbed than released.
What is the energy change symbol?
ΔH.
What is the sign of ΔH for exothermic reactions?
Negative.
What is the sign of ΔH for endothermic reactions?
Positive.
Why is ΔH negative in exothermic reactions?
Energy is transferred to the surroundings.
Why is ΔH positive in endothermic reactions?
Energy is absorbed from the surroundings.
What happens when chemical bonds break?
Energy is absorbed.
Is bond breaking endothermic or exothermic?
Endothermic.
What happens when chemical bonds form?
Energy is released.
Is bond making endothermic or exothermic?
Exothermic.
Why do chemical reactions involve both bond breaking and bond making?
Old bonds must be broken before new ones can form.
When is a reaction exothermic in terms of bond energies?
When more energy is released making bonds than absorbed breaking bonds.
When is a reaction endothermic in terms of bond energies?
When more energy is absorbed breaking bonds than released making bonds.
What is bond energy?
The energy needed to break one mole of a particular bond.
What is meant by the overall energy change?
The difference between energy absorbed and energy released.
What is the equation for calculating energy change using bond energies?
Energy change = Energy needed to break bonds − Energy released when bonds form.
Why are bond energy calculations only estimates?
Bond energies are average values.
What is a catalyst?
A substance that increases the rate of a reaction without being used up.
How does a catalyst speed up a reaction?
It provides an alternative reaction pathway with a lower activation energy.
Does a catalyst change the energy of the reactants or products?
No.
Does a catalyst change ΔH?
No.
What effect does a catalyst have on activation energy?
It lowers it.
Why does lowering activation energy increase reaction rate?
More particles have enough energy to react.
How does a catalyst appear on a reaction profile?
It lowers the peak but reactant and product energy stay the same.
What are enzymes?
Biological catalysts.
Why are enzymes important?
They allow reactions to happen quickly at body temperature.
What is a chemical cell?
A device that converts chemical energy into electrical energy.
How does a chemical cell work?
A redox reaction causes electrons to flow through a circuit.
What type of reaction occurs in a chemical cell?
A redox reaction.
What is the energy transfer in a chemical cell?
Chemical energy to electrical energy.
Give examples of non-rechargeable cells.
Alkaline batteries.
Give examples of rechargeable cells.
Lithium-ion batteries.
Why are rechargeable batteries useful?
They can be recharged and reused many times.
What happens during battery charging?
Electrical energy is converted into chemical energy.
What happens during battery discharge?
Chemical energy is converted into electrical energy.
What is a fuel cell?
A cell that continuously converts fuel and oxygen into electricity.
What are the reactants in a hydrogen fuel cell?
Hydrogen and oxygen.
What is the only chemical product of a hydrogen fuel cell?
Water.
What is the overall equation for a hydrogen fuel cell?
2H₂ + O₂ → 2H₂O.
Why do hydrogen fuel cells produce little pollution?
Their only chemical product is water.
Why are hydrogen fuel cells considered environmentally friendly?
They do not produce carbon dioxide during use.
What are the disadvantages of hydrogen fuel cells?
Hydrogen storage is difficult and hydrogen production often uses fossil fuels.
Why is hydrogen difficult to store?
It is highly flammable and requires high pressure or very low temperatures.
What is meant by renewable energy?
Energy from sources that are naturally replenished.
How is hydrogen sometimes produced?
By electrolysis of water.
Why is hydrogen production by electrolysis expensive?
It requires a lot of electricity.
How is most industrial hydrogen currently produced?
From natural gas.
Why is producing hydrogen from methane less environmentally friendly?
It releases carbon dioxide.
Describe the required practical investigating temperature changes.
Measure the initial temperature, mix reactants in an insulated container, stir, record the highest or lowest temperature reached and calculate the temperature change.
Why is a polystyrene cup used in the required practical?
It reduces heat transfer to the surroundings.
Why is a lid placed on the cup?
To reduce heat loss.
Why should the solutions be stirred?
To ensure even temperature throughout.
Why should the thermometer not touch the sides of the cup?
To improve accuracy.
What measurement is recorded in the energy changes practical?
The temperature before and after the reaction.
How do you calculate the temperature change?
Final temperature − Initial temperature.
What indicates an exothermic reaction in the required practical?
The temperature increases.
What indicates an endothermic reaction in the required practical?
The temperature decreases.
Why are repeat results taken in practical work?
To improve reliability.
Why should the same volumes and concentrations be used when comparing reactions?
To ensure a fair test.
What variable is measured in the required practical?
Temperature.
Name two control variables in the required practical.
Volume of solutions and concentration of solutions.
What is the independent variable in the required practical?
The reactants used.
Why is insulation important in energy experiments?
It reduces energy transfer to the surroundings.
Explain why combustion reactions release energy.
More energy is released making carbon dioxide and water than is absorbed breaking the reactant bonds.
Explain why thermal decomposition requires heating.
Energy is needed to break the chemical bonds.
Explain why catalysts do not affect the overall energy change.
They only lower activation energy and do not change the energy of reactants or products.
Explain why bond breaking is always endothermic.
Energy must be supplied to overcome the attractive forces holding atoms together.
Explain why bond making is always exothermic.
Energy is released when new stable bonds form.
How can you identify an exothermic reaction from a reaction profile?
The products are lower in energy than the reactants.