Energy Changes
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51 Terms
What is an exothermic reaction?
A reaction that transfers energy to the surroundings, usually causing a temperature increase.
What is an endothermic reaction?
A reaction that takes in energy from the surroundings, usually causing a temperature decrease.
Give three examples of exothermic reactions.
Combustion, oxidation and neutralisation.
Give two examples of endothermic reactions.
Thermal decomposition and some reactions between acids and alkalis.
What happens to the temperature of the surroundings during an exothermic reaction?
It increases.
What happens to the temperature of the surroundings during an endothermic reaction?
It decreases.
What is a reaction profile diagram?
A diagram showing the energy of reactants and products during a reaction.
What does the vertical axis show on a reaction profile?
Energy.
What does the horizontal axis show on a reaction profile?
Progress of reaction.
What is activation energy?
The minimum energy particles need for a reaction to occur.
How is activation energy shown on a reaction profile?
The energy difference between the reactants and the peak.
How does a reaction profile show an exothermic reaction?
The products are at a lower energy level than the reactants.
How does a reaction profile show an endothermic reaction?
The products are at a higher energy level than the reactants.
What is the overall energy change of an exothermic reaction?
Negative, because energy is transferred to the surroundings.
What is the overall energy change of an endothermic reaction?
Positive, because energy is taken in from the surroundings.
What is the equation for overall energy change using reaction profile energies?
Energy of products − energy of reactants.
What happens to bonds during a chemical reaction?
Existing bonds break and new bonds form.
Is energy absorbed or released when bonds break?
Energy is absorbed.
Is energy absorbed or released when bonds form?
Energy is released.
How do you calculate energy change using bond energies?
Energy change = energy needed to break bonds − energy released when bonds form.
Why are bond-breaking processes endothermic?
Energy must be supplied to overcome the forces holding atoms together in bonds.
Why are bond-forming processes exothermic?
Energy is released when new bonds form.
When is a reaction exothermic using bond energies?
When more energy is released forming bonds than is needed to break bonds.
When is a reaction endothermic using bond energies?
When more energy is needed to break bonds than is released forming bonds.
What does a negative bond-energy calculation result mean?
The reaction is exothermic.
What does a positive bond-energy calculation result mean?
The reaction is endothermic.
Why can bond energies only give an estimate of reaction energy change?
Bond energies are mean values averaged across different compounds.
What is the bond-energy calculation for H₂ + Cl₂ → 2HCl?
Energy change = energy to break H–H and Cl–Cl bonds − energy released forming two H–Cl bonds.
A reaction breaks bonds requiring 500 kJ and forms bonds releasing 700 kJ. Calculate ΔH.
500 − 700 = −200 kJ/mol
A reaction breaks bonds requiring 900 kJ and forms bonds releasing 650 kJ. Calculate ΔH.
900 − 650 = +250 kJ/mol
Why does a reaction release energy overall even though bonds must be broken?
Bond formation releases more energy than bond breaking absorbs.
Why does a reaction absorb energy overall even though bonds form?
Bond breaking absorbs more energy than bond formation releases.
What is the difference between an energy level diagram and a bond-energy calculation?
An energy level diagram compares reactant and product energies
What is the specific heat capacity equation?
Energy transferred = mass × specific heat capacity × temperature change (E = mcΔT).
How do you calculate temperature change?
Final temperature − initial temperature.
What units are used for energy, mass and specific heat capacity in E = mcΔT?
Energy: J
A 2 kg substance with c = 500 J/kg°C warms by 10°C. Calculate energy transferred.
E = 2 × 500 × 10 = 10,000 J.
A substance receives 12,000 J, has mass 2 kg and c = 600 J/kg°C. Calculate its temperature change.
ΔT = 12,000 ÷ (2 × 600) = 10°C.
Why can a temperature change be used to identify whether a reaction is exothermic or endothermic?
A rise suggests energy transferred to surroundings
Why should a reaction mixture be insulated during a temperature-change experiment?
To reduce energy transfer to or from the surroundings.
How can heat loss affect a measured temperature rise?
The measured rise is smaller than the true rise, reducing accuracy.
Why should the same volumes and concentrations be used when comparing reactions?
To make it a fair test by controlling variables that affect energy transfer.
How does a catalyst affect activation energy?
It provides an alternative reaction pathway with lower activation energy.
Does a catalyst change the overall energy change of a reaction?
No
What is the difference between activation energy and overall energy change?
Activation energy is the minimum energy needed to start reaction
How can you identify the more exothermic reaction from reaction profiles?
It has a larger drop in energy from reactants to products.
How can you identify the more endothermic reaction from reaction profiles?
It has a larger rise in energy from reactants to products.
Why is combustion usually exothermic?
Energy released forming product bonds exceeds energy needed to break reactant bonds.
Why is thermal decomposition usually endothermic?
Energy must be supplied to break down a compound.
What is the key Grade 9 explanation for energy changes in reactions?
Compare energy absorbed breaking bonds with energy released forming bonds
What mistake should you avoid when calculating energy change from bond energies?
Do not subtract energy needed to break bonds from energy released