Drawing and reading reaction profiles for both types of reaction, what activation energy means, and how a catalyst changes the picture.
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1.What is a reaction profile?
A diagram showing the energy of the reactants and products through the course of a reaction.
2.What is on each axis of a reaction profile?
Energy on the vertical axis and progress of reaction on the horizontal axis.
3.Describe the reaction profile of an exothermic reaction.
The products are at a lower energy than the reactants, so the line finishes below where it started.
4.Describe the reaction profile of an endothermic reaction.
The products are at a higher energy than the reactants, so the line finishes above where it started.
5.What is activation energy?
The minimum amount of energy that particles must have in order to react when they collide.
6.Where is activation energy shown on a profile?
It is the height from the reactants up to the top of the curve.
7.What does the peak of the curve represent?
The energy of the particles at the point of reacting, when bonds are being broken and formed.
8.How is the overall energy change shown on a profile?
It is the vertical difference between the level of the reactants and the level of the products.
9.Why does an exothermic reaction still need activation energy?
Bonds in the reactants have to be broken first, and that requires an input of energy even though more is released later.
10.What is a catalyst?
A substance that speeds up a reaction without being used up or chemically changed itself.
11.How does a catalyst work?
It provides a different pathway for the reaction with a lower activation energy.
12.How does a catalyst appear on a reaction profile?
As a second curve with a lower peak, starting and finishing at the same energies as the original.
13.Does a catalyst change the overall energy change?
No. The reactants and products are unchanged, so the energy difference between them is the same.
14.Does a catalyst appear in the balanced equation?
No, because it is not used up during the reaction.
15.Why does lowering activation energy speed a reaction up?
A greater proportion of colliding particles have enough energy to react, so more collisions are successful.
16.What are enzymes?
Biological catalysts, which catalyse reactions in living organisms.
17.If the products are lower in energy than the reactants, what type of reaction is it?
Exothermic.
18.A profile shows reactants at 100 and products at 60. What is the energy change?
A decrease of 40, so the reaction is exothermic.
19.A profile shows reactants at 50 and products at 90. What is the energy change?
An increase of 40, so the reaction is endothermic.
20.Why is the activation energy peak always above both the reactants and the products?
Bonds must be broken before new ones form, and breaking bonds always requires energy first.
21.What does a low activation energy suggest about a reaction?
That it happens readily, because most colliding particles have enough energy to react.
22.Why can some reactions only start once heated?
The particles do not have enough energy to overcome the activation energy at room temperature.
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