If a mixture of gaseous hydrogen and iodine vapors is made to react at k in a closed vessel for about 2 - 3 hours, gaseous hydrogen iodide is produced according to the following equation:.
But along with gaseous hydrogen iodide, there will be some amount of unreacted gaseous hydrogen and gaseous iodine left. On the other hand if gaseous hydrogen iodide is kept at K in a closed vessel for about 2 - 3 hours it decomposes to give gaseous hydrogen and gaseous iodine. In this case also some amount of gaseous hydrogen iodide will be left unreacted. This means that the products of certain reactions can be converted back to the reactants.
What conclusion could be drawn from this observation? We are told that the starting substance in this reaction is an alcohol and that it is ignited. When a substance is ignited, we assume that it is burning in air.
In other words, we assume that it is undergoing a combustion reaction with oxygen in the air. We are told that there are two different gases collected after the alcohol is ignited. Since the two gases are different from each other and different from the two starting substances alcohol and oxygen , we can deduce that a reaction did indeed occur.
As the initial substance, the alcohol, was not collected at the end of the reaction, we can also assume that all of the reactants were converted to products and that the products did not recombine to form the alcohol. We can conclude that the reaction is irreversible. So, the correct answer is A, the reaction is irreversible. However, in reality, many reactions can also proceed in the opposite direction. So, we can write the equation with two half-arrows instead, each pointing in an opposite direction.
We call this a reversible reaction. In such a reaction, both the forward and reverse reactions occur simultaneously. A reversible reaction is a reaction that proceeds in both directions; reactants react to form products, and the products react to reform the reactants. However, it is important to note that, in principle, all reactions are reversible. However, the conditions necessary for the backward reaction to occur are often very difficult to achieve.
For example, the backward reaction for the combustion of propane would need very specific conditions to occur. These conditions would not be easily achievable in a standard laboratory, and that is why we say that only the forward reaction occurs for combustion. In a reversible reaction, the reactants can react to form products, and the products can react to reform the reactants. The correct answer is E, a reversible reaction is a chemical reaction that can proceed in both directions.
Reversible reactions are more noticeable to us when they are incomplete, in other words, when not all of the reactants are converted to products. An example of a reversible reaction is when ammonium chloride, a white solid, is heated and decomposes into two different products, ammonia gas and hydrogen chloride gas.
Sometimes the mouth of the test tube is plugged with cotton wool to prevent cooling vapors from escaping, but allowing lighter gases in the air to escape. In this way, the reaction components are confined within the test tube and the gaseous products can react with each other. This reaction shows us that heat energy input or removal is one of the conditions that can influence the direction of a reversible reaction.
Removing heat energy by cooling the chemicals will drive the reverse reaction. It is important to remember, though, that not all forward reactions are endothermic and not all reverse reactions are exothermic.
This means that this reaction absorbs heat energy from the surroundings to occur. The amount of heat energy transferred from the surroundings to the reactant during this dehydration process forward reaction is the same as the amount of energy transferred from the product to the surroundings during the hydration reaction of the anhydrous product C u S O 4 s.
This backward reaction is therefore exothermic and so the correct answer is D, the reverse reaction is exothermic. Bond breaking is typically an endothermic process as energy is required to break the bonds in the reactants. The reactant atoms then rearrange to form the products. Forming bonds is typically an exothermic process and energy is released.
We call an overall reaction endothermic if the overall amount of energy absorbed is greater than the overall amount of energy released. Conversely, in an exothermic reaction, the overall amount of energy released is greater than the overall amount of energy absorbed. It contains waters of crystallization, and so we call it a hydrated salt. If we heat some of these hydrated copper sulfate crystals, we would notice a white vapor being given off and the crystals turning into a white powder.
The vapor is steam, or gaseous water. We can observe the reverse reaction too by adding a few drops of water to the white anhydrous powder. The white powder would turn blue again as shown in the photograph below.
This means that the products can be changed back into the original reactants. This is not obvious when a reaction 'goes to completion', leaving very little reactant, or none at all. Examples of reactions that go to completion are:. It is more obvious in reactions that do not go to completion that the reaction is reversible. This is the case when the reaction mixture contains both reactants and products. Ammonium chloride is a white solid. It breaks down when heated, forming ammonia and hydrogen chloride.
When these two gases are cool enough, they react together to form ammonium chloride again. This reversible reaction can be modelled as:. It is used in equations that model reversible reactions:.
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