Chemistry Model Answers

"The job of a catalyst is to speed up reactions. Only a small amount is needed as the catalyst is not actually used up in the reaction. By lowering the activation energy and providing an alternate route for the reaction, the reaction is sped up" 

Hints

• If you can relate science into something you're familiar with, the idea is that you'll learn it much quicker. That being said, let's rephrase this paragraph into something more familiar. The catalyst acts as a short-cut. Think of the reaction as being the highway on a busy road. There are dozens and dozens of cars (particles) that are driving past but only those with the right amount of fuel (activation energy) can get past. But here's the key, just like any destination there are a large amount of ways to get there - they're usually called short-cuts. The catalyst that we're talking about here is that short-cut. So with the catalyst the cars are able to get to their destination even without the right amount of fuel because the short-cut is well... for lack of a better phrase: shorter.

"Initially only the forward reaction is favoured because only the reactants are present however in a reversible reaction the products revert back to being reactants. The rate at which the forward reaction occurs decreases while the rate at which the backward reaction occurs increases until both the rates are equal hence dynamic equilibrium"

Hints

• At first glance, that paragraph seems like a nightmare. But let's break it down. The first point is that normally in reactions, the forward reaction is favoured. Common sense really, usually it's much faster to make something than it is to break something. The two points in bold which talk about an increase and a decrease are basically talking about the speed of the reactions. Let's rephrase it: there are two guys on bicycles. One of them, let's call him Bill, is in front (the forward reaction) and the other is lagging behind (the backward reaction), he's called Ben. Now to become equal (dynamic equilibrium) Bill needs to slow down and Ben needs to speed up, so they do this until they are side by side. Once they are side by side, they are equal meaning that they are riding their bikes at the same speed - which is the idea of dynamic equilibrium. 
• Also remember that there are different and varying conditions that are needed for a dynamic equilibrium to occur. Both forward and backward reactions are present in unchanging amounts - if you can't understand then think of a man on a treadmill, neither the man nor the treadmill have stopped and the man is likely to stay there forever. 

"Molecules are attracted to the stationary and mobile phase by varying amounts, molecules move between the two phases therefore there is dynamic equilibrium. If the molecules are more attracted to the stationary phase then there is fewer and slower movement however if they are more attracted to the mobile phase there is more and faster movement" 

Hints 

• This is quite a simple model answer. This is because if you can remember half of the model answer then you can pretty much work out the other through logic (i.e. being hungry means eating more, therefore not being hungry means eating less). So let's break it down: the first line speaks about dynamic equilibrium but this isn't anything to worry about if you don't remember the model answer for that because you just have to know the basic idea of it, if you're in the middle of something then you're at equal distance from both and it's the same concept here. Rephrasing this answer gets you something like this; take a physically fit person (mobile phase) and an unfit person (stationary phase). If you're more like that fit person then you'll be more active - this means you'll be moving a lot more and a lot faster than the unfit person. You get the reverse if you're more like the unfit person. 

"In energy reactions, energy is required to make bonds while energy is released upon breaking bonds. In an exothermic reaction more energy is released than is required to break the bonds this often results in the release of heat and/or light to the surroundings. Because more energy is released exothermic reactions are often denoted by a negative enthalpy change" 

Hints 

• Before I talk about this one I'd just like to say that if you're wanting the model answer for endothermic reactions it's pretty much the complete reverse of this one. The reason I did exothermic is because it's usually more common because of combustion reactions. 
• When a chemical reaction happens, energy is transferred to or from the surroundings and often there is a temperature change. To rephrase is, imagine a bonfire, when this bonfire burns it transfers heat energy to the surroundings. Objects near a bonfire become warmer by comparison to objects far away from the bonfire. 

"This is the general reaction of an exothermic reaction;

• A + B <--> C + D     (negative delta energy)

This is a reversible reaction, depicted by the reversible sign. It is an exothermic reaction because there is a negative delta energy change. If you increase the temperature of the exothermic reaction, the equilibrium shifts to the left hand side, in other words the backward reaction is favoured to oppose the change in the reaction."

• To have the answer for an endothermic reaction, the answer is the exact opposite. Much like the model answer for "Exothermic Reactions", card 4 of 5. Instead of the equilibrium shifting to the left hand sife, the equilibrium shifts to the right hand side to oppose the change.
• Rephrase this answer, think of it instead as two tasks. Doing homework (A + B) is the forward reaction, playing games (C + D) is the backward reaction. The reversible sign is the child who you are telling to do the task. Much like the sign, the child can go both ways; the child can end up doing either task. But also, like a child if you tell it do one thing (increase the temperature) it will do the opposite. Telling it to do their homework will bring an increase in them wanting to play games.