What is the relationship between rate and time?
A high rate of reaction is when a reaction is over in a fraction of a second. As the time taken for the reaction to be completed increases, the rate of reaction decreases.
Fast and slow
Some reactions are fast and some are slow. Look at these examples;
Silver chloride precipitating, when solutions of silver nitrate and sodium chloride are mixed. This is a very fast reaction.
Concrete setting. This reaction is quite slow. It will take a couple days for concrete to harden.
Rust forming on a heap of scrap iron in a scrap yard. This is usually a very slow reaction.
It is not always enough to know just that a reaction is fast or slow. For example, in a factory that makes products from chemicals, the chemical engineers need to know exactly how fast each reaction is going, and how long it takes to complete. In other words, they need to know the rate of reaction.
Rate of reaction depends on four things;
- Temperature
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Concentration – (or pressure for gases)
- Catalyst
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Size of particles – (or surface area)
Speed of reaction
There are two ways speed of reaction can be observed; either by how quickly the products are forming, or how quickly the reactants are used up.
Speed of reaction can be measured in three ways;
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Precipitation- this is when the product of the reaction is a precipitate which clouds the solution.
E.g. mark a cross on a piece of paper, put it under container carrying the solution and measure how long it takes for it to disappear.
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Change in mass (usually gas given off) - Any reaction that produces a gas is carried out on a mass balance, as the gas is given off the mass disappearing is measured.
3) The Volume of gas given off- Gas syringe is used to measure the volume of gas given off.
Collision Theory
Collision theory perfectly explains reaction rates. The rate of reaction depends on how often and how hard the reaction the reacting particles collide with each other. Basically the particles have to collide hard enough in order to react.
More Collisions increases the Rate of Reaction
Increasing rate of reactions can be explained in terms of increasing the number of collisions between the reacting particles;
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Temperature increases the number of collisions
As the temperature increases the particles move quicker. This means that the particles are going to have more collisions.
COLD Hot
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Concentration (or pressure) increases the number of collisions
The more concentrated the solution, the more particles of reactant knocking about between water molecules. This makes the collisions between the important particles more likely. In a gas, the higher the pressure, the more molecules squashed up. This means that there are going to be more collisions.
3) Size of solid particles (or surface area) increases collisions
If you break up a solid into smaller pieces it will increase the surface area. This means the particles around it in the solution will have more area to work on so there’ll be more useful collisions.
4) Catalyst increases the number of collisions
A catalyst gives the reacting particles a surface to stick to where they can bump into each other. This increases the number of collisions.
A faster collision increases the rate of reaction.
The higher the temperature increases the energy of the collisions. This is because it makes particles move faster.
Reactions only take place if the particles collide with enough energy. The higher the temperature the more particles the more particles collide with enough energy to make the reaction happen (initial energy) - known as the activation energy. It is needed to break the initial bonds.
Prediction
My hypothesis is that the higher the concentration of the hydrochloric acid increases, the faster gas will be given off.
I know this from testing in my trial experiments.
Method
Since I will be working with strong acids, I will wear safety glasses whilst conducting my experiment.
I will use Marble chips all of a small size. The amount of acid I have decided on is 10ml. I can measure the speed of the reaction by how much gas is given off.
I can measure this in cm3 in a gas syringe. When the experiment takes place, I will take the reading when the gas syringe reaches 40cm3. This will tell me how long it takes in seconds to get to 40cm3.
To be sure I can rely on my results I will repeat the experiment twice or three times if necessary, because there is a chance that it may be inaccurate.
I learnt from the trial experiments that it is quite hard to start a stopwatch, pour acid and marble chips together all at the same time, so I will work in a group while carrying out the experiments. 5s2tq
If the experiment goes on for a ridiculous time, I will cut it off. I don't think it is necessary to have the experiment go over three minutes, so I will cut it off then. Austen.
It is important to keep the starting temperatures constant for all the reactions as this can affect the results by giving the particles more energy (and making it faster). 2B4UbDSN from
Results
Table of results.
Graphs
Analysis of Results.
From the experiment, I have found that an increased concentration of acid increases the rate of the reaction.
I think that the best way of displaying these results is by drawing a line graph. In it, I have used the average of the two sets of results. The pattern I can see is that when the reaction first starts, the reaction is fastest, from then on, the reaction slows. We can see this because the gradient of the curve is steepest at the beginning. Heidegger.
The cause of this curve is that at the start of the reaction, there are the most amounts of acid molecules that are colliding with the marble.
The statement that I outlined in my prediction was accurate. Therefore I can be satisfied with my results. Each time I conducted the experiment the results were consistent. I was more then content with my results and the way I approach collating them.
Bibliography
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