Catalyst
catalysts are useful in a reaction as it speeds up the rate of reaction without being used up. Most catalysts are there to speed up the rate of reaction, however some can slow them down. The ones that speed up the rate of reaction are called activators, and those that slow down the rate of reaction are called inhibitors. For example, in the reaction where the enzyme (biological catalyst) breaks down hydrogen peroxide (H2O2) into water (H2O2) and oxygen (O2), glycerine is sometimes added, this is in order to slow down the rate at which hydrogen peroxide is broken down during storage.
Most of the catalysts that are used are transition metals and their compounds, such as the making of margarine, where a nickel catalyst is used. The nickel catalyses an addition reaction, between a double bonded hydrocarbon (alkenes), the oil, and hydrogen. The result is a solid fatty product, which is margarine. By controlling the rate of reaction (i.e. how much catalyst is used) you can also control the solidity of the margarine.
A catalyst allows a substance to react more easily by reducing the activation energy. This is where the energy needed in order to break the bonds is reduced. Therefore the particles require less energy to react, and the reaction occurs faster. Catalysts can be compared to getting from a-b in a car. The normal way would be by going through small roads, however using the motorway is like using a catalyst. This is as it takes less energy (petrol) to get there as well as far less time than compared to taking the smaller roads.
Temperature
when particle collide with each other, they do not always react. This is, as they do not have the sufficient kinetic energy for them in order to stretch or beak the bonds in order to form the products. In some reactions, only the particles with high energy can react. This sort of situation can be compared to a car crash; if two cars hit each other at low speeds, then hardly any damage will be done, however, if the cars hit each other at a higher speed, then a lot more damage would be done to both cars.
Mass of calcium carbonate chips
when you increase the mass of the chips, it means that there are more particles present for the hydrochloric acid to collide with. This would cause more collisions, which means a faster rate of reaction.
Surface area of calcium carbonate chips
in a reaction; if one of the reactants is a solid then the surface area of the solid will affect the rate of reaction. This is because the only particles that can collide with each other are the ones at the solid-liquid interface. This is the area in which the surfaces of the marble chips come in contact with the hydrochloric acid. This would therefore mean that the larger the surface area of the marble chips, the more collisions there would be, which a higher rate of reaction is.
Diagram A and B are marble chips with the same masses. Diagram be has a higher surface area, and as you can see, there are more marble particles exposed to the surrounding, which would mean that there would be a larger amount of collisions in a given amount of time.
If you have a two fixed masses of hydrochloric acid, which have both been crushed up, the one with the smaller particles will have a higher rate of reaction, due to the fact that there is a higher surface area exposed to the hydrochloric acid. A good example to compare this to would be slicing a loaf of bread, then spreading butter on it. The more thinner you cut each slice of bread, the more you can get which means that you would be able to spread more butter. This type of theory is known as the ‘’bread and butter theory’’ and is often used in biology. The higher surface area of marble chips there is that is in contact with the hydrochloric acid, the higher the rate of reaction, and the more products that are formed in a given amount of time.
Light
Some chemical reactions are affected by the presence of light. One example of a reaction that depends on the presence of sunlight is photosynthesis, which only takes place when sunlight falls on a plant, which contains green pigmentation called chlorophyll. Another example of a reaction that only takes place in the presence of light is that which occurs when a photograph is taken. Inside the camera is a transparent film, which a clear plastic film that is covered in an emulsion of a layer of gelatine which is filled with millions of tiny crystals of silver halides, especially silver bromide (AgBr). The emulsion that is used is similar for both colour and black and white film. The only difference is that the colour film contains three layers of emulsion which all contain different dyes.
When light falls hits a silver bromide molecule, silver cations (Ag+) accepts an electron from the bromine ions (Br-) creating a silver atom.
Ag+ + e- Ag
Silver ion + Electron Silver atom
The bromine atoms that are produced are then trapped in the gelatine. The more light that hits the photographic film the greater the amount of silver that is deposited.
As you can see, most of the ways of increasing the rate of reactivity involve increasing the number of successful collisions within the reaction itself. Once you do this the rate of reaction should increase. However, just increasing the number of collisions doesn’t necessarily mean that the rate of reaction is increased. This is because in order to increase the rate of reaction you need to increase the number of successful collisions. When collisions occur, if they do not have enough energy, which is in the form of kinetic energy, then they would repel each other. However if they do have enough energy, then they would combine, which is a successful collision.
The way in which the rate of reaction is calculated is by using the following formula:
Reaction rate= change in amount (or concentration) of a substance/products produced
Time taken
Therefore, in this case, I will be measuring the rate of reaction by measuring the quantity of products formed. The formula would then be:
Reaction rate= carbon dioxide produced
Time taken
Using this method you can work out the average rate of reaction between calcium carbonate and hydrochloric acid in the time given for the experiment. The rate at which it reacts would be measured in x grams/minute.
Aim
The aim of this experiment is to see how the rate of reaction can be affected by a factor [variable]. The variable that we chose was the concentration of the acid, so the aim is to see how the concentration of hydrochloric acid can affect the rate of reactivity.
Prediction
Substances are prone to burn much faster in the presence of oxygen. For example a charcoal in a barbeque would often be burning quite slowly with a red glow. However, when you blow on t, the coal would become even hotter as it gets more red, and my even burst into flames. This is as you are blowing a higher concentration of oxygen onto it. Another example is acetylene, which burns extremely rapidly in the presence of pure oxygen. The energy produced, in the form of heat, is enough to burn through metal, which is what it is used for:
C2H2 + 2.5O2 2CO2 + H2O + heat
Acetylene + Oxygen Carbon dioxide + Water + heat energy
Chemical reactions occur when successful collisions occur between particles. This is when particles gain enough speed that when they collide with each other they join together. Successful collisions between acetylene molecules and oxygen molecules occur when pure oxygen is used instead of air. Therefore, the rate of reaction increases the rate of reaction as well as increasing the amount of heat given off. From this, I can therefore predict that as you increase the concentration of the hydrochloric acid, you would also be increasing the rate of reaction.
I predict that the shape of the graph would look like so:
This is as the rate of reaction at the beginning would be quite rapid. This would eventually slow down due to the fact that the calcium chloride and the hydrochloric acid are being used up. I therefore predict that the shape of the graph would eventually level off, and all of the lines (the concentrations) should end up at the same level in the end.
Preliminary Investigation
There are two methods that may be used to measure the rate of reaction, which are:
- How fast the reactants are used up
- How fast the products are formed
The method we are using is to measure how fast the products are formed; in this case how fast the carbon dioxide is formed.
Preliminary experiments were performed beforehand to understand how the actual experiment should be carried out, decide on the quantities of substances used as well as comprehend the observations in the experiment, i.e. effervescence, in order to acknowledge the reaction.
First, we had to decide on the concentration of hydrochloric acid used. The fist concentration that was tried was 3 molar, which I decided was too fast. This was as the reading on the gas syringe went above the maximum level quite rapidly, which is not sufficient as it means that the results would not be adequate enough to identify a trend, which is the main aim of this investigation. The next concentration level that was tried was 2 molar, which I decided was adequate enough for the maximum concentration. Even though the reading in the gas syringe did go above the maximum, I did keep in mind that I was going to be using a larger gas syringe for the actual experiment. Therefore the maximum concentration of acid that is going to be used is 2 molar, which is the variable. We are then going to reduce this to 1.8 molar, 1.6 molar, 1.4 molar, 1.2 molar and then finally 1 molar. I believe that these sets of variables would be adequate enough in order to identify a trend. After deciding on the quantities used, I then had to decide what volume of acid I was going to be using. The first volume I tried was 0.5ml, which was too slow. I then tired 10 ml, which was again proving to be quite a slow reaction. So I then doubled the volume of the acid, which I believed was satisfactory, as the rate of reaction was good, and the amount of co2 produced was satisfactory, as it did not go over the syringe limit.
I then had to decide what mass of calcium carbonate chips I had to use. The first mass that I had tried was 0.5grams, which produced a far too slow rate of reaction. I then tried 2 grams of calcium carbonate, which was too fast of a reaction, and the amount of CO2 produced was off the reading on the gas syringe, so I halved it to 1 gram, which was satisfactory.
The amount of time I am going to use is 90 seconds (one and a half minutes) as I believe that this is a sufficient amount of time in which I will be able to identify any trend between the concentration of hydrochloric acid and the amount of carbon dioxide produced.
Here are the results for the volume of acid:
Concentration of acid: 1M
Mass of marble chips: 1.0 gram
Volume of acid used: 5 ml.
Concentration of acid: 1M
Mass of marble chips: 1.0 gram
Volume of acid: 10 ml
Concentration of acid: 1M
Mass of marble chips: 1.0 gram
Volume of acid: 20ml
Here are the results for the mass of marble chips:
Concentration of acid: 1M
Mass of marble chips: 0.5 grams
Volume of acid: 20ml
Concentration of acid: 1M
Mass of marble chips: 2 grams
Volume of acid: 20ml
Concentration of acid: 1M
Mass of marble chips: 1 gram
Volume of acid: 20ml
Here are the results for the concentration of hydrochloric acid:
Concentration of acid: 2M
Mass of marble chips:1 gram
Volume of acid: 20ml
Concentration of acid: 1M
Mass of marble chips: 1 gram
Volume of acid: 20ml
The overall result for the preliminary investigation using the correct quantities would be:
The graph for the preliminary investigation looked like this:
