Chemistry: how much energy is transferred as heat when acids react with alkalis?
In this investigation, I will be looking at the heat levels during the process of neutralisation, where you react and acid and an alkali together.
What are acids?
An acid is a chemical substance that neutralizes alkalis, dissolves some metals, and turns litmus red. Acids can come in each of the three states when they’re pure. They can be solids, such as citric acid, they can be liquids, for example sulphuric acids, or they can be like hydrochloric acid, which is a gas. To identify if a solution is acidic .You can use the pH scale, and some litmus paper. On a pH scale, an acid will be less than 7 – the lower the pH number, the stronger the acid. To use it, you have to see what colour the paper turns, and then compare it against the scale. The darker the colour, the lower the pH number, and the lower the pH number, the stronger the acid. There are two different kinds of acids: organic and mineral acids. Organic acids are molecular and consist of carbon and oxygen atoms. Mineral acids come from inorganic/mineral sources. All pure acids are mineral acids.
Throughout the group of acids, they all show some common properties. The common properties are: they’re corrosive to metal and skin, they turn litmus paper red, they have a pH of less than 7, they contain H+ ions and they’ll donate these during a reaction.
What are alkalis?
An alkali is a chemical compound that neutralizes or effervesces with acids and turns litmus blue. They are sold as tablets to treat things such as heartburn, as they neutralise the acid. Common alkalis include sodium hydroxide, potassium hydroxide and calcium hydroxide. They are identified in the same way as acids are – by using a pH scale and litmus paper. However, they will have a pH of anything above 7, up to 14. They also turn the paper blue – the darker the colour, the stronger the alkali. Like acids, there are some similar properties that are shown across the entire group. They all produce heat when they’re heated, they contain OH- ions and will donate them when they react and they all have pH numbers of 8 or above.
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How does neutralisation work?
Neutralisation occurs when an acid reacts with an alkali. The products are always a form of salt, e.g. NaCl, and water. This is because when the acids dissolve, they donate H+ ions and the alkalis donate OH- ions, which react to form H20, as shown in this equation:
Hydrogen +Hydroxide Water
H+ (aq) + OH- (aq) H20 (l)
The salt is always formed when the positive ion from the alkali reacts with the negative ion from the acid. For example, when you react Sodium Hydroxide and Sulphuric Acid, the positive ion from the alkali (sodium) reacts with the negative ion from the acid (chlorine). This then forms sodium chloride, as shown in this reaction:
Sodium + sulphur Sodium sulphate
Na + S NaSO4
So the whole neutralisation equation for sodium hydroxide and Sulphuric acid is:
Sodium Hydroxide + Sulphuric Acid Sodium Sulphate + Water
NaOH (aq) + H2SO4 (aq) Na2SO4 + H20
There are two types of energy changes. These are endothermic and exothermic energy changes. Exothermic reactions give out heat energy to the surroundings, causing the heat of the surrounding area to rise. Endothermic reactions take in heat from the surrounding are, so the temperature falls. The energy changes in a reaction can be shown in an energy level diagram:
You can see that in the first diagram, the products have more energy than the reactants did, so it has taken heat in from its surroundings, making it an endothermic reaction. The other diagram looks more complicated, but if you ignore the part about activation energy and the curve, you can see that the reactants had more energy than the products, so energy has been given out to the surroundings. An example of an exothermic reaction is burning, and an example of and endothermic reaction is electrolysis.
Measuring energy changes
Whenever a reaction takes place, so do a number of energy changes. They occur due to the bonds of the reactants breaking and the bonds of the products forming. When bonds are breaking, an endothermic energy change is occurring, because you have put the energy in. when bonds are forming, it’s an exothermic reaction because energy is given out when the bonds are made. We classify the whole reaction as endothermic or exothermic based on the net energy output. If the level of energy change in the products is positive, it’s an endothermic reaction, and if it’s negative, it’s an exothermic reaction. We can work out if a reaction is exothermic/endothermic by using this formula:
Energy change (J) = M x S x T.
In the equation, the energy change is measured in joules; M is the volume of the liquid in the reaction mixture, which is measured in cm3. S is the specific heat capacity of water, which is 4.2J/g◦C, and T is the temperature change measured in ◦C.
I predict that the reaction of sodium hydroxide and Sulphuric acid will be an exothermic reaction, and as I increase the volume of acid, the amount of energy released will be increased, up to a point, when it will level off. I believe this will happen because as I add more Sulphuric acid, there will be more successful collisions and more molecules to form bonds, which will release more energy than it takes in. I already know that the reaction is exothermic, as I have researched the bond enthalpies online. However, this will only work up to a certain point, as eventually, all of the sodium hydroxide ions will be used up, and I will just be left with an excess of Sulphuric acid.
If you look at the equation above, you can see that M is equal volume of liquid in the reaction mixture, so increasing the volume will increase the amount of energy released.
For my experiment, I will have an independent variable, a dependant variable and a number of control variables. My independent variable will be the thing in my experiment that I will be changing, which will be the volume of acid. I am changing this because it will help me prove my hypothesis true or false, as it’s a key part of my hypothesis. My dependent variable is what I will measure, which will be the change in temperature. My control variables are as listed below, as are the reasons I have to control them:
- Volume of alkali – it is important to control the volume of alkali, as an increase in the volume will have the same effect as increasing the volume of acid, which will increase the energy release. To control it, I will use 25cm3 of sodium hydroxide each time.
- Amount of insulation – keeping this the same is very important as if it is changed, it will allow a different amount of heat to escape, which will affect my results, ruining the experiment. To keep it the same, I will use the same insulating equipment each time, so that they have the same properties.
- Temperature – keeping this the same is important for two reasons. One, it will increase the rate of reaction, which could result in an increase in energy release, which would affect my results. The second reason is because I am measuring the temperature, so it will affect the reading on the thermometer if it increases/decreases. In order to keep the temperature the same, all of my experiments will be conducted at room temperature.
- Concentration – keeping this the same is crucial as the higher the concentration, the more particles in a fixed space. This will increase the number of collisions and bonds making/breaking, which will affect the energy release and therefore, my results. In order to prevent this, I will keep the concentration at 2M.
- Equipment used – this links with the insulation control variable. Different pieces of equipment will have different insulating properties, and I need to use the same things so that one set of results isn’t different to the other sets of results. All I need to do to control this is to use the same things every time.