How do we use matter?
Define Exothermic and Endothermic Reactions
Exothermic reactions refer to reactions that release energy, usually in the form of heat or light. The reason why energy is released is because the total energy in the products is less than the total energy in the reactants
Endothermic reactions refer to reactions that absorb energy, usually in the form of heat or light. The reason why energy is released is because the total energy in the products are more than the total energy in the reactants
Explain incomplete & complete combustion
Combustion is the scientific word for burning. In a combustion reaction, a substance reacts with oxygen from the air and transfers energy as light and heat to the surroundings.
Define Complete Combustion:
Complete Combustion occurs when the reaction of combustion holds a sufficient amount of oxygen to carry out the reaction.
Define Incomplete Combustion:
Incomplete Combustion occurs when the reaction of combustion holds an insufficient amount of oxygen to carry out the reaction. Incomplete Combustion can lead to Carbon Monoxide, which is poisonous to humans.
A hydrocarbon is any class of organic chemicals made up of only the elements hydrogen and carbon. Examples are Methane, Butane, Propane, etc.
State word & balanced chemical equations for complete combustion.
Hydrocarbon + Oxygen → Carbon Dioxide + Water
Hydrocarbon + O2 → CO2 + H2O
State word & balanced chemical equations for incomplete combustion.
Hydrocarbon + Oxygen → Carbon + Water + Carbon Monoxide
Hydrocarbon + O2 → C + H2O + CO
State examples for complete and incomplete combustion
As you can see below in the equation tables, one visible difference is that for complete combustion, you will see CO2, whereas for incomplete combustion you will see CO + C
Examples of Complete Combustion:
Examples of Incomplete Combustion:
Link Combustion Equation to Exothermic Reactions.
Exothermic reactions refer to reactions that release energy, usually in the form of heat or light. The reason why energy is released is that the total energy in the products is less than the total energy in the reactants. Because of this, all combustion reactions are considered exothermic reactions.
Define the term flashpoint; ignition temperature.
Flash Point is the lowest temperature at which a chemical can vaporize itself to a form of ignition mixture in the air.
An ignition temperature of a substance is the lowest (minimum) temperature at which the substance starts combustion.
Explanation about where the energy comes from in chemicals
The energy in the chemicals are stored in the chemical’s bonds, therefore when these bonds are released or formed, energy is released or absorbed.
Define the term exothermic reactions and endothermic reactions while giving examples of each.
Exothermic Reactions are reactions in which heat is released, this causes the surrounding temperature to increase. Endothermic Reactions are reactions in which heat is absorbed by the surrounding heat, which causes the surrounding temperature to decrease.
Examples of Exothermic and Endothermic Reactions:
Labelled Diagrams of Exothermic and Endothermic Reactions.
Define Potential Energy:
Potential Energy is the amount of energy that is stored in an object.
Define Reaction Progress:
Reaction Progress is the timeline of the reaction.
A reactant is a substance that is there in the start of a chemical reaction.
A product is a substance that is created at the end of a chemical reaction.
Define Activation Energy:
Activation Energy is the minimum amount of energy that is required for the reactants to undergo a chemical reaction.
The amount of energy that is released during the reaction.
Enthalpy is defined as the total amount of heat energy possessed by a chemical substance. In the form of equations, enthalpy is written as H
Define Enthalpy Change:
Enthalpy Change is the change in potential energy as a result of bond changes between particles in a chemical or physical reaction. In the form of equations, the change of enthalpy (with reference from above) is written as ΔH
Deduce, from an enthalpy level diagram the sign of the enthalpy change for the reaction.
Identify the importance of energy changes in chemical reactions, the importance of exothermic reactions and how humans use fuel.
Energy change is important as it is present in almost all chemical reactions, therefore finding out the energy changes in a specific reaction can be important to determine if the reaction is exothermic or endothermic.
Many endothermic reactions are used in everyday life such as photosynthesis. They are also useful for forensic sciences, specifically fire and explosion investigation.
Many exothermic reactions used help us significantly, such as burning fuels such as paraffin, coal, propane and butane for energy, because the chemical changes that take place during the reaction release huge amounts of energy, this is used for power and electricity. We depend on those fuels to heat our homes, run our vehicles and provide us with electricity.
Define the terms average bond enthalpy; standard enthalpy change of reaction (∆HѲ)
Average bond enthalpy:
The amount of energy required to break the covalent bonds of 1 mole of gas molecules into gas atoms.
Standard enthalpy change:
When the reaction only occurs in standard temperature and pressure, whereas enthalpy change has no specific constraint.
The relationship between temperature change, enthalpy change and the classification of a reaction as endothermic and exothermic.
When it comes to chemical reactions, it always includes an energy transfer. When it comes to exothermic reaction, it releases energy when the reaction is completed, this is signified by a negative change enthalpy and the temperature change will increase as it releases heat. Endothermic Reactions absorb the surrounding temperature allowing the enthalpy change to be positive and the temperature change to decrease as it absorbs heat.
The specific heat capacity is the definition of the calorie – the amount of energy needed to increase 1 gram of a certain material by 1 degree Celsius. The calorie is equivalent to 4.186 joules, and was specifically made so that water is equal to 1 calorie.
In simple terms, the specific heat of a substance is the number of calories needed to raise the temperature of a 1 gram substance by 1°C.
Define Heat Capacity
Heat capacity in simple terms refers to the amount of heat energy needed to be supplied to a certain mass of a material to increase/decrease its temperature.
Define the Specific Heat Capacity of an object/atom
Specific Heat Capacity is much like heat capacity, except it’s specific to the amount of energy to increase 1 gram of a certain material by 1 degree Celsius.
- For Calories, the unit is often cal/g°C and kcal/g°C
- For Joules, the unit is often J/g°C and kJ/g°C
- For atoms, the unit is often J/mol°C or kJ/mol°C
The equation for Specific Heat Capacity is Q=mcΔT, in which Q represents heat energy, m represents mass of substance, c represents specific heat capacity and Delta T represents change in temperature.
Questions about Specific Heat Capacity:
- How many joules of heat are needed to raise the temperature of 10.0 g of aluminum from 22°C to 55°C, if the specific heat of aluminum is 0.90 J/g°C?
Since the question asks for Q (heat), there is no need to manipulate the equation. We can identify that m (mass) is 10g, the ΔT (temperature change) is 55-22=33, and c (specific heat) is 0.9. All we have to do is just plug our values into the equation as so: Q=10 x 0.9 x 33, and we get our answer of 297J
- What is the specific heat capacity of silver metal if 55.00 g of the metal absorbs 47.3J of heat and the temperature rises 15.0°C?.
Since this question asks for c, we manipulate the equation to make c the subject, by making it c = q/(mΔT). We plug our values into the equation as so: c = 47.3/(55 x 15) and we get our answer of 0.57J/g°C
Evaluate the results of experiments to determine enthalpy changes.
In a simple calorimetry experiment, the mass of the water and the temperature change is already recorded while the specific heat capacity remains constant for water at 4.18 J/g°K. The same equation (Q=mcΔt) can be applied to calculate the heat difference or the enthalpy change of the reaction.
Note: Since combustion is exothermic the sign of the enthalpy change for the fuel must be negative.
Students should be aware of the assumptions made and errors due to heat loss.
Heat loss is quite evidently present in most experiments and 100% efficiency can not be achieved even with numerous precautions such as draught shields and laggings, therefore, these limitations are ignored and it is assumed that there is no heat loss when drawing out conclusions. However, scientists always are mindful that the theoretical values are always inconsistent and significantly lower for theoretical values then standard values, and thus these experiments are only useful for comparison purposes such as determining the best fuel.
Evaluate the use of a calorimeter for measuring the energy transferred.
Energy is lost during the practical from the spirit burner as the process of the calorimetry is not 100% efficient, energy can be lost at various stages such as from the radiation from the spirit burner to the surrounding, or the conduction of heat from the metal to the water can result in energy being diverted from heating the water. As a result, the practical values are always lower than the theoretical values and thus a calorimeter is not accurate although ideal with the given constraints.
Create thermochemical equations for different reactions
Thermochemical equations simply refer to chemical reactions that signify the enthalpy change. Use the knowledge from these revision notes to calculate the enthalpy change and note it with the chemical reaction.
Explain, in terms of average bond enthalpies, why some reactions are exothermic and endothermic reactions.
Bond Enthalpy is the energy required to break a chemical bond or is the measure of bond strength in a chemical bond. This links to endothermic and exothermic reactions as when breaking bonds requires energy and making bonds releases energy. This shows that as a product, if the bonds break it will be an endothermic reaction as it requires energy and when it makes bonds it will be an exothermic reaction as it releases energy.
State Hess’ Law
Hess’s law states that regardless of steps of a reaction, the total enthalpy change for the reaction is the sum of all changes. Therefore, the enthalpy change accompanying a chemical change is independent of the route by which the chemical change occurs.
(e.g. in a normal reactant to product reaction, if it were exothermic, the reactants would go down by -200kJ. If there was an added step, the reactants would go up by 100kJ but go down by -300kJ, thus still equalling -200kJ)
NOTE: everything left of the chemical equation (reactants) is always positive, whereas everything right (products) is always negative in calculated bond enthalpies
Calculate simple enthalpy changes by applying Hess’ Law using bond energies
- Count the number of different bonds for the reactants.
- Find the sum of the energy required to break the bonds of the reactants using bond energies, or simply multiply each type of bond by the bond energy.
- Repeat steps 1-2 for the products.
- The simple Enthalpy change = Energy required for Reactants – Energy released by-products (the unit is the same as the unit for bond energies)
Example of Hess’ Law and Bond Enthalpy Questions:
Structure of the molecules
- Count all the bonds in the molecules diagram:
1(C=C) | 4(C-H) | 1(Br-Br) → 4(C-H) | 1(C-C) | 2 (C-Br)
- Look at the Bond Energies Table and find the elements’ respective Bond Energies:
612 + 4(412) + 193 – 4(412) – 348 – 2(276)
- Find the sum of the bond energies:
ΔH = 612 + 1648 + 193 – 1648 – 348 – 552
ΔH = -95kJ/mol
In relation to Hess’s Law, the thermochemical cycle is a process that signifies what happens during any chemical reaction. It goes from pure reactants, which get broken down into raw elements. Then these elements are reformed into the products
Do Questions with each step of the Thermochemical Cycle:
NH3(g) + HCl (g) → NH4Cl(s)
- First count each atom on the reactant side (if the equation isn’t balanced yet, balance it first)
N = 1 | H = 4 | Cl = 1
- Next, break reactants into separate atoms. Since these are gaseous chemicals, ensure you are aware of the diatomic atoms.
N2(g) | H2(g) | Cl2(g)
- Because these atoms are diatomic, we have to make sure they are equal to the number of atoms on the reactants (refer to step 1). This is done by adding a coefficient to the atoms.
½N2(g) + 2H2(g) + ½Cl2(g)
- Identify the Enthalpy Change of Formation
The enthalpy change of formation is the enthalpy needed in order to form a chemical compound or a set of compounds.
Looking back at the original equation, forming reactants from raw elements would be -46+-92 equalling to -138kJ/mol. Hence, to break the reactants into their individual elements, you would need +138 kJ/mol.
And as the last row of the table signified, to go from the pure elements to the product, you need -314 kJ/mol
- Make your Thermochemical Cycle!
What is Extrapolation
Extrapolation is the process of estimating and finding missing data points on the graph. Extrapolation is useful in events when there is a lack of data in certain points on a graph, and filling it in would reveal a crucial aspect of the data.
An example of this is an Endothermic Graph. In the event that the ΔT is not recorded when the reactants transform into the product, extrapolation can help to find the ΔT.
How to Extrapolate:
The diagram below easily shows how to extrapolate a regular exothermic reaction graph to find ΔT. The black lines represent the exothermic time-temp graph, and the blue lines are the estimation. All that is needed is for you to draw a few lines estimating where the product line would begin, and the reactants line ends.
- Extend the lines backward in a way that it reaches the missing x-axis point (as shown in the blue lines). This should mainly be a line of best fit dependent on only the right side of the graph (products)
- Draw a line upwards from the x-axis at the missing point.
- Your ΔT is the end of your reactants to the start of your estimated product line of best fit.
Calculate the maximum energy produced in chemical reactions
Extrapolation is known as an estimation of a data point out of the range of the data points provided. In the following case when the data is extrapolated backward the maximum energy is obtained where the person can use the trend of the graph to visualize the latent energy this is obviously inaccurate but it simultaneously presents a plausible estimate of where the data points can be located in. This can thus be utilized to find a rough temperature change in order to get an estimated value for the bond enthalpy change.