Exothermic & Endothermic (Edexcel IGCSE Science (Double Award): Chemistry): Flashcards
Exam code: 4SD0
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Define exothermic reaction.
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Cards in this collection (35)
Define exothermic reaction.
An exothermic reaction is a reaction that releases heat energy into the surroundings, causing the temperature of the reaction mixture and its surroundings to increase.
Define endothermic reaction.
An endothermic reaction is a reaction that takes heat energy in from the surroundings, causing the temperature of the reaction mixture and its surroundings to decrease.
Why does the temperature of the surroundings rise during an exothermic reaction?
In an exothermic reaction, heat energy is released into the surroundings. The products have less energy than the reactants, and the difference is transferred as heat, raising the temperature of the surroundings.
True or False?
Combustion reactions are endothermic.
False.
Combustion reactions are always exothermic. They release heat energy into the surroundings, causing the temperature to increase.
Neutralisation reactions always give energy ...........
Neutralisation reactions always give energy out.
What does the law of conservation of energy tell us about chemical reactions?
The law of conservation of energy states that energy cannot be created or destroyed, only transferred. In a chemical reaction, any energy released to the surroundings is equal to the drop in energy content of the products compared to the reactants.
True or False?
Displacement reactions are always exothermic.
False.
Displacement reactions can be either exothermic or endothermic. Unlike combustion and neutralisation, they do not always give out energy.
How can you use temperature data to identify a reaction as exothermic or endothermic?
If the final temperature is higher than the initial temperature, the reaction is exothermic. If the final temperature is lower than the initial temperature, the reaction is endothermic.
Define calorimetry.
Calorimetry is an experimental technique used to measure the heat energy change of a chemical reaction by recording temperature changes in a known mass of substance.
Why is a polystyrene cup used as a calorimeter for reactions in solution?
A polystyrene cup is a good insulator, which minimises heat loss to the surroundings and gives a more accurate measurement of the temperature change.
True or False?
In calorimetry calculations, the density of the solution is assumed to be the same as pure water (1 g/cm3).
True.
This assumption allows the mass of the solution to be calculated directly from its volume, simplifying the use of the formula q = mcΔT.
Define specific heat capacity.
Specific heat capacity is the energy needed to raise the temperature of 1 g of a substance by 1 °C. The specific heat capacity of water is 4.18 J/g/°C.
In the formula q = mcΔT, the ΔT represents the ...........
In the formula q = mcΔT, the ΔT represents the temperature change (in °C).
What are the two main sources of error in a simple combustion calorimetry experiment?
The two main sources of error are heat losses to the surroundings and incomplete combustion of the fuel, both of which cause the measured energy value to be lower than the true value.
True or False?
Simple calorimetry can be used to measure enthalpy changes for dissolving solids in water.
True.
Calorimetry in solution can be used for dissolving, neutralisation and displacement reactions by measuring the temperature change when the substances are mixed in an insulated container.
What are the key steps in a combustion calorimetry experiment?
Measure a fixed volume of water into a copper can and record the initial temperature
Weigh the spirit burner, burn the fuel to heat the water, then record the final temperature and re-weigh the burner
Use q = mcΔT to calculate the energy released
The specific heat capacity of water is .......... J/g/°C.
The specific heat capacity of water is 4.18 J/g/°C.
Define heat energy change (Q).
Heat energy change (Q) is the energy transferred as heat during a chemical reaction, calculated using the formula q = mcΔT and measured in joules (J).
What does each symbol in q = mcΔT represent?
In q = mcΔT: q is the heat energy change (J), m is the mass of the substance being heated (g), c is the specific heat capacity (J/g/°C) and ΔT is the temperature change (°C).
If the temperature of the reaction mixture increases, the reaction is exothermic and Q has a .......... sign.
If the temperature of the reaction mixture increases, the reaction is exothermic and Q has a positive sign.
True or False?
A temperature change of 12 °C is equal to a temperature change of 12 K.
True.
The size of a degree Celsius is identical to the size of a kelvin, so a temperature change in °C is numerically the same as a temperature change in K.
A student heats 100 g of solution using a displacement reaction. The temperature rises by 7.5 °C. Calculate Q in kJ. (c = 4.18 J/g/°C)
q = mcΔT
q = 100 g x 4.18 J/g/°C x 7.5 °C
q = 3135 J
q = -3135 J (exothermic, temperature rose)
q = -3.1 kJ (to 2 s.f.)
Define molar enthalpy change (ΔH).
Molar enthalpy change (ΔH) is the heat energy change per mole of substance reacting, calculated as ΔH = Q/n and measured in kJ/mol.
How do you calculate molar enthalpy change from Q and the number of moles?
Molar enthalpy change is calculated using ΔH = Q/n, where Q is the heat energy change in kJ and n is the number of moles of the limiting reagent. The units of ΔH are kJ/mol.
To convert a heat energy change from J to kJ, you divide by ...........
To convert a heat energy change from J to kJ, you divide by 1000.
True or False?
If the temperature of the water rises during a calorimetry experiment, ΔH is positive.
False.
If temperature rises, the reaction is exothermic and ΔH is negative by convention, reflecting that the system has lost energy to the surroundings.
Why must ΔH be given a negative value when the temperature of the solution increases?
A rising temperature means the reaction releases heat to the surroundings, making it exothermic. By convention, exothermic reactions have a negative ΔH because the system loses energy.
What is the aim of the temperature changes practical using HCl and NaOH?
The aim is to perform a calorimetry study of the reaction between HCl and NaOH to investigate how the volume of acid added affects the temperature change of the mixture.
What is a polystyrene (Styrofoam) calorimeter?
A polystyrene (Styrofoam) calorimeter is an insulated container used in calorimetry experiments to minimise heat loss to the surroundings and improve the accuracy of temperature measurements.
A lid is placed on the calorimeter to prevent ...........
A lid is placed on the calorimeter to prevent heat loss (to the surroundings).
True or False?
The mixture must be stirred after adding the acid to the alkali in the calorimeter.
True.
Stirring ensures the reactants are fully mixed and that an even, accurate maximum temperature is recorded throughout the solution.
What four types of reaction can be investigated using temperature change experiments?
Salts dissolving in water
Neutralisation reactions
Displacement reactions
Combustion reactions
How do you use the results from the HCl/NaOH practical to find the volume of acid that causes the biggest temperature change?
Plot a graph of temperature against volume of acid added and draw a line of best fit. The volume corresponding to the highest temperature on the graph is where the greatest energy change occurs.
The .......... the temperature difference, the more energy is absorbed or released in the reaction.
The larger the temperature difference, the more energy is absorbed or released in the reaction.
True or False?
The temperature change practical method can only be used to study neutralisation reactions.
False.
The method can be used to study a range of reactions including neutralisation, displacement, dissolving solids in water and combustion reactions.
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