Question: What is thermochemistry?
Answer: a branch of chemistry concerned with the quantities of heat released or absorbed during chemical reactions
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Question: What is a system?
Answer: the chemical reaction or physical change being examined
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Question: What are some examples of systems?
Answer: chemical energy in a food chain, energy transfers between reactants and products
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Question: The energy of a system is described in terms of temperature, thermal energy, and heat. Describe temperature.
Answer: Temperature is a measure of the average kinetic energy of the particles in a sample of matter. The greater the average kinetic energy of the particles in a sample of matter, the higher the temperature of that matter.
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Question: The energy of a system is described in terms of temperature, thermal energy, and heat. Describe thermal energy.
Answer: Thermal energy is the measure of the total kinetic energy in a sample. It is different than temperature because it represents all the kinetic energy of every particle available in the system added together.
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Question: The energy of a system is described in terms of temperature, thermal energy, and heat. Describe heat.
Answer: Heat is the transfer of thermal energy from one substance to another due to the temperature difference between the two substances. A sample of matter can have a certain amount of thermal energy, but the matter does not have heat.
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Question: How does heat flow?
Answer: Heat flows spontaneously from matter at a higher temperature to matter at a lower temperature.
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Question: What controls energy flow?
Answer: Energy flow depends on the temperature difference between substances, not on the difference in total thermal energy of the two substances.
This flow of thermal energy will continue until both substances are at the same temperature.
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Question: What are the heat and thermal energy units of measurement?
Answer: joule (J)
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Question: What symbol is used to denote heat?
Answer: q
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Question: What is enthalpy?
Answer: total heat content of a system equal to the internal energy of the system plus the product of pressure and volume
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Question: Chemists use enthalpy instead of heat to describe the energy transfer. Why?
Answer: We are not just interested in the energy being released, but also the affect that energy has on the pressure and the volume of the system we are studying.
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Question: What equation is used to represent enthalpy?
Answer: H = U + P x V
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Question: When heat flows into matter, what happens?
Answer: enthalpy and temperature increase
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Question: When heat flows out of matter, what happens?
Answer: the enthalpy and temperature decrease
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Question: Explain the difference between reactions that increase in energy and those that decrease in energy.
Answer: Reactions that absorb energy from their surroundings, resulting in a net increase in energy and +q, are called endothermic.
Reactions in which there is a release of energy to the surroundings, resulting in a net decrease and −q, are called exothermic.
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Question: The amount of energy absorbed by an endothermic reaction depends on what factors?
Answer: the amount of reactants and the difference in the potential energy of the reactants and products
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Question: List examples of endothermic reactions.
Answer: sweating, baking a cake, photosynthesis
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Question: The amount of energy released by an exothermic reaction depends on what factors?
Answer: the amount of reactants as well as the difference in the potential energy of the reactants and products
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Question: List examples of exothermic reactions.
Answer: coffee cooling, exploding rocket, chemical heat packs
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Question: What controls changes in enthalpy?
Answer: The change of enthalpy (△H) in a system is equal to the heat gained or lost between the system and its surroundings under constant pressure.
When heat is gained in a system, q is positive. When heat is lost from a system, q is negative.
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Question: What causes a negative change in enthalpy?
Answer: In an exothermic reaction, the enthalpy of the reactants, what you begin with, is greater than the enthalpy of the products, what you end up with.
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Question: Describe the exothermic reaction demonstration in the video.
Answer: In an endothermic reaction, the reactants have less enthalpy than the products. In this case, it takes enthalpy to break chemical bonds.
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Question: What causes a positive change in enthalpy?
Answer: tracks the potential energy of a system over the course of a reaction or process
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Question: Describe the endothermic reaction demonstration in the video.
Answer: In an exothermic reaction, the products have less potential energy than the reactants had. This is why the potential energy diagram for an exothermic reaction starts at a higher energy value and ends at a lower energy value.
In an endothermic reaction, the products end up with more stored potential energy than the reactants. Energy is absorbed by the reaction to provide enough energy to form the products.
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Question: What is a potential energy diagram?
Answer: Due to the decrease in enthalpy between reactants and products, potential energy diagrams for endothermic reactions have a positive, or upward, slope that represents a +q or +△H. Conversely, in exothermic reactions, there is a decrease in enthalpy between reactants and products, which creates a negative, or downward, slope on a potential energy diagram and a -q or -△H.
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Question: How do potential energy diagrams differ between endothermic and exothermic reactions?
Answer: the minimum amount of energy colliding particles must have for a chemical reaction to occur
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Question: Compare the slope of the line on a potential energy diagram for an endothermic reaction and an exothermic reaction.
Answer: On a potential energy diagram, the activation energy is represented by the positive _______, or _____, that follows the reactants.
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Question: What is activation energy and why is it needed?
Answer: For a reaction to occur, enough ________ must be provided to the system to meet the ______________ requirement.
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