Questions tagged [heat]

For questions about heat - a form of transfer of energy from one body to another or from one body to its surroundings. Also see these tags: [thermodynamics], [enthalpy], [energy], [temperature].

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23 views

How to calculate the temperature change when CaCl2, NaCl, and water added together at an initial temperature of 298 K?

My question: How to calculate the temperature change when adding $\pu{0.1 kg}$ of $\ce{CaCl2}$ and $\pu{0.1 kg}$ of $\ce{NaCl}$ to $\pu{1 kg}$ water with an initial temperature of $\pu{298 K}$? I ...
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1answer
19 views

Steam rises when the heat supply stops

When water boils and you suddenly shut the heat off, a lot of steam arises from it just after you shut the heat off. Why this happens, I thought that maybe water condenzes very fast in small droplets ...
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Determining heat capacity of gasses

I need a little help on answering a question which popped into my head while I was reading about gas laws: Suppose that I have three canisters, each holding a different gas of unknown identity [...
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60 views

Hess's Law and ΔH

Target equation: $\ce{Mg(s) + 1/2 O2 -> MgO(s)}$ $$ \begin{align} \ce{MgO(s) + 2 HCl(aq) &-> MgCl2(aq) + H2O(l)} &\quad ΔH &= \pu{-1300 kJ} \tag{1}\\ \ce{Mg(s) + 2 HCl(aq) &-&...
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79 views

Why could it be assumed that the standard enthalpy change is independent of temperature down to 25 °C?

For the reversible reaction of pyruvic acid to 2,2-dihydroxypropanoic acid, we calculated enthalpy using NMR and extrapolated from area (integration) and peaks etc. We then plotted $\ln K$ vs $1/T.$ ...
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1answer
53 views

Why does ice get colder when you add salt? [duplicate]

I know that when you put salt on ice it may turn to a slush and it does feel cold. What is the reason for this?
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1answer
55 views

How do I know which alkane has the lowest combustion heat?

I'm asked which one of the alkane has the lowest heat of combustion (note: they question says heat, not enthalpy) and according to the answers, the right one is d. I'm trying to understand why d ...
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1answer
60 views

Can the human body withstand the heat from setting plaster?

There was an incident a couple of years ago where a teenage girl during an art lessen got her hands stuck in a bucket with plaster. As the plaster set, the heat from the chemical process caused her ...
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Why are IUPAC's definitions of exo- and endothermic disconnected from the direction of heat flow?

I'm wondering if anyone can provide a rationale for IUPAC's definitions of exothermic and endothermic, since they disconnect these terms from the direction of heat flow. Specifically, here are IUPAC'...
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57 views

Enthalpy and heat relationship

They say that enthalpy of reaction is the heat when at constant temperature and pressure. So if enthalpy is $$\mathrm dH = \mathrm dU + p\,\mathrm dV = T\,\mathrm dS - p\,\mathrm dV + p\,\mathrm dV + ...
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79 views

Why is the product of the reaction “Bi + O2” Bi2O3 instead of Bi2O5?

I am stuck on a homework question that states: A piece of solid bismuth is heated strongly in oxygen. The answer key states the answer as $\ce{Bi + O2 → Bi2O3}$. However, it is to my understanding ...
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1answer
104 views

Why does boiled water have a different taste as compared to cool water? [closed]

I have noticed it frequently and decided to check with some of my peers and friends about this and they too have felt such a thing. The taste comparison occurs after the boiled water is cooled down to ...
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How to derive this temperature-pressure-specific volume relationship?

(for isentropic, adiabatic, ideal gas flow) $$\frac{T_x}{T_y} = \left(\frac{p_x}{p_y}\right)^{\frac{(\gamma-1)}{\gamma}} = \left(\frac{V_y}{V_x}\right)^{\gamma-1}$$ where $V$ is the specific volume, $...
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126 views

Is it safe to boil water in a foil pan on an electric burner?

Is it safe to boil water in a foil pan on an electric burner? Can the metal leach into the water? I've heard some reports recently about avoiding aluminum foil due to it leaching into food. Photos ...
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What is total enthalpy?

I've seen that it's defined as $h_0 = h + \frac{v^2}{2}$, where $h_0$ is the total enthalpy per unit mass. I can see that the $\frac{v^2}{2}$ term probably has something to do with kinetic energy, but ...
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81 views

Could Calcium Oxide be made from CaCO3 in a bonfire?

I have some chalk and I need to turn it into CaO. Would a big bonfire be hot enough to make it? If anyone has personal experience with this reaction please share, whether succesful or not
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Heating of cyclic dicarboxylic acid [closed]

How do we predict the products formed when we heat cyclic dicarboxylic acids? I know that in case of aliphatic dicarboxylic acids we apply Blanc's rule and predict the product formed based on the ...
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1answer
36 views

How hot will a carbon electrode get in an electrolysis setup?

For the electrolysis of sulfuric acid, I plan to use carbon graphite electrodes at voltages up to 6 V. I would like to know if my carbon electrodes would get considerably hot or warm (45 °C or higher)....
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Exothermic: enthalpy or heat?

I am used to thinking of exothermic reactions as being defined as having negative $ΔH.$ However, I also am used to hearing/thinking of exothermic reactions as "releasing heat". Since change in ...
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3answers
163 views

Why doesn't beryllium, an elemental solid, obey Dulong and Petit's law?

Beryllium has a molar heat capacity of $\pu{17.7 J mol-1 K-1},$ which is quite far from the $\pu{25 J mol-1 K-1}$ as predicted by Dulong and Petit's law. What is the reason for this discrepancy?
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1answer
128 views

How to get the efficiency of a heat engine which undergoes an elliptical cycle?

An ideal diatomic gas undergoes an elliptic cyclic process characterized by the following points in a $PV$ diagram: $$(3/2P_1, V1)$$ $$(2P_1, (V1+V2)/2)$$ $$(3/2P_1, V2)$$ $$(P_1, (V1+V2)/2)...
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26 views

How would a temperature profile in wall change if the rate of heat transfer increases

Given the system below, how would the temperature profile in the wall change if the heat transfer in Fluid II was very fast? If the heat transfer in Fluid II is very fast, then the T-profile ...
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1answer
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Prolonging the exothermic reaction between calcium oxide and water

Is it possible to generate 10-minutes of heat using very small amounts of CaO and water? This is an exothermic reaction. I've done this experiment at home, but it is very short-lived. I'm trying to ...
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1answer
110 views

Negative Molar Specific Heat of a Gas

It is known that the specific heat of a gas is process dependent. So it must be theoretically possible to have a negative value for a gas according to the following equation (for polytropic process, $...
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1answer
93 views

Current and reversible heat in battery reaction

In the book Battery Technology Handbook by Kiehne [1] (Google Books) on page 21, equation (32): The reversible heat effect per time unit can be related to current flow, because each multiple of the ...
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151 views

Energy balance in battery thermodynamics

I am trying to understand the derivation of a general energy balance in battery thermodynamics. The following relation is frequently found to determine the heat generation of a battery: $\dot{Q} = \...
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1answer
41 views

Change in heat of photosynthesis

Consider a plant that uses the energy it receives from the sun to do work (to undergo photosynthesis) to produce glucose. The minimum amount of work this plant can do to produce 1 mole of glucose is ...
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110 views

Why mixing water with ethanol produces heat energy? [closed]

Sorry if I'm asking something stupid but I'm trying to understand where this energy gets from. As far as I could find, it is not a reaction as both chemicals left unchanged, so it might have something ...
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90 views

Calculate the energy in the form of heat (in kJ) required to convert 325 grams of liquid water at 20.0 °C to steam at 115 °C

Calculate the energy in the form of heat (in kJ) required to convert 325 g of liquid water at 20.0 °C to steam at 115 °C. Assume that no energy in the form of heat is transferred to the environment. (...
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Derive expression for average heat flux in heat exchanger

The single-tube heat exchanger is used to increase the temperature of fluid I from $T_{b1}$ to $T_{b2}$. The heating oil (fluid II) used for this task enters at temperature $T_{02}$ and leaves at ...
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Simplifying temperature profile

I am given the temperature profile below and the two equations: $$Q = (2\pi R_0)LU_0(T_{1\infty}-T_{2\infty})\tag1\label1$$ $$\frac{1}{U_0R_0} = \frac{1}{h_1R_0} +\frac{\ln(R_\mathrm a/R_0)}{k}+ ...
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1answer
78 views

Derive expression for heat flow from oil bath to metal sphere

A spherical ball of iron is put into a hot bath of oil. The iron ball is initially at a low temperature and heat transfer from the oil to the iron will increase the temperature of the iron ball. ...
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When a candle burns, why does the top of wick glow if bottom of flame is hottest?

Totally an elementary question. Staring at a candle, it appears that the bottom of the wick is dark whereas the top glows. However the bottom of the flame (the blue) is the hottest. Is the reason ...
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1answer
535 views

Heat of formation of aqueous H⁺

In my book it is given that by convention heat of formation of aqueous $\ce{H+}$ is taken to be zero. Heat of formation is energy released or absorbed when 1 mole of a compound is formed from its ...
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4answers
176 views

How does the room gets heated by leaving a fridge open?

Question: If the door of the refrigerator is kept open, the room in which the refrigerator is kept: gets cooled gets heated neither gets cooled nor gets heated gets cooled or heated ...
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187 views

Is heat just a change in temperature?

According to my chemistry textbook, heat is defined as "Energy that is exchanged because of a difference in temperature or a change in phase." My textbook also says, "Heat is a form of energy. ...
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2answers
126 views

Find out of the friend is getting a good deal [closed]

I have a strange chemistry question that I've never came across before, and I have no idea about how to solve it. Your friend comes across a good deal to purchase a gold ring. She asks you for ...
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1answer
373 views

Are the vibrational modes of CO2 active at room temperature?

I was doing an experiment whereby I had to measure the specific heat ratios of certain gases. Carbon dioxide came out to be around 1.3, and checking with the accepted values this is close. My question ...
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75 views

Determine heat flux from temperature profile

The heating element and the insulator are of equal thickness L. Heat transfer in the air film adjacent to the heater is assumed negligible. I've noticed that I find these type of problem the ...
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2answers
147 views

Heat transfer through layered wall

An electrical heater is placed between two layers of insulation. The three layers (the heater and the two layers of insulation) have the same thickness. The figure below shows different temperature ...
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80 views

Adiabatic flame temperature of a blast furnace gas

Calculate the adiabatic flame temperature of combustion of a blast furnace gas analyzing $25\%$ $\ce{CO}$, $10\%$ $\ce{CO2}$, and rest nitrogen. Use theoretical amount of preheated air at $\pu{1000 °C}...
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148 views

Determine surface area of heat exchange tube

A heat exchanger is used to cool down a stream of $\pu{20 kg s-1}$ of water from $\pu{80 °C}$ to $\pu{60 °C}$. The cooling water enters at a rate of $\pu{12 kg s-1}$ and $\ce{20 °C}$. The overall heat ...
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1answer
74 views

Equation for overall heat transfer coefficient

The heat flow (in $[\pu{J/s}]$) through the wall of a tube of length $L$ and inner radius $R_0$ is $$Q = (2πR_0L)U(T_{1\infty} - T_{2\infty}),$$ where $T_{1\infty}$ is the bulk temperature ...
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Why total heat of hydrogenation of 1,3-cyclohexadiene is more than that of benzene?

I'm told that heat of hydrogenation (HOH) is directly proportional to number of π bonds and inversely proportional to stability. So, is the aromaticity responsible for this? Also, what is the general ...
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164 views

Does dissolving something in water change the specific heat of the solution?

I’m a little confused on this topic. If I have a solid that I dissolve in water and that undergoes an exothermic reaction, can I assume that the specific heat $c$ of the solution instantaneously after ...
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1answer
274 views

Meaning of red heat

I have two reactions below. What is the difference between red heat and normal heat? Is it related to iron as a catalyst when it is red hot?
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79 views

Heat of formation method

For enthalpy calculations why is it that in some cases the "heat of formation method" is used? I know that $ΔH_f$ represents the heat required to form the compound at standard temperature and pressure ...
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2answers
189 views

Is paper aging(oxidation) and paper burning(rapid oxidation) the same thing?

I woke up today with a question of what is fire; looked up the definition and the first response was that fire was rapid oxidation. I was curious what the difference was between rapid oxidation and ...
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1answer
89 views

Is it safe to heat fluorite (CaF2) to 140 °C?

For the manufacture of the optical system, we need to heat the fluorite ($\ce{CaF2}$) lenses to $\pu{140 °C}$. Is it possible: for some poisonous gas to be emitted? ($\ce{F2, HF}$ or other) for ...
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1answer
69 views

Why do we use Δ to mean heating?

To express heating, we usually use $\Delta$ over reaction arrows like \begin{align} \ce{CaCO3 ->[\Delta] CaO +O2}. \end{align} Why do we use $\Delta$ to mean heating during chemical reactions? I ...